Laser Ranging Newsletter of the SLR/LLR Subcommission of the CSTG

International Coordination of Space Techniques for Geodesy and Geodynamics

May 1996

CONTENTS

INTRODUCTION

SUBCOMMISSION NEWS

• Integration of LLR into the Subcommission
• Status of the SLR Network
• Erik Vermaat Leaves DUT
• CSTG SLR/LLR Subcommission Meetings in 1996
• SLR/LLR Subcommission Steering Committee
• SLR/LLR Subcommission Now on the Web

EDITORIALS/OPINIONS

• Prof. Beutler's Comments on the Future of SLR
• New Applications for Lasers

SLR MISSIONS AND PROGRAMS

• New SLR Missions in 1995
• The GFZ-1 Mission
• The ERS-1/2 Missions
• METEOR 2-21/Fizeau and RESURS-O1-3 Tracking
• GPS Tracking Status and Campaign Announcement
• Western Pacific Laser Tracking Network (WPLTN)
• Overview of Upcoming SLR Missions
• Western Pacific Laser Tracking Network Satellite (WPLS)
• The TiPS Mission
• SUNSAT
• Advanced Earth Observing Satellite (ADEOS)

TECHNOLOGY/STATION REPORTS

• Portable Satellite Laser Ranging (PSLR) System
• Starfire Optical Range
• French Transportable Laser Ranging System (FTLRS)
• Saudi Arabian Laser Ranging Observatory (SALRO)
• Status Of The New Zimmerwald SLR Station
• Eyesafe Systems
• MOBLAS Single Operator Automation System (SOAP)

SLR DATA AND ANALYSIS

• Changes in SLR Data Format
• CSR Announces WWW Home Page
• Improvements to the Normal Point Data Management System
• Discontinuation of Full-Rate Data Archiving
• Review of SLR Prediction Data
• New SLR On-Site Normal Point Reports
• Station Height Accuracy
• Review of Satellite Data Use

MEETING SUMMARIES AND ANNOUNCEMENTS

• CSTG SLR Subcommission Meeting (Berne, Switzerland)
• WEGENER Board Meeting (Bologna, Italy)
• WEGENER and LAGEOS-2 Meeting (Porto, Portugal)
• Tenth International Workshop on Laser Ranging Instrumentation

OTHER BUSINESS ITEMS

• Meetings Schedule
• Publications
• Acronyms
• SLR Newsletter Information

INTRODUCTION

This Newsletter is an activity of the Subcommission on Satellite and Lunar Laser Ranging, a subcommission of the CSTG (International Coordination of Space Techniques for Geodesy and Geodynamics). The CSTG has been organized within the IAG, the IUGG, and COSPAR.

The purpose of this Newsletter is to facilitate the dissemination of information within the SLR/LLR community. Manuscripts and suggestions for articles to be published are welcome, as are other suggestions for improvement of the Newsletter.

As described in the first Newsletter (September 1986), the subcommission organization includes several subcommittees. The Newsletter provides a status report for some of the subcommittees. Other reports will be included in subsequent issues.

Please return the last page of this issue to be added to or removed from the mailing list.

SUBCOMMISSION NEWS

INTEGRATION OF LLR INTO THE SUBCOMMISSION

Peter Shelus/University of Texas at Austin
John Degnan/NASA GSFC

For largely historical reasons, lunar laser ranging (LLR) and artificial satellite ranging (SLR) have long been considered to be separate observational and scientific disciplines. This is understandable when one examines the techniques of laser ranging that were being practiced from the 1960s through the early 1980s.

LLR is a child of the NASA Apollo program to land a man on the Moon. The science originally derived therefrom concerned mainly the dynamics of the Earth-Moon system and some of the tenets of general relativity and gravitational theory. The Moon, because of its distance, proved to be an extremely challenging target. Even today, almost 30 years after Apollo 11, only two laser stations are routinely obtaining lunar ranges. The Moon is a trillion times harder to range than TOPEX/Poseidon. Many attempts at LLR enjoyed only marginal results and most ended in failure. Although proving to be an excellent LLR station, budgetary considerations in the late 1980's kept the LURE Observatory in Hawaii from continuing as an LLR site. However, the science being obtained from LLR continues to make the effort worthwhile.

On the other hand, the SLR technique was originally oriented toward probing the gravitational figure of the Earth and the intricacies of atmospheric drag. It was a natural outgrowth of artificial satellite technology. Targets are relatively close and the return signal strength can be quite strong, especially for the lowest artificial satellites like BE-C, STARLETTE, and AJISAI. SLR stations could be built modest in size and, therefore, relatively inexpensively. Some dozens of artificial satellite stations around the world have passed through several generations of evolving technology until we have reached the sub-centimeter stations of the present era.

The two disciplines, LLR and SLR, did overlap somewhat in the early days of operations in their joint quest for precise Earth orientation parameters, plate tectonic activity, accurate station locations, and reference frame ties. And, as higher and higher artificial satellites with retroreflector packages were launched and as SLR stations strove for more robustness and better cost efficiency through lower power lasers and smaller optics, the differences between SLR and LLR began to blur somewhat by the mid-1980s.

Both presently active LLR stations, the McDonald Laser Ranging Station (MLRS) in the mountains of west Texas in the USA and the Observatoire de la Cote d'Azur (OCA) on the Calern Plateau near Grasse, France are each capable of both SLR and LLR. In recent years, data yield has increased; the MLRS has acquired over 430 data points in 1995, a level not seen in Texas since the McDonald 2.7 meter telescope days. With some modification, it is highly probable that several presently operating SLR-only stations could range the Moon. A new SLR station being built at Matera, Italy will have LLR capability. Further, the next generation of SLR stations now on the drawing board, like SLR2000, will use the single photoelectron (SPE) technique extensively. SPE was developed and refined under LLR and later applied to SLR to extract the necessary science in a very low signal-to-noise environment.

This merging of the two laser ranging techniques within the CSTG SLR Subcommission was initially proposed at XXI-th IUGG General Assembly held in Boulder in early 1995 with the formation of the CSTG Subcommission on Satellite and Lunar Laser Ranging (SLR) and Lunar Laser Ranging (LLR). The current chair of this subcommission is John Degnan from NASA/GSFC. LLR was formally integrated into the Subcommission by a vote of members at the December 1995 meeting in Berne. The position of LLR Representative to the Subcommission Steering Committee was created and Peter Shelus, from McDonald Observatory, University of Texas at Austin, was elected to the post by the membership.

STATUS OF THE SLR NETWORK

John Degnan/ NASA GSFC

Figure 1 shows the distribution of permanent and mobile SLR sites contributing to the global data set during the two year period 1994-1995. Five new SLR sites were established during this period including the Starfire Optical Research (SOR) Facility in Albuquerque, New Mexico (USA), Riyadh (Saudi Arabia), Mendeleevo (Russia), Sarapul (Russia), and Beijing (China). New mobile sites were established and occupied at Sofia (Bulgaria) by MTLRS-1, Santiago (Chile) by TLRS-2, and LaGrande (Canada) by TLRS-4. As can be seen in the accompanying table from Carey Noll , 46 international SLR sites tracked a record 46,365 satellite passes of 17 artificial satellites during 1995.

The SLR community is in the process of redistributing a portion of its network to achieve a better global distribution, particularly in the Southern Hemisphere. In collaboration with the Centre National d'Etudes Spatiales (CNES), NASA is actively attempting to establish a new MOBLAS site hosted by the University of the Pacific on the island of Tahiti in French Polynesia by the end of the current calendar year. It is hoped that a similar collaboration can be realized between NASA, the GeoForschungsZentrum (GFZ) in Germany, and the Foundation for Research and Development (FRD) in South Africa to establish a second cooperative MOBLAS site at the Sutherland Observatory north of Capetown. NASA is presently searching for a host country to take over operations of the TLRS-4 system. Italy continues with its development of the state-of-the-art Matera Laser Ranging Observatory (MLRO), while the Institut fur Angewandte Geodasie (IfAG) in Germany has begun integration of its multi-technique TIGO system for a probable field deployment in 1998. If funding permits, the Italian Space Agency hopes to send the TLRS-1 system to Malindi in Kenya and possibly to future sites in the Indian Ocean, while IfAG in Germany is investigating the possibility of sending the MTLRS-1 system to Firuza in Turkmenistan and to other sites in the Orient, notably China. A bilateral agreement is also being pursued to move the Dutch MTLRS-2 system to Indonesia in 1997 for a 1 to 2 year occupation. Following final testing, the new French transportable is expected to occupy the island of Ibiza to the south of Spain to better support oceanographic missions and altimeter calibration activities. The Zimmerwald station in Switzerland has also been undergoing major upgrades, and Polish Academy of Science continues to work toward establishment of a new site in Tunisia. NASA, EUROLAS, and WPLTN are also collaborating with colleagues in the former Soviet Union and China in the upgrade of several key SLR stations on the Asian continent. At the Subcommission meeting in Berne, both Australia and Japan announced active proposals to augment their national SLR networks to as many as four or five stations each by the year 2000. The new stations would be of the "Keystone" type designed by EOS Inc. in Australia. The rapid transmission of high precision normal point data to the IERS Quick-look Analysis Centers at the University of Texas and the University of Delft combined with more comprehensive satellite force orbit models has greatly improved the effectiveness and timeliness of the SLR data quality control function. The weighted fits of LAGEOS-1 and -2 orbits from UTCSR, for example, routinely fall into the one to two centimeter range, and centimeter level biases can be quickly detected and reported back to the stations. Most SLR station positions are determined very well, but a few are still at issue due to both data quality and quantity problems. Unresolved orbital biases indicate that there may be a center-of-mass problem on AJISAI on the order of a few centimeters.

Like most scientific endeavors in recent years, much of the international SLR network was faced with continuing financial pressures during the past year. The NASA SLR network, for example, coped with rather harsh budget cuts on the order of 35% by closing down its remaining transportable operations, introducing more automation into its field and data operations, and eliminating specialized data products as well as engineering programs, reporting, and routine processes of marginal value. In spite of these fiscal pressures, the international network integrated five new satellites into its routine tracking schedules in 1995, successfully supported the launches of GFZ-1 and ERS-2, and set new data records. In addition, some positive developments were spurred on by the financial pressures. These included the adoption and implementation of a common acquisition data set (i.e., "tuned" IRV's), the adoption and implementation of on-site normal points as the sole SLR data product, and an automated data processing and archiving system which gets millimeter quality data into the scientific users' hands within hours instead of months.

