From Pascal.R.Willis@jpl.nasa.gov Fri Feb 28 12:36:29 2003 Date: Wed, 26 Feb 2003 15:50:01 -0800 From: Pascal Willis To: Daniel Gambis , Richard Gross , Zuheir Altamimi , Meise Barbara Cc: Martine Feissel , Gilles Tavernier , Jean-Paul Berthias , Carey Noll , Jean-Jacques Valette , Laurent Soudarin , Serguei Kuzin , CRETAUX@sc2000.cst.CNES.FR, "John C. Ries" Subject: DORIS/EOP precision vs number of satellites + epoch of minimum variance dear colleagues, here are a number of ideas and proposals related to current DORIS/EOP analysis. Some of them could be useful in the preparation of becoming a new IAG Service in Sapporro. 1) number of DORIS satellites actually used in computation during the recent IDS Analysis Workshop in Paris, I was surprised to see that all groups when showing DORIS accuracy for EOP did not distinguish between solutions using 2, 3 or more DORIS satellites. When doing point positioning solutions, the precision and the accuracy of the results can be easily linked with the number of DORIS satellites used in the estimation. In my opinion, the EOP parameters precision should also improve with this parameter : A 5-6 satellite solution should provide far better results than a TOPEX-only solution. In the past submission of EOP, I have put the statistics of the number of measurements per satellites in the individual sinex files. Unfortunately, nobody seems to use it. There is nobody to blame as I must admit that it is not very easy to look though all these individual sinex solutions to derive the number of DORIS satellites actually used in the computation. I have then prepared for you a synthetic file called mjd_sat.txt containing the following basic information : First column is mjd. The second column gives you the number of satellites. The 3rd and eventually other columns give the names of the satellites (for more extensive studies), using alphabetical order. 48990.5 2 SPOT2 TOP 48991.5 2 SPOT2 TOP 48992.5 2 SPOT2 TOP 48993.5 2 SPOT2 TOP 48994.5 2 SPOT2 TOP 48995.5 2 SPOT2 TOP 48996.5 1 TOP 48997.5 2 SPOT2 TOP 48998.5 2 SPOT2 TOP 48999.5 2 SPOT2 TOP ..... 52574.5 4 ENVIS1 SPOT4 SPOT5 TOP 52575.5 4 ENVIS1 SPOT4 SPOT5 TOP 52576.5 4 ENVIS1 SPOT4 SPOT5 TOP 52577.5 4 ENVIS1 SPOT4 SPOT5 TOP 52578.5 4 ENVIS1 SPOT4 SPOT5 TOP 52579.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52580.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52581.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52582.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52583.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52584.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52585.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52586.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP 52587.5 5 ENVIS1 SPOT2 SPOT4 SPOT5 TOP You can get it at IGN by anonymous ftp : ftp lareg.ensg.ign.fr cd incoming get mjd_sat.txt I would be very interested if one group could provide DORIS/EOP statistics by number of satellites (1,2,3,4 and 5) and not as global result of all types of observations. If you find out that such an approach makes a difference in the statistics, it would then make sense to define a format for such a file and ask all DORIS Analysis groups to provide this information in liaison with their EOP series. 2) epoch of comparison of EOP On the other hand, I would like to raise another technical point concerning the availability of estimated velocities for the polar motion in the EOP sinex file. In my opinion, the comparisons of EOP should not be done at the Modified Julian Days given in the EOP file but a pre-processing should be done at this step in order to define the epoch of minimum variance of X and Y polar motion. In the case of DORIS, like SLR and VLBI, the observations are not continuous all around the day (by opposition to GPS). For example, if only the second half of the daily data are available (between 0.5 and 1.0), it is best to compare the polar motion parameters at 0.75. Comparing at 0.5 will give worse result and will not be a good estimator of the DORIS accuracy. The proper mathematical way to do that is to look for the epoch of minimum variance (minimum of a very simple quadratic form) that will determine the mean observation value. By doing so, you ensure that the uncertainty in the polar motion velocity does not affect the accuracy of the polar motion parameter. It is exactly the equivalent of comparing the stations coordinates at a given reference epoch (like 1997.0) instead of looking for the mean observed period (using the minimum variance) that could be far away in 2001. When doing the comparison when DORIS observations are available in 2001 (just an example), results are better because they are not extrapolated. If EOP analysis groups do not use such a strategy, it then makes sense for me to do an internal post-processing to provide EOP at non-fixed epoch or to fixed certain EOP parameters (like polar motion velocities and UT1-UTC related parameters) Would such a proposed preprocessing look easy enough to implement or should I submit different types of solutions as proposed above (irregular epochs or estimating less parameters)? 3) outliers I would be also very interested to have access to the list of outliers that you found in your different analysis. That would help me a lot in understanding problems in the current DORIS data processing and it should also help me submit more reliable solutions in the future. My first guess is they are related to single-satellite solutions but I may be wrong and there may be some more. 4) different DORIS results from Analysis Centers It was also very surprising for me to see in the view-graphs presented by Daniel Gambis the different types of results of DORIS determination of polar motion. Some seemed to be noisier but more stable on the very long-term while others seem very precise on short-term but but noisier on longer-term showing some very systematic errors changing with time. I wonder if someone should not spend some time in understanding the different strategies used by all DORIS groups in order to be able to define better analysis strategies for the future. It might be presently one of the limiting factor of the accuracy of DORIS/EOP solutions. Looking forward for your comments and suggestions Pascal