Switch Between TOPEX-A and TOPEX-B
TOPEX-A altimeter operated from cycles 1-235. In mid 1997, it was determined that there was a degradation in the point target response which impacted the significant wave height. In February 1999, TOPEX-A was turned off and the redundant backup input altimeter, TOPEX-B altimeter, was turned on. Subsequent calibration to assess the data resulted in keeping TOPEX-B on for cycle 236 and all subsequent cycles. Cycle 235 was the last cycle using TOPEX-A.
Errors in MGDR-B
In the MGDR version B data cycles 1-190 there were some small errors, mostly the incorrect default values were placed in the data, but the flags are correct. A full list of the errors can be found at ftp://podaac.jpl.nasa.gov/allData/topex/L2/mgdrb/docs/uhmgdrb/mgdrberrors.doc
Early Pointing Angle Problems
From cycle 1 through cycle 8 pass 189 (23 September through 8 December 1992, day of year, DOY, 343), the satellite attitude control system was not properly calibrated. This resulted in pointing the altimeter relatively far from nadir (typically 0.3, but up to 0.6) with a sinusoidal signature over each pass much of the time. Since the pointing angle/sea state corrections are less accurate for angles larger than about 0.3 and the geometric pointing correction cannot be done, these data will be less accurate than later data. The pointing calibration was improved on 1992 DOY 353 and 357 and 1993 DOY 046, but these changes were minor compared to the initial calibration on 1992 DOY 343. The user may want to begin analysis on data beginning with cycle 11.
A list of calibrations for the TOPEX backscatter can be found at ftp://podaac.jpl.nasa.gov/allData/topex/L2/gdr/docs/sigma0_calibration/.
The calibrations are provided to improve the sigma0 values provided by the project that are found in the GDRs. If you are using the MGDRs, the calibrations have been applied to cycles 1-132, otherwise you need to apply the calibrations to the other cycles yourself. If you are applying the GPC correction to the GDR, it contains the calibration corrections.