Although SLR is facing increased competition from radio techniques (such as GPS, GLONASS, DORIS, and PRARE) for many of its traditional scientific duties, SLR's continuation as an integrated space geodetic technique has received strong endorsements from various international bodies including the Full CSTG Commission , the International Earth Rotation Service (IERS), and the International GPS Service for Geodynamics (IGS). Both the CSTG and IERS recognize the unique role of SLR in defining the origin and scale of the Terrestrial Reference System and LLR in determining the obliquity of the ecliptic and the right ascension origin for the Solar System Dynamical Frame. In a recent draft of the IERS Five Year Strategy, SLR was cited as "required" for position of the geocenter, long-term polar motion, and long term maintenance of the Terrestrial Reference Frame and "useful" for rapid determination of UT1-UTC and tidal variations in Earth Orientation Parameters. The IERS Bulletin "Plans and Needs for 2000" dated May 1995 further recognizes LLR as "the primary source of information for studying the dynamics of the Earth-Moon system" and as a source of near real-time information on UT0-UTC. Because of its high accuracy and unambiguous range data, SLR has also clearly played an important role in the calibration and testing of all of the aforementioned spaceborne radionavigation systems. At its December 1995 meeting, the IGS Governing Board called for increased tracking of the GPS-35 and GPS-36 satellites, and the Subcommission is developing a plan and schedule for an extended GPS campaign later this year, probably in November when GPS is in a night-time status at most sites capable of tracking it. Since returns from GPS are largely at the single photoelectron level for most systems, detection thresholds must be set fairly low or compensating amplifiers used to achieve large rates of return.

Interest in the SLR technique has increased dramatically in recent years. The number of missions supported by the SLR technique continues to grow at a rapid pace, placing additional demands on the international network. Figure 2 shows the recent historical growth in space missions from 1986 and the projected growth through the year 2000. More detail on recent and upcoming missions can be found in the SLR Missions and Programs Section of this newsletter.

With the assistance of Mike Pearlman and various members of the SLR/LLR analysis community, I have put together the accompanying table which traces the technological progress and scientific contributions of the SLR and LLR techniques since their inception in the 1960's. We welcome updates from the SLR/LLR community if we have left anything out.

ERIK VERMAAT LEAVES DUT

Michael Pearlman/SAO

With mixed feelings we received the news that Erik Vermaat will no longer be involved in the world of Space Geodesy and will be moving on to new areas and interests in New Zealand. There was sadness that his friendly collaboration and very valuable contribution to Space Geodesy will be sorely missed, but there was also a tinge of envy at his opportunity to move to exciting new pastures. EUROLAS owes a great debt of gratitude to Erik for his hard work for four years as the founding Secretary of EUROLAS, during which time he was largely responsible establishing EUROLAS as a recognized and viable network participant in the international SLR scene. Erik has also been a very active member of the CSTG SLR Subcommission, WEGENER and almost every other SLR working group for as far back as any of us can remember.

Is anybody thinking about an SLR system in New Zealand?

CSTG SLR/LLR SUBCOMMISSION MEETINGS IN 1996

John Degnan/ NASA GSFC

Two meetings of the SLR/LLR Subcommission are planned for 1996. The first is scheduled for the afternoon of 06 June 1996 in Porto, Portugal following the WEGENER meeting on June 3 through 5 and preceding the LAGEOS-2 Investigators' Meeting on June 7. A second meeting will be held in conjunction with the Tenth International Workshop on Laser Ranging Instrumentation scheduled for November 11-15 in Shanghai, China.

SLR/LLR SUBCOMMISSION STEERING COMMITTEE

Michael Pearlman/SAO
John Degnan/NASA GSFC

The membership of the Steering Committee of the SLR Subcommission is presently made up of:

• United States - 2 members, presently John Degnan/NASA GSFC (chairperson) and Michael Pearlman/SAO (recorder)
• EUROLAS - 3 members, presently Werner Gurtner/AIUB (who replaces Erik Vermaat), Wolfgang Schlueter/IfAG, and Andrew Sinclair/RGO
• China - 1 member, presently Yang Fumin/Shanghai Observatory
• At Large - 3 members (including one member from either Australia or Japan), presently Richard Eanes/UTCSR, John Luck/Orroral Observatory, and Ron Noomen/DUT
• LLR - 1 member, presently Peter Shelus/UT

At the CSTG Subcommission Meeting in Berne, the membership agreed that:

• The current Steering Committee organization should continue.
• Two of the positions, that of China and the at-large member from Australia or Japan, may be reallocated to the WPLTN, provided the CSTG members from China, Japan, and Australia agree.
• Formal term limits for Steering Committee members were not necessary. Each of the regional networks (EUROLAS, US/NASA, WPLTN, LLR) should continue to choose its own representatives and decide when to change.
• Members-at-Large should be elected (or reelected) periodically at the Subcommission meeting held in conjunction with the Laser Workshop.

At the upcoming Laser Workshop in Shanghai we also need to discuss our policy regarding the terms of Chairperson and the Recorder.

SLR/LLR SUBCOMMISSION NOW ON THE WEB

Carey Noll/ NASA GSFC

At the most recent Subcommission meeting in Berne, John Degnan announced plans to create a CSTG SLR/LLR Subcommission Home Page on the World Wide Web (WWW) and that the host computer would be the Crustal Dynamics Data Information System (CDDIS). This home page, shown in Figure 3, has recently been implemented and can be located at URL http://cddis.gsfc.nasa.gov/cstg/cstg_slr.html. The CSTG SLR/LLR Subcommission home page provides up-to-date information about the subcommission, charter, and steering committee, as well as on-line access to previous issues of the newsletter. Furthermore, the SLR/LLR Subcommission home page provides links to related resources, such as the home pages of the CSTG and IUGG, and web pages for SLR networks, systems, and satellites, as well as numerous science applications. Users are encouraged to view this page, make comments, and contribute useful URLs for inclusion in the supporting organizations or SLR systems and applications sections. It is hoped that the creation of a CSTG SLR/LLR Subcommission Home Page will permit more rapid dissemination of information within the SLR/LLR community than can currently be accomplished through newsletters and workshops.

EDITORIALS/OPINIONS

PROF. BEUTLER'S COMMENTS ON THE FUTURE OF SLR

Michael Pearlman/SAO

Prof. Gerhard Beutler (AIUB), the new President of CSTG, briefed the attendees of the CSTG SLR Subcommission meeting in Berne on the new CSTG organization and his philosophy for the next four years. The CSTG has been expanded to include (1) a new Subcommission on Precise Satellite Microwave Systems, chaired by Pascal Willis (France) and (2) a Project on Coordination and Combination of Space Geodetic Techniques to be chaired by Tom Herring (USA).

Prof. Beutler's main theme for the next four years is to provide a higher, broader point of view recognizing that we need to consider space geodetic techniques as an integrated system from the standpoint of both data acquisition and analysis. A fundamental concept within this theme is a Network of Fundamental Geodetic Reference Stations and Station Clusters to provide this integration and as a basic tool for improvement in measurement capability. He stressed the need for SLR to develop a clear concept of a network of cooperating stations, and in particular that stations in close proximity (such as Europe) should work together so that, in effect, they constitute a regional Observatory, with well defined scientific goals, and with collaborative scheduling of observing to ensure best region-wide achievement of these goals. Improvement of data quality was vital, and each region should provide its own effort for data quality control.

At the IUGG meeting in July, a proposal to establish a Fundamental Reference and Calibration Network for Space Geodesy Applications was rejected by the IUGG Executive Committee, in part because the need was not clearly defined. At the Executive Committee Meeting in November, however, authority was given to organize an IUGG/IAG Working Group to develop the scientific and operational/logistical rational for a Network of Fundamental Geodetic Reference Stations and Station Clusters to support and integrate the separate networks. The concept of clusters was introduced to provide intra- and inter-technique redundancy for effective reduction of measurement errors in order to reach 3D mm accuracies. The proposal for the combination of global network and regional clusters attempts to include all participants in order to maximize the geographic coverage and to offer technological and operational assistance to those groups needing help.

Prof. Beutler discussed the role of SLR within the space geodesy complex. He made the following points:

1. All techniques (SLR, GPS, VLBI) have contributed to determination of the x- and y- components of polar motion.
2. SLR and GPS do not presently provide the UT1-UTC series with good long-term stability (beyond 40-50 days), nor do they contribute to the establishment of the celestial reference frame.
3. Time resolution for Earth rotation measurements is getting critical. VLBI and SLR observations are not dense enough. VLBI determinations of polar motion are based on a once per week, 24 hour observation by a five station network. SLR determinations have a resolution of three days. Only GPS provides daily values, but isolation of bias effects with GPS alone will be very difficult.
4. Each of the space techniques has its own incipient error sources which will alias geophysical results. The combination of space geodetic techniques (measurement and analysis) is a very powerful means of understanding and isolating these error sources from geophysical observables. The cluster concept (Europe, WPLTN, etc.) of several stations and several techniques in the same region will give that redundancy.
5. The SLR/LLR Subcommission objectives should be linked with the IGS.
6. SLR is very important for static and changing gravity field modelling. Because of its insensitivity to water vapor and ionospheric effects, SLR also brings a unique capability for height determination.
7. LLR is a very good technique for measurement of UT1; in principle, SLR of adequate density on high satellites should also provide a good estimate of UT1 with better time resolution.
8. The network configuration should be driven by our requirements, including our required system performance.

We need to form a working group to develop the rationale for the Network of Fundamental Geodetic Reference Stations and Station Clusters to provide this system integration and as a basic tool for improvement in measurement capability.

Discussion and Other Contributions Other considerations suggested for inclusion in the plan were: more efficient SLR systems, improved SLR array configurations to support mm accuracy, and improved temporal and geographic coverage.

In support of the Networks concept and SLR participation in CSTG, Dr. Horst Montag (GFZ) discussed the relative strengths and weaknesses of SLR, VLBI, and GPS (plus the other radio tracking techniques) and argued the importance of exploiting the unique advantages of each and the need to use the entire complex of techniques to identify and eliminate the incipient error sources. There is a strong need to organize the SLR tracking to optimize results.

VLBI provides the best UT1 coverage, but adequate data for one day results are not available in general. There is plenty of GPS data, but the GPS force models are not known well enough to rely solely on that technique. GPS data enhanced with SLR would provide better results. Based on the lunar experience, high satellites like ETALON should provide good measurements for UT1. Low inclination satellites are needed for gravity field model development.

NEW APPLICATIONS FOR LASERS

Michael Pearlman/SAO

In this age of very tight budgets, the SLR community has to search for a broader spectrum of customer for our services. John Luck spoke on several new applications for laser ranging stations, including time transfer, LIDAR, and space communications.

In the area of time transfer, applications were split into (1) passive applications such as orbit determination and support for radio measurements (GPS), (2) reflective applications such as time transfer off of AJISAI, and (3) active applications such as the LASSO Experiment. Time transfer requirements are now at the 35 psec level in order to be comparable to current time standards over a period of a few days.

