ILRS NEWG Meeting (Nice 2002)

Nice, France
April 24, 2002


Graham Appleby, John Degnan, Buddy Donovan, Werner Gurtner, Julie Horvath, Van Husson, Toshi Otsubo, Carey Noll, Mike Pearlman, Ivan Prochazka, Randy Ricklefs, Ulli Schreiber, Wolfgang Seemueller, Peter Shelus, Scott Wetzel

EUROLAS Engineering Meeting:

A meeting on Detecting and Eliminating Errors in the EUROLAS Network was held at Herstmonceux on March 11 and 12. The notes from the meeting are on the ILRS web site:



  • Long term goal should be mm accuracy
  • Detect and reduce systematic errors
  • Reduce Random errors (to better expose systematic effects)
  • Stations have primary responsibility for data quality (good onsite data screening handling techniques)


    • Residual time walk determination with signal strength measurements or return rate
    • Need to properly tune the device to accommodate pulse length dependence
    • Carefully check alignment
    • Request stations to test SPADs at high and low signal strength modes to quantify change in bias
  • PMT
    • Residual time walk determination with signal strength
    • Avoid time walk with single photon operation
    • Be aware of gating issue

Counters and Timers

  • SR620 counters from several EUROLAS stations compared at Herstmonceux
    • Tests conducted over full dynamic range
      • Individual characteristic for each counter determined
    • All counters referred to PPET through earlier comparison at Herstmonceux
      • Characteristics available for application to individual counters
  • Some stations will perform ranging tests on satellites at both high and low signal strengths
    • NERC will analyze the data
  • TU Prague will spearhead a proposal to the EU for a PPET standard to calibrate the SLR Network 

Data Reduction

  • Carefully check and monitor system noise - key indicator of problems
  • Compute and monitor other pass parameter - skew, kurtosis, etc
  • Monitor return rates as a means of checking performance and signal strength
  • Introduce minimum number of returns per normal point
    • Daytime - 6 points
    • Nighttime - 3 points
  • Leave provision for low repetition rate systems
  • Reintroduce full rate date flow and archiving
    • No real time requirement
    • Include calibration data
  • Introduce provision for withdrawal by the stations of poor data from the Data Centers (DFP W/G)
  • Daily short and long arc quality check by NERC

"Near Real-Time" Distribution of Time Bias and Station Status through AIUB

  • Expand usage throughout ILRS network
  • Speed up time bias updates by expediting data flow
  • Include time biases for all prediction sets used

Other Items

  • Exchange stations personnel to spread knowledge
  • Circulate a reference barometer around the EUROLAS network for calibration

Site Logs

Van Husson reported that site logs from forty-four stations have now been archived. Logs from Maidanak and Komsomolsk are still missing. The logs are available on an Excel spread sheet data base with auto filter search capability. (see ftp://cddis.gsfc.nasa.gov/reports/slrlog/). Where possible, manufacturer's specifications have been included. All stations are requested to review their file in the site log archive and report any errors to Van. Stations are also requested to keep their file in the site logs up to date.  New fields can be added as the need arises. Historical station information will remain available in the old configuration files. Van is also trying to obtain site log information from deactivated stations. A file of engineering corrections (range, timing, meteorological, etc.) is also being assembled. These include corrections determined at the stations (instrument, calibration, etc), but not yet applied to the data. This facility will be available on line for the analysts to use in data processing. It will avoid the need for stations to update data already submitted the Data Center files whenever such biases are discovered. New data should of course be properly corrected. The site logs also contain the site tie information, which the ISGN is trying to organize. We need to implement a means of recording the site tie chronology and maintaining these files in an updated manner.

Cloud Cover Camera

John Degnan presented some information about the cloud cover monitor which is part of the comprehensive meteorology package for the SLR 2000. The camera relies on a 120 by 120 pixel uncooled IR camera that detects clouds by temperature differences (clouds are warmer than clear sky). The monitor can see down to 10 degrees altitude and a full sky map take only about 15 seconds. The cost is presently about $75K, but should be reduced to about $30K if a volume purchase can be organized.  Contact John Degnan for more details and to express interest.

"Near Real Time" Distribution of Prediction Time Biases 

NERC is presently computing time bias functions using the most recently submitted SLR data accessed through the Data Centers. The time bias functions (including drag functions) are available through a server at AIUB. Stations with proper client software (EUROSTAT or just TELNET) can access and display the functions and use them to update their predictions. Although only a one or two prediction sources are used by stations for each satellite,  time bias functions for all of the commonly used predictions are provided by this service. It behooves us to speed up the cycle (data submission, processing, etc.) as much as possible to improve the predictions, primarily on the low orbiting satellites.

Knowledge Base

The ILRS Bibliography has been established and the Site Logs are in place. The text of many of the recent articles in the ILRS Bibliography are accessible in electronic format. The next step in the development of the knowledge base is to improve the means of searching both the ILRS and linked sources for specific topics. As a start, a list of key words should be developed for each of the articles within the ILRS bibliography.

Action - Van Husson, John Degnan, and Mike Pearlman: Develop a plan to improve the search capability