[RADS] J1 GDR-B to GDR-C conversion

[Remko Scharroo]

Dear RADS users,

As most of you know, the Jason-1 project has adopted new standards for  
the processing of Jason-1 data, which are to be consistent with the  
standards for Jason-2. Additionally, some other updates were included  
that were long overdue. Earlier I explained how this impacts the RADS  
data base, and that the only thing I could do to harmonise the data  
set was to replace the SSB. All other things remained the same.

Last week Nicolas Picot and Shailen Desai provided correction data for  
GDR-B that allows expert users to update the GDR-B data to GDR-C  
standards. Some information was missing though, which, on my  
instigation, have since provided by the Jason-1 project.

Below follows a list of changes to the Jason-1 data that have been  
uploaded last night to the server in Delft. This makes all Jason-1  
data consistent and conforming to the GDR-C standards. In the next  
months I expect the official GDR-C cycles to come in in bunches which  
I will process to replace the current patched data. The differences  
between the two (GDR-C and patched GDR-B) are, however, minute, as I  
have verified by comparing patched and official releases of those  
cycles for which I have both the GDR-B and GDR-C.

The Delft server was synchronised just an hour ago.

** EIGEN-GL04C orbit **

The Jason-1 GDR-C sports, consistent with Jason-2, an orbit based on  
the EIGEN-GL04C gravity field, including time-varying coefficients.  
This orbit is significantly different form the EIGEN-CG03C orbit that  
it replaces. There was some confusion in the beginning as for which  
time-varying coefficients to include, but that is now settled in the  
standards that are referred to as GDR C' (GDR C prime).

Orbits provided in the SP1 format have been interpolated and put on  
all Jason-1 cycles 1-239. The remaining cycles already had the new  
orbit. I verified that the interpolation of the orbit yields the same  
results as on the GDRs within 1 mm.

To better distinguish the old and new orbits, I renumbered the EIGEN- 
GL04C orbits to field 414. The EIGEN-CG03C orbits are available as  
field 412 (only for those cycles that are not directly computed with  
GDR C, i.e. cycle 1-207).

To be consistent, the orbits on the J2 data were renumbered from field  
412 to 414 as well.

** JMR CORRECTIONS **

JMR correction files were provided by Shailen Desai. This includes  
some minor adjustment to calibrations and the handling of the side  
lobes of the antenna pattern. This updates the brightness  
temperatures, and all related parameters: wet tropospheric correction,  
radiometer land mask, liquid water vapour content, and the atmospheric  
corrections to sigma0.

** INSTRUMENTAL CORRECTIONS **

After retracking, SWH, range and sigma0 are corrected based on so- 
called Look-Up Tables that list those corrections as a function of the  
measured SWH. Between GDR-B and GDR-C these LUTs were changed. The  
corrections differ by up to 5 cm in SWH and 4 mm in range. Of course,  
these changes also affect the SSB and ionosphere correction.

** SEA STATE BIAS **

The sea state bias was changed significantly between GDR-B and GDR-C.  
The former SSB model was, in fact, based on GDR-A data which used a  
different retracker (MLE3) than GDR-B and GDR-C (MLE4). The current  
SSB model is based on GDR-B data, and should be consistent with GDR-C  
since it uses the same retracker. Alas, the LUT changes, also  
functions of SWH were not considered in the SSB, nor were the new  
orbits, so small corrections to this model may be needed in the future.

** WIND SPEED **

A minor change in wind speed and a more significant change in SWH  
changes the wind speed according to the Vandemark et al. algorithm.

** IONOSPHERE CORRECTION **

The change in Ku and C-band range as well as change in SSB requires a  
change in dual-frequency ionosphere correction. The ionospheric  
correction is recomputed based on the new ranges, new SWH, new wind  
speed, and new SSB correction, and then smoothed.

Cheers,
Remko