Closely coupled with the time transfer are chronometric applications which include the behavior of clocks in space, including the upcoming Hydrogen Maser Clock (HMC) experiment being built by SAO for flight in the MIR in 1997. Programs under consideration for measurement of fundamental or relativistic parameters include flights around the sun using optical transponders and on-board clocks to measure gravitational effects. The development of good optical transponders for space would greatly increase ranging distance and time transfer capability, with the possibility of making lasers a basic tool for interplanetary measurements.

SLR MISSIONS AND PROGRAMS

NEW SLR MISSIONS IN 1995

John Degnan/NASA GSFC

Five new satellites were added to the operational SLR constellation in 1995 bringing the total number of active satellites to 17. These included GFZ-1 (Germany), ERS-2 (ESA), GLONASS-63 (Russia) and GLONASS-67 (Russia). and RESURS-3 (Russia). A brief summary of each mission follows:

GFZ-1 (Germany) GFZ-1 (COSPAR ID: 8601795) is the first satellite mission designed and funded by the GeoForschungsZentrum Potsdam (GFZ), Germany. The satellite was fabricated in Russia with launch and deployment by the Russian RKK Energia organization. On 9 April 1995, the satellite was launched from Russia on the PROGRESS cargo ship which docked with the MIR Space Station on April 11. At an altitude of 396 Km with an inclination of 51.6 degrees , it is the lowest satellite tracked by the global SLR network and is contributing to our understanding of the Earth's gravity field. The 215 mm diameter ball has 60 retroreflectors imbedded in its surface and weighs 20.630 Kg. The theoretical center of mass correction is 58.5 + 1 mm.

ERS-2 (ESA) ERS-2 is the second of the European Space Agency's (ESA) Earth Remote Sensing Satellites designed to ensure continuity of sea and ice surface measurements begun by ERS-1 in 1991. The satellite was launched on an Ariane rocket on 21 April 1995 into a nominally circular orbit (Perigee: 785 Km, eccentricity: 0.0008; Inclination: 98.5&degree;). The spacecraft payload consists of seven instruments:

1. an active microwave instrument , operating in C-band either as a Synthetic Aperture Radar (SAR) or as a wave and wind scatterometer
2. a radar altimeter
3. a global ozone measurement experiment (GOME)
4. a microwave sounder
5. a Precise Range and Range Rate Equipment (PRARE) which provides additional onboard tracking capability
6. the laser retroreflector array (same as ERS-1: one nadir cube plus eight cubes in a ring)
7. an Along Track Scanning Radiometer (ATSR)

GLONASS-63 and 67 (Russia) The Phase I constellation of radionavigation satellites was established in 1990. Twenty-one satellites make up the operational constellation . Orbit inclination angle is 64.9 degrees; perigee is 19,140 Km. Although all of the Russian GLONASS satellites are equipped with retroreflector arrays, GLONASS-63 and -67 were selected by the Russian Space Agency for international tracking in return for their tracking of the GPS-35 and 36 satellites. The planar reflecting panels are large (120 cm by 120 cm) and contain 396 reflectors. As a result, signal levels are fairly large compared to GPS returns from the same altitude.

RESURS-3 (Russia) The Russian Space Agency (RSA) has requested the international community to track the RESURS-3 satellite. and to provide signal strength information along with ranging data. The satellite uses Fizeau style reflectors similar in design to those used on METEOR-2, and RSA is interested in obtaining engineering data on their performance. To be useful to the engineering analysis, relative signal strengths must be accurately recorded and corrected for any variable attenuation used in the system.

THE GFZ-1 MISSION

Rolf Knig and Franz-Heinrich Massmann/GFZ, D-PAF

On 19 April, 1996, GFZ-1 completed its first year in orbit after about 5700 revolutions. During its first year, GFZ-1 lost about 7 km of height which is less than previously estimated. It may well happen that we will arrive at the nominal lifetime with another three years of operations remaining. The qualification of the GFZ-1 mission at its current altitude for the enhancement of the resolution and the accuracy of the mesoscale part of the gravitational spectrum has been given proof. As the orbit decays through lower altitudes, the mission will become more and more interesting due to the varying resonance regimes coming into effect.

Of the global SLR network of about 45-50 stations, 25 stations have succeeded in tracking GFZ-1. Due to its low altitude of less than 400 km, the passes over the individual stations are very short (3-5 minutes) and the station hardware must be fast enough to follow the satellite continuously. Furthermore, the quality of the orbit predictions is poorer than for ERS-1/2, due to the stronger dependence on the predicted solar and geomagnetic activity and the increased drag at the lower altitude. The tracking statistics clearly show a monthly (see Figure 4 below). There is also a wide range in the number of observed passes, i.e., from 1 to 317 for the period April 95 through April 25, 1996. Stations with extremely good contributions are Potsdam (317 passes) and Herstmonceux (277), followed by Yarragadee (181), Monument Peak (168), Grasse (146), and Graz (134).

THE ERS-1/2 MISSIONS

Franz-Heinrich Massmann/GFZ, D-PAF

The two ESA Remote Sensing Satellites (ERS-1/2) are operating with great success:. After almost five years ERS-1 is still functioning normally. As this considerably exceeds the expected lifetime of two to three years, this performance has to be considered outstanding. Since the end of March 1995, ERS-1 is again flying in the 35 day repeat cycle. Up to 29 April, 1996, ERS-1 has been providing the nominal ESA service. At the end of the ERS Tandem Mission ERS-1 will be deactivated, probably beginning June 1996. The satellite will be reactivated from time to time over Kiruna for checking and maintenance only. The ERS-1 orbit will be maintained as it has been done so far. Some specific tandem SAR campaigns might be planned in the near future. This means that SLR tracking will be discontinued at the beginning of June 1996, but there might be short campaigns were it has to be reactivated in order to allow a precise orbit determination for SAR interferometric applications.

On 21 April, 1995, ERS-2 was successfully launched. ERS-2 was placed into the same 35d repeat orbit as ERS-1, but with a one day offset (later). The commissioning of the SAR and Radar Altimeter was completed by August 1995, and PRARE by January 1996. The GOME commissioning will probably be completed in summer 1996. After some initial problems with the Wind Scatterometer operations, a work-around solution has been found and routine operations will start in May 1996. The ATSR-2 had been working until December 1995 but had to be switched off and there is little hope for any recovery.

While for ERS-1 precise tracking data is available from the global SLR network only, both PRARE range and Doppler data can be used for ERS-2. Despite still some details to be improved in the data processing and modeling, the PRARE data has demonstrated its high quality. The noise of the range measurements is about 2-3 cm (full rate) and 0.6-1.0 cm (15 second normal points), and for the Doppler observations 0.06-0.20 mm/second (full rate) and 0.01-0.02 mm/second (15 second normal points). Today's network consists of about twenty stations with a good global distribution, including a good North/South balance, forming an ideal completion of the SLR network.

Due to the unique opportunity to operate two satellites with similar characteristics in 'tandem', ESA supported a Tandem Mission of nine month duration (August 17, 1995 through May 1996). Special attention is given to the synchronization of the two orbits. In general the ground tracks were kept within 120 to 250 m of each other. The SLR tracking for both ERS satellites is of high intensity and quality, as being reported in GFZ's weekly and monthly SLR reports. Also the ERS-2 SLR tracking yield is the same as for ERS-1, resulting in about 12 passes per day.

The ERS-1 SLR data has been used together with altimeter crossover data to compute an updated Earth gravity model. Data from almost all ERS-1 repeat cycles have been added to the GRIM4-S4 satellite-only model. The resulting new model EGM2b (PGM055) allows for the computation of high quality orbits for the altimetric and SAR interferometric user groups (7-9 cm radially).

METEOR 2-21/FIZEAU AND RESURS-O1-3 TRACKING

Dan Nugent/ATSC

On 31 August, 1993, METEOR 2-21, a Russian made meteorological satellite with three-axis stabilization, was placed into a 930 kilometer, 82.5 degree inclination orbit by a Russian launch vehicle. On 04 November, 1994, RESURS-O1-3, a Russian Earth sensing satellite, was injected into a 640 kilometer, sun-synchronous orbit at a 98 degree inclination. Both of these satellites carry experimental packages designed to measure the speed of light in a moving reference frame, i.e., the Fizeau effect.

The experiment is comprised of two quartz and two hollow retroreflector corner cubes designed and installed so that, to a ground based observer, only one cube is visible at any given moment. The impact of configuration on a satellite laser ranging system's tracking is seen as a variance in the mean receive laser pulse amplitude between the ascending and descending segments of a given pass.

With the primary support requirement for these missions being the collection of receive laser pulse amplitude and given that most stations within the global satellite laser ranging community do not have an absolute receive energy measurement capability, it became imperative that stations maintain a fixed receive path configuration for the duration of each pass. This procedure allows a station to provide a relative receive laser energy reading that can be equated to some measured or calculated reference.

At the request of Dr. Victor Shargorodsky, Dr. John Degnan authorized NASA satellite laser ranging support of the METEOR 2-21 spacecraft in early March 1994. Under the mission name of Fizeau, the first successful pass was obtained by the MOBLAS-7 SLR station at the Goddard Geophysical and Astronomical Observatory in Greenbelt, Maryland on March 12, 1994. As anticipated, the tracking station reported a distinct variance between ascending and descending average return signal strengths.

In late December 1995, again at the request of Dr. Shargorodsky, the NASA satellite laser ranging network commenced support of the RESURS-O1-3 mission. The first laser pulse returns were received on December 25, 1995, again by the MOBLAS-7 station at the Goddard Geophysical and Astronomical Observatory in Greenbelt, Maryland. Yet again, the supporting stations reported an appreciable receive laser energy difference between ascending and descending pass segments.

As of the end of April 1996, with a total of 442 normal points for METEOR 2-21 and 129 normal points for RESURS-O1-3 (as shown in Figure 5), the data analysis and collection efforts continue. Engineering interpretation of the results is being carried out by Dr. Shargorodsky's group in Moscow.

GPS TRACKING STATUS AND CAMPAIGN ANNOUNCEMENT

Erricos Pavlis/University of Maryland
John Degnan/NASA GSFC

Since the launch of the two GPS spacecraft, NAVSTAR 35 and 36, Satellite Laser Ranging stations from the international network have tracked the spacecraft within the available time from their overburdened schedule and according to relatively low CSTG priorities. Nevertheless, analysis of the data and comparison of the SLR orbits with those derived from radiometric (GPS) data has given scientists a reliable evaluation tool. These limited and selective tests have validated the accuracy of the GPS-derived orbits at the decimeter level They have also pointed out the possibility of some minor mis-modeling which is manifested as a very small (~1 cm) but persistent (station-dependent) bias. There are however important scientific and engineering experiments which require more than just sporadic tracking. The majority of the experimental goals can be accomplished with some dedicated tracking from a globally distributed set of stations over a period of time on the order of one or two months. It is desirable to have as many sites around the globe tracking over the same time period as possible, to minimize the effects of site outages due to weather.

