Analysis Products

The ILRS provides a service to utilize Satellite and Lunar Laser Ranging data to generate geodetic, geodynamic, and geophysical scientific products.

Currently, seven SLR analysis groups (ASI, BKG, DGFI, ESA, GFZ, JCET and NSGF) provide direct input for the official ILRS products "POS+EOP" (station coordinates and Earth Orientation Parameters—EOPs). This is a result of an ILRS POS+EOP pilot project following an evaluation of the outcome during the AWG meeting in San Fernando. The original five contributors were acknowledged and given an official AC status, although the list was later expanded with the admission of additional groups that underwent strict evaluation. In addition to the adoption of the official ACs, ASI was selected as the official ILRS primary combination center—CC, and DGFI was originally selected as the official ILRS backup combination center, each for a two-year term. When DGFI decided to stop acting as a combination center, the task was undertaken by JCET. These combination centers must follow strict timelines and provide routine products of the highest possible quality based on the combination of the individual solutions provided by each of the ACs. Originally on a weekly basis, official ILRS products from these two combination centers were made available in SINEX format each Wednesday at CDDIS and EDC.

Following the solicitation of more frequent product updates by USNO's EOP Combination and Prediction Division, the ILRS AWG organized a Pilot Project and tested the generation of "rolling" weekly solutions on a daily basis. While in the past the weekly solutions spanned a true week (Sunday 00:00:00 UTC to Saturday 23:59:59 UTC), the new product shifted the first day of the 7-day arc by one day on a daily basis. It was thus shown that the resulting EOP with a 2-day latency were of superior quality for use in USNO"s forecast process. The results of the PP were reviewed during the AWG meeting at Metsovo, Greece in 2009, and it was then decided to proceed with the generation of the "DAILY" series. In early 2012, after a lengthy testing period, USNO accepted the new product and the ILRS AWG switched the DAILY product to become the official POS+EOP product and the WEEKLY series was reserved as a testbed for future PPs and other model enhancement tests. Furthermore, following many requests for the generation of an official orbital product, the AWG launched a PP that led to the adoption of yet another product in early 2016: since May 2016 there is a WEEKLY product of precise orbits derived from the combination of individual products from each AC, for all four satellites used for the generation of the official products, LAGEOS, LAGEOS-2, Etalon 1 & 2. The orbits are provided in an Earth-fixed frame, using the ITRF that is used at the time of generation of these orbits. They are archived in SP3c format on both ILRS DCs.

The ILRS now generates daily, week-long loosely constrained solutions for station coordinates (valid for the mid-point of each 7-day interval) and daily resolution EOPs (x-pole, y-pole and Length-Of-Day (LOD), all of them for 1-day intervals). These results are stored in subdirectories of the form "pos+eop/YYMMDD", where "YYMMDD" is the date (YY=2 digit year, MM=2 digit month, and DD=2 digit day) of the end of each 7-day interval. Within each subdirectory, one finds so-called individual solutions and combination solutions that are explained below.For the individual contributions to the POS+EOP, project filenames are of the form (examples reflect CDDIS archive):

Anonymous ftp service will be discontinued on October 31, 2020 as CDDIS moves to https archive access. Filenames will remain the same but the paths are slightly different. An example is shown for the product below.

https://cddis.nasa.gov/archive/slr/products/pos+eop/YYYY/YYMMDD/CENTER.pos+eop.YYMMDD.v1.snx.Z

ftp://cddis.gsfc.nasa.gov/pub/slr/products/pos+eop/YYYY/YYMMDD/CENTER.pos+eop.YYMMDD.v1.snx.Z

Here, "CENTER" is replaced by the name of the actual contributor. It is the original solution that is generated for this period and by this analysis center, hence "vN", where N is the version number, for the DAILY series this "170" at present, while for the WEEKLY series it is "70". The "version number" changes in general following the adoption of a new ITRF standard. In case an AC or CC decides that a re-computation is necessary, the version number will be increased by one, i.e. v170 ==> v171 or v70 ==> v71, when the product is resubmitted to the archives; solutions for subsequent days and weeks will start with "vN", however. The results are stored in the SINEX format (".snx"). The reader is referred to the COMMENTS section of each solution (in the file itself) or more general explanations of SINEX for further details of the solution and/or the format.

The official ILRS primary product on POS+EOP is labeled as:

ftp://cddis.gsfc.nasa.gov/pub/slr/products/pos+eop/YYYY/YYMMDD/ilrsa.pos+eop.YYMMDD.v1.snx.Z

In a similar fashion, the EOP solutions (w.r.t. ITRF2000) generated by the official ILRS primary combination center are named:

ftp://cddis.gsfc.nasa.gov/pub/slr/products/pos+eop/YYYY/YYMMDD/ilrsa.eop.YYMMDD.v1.snx.Z

The ILRS also has an official backup combination center, and its products are labeled:

ftp://cddis.gsfc.nasa.gov/pub/slr/products/pos+eop/YYYY/YYMMDD/ilrsb.pos+eop.YYMMDD.v1.snx.Z
ftp://cddis.gsfc.nasa.gov/pub/slr/products/pos+eop/YYYY/YYMMDD/ilrsb.eop.YYMMDD.v1.snx.Z

The orbital products are similarly archived as:

ftp://cddis.gsfc.nasa.gov/pub/slr/products/orbits/satellite/YYMMDD/ilrsa.orb.satellite.YYMMDD.vN.sp3.Z
ftp://cddis.nasa.gov/pub/slr/products/orbits/satellite/YYMMDD/ilrsb.orb.satellite.YYMMDD.vN.sp3.Z

Since 2016, the ILRS AWG has changed its name to ILRS Analysis Standing Committee (ASC), in accordance with the IAG-imposed rules about commissions, working groups, committees, etc.

Following the 2015 Workshop in Matera, Italy, the ASC has focused its efforts in quantifying the systematic errors of each contributing system over the entire period covered by the analyzed data that contribute to the development of the ITRF. This effort has produced very significant preliminary results, the most prominent of which is the reduction of the disagreement in the scale of the TRF between the SLR and VLBI realizations. Based on the results of a multi-year effort, the "Station Systematic Error Monitoring Pilot Project—SSEM PP", a new modus operandi has been proposed for adoption for the development of the ILRS ASC contribution to ITRF2020 as well as the production of the operational official products of the future. A complete reanalysis of the 1993 to 2019 period is nearly completed (Oct. 2019). The main outcome from this reanalysis is the detection and adoption of the periods over which each system suffers from a significant systematic error. A mean value of the error will be adopted, cataloged in the "Data Handling File" and pre-applied to the data during the re-analysis for the ITRF2020. It must be understood that the estimated systematic errors are conditional upon the adopted value of GM and the model for the correction of each system for the effect of target signature variations. Changes in either or both of these parameters will require appropriate changes in the adopted systematics to avoid inconsistencies.

Currently (Oct. 2019), the ILRS ASC is in the process of finalizing the SSEM PP, in preparation for the development of the official ILRS contribution to ITRF2020. It should be noted that for this product the ASC will also analyze the data from the LARES satellite (since March 2012), which will become an additional official target contributing to all ILRS products. Other near-term plans include the generation of a series of low-degree spherical harmonics of the gravitational field from SLR data from multiple SLR targets, the inclusion of additional SLR targets and the exploitation of SLR tracking of GNSS s/c for improved EOP and other products.

Historical products include: