GPS. David Smith CENBG/In2p3/CNRS Bordeaux, France. Absolute LAT Timing. Motivation. For pulsar studies, bad event absolute time stamps would be nearly as bad a disaster as gyroscope failures. Many missions have had issues, including recent ones.
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Absolute LAT Timing
Example of “real life” timing problem
CHANDRA pulsar timing problems, cont’d
CHANDRA pulsar timing solution
They recovered, but apparently with some loss of pulsed sensitivity…
LAT pulsar timing tests
Real GPS timestamps in real data
VSC = Virtual Space Craft
It’s just a VME crate with some ordinary and some special modules.
How to build MET from raw data
Slide 9 of Anders Borgland’s talk,
Timestamps in SVAC & Merit Tuples
E-mailed with Anders & Heather about this.
To me, seems that Merit value should be replaced by TTCTSecs+TTicks/diff(Anders’ recipe) but they concluded something different that I didn’t understand.
Nota bene – Joanne Bogaert selects rdb calibration files using these event times.
(What does “plausible” mean?)
<78>smith:borlin57.cenbg.in2p3.fr% /bin/date -u --date="1 Jan 01 00:00" '+%s‘ 978307200
<79>smith:borlin57.cenbg.in2p3.fr% /bin/date -u --date="27 May 06 18:59:38" '+%s‘ 1148756378
Difference is 170,449,178 (MET of begin run).
SVAC recipe shows 170,449,260 82 second difference “plausible”
EvtID TTCTSecs TTCTTicks TTCGemTTix TTPGemTTix diff TTicks/diff
3695052 170449260 21287574 2918647 16473104 19999975 1.064380
3696052 170449263 32400049 29364140 9364165 19999975 1.620004
3697052 170449265 9014258 2255226 15809683 19999975 0.450713
The 20 MHz scaler counts 19999975 times per second instead of counting 20000000. Fine.
TTicks/diff would, naively, never be more than 1 but apparently some PPS's get lost, which is also fine.
(A glance at the status words)
NEXT: quick look at the many status variable...
"FlyWheeling" always zero for the above.
if (TTCincomplete!=0 || TTPincomplete!=0) nHOLA++ ; ten thousand times! Always 65536.
printf(" Current flags: %d %d %d %d %d \n",CFlagsValid, CMissingGps, CMissingCpuPps, CMissingLatPps, CMissingTimeTone) ; Ditto “Previous”
For all 10 events sampled from 10,000 events, here's what you get:
Current flags: 1 1 0 0 0
Previous flags: 1 1 0 0 0
Anders says: features known from the digi reports, and under investigation.
Current events (downstream)
Current events (upstream)
* See ccsds.org
An independent, external muon timestamp?