Date: Sat, 12 Oct 2002 21:08:55 -0500 From: "John C. Ries" To: Escudier Philippe Cc: Berthias Jean-Paul , Gilles Tavernier , Martine Feissel , "Jayles, Christian" , "Sengenes, Pierre" , pascal.r.willis@jpl.nasa.gov, eelco@deos.tudelft.nl, Jean-Michel.Lemoine@cnes.fr, Bruce.J.Haines@jpl.nasa.gov, krchoi@csr.utexas.edu Subject: RE: satellite clock in the SAA region Hi Philippe, > >I would like to come back on two points that you mentionned : >- The effect is clearly non linear and non quadratic when the >satellite enters the SAA. At this time due to the radiation effect >there is a rapid change in the frequency drift so that the slope >goes from -2E-10 / day outside the SAA up to few +10-9/day in the >SAA. >This explains the results you got, which were very useful for us to >get a better understanding of the physical nature of the effect : > * adjusting a slope is somehow efficient for beacons located in the >middle of the SAA (it helps recovering this important drift which >otherwise go in the tropo coefficient and / or station altitude and >orbit solution and /or residuals). > * adjusting a slope is not efficient for beacons located at the >edge of SAA because there, the main effect is a sudden slope change >during the pass, which is an effect going in the station latitude >and/or orbit solution and/or residuals. >Even for station in the middle of SAA as the exact shape is not a >linear drift this slope cannot recover the complete effect, that is >why we consider that beside the workaround that you have proposed >and which proves to be very efficient for POD (underweighting those >beacons), the only way to efficiently compensate from this effect >for those beacons will be to use a radiation model (which may be >very simple, just a function of latitude and longitude when computed >for Jason) that will have to be tuned with the data in a similar way >you are doing for atmosphere model for instance. I hope you or someone can figure something out. Jason-1 would clearly be superior to T/P for geodesy, if not for this effect. In the worst case, only a small number of stations seem to be affected. > >- It seems that there has been some misunderstanding with this >"saturation effect". This saturation should reduce significantly the >effect and not enhance it (it seems that T/P USO evidenced an effect >almost as large as Jason during the first months of the mission, we >would be interested to know if you have some evidence of that). I recall CACB did have some anomalies at the beginning which I had ascribed to some local geophysical effect. Now that we know about this, it seems logical that it could have been the radiation. I have attached part of an old presentation, which shows that there was indeed an anomaly for CACB. This was the one I plotted because it was unusually large. I don't recall seeing anything quite this large on any other station. I note that the effect is mainly in the height and latitude. >At the present time we think that the explanation of this effect >increase since January is due to the fact that during the last >months we have been leaving the solar activity maximum which has >induced a lowering of the SAA which has created a more severe >radiation environment (this can be seen on other equipment of the >satellite such as the star tracker). Well, we can hope that a decrease in the solar activity will help. Thanks for the information. Best regards, John