Date: Tue, 2 Apr 2002 14:32:34 -0600 From: "John C. Ries" To: Henno Boomkamp Cc: Robert Weber , Gilles Tavernier , Martine Feissel , biancale , exertier , Jean-François Crétaux , Jean-Jacques Valette , Jean-Paul Berthias , Jean-Michel Lemoine , kra@ipa.rssi.ru, Laurent Soudarin , Pascal Bonnefond , Pascal Willis , skuzin@inasan.rssi.ru, nzelensk@geodesy.gsfc.nasa.gov, Frank.Lemoine@gsfc.nasa.gov, Bruce.J.Haines@jpl.nasa.gov Subject: Comparing Jason-1 orbits >With regard to the contributed orbits, I would prefer the following: > >(1) Earth-fixed orbit files in SP3 format, preferably 1-day arcs but this is > not critical. If velocity vectors are present in both compared files > of a pair, the velocity differences will also be generated. > > For CHAMP we have installed an orbit repository at CDDIS, it seems > practical to do the same for JASON. > >(2) A small READ.ME file to describe the POD method. For TOPEX and JASON > most orbits will be from dynamic solutions, but what is important in this > case is to indicate > (A) all tracking data that has been used (GPS, DORIS, SLR, altimetry, > ...). > (B) the terrestrial reference frame in which the orbit has >been estimated > >(3) The aim is to have a longer JASON campaign than for CHAMP. JASON POD > is already more mature so that we don't expect a lot of re-processing. I > would suggest a period of at least two repeat cycles, but we might as > well do a full month. If you have already chosen a campaign duration, > please let me know. > It will probably not be a problem to run periods even longer than one > month. Dear Henno, I would be happy to participate in an orbit comparison for Jason-1. We have already started some intercomparisons between a few of us here. There has been some difficulty due to the delay in getting the tracking and uncertainty in the knowledge of the spacecraft attitude. So we need to keep this in mind when we choose the time interval to use for the comparison. Given that we should soon have DORIS for cycles 4 and 5, and the attitude knowledge for cycles 3-5 should be fairly confident, I might propose this as our comparison interval. This is between two maneuvers. If other folks want to do another period or a longer period than this, then there might be some delay before we have the complete information we need. We here at CSR plan to produce orbits based on SLR/DORIS as a reference, as well as GPS-only and combination solutions. Per our previous discussions, we would need to have a clean SLR data set to use for evaluation. Unreliable stations and data need to be eliminated when we are working with orbits that are at the 2 cm level or better. It would be best if we could all agree on one SLR data set for this evaluation to avoid uncertainties we encountered in the CHAMP comparisons. If the data is sufficiently clean, station weighting issues become less important. I would hope to have the altimeter data at some point...the crossovers are also an independent measure, albeit not at the same level as the SLR. But it does help evaluate the relative performance of various orbits. We have been using the POE format mainly, but I don't have any objection to the SP3 format. I should be able to convert to that format. I presume, though, that this implies that the orbits would be in GPS time, per the SP3 format. This might cause the nodes of some orbits to not be on the whole minutes GPS time because they would be on the whole minutes of UTC time. Do you foresee any problems associated with this? I presume you interpolate for the SLR comparisons, but I don't know what you do when you compare the orbits directly and the times do not match. Best Regards, John ________________________________________ Date: Wed, 3 Apr 2002 01:26:13 -0600 From: "John C. Ries" To: Henno Boomkamp Subject: Comparing Jason-1 orbits Dear Henno, A correction to my email...I see from the recently posted sequence of events file that there were in fact two gyro calibration maneuvers during the period of Cycles 3-5. So perhaps these are not optimal cycles to use. We may have to wait for a more benign period. I plan to use all the cycles, so any choice works for me. Best Regards, John