At the December 1995 meeting of the CSTG SLR Subcommission in Berne, Switzerland and at a subsequent meeting of the International GPS Service for Geodynamics (IGS) in San Francisco, data analysts interested in validating and improving the GPS orbits called for more laser tracking data to be provided to the user community. In response to this request, Dr. Gerhard Beutler, President of CSTG, asked the SLR Subcommission in January of this year to schedule a special GPS laser tracking campaign to meet some of these needs.

The NASA SLR network is at present on a schedule consisting of ten eight-hour shifts spread over seven days. During the months of October and November of this year, the Naval Research Laboratory's SLR site at the USAF Starfire Optical Range (NRL@SOR) in Albuquerque, New Mexico, will also be operational. This system has demonstrated its capability to track these targets with the same precision as the best NASA systems. Furthermore, since the NRL@SOR system is not obliged to track the same suite of targets as the NASA network, it can dedicate itself to acquiring long, horizon-to-horizon tracking of the GPS spacecraft from this site. We therefore propose that the special GPS tracking campaign coincide with the planned operations of this highly capable, large aperture NRL@SOR system

The NRL@SOR SLR site's geographical location increases the density of sites in the western United States. It is expected that passes of significant length will be tracked simultaneously by SOR in New Mexico and by MLRS in Texas, by Monument Peak and Quincy in California, and Haleakala in Hawaii. Such dense tracking is required, for instance, by those who are interested in the evaluation of the on-board clock performance. The NASA sites and SOR are already equipped with GPS receivers having precisely known positions with respect to the SLR reference marks. The collected data thus become an additional tool for intercomparison of techniques and reference frames. It is further expected that some of the U.S. sites will be hosting DoD receivers to perform common view time transfer experiments.

Signal returns from the GPS satellites are the smallest of any artificial satellite currently tracked. For most systems, single photoelectron sensitivity receivers are required. We recognize that some participating sites will need some lead time to modify, test, evaluate, and/or adjust their systems and procedures in tracking this difficult targets. NASA, for example, is installing a parallel high sensitivity (low detection threshold) receiver in its MOBLAS stations to increase night-time data rates and has improved daylight performance through the use of improved prediction vectors. We are distributing this call as early as possible to invite all international station managers interested in participating in the campaign to inform us of their intent and to provide lead time for these engineering activities. We hope that international systems that have demonstrated a capability to track GPS will contribute heavily to this special two month campaign by turning over, as much as possible, their tracking responsibilities for other satellites to less capable stations in their respective regions.

NASA and NRL will provide technical guidance based on their experience to date in tracking the two spacecraft and make suggestions for adapting the local systems as much as possible to the peculiarities of tracking these targets. We hope that with some small effort from each station, either through the direct tracking of GPS or the offloading of tracking demands of a more capable station, we will be able to complete a successful campaign and amass a useful data set that is long overdue.

For questions concerning network coordination and hardware issues contact John J. Degnan (jjd@ltpmail.gsfc.nasa.gov) and for data analysis and modeling issues Erricos C. Pavlis (epavlis@ltpmail.gsfc.nasa.gov).

WESTERN PACIFIC LASER TRACKING NETWORK (WPLTN)

John Luck/Orroral Observatory

Inauguration

WPLTN was inaugurated on 11 November 1994, in Canberra, Australia following a resolution passed during the WPLS94 Symposium held in conjunction with the 9th International Workshop on Laser Ranging Instrumentation.

The initial member countries were Japan, China, Russia, and Australia. Subsequently, Saudi Arabia has been admitted as a member. Membership is also being considered for some Asian republics with SLR stations. The geographical scope of WPLTN has been defined to encompass longitudes from Moscow, Russia to Wellington, New Zealand.

Founding Resolutions

The WPLS94 Symposium resolved:

1. That the Western Pacific Laser Ranging Network be established with the following initial characteristics, organizations and powers:
   • Initial member countries comprising Japan, China, Russia and Australia, but with all Western Pacific countries eligible to join.
   • An effective establishment date of November 11, 1994.
   • A steering committee comprising two delegates from each participating country, elected at this forum, but to be ratified in writing within 120 days by the appropriate authority in each respective country, and to be subject to replacement by that authority at any time.
   • Working Groups appointed by the Steering Committee, with tasks to include defining network protocols, internal data standards, system configuration requirements, and interface and control protocols.
   • A Secretariat to be located for two years at a time in a participating country, with the initial host country for the Secretariat to be Japan, with the Secretariat functions to be performed by the Communications Research Laboratory (CRL).
2. That the Steering Committee shall be charged with the responsibility of preparing the mission statement and strategic plan of the WPLTN, for ratification by the individual member countries through their appropriate authority. The Steering Committee shall meet as often as necessary.
3. That the WPLTN shall meet not less than annually, and whenever possible in conjunction with the International Workshop on Laser Ranging Instrumentation.
4. That WPLTN tracking priorities be established collectively by member countries, but with no obligation on any national facility to comply with such priorities at the expense of a national priority. WPLTN affiliated stations shall place a high priority on tracking requests from member countries, and in this context will allocate a high priority to the tracking of the Reflector In Space (RIS).
5. That the WPLTN will seek collaboration with the global laser ranging community through organizations such as CSTG, NASA, and EUROLAS.
6. That this Symposium expresses its thanks to colleagues from NASA, EUROLAS, and other organizations for their support in the formation of this Network.
7. That a vote of thanks be registered with the Science and Technology Agency of Japan for organizing and sponsoring this Symposium.

Executive Committee

The Executive Committee is currently:

Mission Statement and Corporate Plan

The following Mission Statement was adopted at the Executive Committee meeting at CRL, Tokyo on February 7, 1995:

"The Western Pacific Laser Tracking Network shall enhance the contribution of laser ranging to space geodesy in the Western Pacific through a total commitment to quality and scientific and engineering excellence."

The following set of goals and strategies was also adopted at that meeting. They are scheduled for review at the next Executive Committee meeting.

Goal: To achieve a critical mass of SLR resources and infrastructure.
Strategies: Act in concert to achieve common program goals; Share resources such as equipment, software, technology and documentation.

Goal: To achieve technical excellence in satellite laser ranging.
Strategies: Maintain a continuous improvement program in equipment and software; Devise and promote new laser measurement techniques; Promulgate standards for calibration, process control, documentation and performance.

Goal: To promote scientific excellence in space geodesy.
Strategies: Organize and coordinate measurement campaigns for key regional phenomena; Establish regional analysis centers for the design and analysis of regional campaigns.

Goal: To enhance international goodwill and mutual understanding.
Strategies: Promote staff exchange between member nations; Initiate joint development programs involving multi-national teams; Priority support for individual national priorities of member nations.

Current Activities

Activities arising from resolutions passed by the Executive Committee meeting hosted by RISDE in Moscow, December 2-3, 1995 include:

1. Analysis Center Comparisons. A Working Group chaired by Dr. Ramesh Govind of AUSLIG has been set up to undertake a program of intercomparison of results provided by analysis centers within WPLTN countries, using common data sets and comparing analysis outcomes such as range biases and station coordinates. Its stated objectives are to "establish the status of current SLR data analysis capabilities of WPLTN members for future developments and enhancements that may be required ultimately meet current world standards"; and to "establish the quality of performance of stations in WPLTN resulting from the above analysis".

   The common data sets chosen are the normal points for LAGEOS-I, LAGEOS-II, AJISAI, and STARLETTE from the global network stored in the CDDIS, for September 1995.

   The participating analysis centers are in AUSLIG, Australia; CRL, Japan; JHD, Japan: Shanghai Astronomical Observatory, China; and Mission Control Center, Moscow, Russia.

   The project will compare the solutions for orbit trajectories, station coordinates, Earth orientation parameters and measurement biases. The report is due at the WPLTN Executive Committee meeting to be held in Moscow on June, 2-4 1996, and will address the several computational procedures used for observable modelling, orbit modelling, station modelling, parameter estimation including how often, and constraints and correlations between parameters.

2. Standardization and Reporting. A Working Group chaired by Dr John Luck of AUSLIG has been set up to standardize the reporting of station performance statistics and to standardize station calibration techniques. Preliminary plans, based on data in the style of NASA's Laser Operations Reports and on presentations on the performance of Orroral, have been circulated to some of the stations for comment.
3. Satellite Launch Program. The meeting resolved that WPLTN shall have its own five-year rolling satellite program to meet the special needs of the Network. A separate article on the first satellite, formerly informally designated as WPLTN-1 but currently called WPLS (for Western Pacific Laser tracking network Satellite) has been prepared for inclusion in this Newsletter.
4. GLONASS Ranging. The WPLTN shall increase the priority of ranging to GLONASS satellites, as well as to "Fizeau" equipped satellites, during the first half of 1996.
5. WPLTN Information. Minutes of WPLTN Executive Committee meetings will be provided to CSTG, EUROLAS, and NASA, in draft form when necessary.
6. Globalizing SLR Coverage. Noting the poor coverage of SLR stations in South America and Africa, the Executive Committee endorses the efforts of other networks and agencies to add stations to these areas.

Relationships with Other Organizations

The WPLTN founding resolutions, and subsequent Executive Committee meetings, have strongly emphasized the need for WPLTN to collaborate with other organizations, with CSTG, EUROLAS, and the NASA network being mentioned specifically.

The CSTG SLR Subcommission has responded by agreeing, at its meeting in Berne, Switzerland on December 5-7, 1995, that: "Two of the Steering Committee positions, that of China and the at-large member from Australia or Japan, may be reallocated to the WPLTN, provided the CSTG members from China, Australia and Japan agree."

OVERVIEW OF UPCOMING SLR MISSIONS

John Degnan/NASA GSFC
Scott Wetzel, Julie Horvath, Alan Murdoch/ATSC

An additional six SLR satellites are expected to be launched by early 1997 increasing the active satellite constellation by 33%, the largest growth ever in a single year. A brief description of each satellite is provided in the following paragraphs. Additional information is provided in subsequent articles on the individual satellites in this newsletter. The following information on launch dates and orbit characteristics represents our best knowledge as of this writing. If new developments have occurred or requirements have changed, please provide the updated information to Scott Wetzel at AlliedSignal Technical Services Corporation (ATSC) via email at WetzelS@thorin.atsc.allied.com.

