From djt@egret.gsfc.nasa.gov Mon Sep 25 16:56:01 2006 Date: Mon, 25 Sep 2006 10:49:38 -0400 (EDT) From: David Thompson To: smith@cenbg.in2p3.fr Subject: CGRO pulsar timing David, I managed to track down the ancient time correction issue for CGRO. Here is the explanatory message (note that it came from none other than Rob Cameron). It does not explain everything, but the key error was the 2.048, which was the length of a major frame of telemetry data. The time was being attached to the wrong major frame. The other corrections are just perturbations on that big one, largely because the timing had to be based on ranging (through TDRS, so the correction was significant) in this pre-GPS era. You will recognize most of the other names on the address list, too. For your historical interest, Dave Thompson ---------------------------- Original Message ---------------------------- Subject: message from Rob Camreon, explaining "all" From: ULMER@OSSENU.ASTRO.NWU.EDU Date: Wed, July 1, 1992 3:25 pm To: djt@sage0.gsfc.nasa.gov -------------------------------------------------------------------------- From: OSSE::CAMERON "Rob Cameron, Code 4153, Naval Res. Lab. 202-7675530" 25-JUN-1992 08:28:52.11 To: KURFESS,JOHNSON,GROVE,LEAS,OSSENU::ULMER,MESSINA,SANDORA CC: CAMERON Subj: explanation of GRO clock correction > TO: INSTRUMENTERS > FROM: BARB MEEHAN > > DATE: 25 JUN 92 > > SUBJECT: CLOCK CORRECTION > > TWO COMMANDS WERE TRANSMITTED TO CORRECT THE CLOCK: > > 1) /UDELDNMS,SECONDS=2 @ 177/105325 > 2) /UDELDNMS,SUBSEC=43109 @ 177/105525 > > IMMEDIATELY FOLLOWING THE S/C CLOCK TIME OF 39325393865.7 > THE CLOCK CORRECTION WAS COMPLETE. Here is my interpretation of this message, from discussion with the FOT. Immediately before the clock updates above, the GRO clock error consisted of two parts: (i) a -45 microsecond error in the on-board clock, based on the old and incorrect understanding of clocks on GRO. This is the clock error which the FOT has customarily tried to keep below 100 microseconds absolute. (ii) a 2.042144 second error, which results from a 2.048 second error in the telemetry interacting with GRO ranging codes through TDRS. The total of these two errors is 2.042099 seconds = net error of GRO clock, and so the total correction applied to the GRO clock was 2.042099 seconds. This total correction was applied in two steps: 1) a 2.00000000 second correction 2) 43109 periods of the GRO 1.024 MHz clock signal = 0.042099 seconds. In OSSE telemetry for day 92/177, 2.000000 seconds was subtracted from the GRO clock between SRCCTs 30565 and 30566, at 10:53:28. A further 42.125 msec (to the nearest 0.125 msec) was subtracted from the GRO clock between SRCCTs 30623 and 30624, at 10:55:27. People who worry about the signs of these errors (was the GRO clock ahead or behind true UTC?) may continue to do so.