Date: Thu, 21 Nov 2002 09:56:18 +0100 From: Jean-Paul Berthias To: Pascal Willis Cc: Nikita Zelensky , ries@csr.utexas.edu, bjh@cobra.jpl.nasa.gov, Yoaz.Bar-Sever@jpl.nasa.gov, sluthcke@xyz.gsfc.nasa.gov, krchoi@csr.utexas.edu, feissel@ensg.ign.fr, Gilles Tavernier Subject: Re: JASON DORIS phase center and SAA effect Hello, What Pascal and Nikita have seen when adjusting the Z offset of the DORIS antenna phase center is perfectly normal: this is again our "old friend", the frequency drift. Actually, if my computations are correct, a Z offset in the on-board antenna phase center has exactly the same Doppler signature as an altitude biais on the station height, with a scale ratio between the two eqal to (R/a)^3 * (1-a/R*cos(alpha))/(1-R/a*cos(alpha)) where R is the Earth radius, a the orbit semi-major axis and alpha the angle between the radius vector to the station and the orbit plane (alpha = 0 for overhead passes). What this means is that solving for a Z offset in the antenna height using Doppler data is identical to solving for a constant altitude offset on all the stations, which amounts pretty much to a scale factor on the global set of coordinates. As we know that there are large pass dependant frequency drifts on Jason, the estimated offset will be the average value of these drifts on the set of xstations included in the solution. Thus, when only stations outside of the SAA are included, the observed offset corresponds to the average drift outside of the SAA. When all the stations are included, the offset corresponds to the global average over the whole network. The drift which is removed from the delivery data is obtained by adjusting a linear frequency model over 10 days (or more precisely a quadratic time model adjusted with pseudo-range data only from master beacons, equivalent to one measurement every few hours). This model is a global average which includes both the slow drift outside of the SAA and the rapid drift over the SAA. The difference between the actual "local" drift and this long term average is still present in the data. This difference induces the perturbations that you see in station altitude, antenna Z offset and scale parameter. The further away you are from a global configuration averaged over 10 days, the more discrepencies you will observe. My recommendation is to leave the DORIS phase center where it is as we have no way of sorting out the different effects. On the other hand, Pascal's results for the other satellites have to be carefully analyzed, in particular for the Spot satellites where the offsets seem to consistently point in the same direction. Best regards, Jean-Paul