Tether Physics and Survivability (TiPS) Experiment

Various groups within the United States are actively preparing for the launch of the Tether Physics and Survivability (TiPS) mission in late June 1996. The experiment, developed by the Naval Research Laboratory (NRL) in the United States, has retroreflector arrays mounted on two spacecraft linked by a four kilometer long tether. The individual spacecraft are named "Ralph" and "Norton" after two characters in a popular American TV series, "The Honeymooners", from the 1950's. SLR range data will be used to study tether dynamics, gravity gradient effects, and the longevity of the tether in the space environment. NASA is modifying its field operational and analysis software to handle the acquisition and tracking of the coupled satellites and will organize the postlaunch tracking support of the international SLR network. The tuned IRV's distributed by NASA will be compatible with existing operational software at the stations. The project is hoping to obtain video coverage from stations during various critical phases of the experiment. Optical magnitude of the tether is expected to be between five and six. (See the June 1995 SLR Newsletter for more information.)

Advanced Earth Observing Satellite (ADEOS)
Retroreflector in Space (RIS)

In August 1996, the ADEOS remote sensing satellite will be launched by an H-2 rocket from the Japanese Space Agency's launch facility on Tangashima Island in Southern Japan with several international oceanic and atmospheric sensors onboard. Scientists expect ADEOS to monitor global ozone density, including polar concentrations, and contribute to our understanding of global warming. The expected lifetime is about three years based on fuel depletion projections. One Japanese experiment, the Retroreflector in Space (RIS), is a single hollow retroreflector, 50 cm in diameter, which will support international experimenters using ground-based LIDAR systems to measure total column content of trace atmospheric species. The cube contains two flat and one curved surface to partly compensate for velocity aberrations. Since the atmospheric LIDAR beams are extremely narrow (measured in arcseconds), easy acquisition of the ADEOS satellite cannot be assured without regular tracking by the international SLR network and the generation of highly accurate orbit acquisition data in support of the SLR and LIDAR communities. In addition, international SLR research groups will use the RIS to evaluate and optimize the next generation of two color laser ranging systems which promise millimeter absolute accuracy range measurements.

GEOSAT Follow-On (GFO-1)

Following the enormous success of TOPEX/POSEIDON, the U.S. Navy's newest GEOSAT oceanographic satellite (GFO-1) will carry an onboard laser retroreflector at the request of the oceanographic community and will be tracked by the international SLR network. SLR will be used to improve the satellite orbit and to calibrate the onboard microwave altimeter and GPS receiver. The target array is being provided by NASA/GSFC and is similar in design to the ERS-1 target.

SUNSAT

The SUNSAT remote sensing mission is sponsored by the Stellenbosch University located in Stellenbosch, South Africa. SUNSAT and Denmark's RSTED magnetic research satellite will be secondary payloads on the Delta II P91-1 mission on 6 March 1997. After the USAF's Argos satellite is released into a 840 km high sun-synchronous orbit, depletion burns will lower the second stage's perigee to 450 km before SUNSAT and RSTED are released simultaneously. The elliptic orbit will drift an hour earlier every seventy days as determined by RSTED's prime mission, which is to map the Earth's magnetic field. The satellite will carry a retroreflector array provided by GSFC as well as the latest GPS TurboRogue flight receiver, developed by JPL to better cope with the negative effects of Selective Availability (SA) and Anti-Spoofing (AS). The laser array is expected to consist of a single ring of eight reflectors located on the tip mass at the end of a two meter magnetometer boom attached to the main spacecraft. As a result, the spacecraft body will block the reflectors and inhibit satellite acquisition at elevation angles above approximately 80 degrees. Besides contributing to ongoing gravity field studies, the experiment will provide an additional intercomparison of laser-derived orbits with the latest differential-mode dual wavelength GPS flight receiver from JPL. Unfortunately, the lack of collocation between the laser array and GPS antenna (which is situated on the main body of the SUNSAT spacecraft) and the flexure of the boom will most likely compromise the quality of the intercomparison.

UoSAT-12

The University of Surrey in England is looking into a possible launch by the Russian Space Agency of UoSAT-12. The proposed payload would carry a single frequency C/A code GPS receiver and a laser array provided by CNES in France. Orbital parameters will be defined once the host mission has been identified, but the altitude and inclination are expected to be in the ranges 600 to 1200 Km and 50 to 80 degrees respectively. Onboard propulsion systems will permit changes in the semi-major axis and slight changes in inclination to examine resonances in the gravity field. The mission and launch date is still in the definition stage.

Western Pacific Laser Satellite (WPLS)

At the last subcommission meeting in Berne, Switzerland, Ben Greene reported that a passive geodetic satellite, similar in design to GFZ-1, will be fabricated and launched into a 835 Km high sun-synchronous orbit by the Russian Space Agency, piggy-backed with the RESURS-O1-4 launch. The WPLS satellite is the first space initiative of the recently formed Western Pacific Laser Tracking Network (WPLTN) which presently coordinates the activities of SLR stations in China, Japan, Australia, and Eastern Russia. The retroreflector design will utilize the Fizeau effect to correct for velocity aberration effects instead of normal dihedral angle spoiling techniques. The theoretical and experimental aspects of the unique optical design were reported on by Victor Shargorodsky et al at the last two International Workshops on Laser Ranging Instrumentation and have been tested on the METEOR-2 and RESURS-3 missions. The reflectors will be recessed in their holders to limit their individual fields-of-view thereby resulting in single cube returns at most orientations of the satellite. This will largely preserve the inherent temporal waveform properties of the incoming laser pulse for two-color and other millimeter accuracy range investigations.

Other SLR missions in the pipeline include the Hydrogen Maser Experiment to be flown on the Russian MIR Space Station in September 1997 and the GFZ Champ mission scheduled for launch in 1998.

WESTERN PACIFIC LASER TRACKING NETWORK SATELLITE (WPLS)

John Luck/Orroral Observatory

At the Western Pacific Laser Tracking Network (WPLTN) Executive Committee meeting in Moscow on December 2, 1995, it was announced by Ben Greene that Electro Optic Systems (EOS) has entered into a joint project with the Russian Space Agency (RSA) to construct and launch a new SLR satellite designed to overcome the limitations of all present targets in relation to millimeter geodesy. Its main immediate purpose is to provide the best possible target for the WPLTN stations and, by implication, the KeyStone Project being constructed by Communications Research Laboratory (CRL) of the Japanese Ministry of Posts and Telecommunications.

Initially, this satellite was referred to as WPLTN-1. It is currently designated WPLS, for Western Pacific Laser tracking network Satellite.

Features

The distinguishing features of WPLS include:

• Only a single corner-cube will reflect on any shot; in fact on average only 0.7 cubes will be active.
• Its response will be optimized for 1.54 micron wavelength, to provide for fully eye safe ranging at any power.
• A new process is being developed to obtain the center-of-mass correction with 0.5 mm accuracy.
• The corner-cube design and material will assume the Fizeau Effect which, if real, will decrease return signal levels dramatically at 532 nm wavelength.

The Satellite

WPLS will be spherical, and generally similar to, although different from, GFZ-1. The retroreflectors will be replaced by 'Fizeau' corner-cubes of refractive index 1.46 made by Prof. Victor Shargarodsky's group in the Russian Institute of Space Device Engineering (RISDE). Each cube will be recessed so that on average 0.7 cubes will retroreflect on any given shot. Design parameters are:

• Diameter: 23.5 cm
• Weight: 24.5 kg
• Number of cubes: 60
• Cube aperture: 28.2 mm
• Center of mass precision: less than 0.5 mm (rms) wrt equivalent reflection plane

Reflection Characteristics

Based on a 'standard' 0.5 meter ground station transmitting 50 mJ pulses, the following return signal strengths may be expected on the assumptions (a) that the "Fizeau Effect" is real and (b) that it is not.

Orbit

The orbit is determined by the availability of launch scenarios. The probable parameters are:

• Altitude: 835 km
• Inclination: 98 degrees, sun-synchronous
• Eccentricity: Circular
• Launch Vehicle: Piggyback with RESURS-O1-4
• Launch Date: Probably First Quarter, 1997

Purposes

The primary purpose, as a very small, dense, spherical, singly-reflecting satellite with a very well-determined center-of-mass correction, in an orbit above most of the atmosphere, is to act as a target with intrinsically very stable range biases for calibrating ground station biases and satellite signatures. It will operate best at eyesafe ranging wavelengths.

WPLS will also be an ideal target for determining medium length baselines using simultaneous range difference and synchronous ranging techniques.

It will verify indisputably the existence or otherwise of the Fizeau effect.

THE TIPS MISSION

Scott Wetzel, Julie Horvath, Paul Stevens, Alan Murdoch/ATSC

The Tether Physics and Survivability (TiPS) satellite was designed and built at the United States Naval Research Laboratory (NRL) located in Washington, D.C. The TiPS mission objective is to study the physics of space tethered systems and to understand survivability of tethers in the space environment. The fully deployed TiPS satellite is comprised of two end bodies connected by a tether that is four kilometers in length, and 2-3 mm in diameter. Each end mass has 18 retroreflectors. The retroreflectors on the larger end mass are coated to reject infrared wavelengths (1064 nm) which will aid in the identification of the individual tip masses in the event they become inverted relative to their normal orientation. The TiPS orbit will be a circular orbit, approximately 1022 km in altitude and 63.4 degrees inclination. TiPS is scheduled for jettison into orbit in June 1996 on a one year mission where both SLR and video recording of the tether dynamics will be monitored.

The role of SLR for TiPS is to provide support of all phases of the mission. These phases are: pre-jettison operations; post-jettison/pre-separation operations; separation operations; post-separation operations; and anomaly operations. SLR will range to each end mass to provide precise position data on each end body and the center of mass (CM). Software upgrades made to the acquisition system will enable the SLR stations to point to the end masses of TiPS as accurately as any other SLR satellite without additional operator interaction. A number of stations will also provide video recording data during critical periods of the mission. This will give a visual image of the separation and tether dynamics of the satellite. In addition, SLR data will be compared against the models for the librations of the end bodies about the system CM and the orbital motion model for the CM about the Earth.

Contribution for the mission is being provided by a large number of organizations in the U.S., Europe, and Australia. TiPS SLR data will be made available on the NASA CDDIS.

SUNSAT

Garth Milne and Sias Mostert/Stellenbosch University John Degnan/NASA GSFC

Introduction

Satellite construction activities at Stellenbosch University, 50 km east of Cape Town in South Africa, are in high gear in preparation for the launch of its 61 kg, 45 x 45 x 52 cm. SUNSAT micro-satellite. Data from the satellite could be of considerable interest to the geodynamics community.

SUNSAT and Denmark's RSTED magnetic research satellite will be secondary payloads on the Delta II P91-1 mission on March 6, 1997. After the USAF's Argos satellite is released into a 840 km high sun-synchronous orbit, depletion burns will lower the second stage's perigee to 450 km before SUNSAT and RSTED are released simultaneously. The elliptic orbit will drift an hour earlier every seventy days as determined by RSTED's prime mission, which is to map the Earth's magnetic field.

