NOAA's Enhanced Geosat GDRs using JGM-3 orbits
 
John Lillibridge

NOAA Laboratory for Satellite Altimetry
1305 East-West Highway
Silver Spring, MD 20910
johnl@bigbird.grdl.noaa.gov

NOAA is the primary producer of altimetric data sets from the Geosat mission, 
in the form of Geophysical Data Records (GDRs).  Initially, only data from 
the unclassified Exact Repeat Mission (ERM) were released.  With the 
declassification of the Geodetic Mission (GM) data in the summer of 1995, we 
began work on a new set of enhanced GDRs for the entire Geosat mission.

The primary enhancement in the new GDRs is an improved orbit, computed by 
NASA/GSFC, based on the latest JGM-3 gravity model.  The errors in the orbit 
are at the 10 cm level, benefitting from a combination of the new gravity model 
and increased Doppler tracking data (also recently declassified).  By contrast, 
the previous release of Geosat GDRs was based on GEM-T2 orbits which had errors 
of 40-50 cm.  The new GDRs allow many altimetric problems to be investigated 
without the need for any orbit error removal.

In addition to the vastly improved orbits on the new GDRs, nearly every other 
field in the data records has been upgraded.  The major enhancements include: 
the addition of a 5 msec timing bias between the range data and orbit; tides 
from the CSR 3.0 model; ionosphere corrections from the IRI95 model; wet and 
dry troposphere corrections based on recent NMC Reanalysis model grids; and a 
sea state bias correction (from Philippe Gaspar at CLS) which is a function of 
wind speed, wave height, and satellite attitude.  Also included with this 
release of the data are ancillary files to correct for oscillator clock drift + 
internal calibration, as well as a non-local globally-averaged inverse 
barometer correction.

An indication of the improvement in the Geosat data can be gained by looking at 
crossover height differences within each 17-day repeat cycle. The T-2 GDRs had  
RMS crossover differences of 40-50 cm (while the GM GDRs were around 1 m); the 
JGM-3 GDRs have crossover differences of only 12-17 cm.  The enhanced Geosat 
GDRs will be available on a set of CD-ROMs from NOAA's National Oceanographic 
Data Center by the end of 1996.