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DORIS Electronic Mail   Mon Oct 19 11:19:11 WETDST 1998    Message Number 0053
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Author: Nikita Zelensky
Subject: NASA/GSFC POD Web site

         DORIS and SLR Data Analysis from the 
NASA/GSFC Precise Orbit Determination Production System Web Site

NASA/Goddard Space Flight Center's Precise Orbit Determination Production
System (PODPS) computes the Precise Orbit Ephemerides (POE) for the 
TOPEX/POSEIDON (T/P) oceanographic satellite. 

Ten day arcs of DORIS and Satellite Laser Tracking (SLR) data are processed
by the GEODYN least-squares program to determine each orbit (POE). Each arc
corresponds to a T/P ground track repeat cycle. These orbits are used 
by the Jet Propulsion Laboratory to form the GDRs. The orbits are believed 
to have 2-3 cm radial accuracy.

In addition to computing the POEs, PODPS also keeps performance histories
for the DORIS and SLR stations by cycle. After each POE is computed, 
the data residuals are analyzed using the Residual Analysis Program (REP). 
REP fits a measurement and timing bias for each pass of residuals. The
average of these biases are saved for each station per cycle. REP also
computes the data post-fit residuals for each pass following the estimation
of the two biases. The post-fit residuals are combined for each station 
within a cycle, and reflect the level of the data noise.

Four performance history plots are kept for each station: 
(1) the number of passes imported and the number of passes actually 
    used in computing the POE.
(2) the GEODYN mean data residuals and the average REP measurement bias.
    Since the DORIS mean residuals are nearly always zero, they are 
    not plotted. The DORIS measurement bias is plotted, but in our 
    processing is considered a nuisance parameter adjusted to facilitate
    improving the timing bias estimate.
(3) the GEODYN RMS fit of the residuals and the REP post-fit RMS residuals.
(4) the average REP timing bias (time tag error).
These plots are updated on a monthly basis.

Station performance plots, POES, and other information can be accessed 
via the PODPS Web site:
 http://cddisa.gsfc.nasa.gov/926/podps/html/welcome.html

However, a direct link to the station plots is also available:
 http://cddisa.gsfc.nasa.gov/926/podps/html/dbase.html

The PODPS Web Site was developed for use with Netscape.

For further information or questions contact:

Nikita Zelensky   (PODPS Team Manager)
 nzelensk@geodesy.gsfc.nasa.gov
 (301) 441-4107
 or
Doug Chinn        (PODPS Analyst)
 dchinn@geodesy.gsfc.nasa.gov
 (301) 286-5940


[Mailed From: Pascal Willis <willis@ensg.ign.fr>]