Background

SUNSAT is Stellenbosch's first satellite, and started as an unfunded dream of Professors Jan du Plessis, Garth Milne, and Arnold Schoonwinkel of the Computer and Control Group in the EE Department. Hardware design started in January 1992 with fifteen masters students, supported by lecturers. The students have graduated and been succeeded by the present team of nearly thirty students in what has now become the Electronic Systems Laboratory in the Department.

Student work on SUNSAT has been supported by numerous engineering companies in South Africa. They encourage the goal of equipping graduate students with practical skills in addition to the theoretical course knowledge of the Masters of Engineering program. Matching funds from the South African Foundation for Research Development, surplus from other research contracts, and scrounged components, facilities, and workshop hours have brought SUNSAT close to completion of flight hardware manufacture in April 1996.

In 1993, the Ariane ASAP launch deposit for the Helios mission could not be raised, and SUNSAT was on stream to become a 'terrestrial satellite'. John LaBrecque of NASA's Geodynamics Group recognized the possibility of using SUNSAT to carry a TurboRogue GPS receiver, and possibly laser reflectors for their research program. The TurboRogue was easily incorporated in an additional tray, and eight laser reflectors were added to 'crown' the gravity gradient boom's tip mass. As an additional challenge, Sias Mostert, SUNSAT's development manager, managed to add a second magnetometer (Skymag) and star camera (Skycam) to the tip mass, opening interesting research possibilities, particularly when paired with RSTED's data.

SUNSAT's Original Mission

Apart from its prime function as a learning experience and carrier for NASA experiments, SUNSAT has two main technical goals - Amateur Radio communication, and Earth imaging.

An Amateur Radio payload supports store-and-forward communications using AMSAT PACSAT protocols at 9600 baud. A 'parrot' which stores and re-transmits audio messages uplinked in the 145 Mhz Amateur band is part of a program of the South African Radio League to stimulate technical interest in children. A 1200 baud AFSK BBS capability will also enable Amateurs only equipped for 2m packet operation to try satellites.

To contribute some new experience and performance to the university microsatellite community, a pushbroom imager is included as the main science experiment. This places challenging requirements on satellite downlinks and the attitude control system. The imaging system was designed for a circular sun-synchronous imaging orbit (with Helios). The RSTED-dictated orbit will drift through the sun-synchronous orbit locations, permitting imaging performance to be evaluated.

Satellite Configuration

To keep costs and technical risks to a minimum, a gravity gradient stabilized configuration similar to the UoSats of the University of Surrey was chosen. The same layered box construction concept is used, with fixed solar panels (31 W each) on the four vertical faces of the 'box'. The 2.4 m folding gravity boom extends from the top of the satellite, and the imager looks out of the bottom (see Figure 6, SUNSAT in its Deployed State).

Attitude Control

The UoSats maintain adequate attitude control for communication and wide-field imaging using only a magnetometer and electromagnetic torquers. SUNSAT adds four small electric reaction wheels for the more precise aiming needed for the pushbroom imager (1km goal). The university-built wheels will only be used when imaging, thereby reducing the total revolutions required from the vacuum-operating bearings. By counting wheel revolutions, short-term commanded rotation of the satellite can be accurately determined without gyroscopes. For precise attitude determination during imaging, the visible Earth horizon and the sun are observed by three light sensors using 2048-element linear CCD arrays.

Imager

The imager concept can produce same-pass stereo images and side-view images. The optical tube assembly contains a 45 degree mirror, and an imaging group producing 15m-wide pixels from 800 km, a penta-prism with dichroic color splitter, and three 3456 pixel linear CCD sensors. The 10 cm aperture optics is being developed by the Lasers and Optics Group of the CSIR (South African Council for Scientific and Industrial Research.)

In the stereo imaging configuration, the satellite moves with the optical tube horizontal and normal to the velocity vector. Forward or rearward views inclined up to 24 degrees from nadir are obtained by rotating the optical tube.

For left/right viewing, the satellite is oriented with the optical tube parallel to the velocity vector. Rotating the tube quickly directs the imager at scenes to the side of the track.

Computers and Downlink.

SUNSAT includes redundant flight control computers, both of which can initiate capture and storage of images in the 64 Mbyte RAM disk. Data can be downlinked later, or in real time to produce continuous image strips when within range of a ground station. The 8W EIRP S-band downlink can operate at up to 40~Mbit/s and produces 14.3 dB E/No at 2000 km slant range for the 4.5 m diameter antenna.

Geoscience Mission of SUNSAT

The SUNSAT mission will also carry the latest flight TurboRogue GPS receiver from the Jet Propulsion Laboratory (JPL). The receiver is designed to overcome many of the Anti-Spoofing difficulties experienced by an earlier version of the receiver flown on the Joint U.S./French TOPEX/POSEIDON oceanographic mission. Because it was not possible to include a nadir viewing laser array on the bottom of the spacecraft, NASA Goddard Space Flight Center is providing eight encapsulated retroreflectors to be integrated by Stellenbosch into the tip mass assembly at the end of the magnetometer boom, as shown in Figure 7. The retroreflectors will be inserted in a single ring configuration azimuthally about the tip mass, and, as a result, the spacecraft body will block laser acquisitions at elevation angles above about 80 degrees. Nevertheless, SUNSAT should permit SLR to provide some meaningful calibration and performance data on the newest TurboRogue receiver as well as additional data useful in the further refinement of the Earth's gravity field.

Conclusion

SUNSAT is a complex satellite for a first attempt, and is likely to teach as much in-orbit as we have had to learn to date. The SUNSAT team is committed to making the most of in-orbit operations, and to contribute maximally to the community that has made its launch possible. A paper on SUNSAT in the 1993 Utah Conference on Small Satellites, and the Web page ESL.EE.SUN.AC.ZA give more information on SUNSAT, (but it receives less effort than completing SUNSAT!).

SUNSAT has originated through friendship, faith, and motivational power of many engineers and scientists in South Africa. The launch and geoscience mission has resulted from new friends in NASA's MTPE and OLS Projects and associated organizations being prepared to take controlled and managed risks that a new satellite group in a far-off land could provide worthwhile interaction. The preparedness of all these professionals to support university activities of this nature motivates students enormously, and is a clear illustration of the spirit of venture and creativity typical of many that make science and engineering their career.

ADVANCED EARTH OBSERVING SATELLITE (ADEOS)

Julie Horvath, Scott Wetzel, Paul Stevens, Alan Murdoch/ATSC

The global Satellite Laser Ranging (SLR) community and the National Space Development Agency of Japan (NASDA) have had a long and productive relationship as partners in the AJISAI satellite mission. Building on this relationship, the SLR community and NASDA will collaborate on a new satellite mission called the Advanced Earth Observing Satellite (ADEOS). ADEOS will carry eight international remote sensing experiments including the Retroreflector In Space (RIS).

RIS is a new design hollow cube-corner retroreflector developed to optimize the pattern of the reflected beam. Two of the mirrors of the retroreflector are flat, while the third surface is curved to compensate for velocity aberration. The RIS was developed by the Japanese Environmental Agency (JEA) to be used for laser long-path absorption measurements of atmospheric trace species and to support two color laser ranging experiments. The retroreflector will also be used for precise range measurements to ADEOS for orbit prediction and determination in support of the atmospheric lidar and laser ranging communities.

The ADEOS mission is scheduled for a August 1996 launch. The ADEOS orbit will be a sun synchronous sub-recurrent orbit with a recurrent period of 41 days. The orbital altitude will be approximately 800 km with an inclination of 98.6 degrees.

SLR will provide ranging data to NASDA for the ADEOS/RIS mission for the generation of the precise orbit predictions necessary to acquire the satellite with narrow laser beams. Due to the orientation of the RIS on the spacecraft, SLR returns from the RIS can only be expected from passes that start in the northwest, north, or northeast and ending in the west, southwest, or south. Returns are also expected for passes that start in the south, southeast or east and end in the northwest, north, or northeast. Of these passes, returns can only be expected from the first half of the pass due to the orientation of the RIS on the satellite (see figures below, Skyplot of Expected Satellite Positions where SLR Returns can be Anticipated at -35&degree; Elevation and Skyplot of Expected Satellite Positions where SLR Returns can be Anticipated at 50&degree; Elevation)

TECHNOLOGY/STATION REPORTS

PORTABLE SATELLITE LASER RANGING (PSLR) SYSTEM

Owen West/SLR Research Pty. Ltd. Janis Balodis/Institute for Geodesy and Geoinformation, U. of Latvia

A Portable Satellite Laser Ranging (PSLR) system, shown in Figure 10, has been constructed and has been tested at Yarragadee, Western Australia, at the MOBLAS-5 site. Atmospheric temperatures within the range +7&degree; to +47&degree; C have been experienced. Four measurement campaigns have been conducted during a one year period commencing in May 1995 and ending in April 1996. The main objective of these campaigns has been to verify the PSLR telescope mount together with its opto-mechanical components, electronics, computer software and GPS steered timing system. Low performance laser and detector systems have been employed on the prototype instrument in the interest of economy and consequently high accuracy and productivity of the system have not been the primary objectives at this stage.

The design of the PSLR has allowed the minimum size and weight for a system which features a 62 cm primary mirror carried on an alt-azimuth mount, driven by stepper motors. The entire system, including the mount drive, angular encoders, laser fire control, gating control, rotating mirror control, range, time, and meteorological interfaces, as well as standardized software including predictions, data processing, normal point computation, reporting, communications, etc., is controlled by a single IBM/i486 compatible PC. The control electronics are incorporated in the PC as extension PCBs. The total weight of the tripod mounted system is less than 100 kg and it can be dismounted into modules and transported in standard cases by almost any vehicle including small cars and light aircraft.

Some data from the PSLR field test results are shown in Table 4 below where the observed satellite passes and number of obtained normal points are shown, as well as the mean internal accuracy (in cm) of the single shot RMS and normal point RMS. The ranging results have been forwarded to NASA for collocation analysis.

Satellites higher than LAGEOS have not been ranged due to the limitations imposed by the emitter/detector systems currently employed. Further improvements of the PSLR are scheduled to take place over the next three months with a further option to upgrade the emitter/detector components prior to deploying the system for operational applications.

STARFIRE OPTICAL RANGE

Charmaine Gilbreath/NRL

In a joint effort, the Naval Research Laboratory (NRL), the USAF Phillips Laboratory (PL), and NASA have established a new Satellite Laser Ranging (SLR) station at the USAF Starfire Optical Range (SOR) in Albuquerque, New Mexico. This powerful new system presents an energy/receiver aperture-to-area Figure-of-Merit of 2886 mJ-m2. Returns have been obtained from satellites as low as 400 km and as high as 20,000 km overhead (NAVSTAR GPS).

