This page contains information concerning tests of the absolute times associated with GLAST LAT events.
Motivation: Important satellite missions, amongst which are USA, Chandra, and CGRO, have had problems with the absolute times assigned to events. See the anecdotes at the end of the page. The 10 uS timing precision required for the GLAST LAT is _essential_for good pulsar science. In addition to extensive testing of the different links in the timing chain, we have proposed an end-to-end test which will be the main focus of this page.
Current projects:
Status of VME GPS in Bordeaux. Actually, it's at the CERN SPS just now, how about that?
Shopping list of things to prepare before going to NRL ; and then to Arizona Draft of list of things to prepare.
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:
At left is cos(theta)sin(phi) vs cos(theta)cos(phi) for those of the first 13000 events having exactly one reconstructed track. I faked a corresponding list of VME GPS time stamps from the muon telescope by taking the LAT times and adding1 µS per event to them (an implausible and catastrophic scenario intended mainly to give some grist to my code). I then pretended that the scintillator paddles were at the place in space corresponding to the spot in the middle plot, which I reconstructed by requiring <100 µs between the LAT time and my fudged time. Note the presence of a few accidentals. Finally, in the plot on the right, I made a spatial cut and plotted the time difference (for a larger sample than for my training sample).
Since the May data had a fair number of strangenesses in the ContextLsf variables, I re-ran the code for some post-TVAC "door open" data from GMT 2006-09-08 11:13:43 (run 77010099). The detector is horizontal. The left-most plot is amusing, I didn't fill the other two. I can explain the funny structure -- can you?
As for the weirdnesses -- they didn't reappear. I spoke with Anders, he said "FSW hasn't changed between the two data sets, those bugs are known and sporadic, you were just lucky on the recent data". Last but certainly not least: here is my code in a gzipped tar file.
Background material:
Links to science requirement document ; to measurements made by J.G. Thayer ; to plot and text made by G. Godfrey ;
Presentation to Spectrum (General Dynamics) made at 22 August I&T meeting by Neil Johnson, prepared by David Smith.
Presentation made in SVAC IA meeting, 16 June 2006 by Smith. It builds on an IA presentation by Anders Borgland on 26 May. The thread continued with presentations by both Anders and Warren Focke on 30 June ; and Warren again on 14 July. You can find all these at the IA meetings page, http://www-glast.slac.stanford.edu/IntegrationTest/SVAC/Instrument_Analysis/Meetings/agenda.html
Link to Bordeaux DAQ note that was prepared for the Beamtest workshop.
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).
XMM: Jean Ballet tells me that two years elapsed before absolute phases were really right, and that there were a series of problems. Recall that XMM has a CCD -- it's not one photon per event. I'd welcome more details on this episode.
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.