From Pascal.R.Willis@jpl.nasa.gov Fri Feb 28 12:33:07 2003 Date: Thu, 27 Feb 2003 11:21:39 -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" , Yoaz Bar-Sever Subject: DORIS/EOP precision vs number of satellites (cont'd) dear all, following my latest message on this subject, I have tried to do a very simple test. I have grouped my EOP results by number of DORIS satellites and I have compared theDORIS polar motion X and Y with the GPS/EOP IGS series. I have removed only very large outliers (which I need to investigate now). In the GPS/DORIS EOP comparison, I have not removed any bias and I give you below the estimated RMS: #satellites #datapoints XPOLE_RMS YPOLE_RMS 1 28 2.69 2.05 2 612 2.53 1.58 3 1517 1.94 1.38 4 49 2.46 1.42 5 94 2.30 1.25 In the Y-component, there is a clear improvement with the number of available satellites. It more or less follow the curve in 1/sqrt(n) except after 4, which may imply some mismodelling for ENVISAT (more probable) or SPOT5. In the X-component, it is also true if you exclude the value for the 3-satellite solution that seems to give better results than expected. In my opinion, this is due to the fact that, with the large number of data point, the basic statistics that I am using is more robust to outlier detection strategy. My number above are probably too pessimistic. On the other hand, the results fo not follow as well the curve in 1/sqrt(n), meaning that there are more systematic error in that component. The Y-results seem to be consistently better than the X-results for a reason that is still unclear to me. It could be interesting to know if the other DORIS analysis group see the same pattern or not. I still wonder how a mismodelling in the orbit could affect more the X-bias than the Y-bias. Suggestions and ideas are very welcome! It could also be interesting to look more closely to the offsets and verify if they could be linked to the presence of a specific satellite in the DORIS solution. That could be a good indicator of present DORIS modelling and could help me resubmit better solutions in the future. Let's now wait for final results done with more sophisticated analysis tools. Best regards Pascal NB: As a comment, the larger residual in the 5-satellite group, only contains 1/2 of the regular TOPEX data and 1/4 of the the regular ENVISAT data. So (in my opinion) the accuracy of the DORIS/EOP can directly be linked with the number of satellites and the number of DORIS data used. When we get more days, the statistics would be more reliable.