NRL designed, integrated, and operates the transmit/receive system at SOR. The USAF designed the optical train through the 3.5 meter telescope and provides tracking and acquisition for the effort. The processing and analysis of scientific data are performed by NRL. NASA provides technical assistance as required.

The new capability has been developed for precise position determination of satellites and for onboard spacecraft system performance verification. This is an experimental facility and runs in a "campaign" mode. Specifically, data on geodetic satellites will be obtained at intermittent times of the year coincident with special experiments.

The system is comprised of a 10 Hz, 300 mJ doubled Nd-YAG laser, with a 250 ps pulse width. The beam divergence is on the order of 70 to 100 microradians. Polarization aperture-sharing enables a monostatic Transmit/Receive configuration on the 3.5 meter telescope facility.

Non-terminator assisted and daytime ranging can be done; however, we are limited to ranging 1.5 hours before sunset to 1.5 hours after sunrise at this time due to direct sunlight restrictions on the telescope itself.

The 3.5 meter telescope combined with the Micro-Channel Plate-enhanced photomultiplier tube used for photon detection, and 300 mJ of energy enables the system to obtain returns from enhanced spacecraft at 22,000 km or higher.

The receiver subsystem also has 1 GHz and 4 GHz oscilloscopes for waveform detection and signature analysis. A GPS-steered Rubidium clock serves as the master clock for the telescope and the Tx/Rx system, and serves as the timing source of the externally-triggered laser.

Officially, the site has been assigned 7884 for a pad marker, 66 for the SLR station ID, occupation 01. The reference point on the 3.5 Meter telescope is:

• x: -1483442.808 (m)
• y: -5019625.640 (m)
• z : 3635692.076 (m)

This position was derived from range data obtained from LAGEOS-1 and is in ITRF-95 epoched 49875.

During May and June 1995, the U.S. Naval Research Laboratory conducted SLR operations at SOR on the satellites LAGEOS-1, LAGEOS-2, TOPEX, GPS-36, ETALON-1, ETALON-2 and GFZ-1 from the facility. Data for these satellites has been released to the international network. The data is available through the CDDIS; contact Carey Noll for the specifics.

It should be noted that time-of-flight measurements are made independently with two Stanford Research SR620 counters (A and B). Typically, both are used and the average is reported (Configuration "4"). Occasionally, only Counter A is used and that is denoted as Configuration "1". The calibration system delay and shift for each pass is constructed from ranging on internal and external targets; however, data will be flagged as "Internal" unless calibrations were done using the external target exclusively.

For more information, contact:

Dr. G. Charmaine Gilbreath
Naval Research Laboratory, Code 8123
Washington, D. C. 20375
Telephone: (202)767-0170
Fax: (202) 404-1226
gilbreath@ncst.nrl.navy.mil

Mark A. Davis, Allied Signal
NRL Code 8123
Ph: (202) 767-2829 (301) 805-3994
Fax: (301) 805-3974
davis@vaporize.nrl.navy.mil

Dr. Robert Q. Fugate
USAF Starfire Optical Range
PL/LIG
3550 Aberdeen Avenue, S. E.
Kirtland AFB, New Mexico 87117
fugate@plk.af.mil

FRENCH TRANSPORTABLE LASER RANGING SYSTEM (FTLRS)

Francis Pierron/Observatoire de la Cote d'Azur
Francois Barlier/CERGA-GRGS

The French Transportable Laser Ranging System (FTLRS) can now be considered in an operational phase at the Grasse Observatory.

In October and November 1995, after having solved many mechanical problems and tuning the system, this station obtained passes on several low satellites (STELLA, STARLETTE, TOPEX/Poseidon, ERS, etc.). Data processing performed by different analysis groups on these measurements seem to be in agreement at the few centimeters level. Because of climatic conditions during the winter, FTLRS was not deployed outside the Observatory during this season. At this time, we intend to install the system again on the same pad near the fixed station and to move into a more intensive period, hopefully tracking LAGEOS-1 and -2 to verify the earlier results.

In the months to come, we plan to deploy FTLRS for an experimental period of two months in the Corsica-Capria area, jointly with tide gauge measurements, in order to calibrate the altimeters onboard oceanographic satellites.

A coordinating group composed of the different French owners and potential scientific users will be established soon; this group will be in charge of defining the missions and finding the financial support for FTLRS operations.

SAUDI ARABIAN LASER RANGING OBSERVATORY (SALRO)

John Guilfoyle/EOS Pty. Ltd.

The Saudi Arabian Laser Ranging Observatory (SALRO) is located some 45 kilometers northwest of Riyadh, in a relatively elevated area, and free from light pollution from the capital city. Site preparations by KACST, in terms of pad and support facilities are, without doubt, the best in the world. The SALRO setup is complete, and achievements include:

• on-site configuration control (subsystem check sheets, etc.)
• scheduled subsystem maintenance carried out by the KACST students
• preparations for, and some satellite tracking, are now the students' responsibility
• site survey and calibration of the weather monitors
• the ability to transmit normal point files via electronic mail (most recently)

Subsequent to formal training, satellite ranging has been the priority for this year. As of 02 April, 1996, full-rate and normal point files have been transmitted to the CDDIS for the following satellite passes:

• GPS-35 and -36: 3
• LAGEOS-1 and -2: 25
• ETALON-2: 2
• STARLETTE: 8

From here on, normal point files will be sent from the site via email as soon as is practicable.

Processing takes place using the NASA software provided by ATSC last year, and delivers results of 12 to 13 mm for STARLETTE and GPS, and lately about 15 mm for LAGEOS.

The major problems faced at this station are:

• Weather. This has been the wettest season in living memory, with innumerable storms and generally poor sky conditions. These conditions are expected to prevail for some time yet, after which time summer will arrive with a vengeance.
• Communications. The Internet is generally not available here, however there is substantial feasibility and establishment work taking place at present. Those in the know expect another six months to pass before the Internet becomes a reliable means of transmitting and receiving data. However, the methods in use do work.

The students in general are handling the transition to operations well, and have coped with the technological and cultural shocks imposed by the arrival of the SALRO and constant proximity of the EOS Installation Team, in a thoroughly professional manner.

STATUS OF THE NEW ZIMMERWALD SLR STATION

Werner Gurtner/Astronomical Institute of Berne

On 01 May, 1995 the Zimmerwald SLR observatory stopped ranging activities for system upgrading. The old telescope, the dome, the laser, and most of the electronic equipment were removed. A new dome which allows ranging down to 15 to 20 degrees elevation in any direction was installed at the end of June 1995. Most of the electrical wiring in the observatory was replaced, and minor renovations on the building were performed.

As planned, the new telescope from TELAS arrived at Zimmerwald on 03 July, 1995 and was immediately assembled and mounted. The telescope control system hardware and software were installed during August and tested as far as possible.

Unfortunately major delivery delays from three subcontractors developed:

• The new Titanium-Sapphire laser system was not delivered until the end of January 1996 due to problems in the development and production of this prototype system. The laser is now installed, but it is not yet working satisfactorily with respect to stability and ease of operation.
• The major optical components (one meter primary mirror, secondary and tertiary mirrors) should have been tested and accepted in the Belgian subcontractor's factory by mid-April 1996. Currently the mirrors are being coated at the observatory in Nice, France. Delivery is expected at Zimmerwald by the end of April or beginning of May.
• Three of the four focal reducers for the guiding TV camera and the astrometric CCD cameras were ready in January 1996. The fourth, a wide-angle reducer, cannot be delivered before summer 1996 due to a shortage in the availability of some very special Schott glass (this delay will not affect SLR operations).

In May and June final system integration will be done; we expect to be operational again some time this summer.

EYESAFE SYSTEMS

John Degnan/ NASA GSFC

NASA has continued its analysis and development of the SLR 2000 system, an unmanned sub-centimeter accuracy, SLR station. The goal is to achieve comparable range accuracy to current single color systems at a much reduced cost. The system proposes to achieve eyesafe operation at 532 nm simultaneously with adequate signal return rates by: (1) reducing the energy per pulse to less than 100 microjoules at 532 nm; (2) filling the common transmit/receive aperture (approximately 30 cm) with the transmitted pulse; (3) increasing the pulse repetition rate from a nominal 5 Hz to 2 Khz; and (4) tightening up the beam divergence from about 30 to 10 arcseconds. Recent simulations have demonstrated the ability of the system to rapidly distinguish actual range returns from background and detector noise for mean signal strengths as small as 0.0001 photoelectrons using correlation range receivers adapted from the Poisson post-detection filters used in lunar laser ranging. The time required to acquire a satellite typically ranges from 1 to 30 seconds. Commercial sources for most of the key subsystems can be identified and those few that cannot are currently under development.

Recently, in a news release, the Tokyo CRL SLR station reported the first ranging to satellites at an "eyesafe" wavelength of 1.54 microns using a germanium SPAD built by the Czech Technical University in Prague. Last January, the station simultaneously collected data at 532 and 1540 nm. Other technical details were not available as of this writing.

MOBLAS SINGLE OPERATOR AUTOMATION PROJECT (SOAP)

Win Decker/ATSC

NASA/SGAPO has initiated the Single Operator Automation Project (SOAP), which was conceived to enable the NASA MOBLAS, and similar, stations to operate effectively and safely with one operator per tracking shift. A single operator per shift will enable an increased level of satellite tracking support by making more effective use of personnel resources. The SOAP is seen as a stepping stone towards the goal of completely automated ranging as envisioned in NASA's SLR 2000 concept.

A NASA/ATSC team, chaired by Dr. John Degnan, has been examining the engineering, maintenance, personnel, and safety issues and operating procedures associated with enabling a single operator to support a normal nine-hour tracking shift. MOBLAS systems currently operate with two crew members per tracking shift. The NASA/ATSC team is investigating a method to address the engineering and safety constraints associated with current operations, primarily through changes in operating procedures and in improved software, rather than by major expenditures in field hardware.

The Final Design Review for SOAP is scheduled for June 1996, with development to begin immediately thereafter. The prototype installation and testing at MOBLAS-7 is scheduled to occur in late July 1996. Installation in MOBLAS-4 will take place concurrently with that of the MOBLAS Upgrade Program in late August 1996, and installation in MOBLAS-6, MOBLAS-8, and MOBLAS-5 is expected to be completed by the end of 1996. Development, testing, and installation will occur on a parallel schedule at HOLLAS and MLRS.

SLR DATA AND ANALYSIS

CHANGES IN SLR DATA FORMAT

Michael Pearlman/SAO

At the CSTG SLR Subcommission Meeting in Canberra in November 1994, Van Husson proposed a revised CSTG SLR data format for field generated normal points (FGNPs) to accommodate greater resolution and additional information. This format was discussed, iterated, and revised through e-mail and finally at the EUROLAS meeting in Munich (March 1995). The results of these discussions were combined by Andrew Sinclair into a draft format revision document, which was sent to the Western Pacific Laser Tracking Network (WPLTN) and NASA for further consideration.

