[IGSMAIL-5890]: recent problems with PRN16 orbit predictions

[Jim.Ray]

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IGS Electronic Mail      30 Jan 09:27:12 PST 2009      Message Number 5890
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Author: Jim Ray & Jake Griffiths

In contrast with the rather optimistic view we gave in IGSMail #5874
(igscb.jpl.nasa.gov/mail/igsmail/2009/msg00000.html), starting about
Tuesday, 27 Jan 2009 (1516_2), IGS Ultra-rapid orbit predictions for
the GPS II-R satellite PRN16/SVN56 have suffered large, atypical errors.
The statistics for the first 6 hr of recent predictions are shown below
for that satellite and for the constellation means.  The overall
constellation WRMS was only seriously affected for IGU 1516_2_12, being
more than 7 times worse than normal.

=======================================================
IGU orbit prediction stats (1-D) for PRN16/SVN56 & full
GPS constellation w.r.t. IGS Rapid orbits
[computed over 6-hr intervals HR to (HR+6)]
=======================================================
                        PRN16    ALL    ALL
                         WRMS   WRMS   MEDI
              WEEK D HR  (mm)   (mm)   (mm)
              ---- - --  ----   ----   ----
              1516 2 00    34     21     17
              1516 2 06   127     35     18
              1516 2 12  1046    172     16
              1516 2 18   158     27     20
              1516 3 00   796     40     18
              1516 3 06    81     21     13
              1516 3 12   145     25     19
              1516 3 18    77     17     13
              1516 4 00    31     18     14
              1516 4 06    88     29     17
              1516 4 12    13     19     13
              1516 4 18    34     18     12

              recent means        24     18
=======================================================

The BRDC orbits for PRN16 seem to have suffered also during the
same period, esp on day 3:

=======================================================
WRMS BRDC orbit prediction residuals (1-D) for PRN16/SVN56
w.r.t. IGS Rapid orbits -- computed over 24-hr intervals
=======================================================
                           PRN16   All
                    WEEK D  (mm)  (mm)
                    ---- -  ----  ----
                    1516 0   946  1031
                    1516 1   794   907
                    1516 2  1259   909
                    1516 3  1915   955
                    1516 4   706   954
=======================================================

There has been no significant degradation in the directly estimated
IGS orbit products using actual tracking data for PRN16, only for the
orbit predictions.

PRN16 is approaching the end of its eclipse season but we did not
see similar problems during earlier periods in this same eclipse
phase.  These recent prediction problems are quite unusual for any
satellite but esp for II-R and II-RM spacecraft.

As you can see above, the median GPS orbit prediction residuals have
remained very low and stable throughout this interval, which means
that users should be able to detect and delete such instances of
anomalous satellite behavior by examining and editing their own data
fits (assuming simultaneous observations of at least 5 SVs).