From jim.ray at noaa.gov Wed Jul 1 11:06:26 2009 From: jim.ray at noaa.gov (Jim.Ray) Date: Wed, 01 Jul 2009 14:06:26 -0400 Subject: [IGSMAIL-5965]: change to exclude some IGU clock predictions Message-ID: <4A4BA5A2.3000706@noaa.gov> ****************************************************************************** IGS Electronic Mail 01 Jul 11:06:47 PDT 2009 Message Number 5965 ****************************************************************************** Author: Jim Ray & Jake Griffiths (ACC) Starting with the IGS Ultra-rapid (IGU) combination today for 1538_3_12 we have implemented a new option to exclude clock predictions for certain satellites. Recall that in each IGU SP3 product file, the first 24 hr period provides satellite orbit positions and clock values based on near real-time observations; the second 24 hr period provides predicted orbit and clock values extrapolated from the earlier data. This change affects only the second half of predictions. While the IGU orbit predictions are of very high quality, normally with errors at the few cm level (IGS Mail 5874 at http://igscb.jpl.nasa.gov/mail/igsmail/2009/msg00000.html), the IGU clock predictions are only about the same level as the broadcast nav clocks. So real-time point positioning using the IGU predicted products performs not much better than BRDC. (On the other hand, near real-time positioning using the first half of each IGU SP3 file should perform nearly as well as the IGS Rapids.) The change today will eliminate predictions for the most problematic satellite clocks and should greatly reduce infrequent disruptions due to unexpected clock resets. A prime motivation for this change is the realization that most users of IGU clock predictions seek a stable time scale reference, not accurate inter-satellite clock offsets (which are being developed as a separate real-time product by the IGS Real Time Pilot Project). Starting today, IGU clock predictions are excluded for: * all 7 IIA SVs with Cs clocks -- PRNs 3, 8, 9, 10, 24, 27, 30 (usually all highly unstable) * all SVs set UNUSUFN -- presently PRNs 1, 25 * any SV during MX events -- presently none Clocks on the unhealthy UNUSUFN satellites are sometimes changed with no notice and MX events usually deal with clock issues, so eliminating these should improve the reliability of the IGS clock predictions. The IIA Cs clocks are simply much less stable than other SV clocks. As the very old satellites continue to be replaced, the number of IGU clock predictions will increase. We expect that the RMS of the remaining IGU clock predictions will improve from about 2 to 4 ns now to well below 1 ns. An ongoing problem with the IGU combinations is that the summary reports are incomplete. Therefore we will be working to enhance these reports so that they give a comprehensive account of the dynamic variations that occur in the combinations, including clock exclusions.