Although a major change in the SLR CSTG Normal Point Format may eventually be required, it was decided at the meeting in Berne not to make a major revision at this time, due to general financial and other pressures at the moment. However, two small changes are needed to accommodate current operational issues and should be introduced immediately by the stations.

Laser Wavelength (Header Record, columns 21-24)

The French Transportable Laser Ranging System (FTLRS) and the Lunar Ranging System at Grasse are now operating and submitting data in the near infrared (1.06 microns). Other groups are also planning to use infrared wavelengths, some as part of an eye-safe mode of operation. The current CSTG normal point format, which has four characters for wavelength specified in units of 0.1 nm (100.0 to 999.9 nm), cannot accommodate the infrared. The change, to be implemented immediately, will allow the units to be either 0.1 nm or 1.0 nm, depending on the wavelength value, as follows:

So, 0.5321 microns will give "5321" (no change from present) and 1.0641 microns will give "1064"

This wavelength modification is necessary for systems that are ranging with wavelengths greater than 1 micron. FOR SYSTEMS OPERATING BELOW I MICRON THERE IS NO CHANGE, and so there is no loss of compatibility with existing data. It is considered that wavelengths lower than 0.3000 microns or greater than 2.999 microns are unlikely to be used for laser ranging in the foreseeable future.

This choice of format change was made to preserve the four most significant figures in both cases. If a quoted data value of 1064 is misinterpreted as being a wavelength of 0.1064 instead of 1.064 microns, then the range error will be 8 meters or greater, and should be easily detected.

Data Release Record (Normal Point Data Record, column 48)

Occasionally stations wish to release a corrected or updated version of their field generated normal points for certain passes. Some data centers have found it difficult to cope with more than one issue of the same data. To ease the data handling procedures, we are adding a sequence flag or index to highlight successive data releases.

At present this character in column 48 is unused. It will be used as follows:

Operational groups are urged to ensure that the data are correct the first time. The sequence index makes it easier to differentiate data releases, but subsequent releases can lead to costly reprocessing of data.

Clarification of Format Definitions

There has been some confusion about the use of the terms RMS, standard deviation, and sigma in the SLR formats. We have agreed to consistently use RMS. Data producers and users should please take note.

CSR ANNOUNCES WWW HOME PAGE

Richard Eanes/UTCSR

A home page on the WWW for the SLR work at the University of Texas Center for Space Research has recently been implemented. The information can be hyperlinked through the CSR home page (http://www.csr.utexas.edu) or directly at http://ftp.csr.utexas.edu/slr.html. Although the page continues in development, users can now download the current weekly report on SLR network performance (as well as past issues), high satellite orbit predictions, and Earth orientation results. Plots of all LAGEOS-1 and LAGEOS-2 range and time bias solutions for every pass in 1995 are also available. Hypertext links to this page are available through the CDDIS and CSTG SLR Subcommission home pages on the WWW.

IMPROVEMENTS TO THE NORMAL POINT DATA MANAGEMENT SYSTEM

Oscar Brogdon, Van Husson/ATSC
Carey Noll/NASA GSFC

In July 1995, AlliedSignal Technical Services Corporation (ATSC) mapped the flow of global normal point data to determine if streamlining the process could speed up delivery. A goal was to eliminate any non-value added steps and fully automate the normal point data delivery system. Previously, the normal point data delivery process was done manually and data were delivered five days a week, Monday through Friday, with data received over the weekend processed on the following Monday. ATSC redesigned the normal point data delivery process and developed an automated batch procedure. The new program checks for format integrity and sort/merges data into the recommended file structure then delivers the normal point data to the Crustal Dynamics Data Information System (CDDIS) on a daily basis, 365 days a year.

The flow of data from NASA SLR sites were received at NASA /ATSC headquarters in a variety of ways including ftp, E-mail and Internet transmission. The data were also compressed using different compression algorithms. In addition, the data were placed in multiple directories, arrived in two different formats and two different types of data, normal point and sampled data. ATSC has now standardized the NASA SLR compression algorithms (ZIP), the flow (through ftp to a central account at ATSC), and format (CSTG normal point format) of the data.

The EURopean LASer (EUROLAS) stations provided data to the European Data Center (EDC), which in turn provided 4 daily files to NASA/ATSC and the CDDIS. In addition, several EUROLAS stations also provided their data directly to NASA/ATSC headquarters which led to redundant data. There were similar problems with stations in the Western Pacific Laser Tracking Network (WPLTN). The following milestones have been achieved through the excellent cooperation between ATSC, CDDIS, EDC, EUROLAS and WPLTN:

• August - September 1995 - Eliminated redundant sources of normal point data from the global community.
• October 1, 1995 - Discontinued archiving sampled data.
• October 26, 1995 - On-line ORACLE database of normal point accountability implemented at ATSC.
• November 1, 1995 - The elimination of Divided Difference Noise Analysis (DDNA) messages.
• November 14, 1995 - EUROLAS provides 1 daily normal point file versus 4 files per day.
• November 15, 1995 - FTP of normal point data for ORRORAL and CRL initiated.
• November 22, 1995 - Automated batch data processing and delivery procedure initiated providing 1 global normal point file and individual satellite files within 24 hours of data receipt weekdays, holidays and weekends at 7:00 a.m. EST.
• December 8, 1995 - Provide NASA GFZ-1 normal point data hourly to CDDIS.
• December 21, 1995 - Provide NASA and WPLTN normal point data to CDDIS for use by EDC.
• January 6, 1996 - TLRS-2 provides normal point data.
• January 11, 1996 - Simosato provides normal point data.
• January 11, 1996 - All operational SLR systems providing normal point data for the first time.
• May 8, 1996 - SALRO delivers normal point data through e-mail to the CDDIS.

The chart below in Figure 11 shows the dramatic improvements in the delivery of data to CDDIS. Occasionally there are outliers caused either by computer network problems or data formatting problems.

Future plans include the elimination of email as a means of transmission of the data. Email will be replaced by ftp as different sites acquire this capability. EDC hopes to deliver their EUROLAS file several hours earlier in the day so that the daily data set will contain the most recent data.

DISCONTINUATION OF FULL RATE DATA ARCHIVING

Michael Pearlman/SAO

At the CSTG/EUROLAS meeting in Berne, Switzerland in early December, we agreed that the SLR data centers should cease routine archiving and transmission of full-rate data on April 1, 1996.

It was the unanimous opinion that field generated normal points (FGNP) have become the primary SLR data product. None of the groups at the meeting, or those contacted afterward, including the ERS-1, ERS-2, GFZ-1 and TOPEX/Poseidon representatives, expressed any need for full-rate data, except for specialized activities such as collocation, the Fizeau cornercube experiments, or the imminent TiPS experiment. At the moment, there are only a few organizations left that receive full-rate data shipments from the CDDIS, and this appears to be just a matter of inertia. These groups have been informed of the new policy and have either had no objection or have not responded.

Stations will be requested to maintain their own full-rate files for one year, and they are then free to dispose of them. Those wishing to use the full-rate data are welcome to contact the stations. Full-rate data for the specialized activities will continue to flow through the customary channels and be archived as in the past for the users.

Traditionally, FGNPs have had preliminary timing and barometric pressure data, while final timing and pressure corrections have waited for their corresponding full-rate data. In spite of this, none of the participants at the Berne meeting has found the full-rate data to be measurably better than the FGNPs for any application. However, operating groups must realize that the best possible timing and barometric pressure corrections should be included in the FGNP, since this will now be the final product.

Special needs for full-rate data should be coordinated through the CSTG Subcommission. Requests for special needs should be made to John Degnan, Chairperson of the Subcommission.

REVIEW OF SLR PREDICTION DATA

Michael Pearlman/SAO

It was reported that some earlier incompatibilities of the reference frames used for IRV orbit integrators had been resolved. In the interest of standardization, NASA has changed its prediction software to include polar motion in its orbital elements, and now everyone is using the system described in the June 1995 issue of the CSTG SLR Subcommission Newsletter.

The remaining differences between the NASA and RGO predictions now are:

1. NASA uses a slightly more elaborate model for the lunar perturbations, which will account for only a few meters difference in satellite position.
2. IRVs generated by the University of Texas, RGO, and GFZ use degree and order:

• 7 for high satellites (LAGEOS and higher)
• 18 for low satellites

whereas NASA/ATSC uses degree and order:

• 10 for high satellites
• 16 for low satellites
• 18 for GFZ-1

Users of these IRV sets should check that the appropriate degree and order are used in their integrator program. The original integrator package IRVINT from the University of Texas was designed just for LAGEOS, and the value of 7 was hard-coded into a data statement in subroutine GEOSET. Presumably users of this package must many years ago have made appropriate modifications in order to use the package for low satellites. The ATSC variant of IRVINT, PCTIVAS, specifies the degree and order of the field in file SATTEL.PRM. For the RGO integrator, ORBIT, the degree and order are specified in a file ORBIT.DAT. The effects of using the wrong degree and order are not large, perhaps 50 to 100 meters, but are better avoided, since it will introduce periodic degradations in the local data fit for screening procedures.

NEW SLR ON-SITE NORMAL POINT REPORTS

Carey Noll/NASA GSFC

The CDDIS has recently created a new report format for SLR on-site normal point data holdings. These reports summarize all data received on a weekly and monthly basis. They are sent electronically every Friday and are accessible on the CDDIS through anonymous ftp in the directory ANON_DIR:[REPORTS.SLRWEEK] and the WWW, at URL ftp://cddis.gsfc.nasa.gov/reports/slrweek/. Additional reports generated by or received by the CDDIS can be accessed in subdirectories of the anonymous ftp directory ANON_DIR:[REPORTS] and through the WWW at URL ftp://cddis.gsfc.nasa.gov/reports/. Those interested parties who wish to receive this report electronically each week should contact Carey Noll (noll@cddisa.gsfc.nasa.gov).

STATION HEIGHT ACCURACY

Peter Dunn/Hughes-STX

The scientific applications of SLR data cover a variety of areas. The accurate satellite position defined by the SLR network enables us to improve the gravity model of the Earth and to investigate other force model effects on the orbit. The network also allows high resolution of Earth orientation parameters from observations of LAGEOS and ETALON. The scale of the measurements allows very accurate definition of the center of mass of the Earth, as well as its scale and gravitational constant. Individual stations can define specific position information to yield tectonic motion measurements and deformation in certain regions, and to monitor height variations to improve measurements of sea level.

The best current SLR systems provide range measurements with an accuracy at their noise level of a few millimeters, but the satellite position determined from these observations is limited by force and Earth model errors to a few centimeters. On the other hand, the influence of model error on station position is low enough to determine the three-dimensional location of th