Date: Fri, 30 Jan 2009 12:27:07 -0500 From: "Jim.Ray" Subject: [IGSMAIL-5890]: recent problems with PRN16 orbit predictions To: IGSMail Reply-to: jim.ray@noaa.gov Message-id: <4983386B.60600@noaa.gov> Content-type: text/plain; charset=ISO-8859-1; format=flowed Content-transfer-encoding: 7bit User-Agent: Thunderbird 2.0.0.19 (Windows/20081209) Sender: owner-igsmail Precedence: bulk ****************************************************************************** IGS Electronic Mail 30 Jan 09:27:12 PST 2009 Message Number 5890 ****************************************************************************** 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).