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Status of VME GPS in Bordeaux . Actually, it's at the CERN SPS just now, how about that?--
- We latch times with a trigger signal, and interuppt the processor to provoke readout with the same signal, and all that works nicely.
- We compare our times with http://nist.time.gov/timezone.cgi?UTC/s/0/java and are convinced that our dates are accurate at the 1 second level.
- We are hatching schemes to validate our dates to the microsecond level, either using a reference at an observatory or with a completely different GPS syste. A possible example is http://www.rfsolutions.co.uk/acatalog/GPS_Evaluation_Kit.html
- We got a spare VME GPS module from our former CAT now HESS colleagues in Paris it, we tested it, and it works.
Shopping list of things to prepare before going to NRL ; and then to Arizona Draft of list of things to prepare. We could be ready to travel on very short notice now.
Data Analysis:
Using FSW muons acquired at NRL on 27 May, 2006 (run 77005390) I practiced how I plan to analyse the data we'll acquire. Here is the output plot:
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Anecdotes: Here are timing failures on four major missions, in chronological order.
Compton GRO: This was in the days before GPS. An on-board clock was set to absolute time using a reference like the one at the Bureau of Standards in Colorado, sent up to the instrument keeping close track of transit times. Events were assembled into packets on board, and the packets were grouped into a "major packet", to which a time stamp was afixed. These packets were sent to the ground by telemetry.The problem was that the time stamp was from the preceding packet! And so the time was off by more than a second. At first the engineers didn't quite believe the scientists, and the scientists weren't confident enough (yet) to be pushy. Further, once the time stamp was right, there was still an absolute phase problem, because the dispersion measure of Vela had changed by more than a millisecond (!) since the days of SAS. (Thanks to Dave Thompson for this story).
USA: The GPS kept getting stuck once on orbit and had to be reset a few times a day. The reason was that sometimes the satellite would go through GPS beams so intense that it confused the receivers. Furthermore, the speed with which the satellite would move relative to GPS's took it far from the design-regime considered for ground-based GPS's. Note that our proposed GLAST test would not have caught this problem. (Thanks to Michael Lovelette for this story).
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CHANDRA: For one of the two instruments on board, the HRC, the time stamp of a given event was that of the previous event. On board filters remove events, so obtaining the right date for a given event was impossible. The solution was to reduce the trigger area significantly, so as to reduce the event rate, so as to allow transmitting all events to the ground. This story documented in e.g. S. Murray et al, ApJ 568:226-231 (2002) and references therein.
Compton GRO: This was in the days before GPS. An on-board clock was set to absolute time using a reference like the one at the Bureau of Standards in Colorado, sent up to the instrument keeping close track of transit times. Events were assembled into packets on board, and the packets were grouped into a "major packet", to which a time stamp was afixed. These packets were sent to the ground by telemetry.The problem was that the time stamp was from the preceding packet! And so the time was off by more than a second. At first the engineers didn't quite believe the scientists, and the scientists weren't confident enough (yet) to be pushy. Further, once the time stamp was right, there was still an absolute phase problem, because the dispersion measure of Vela had changed by more than a millisecond (!) since the days of SAS. (Thanks to Dave Thompson for this story). Thanks further to Dave for digging up this old e-mail that details the story further.