[time-nuts] GPS Receiver Self-Test Gotchas

Randy Warner Randy at synergy-gps.com
Tue Aug 8 22:06:03 UTC 2006


 
Guys,

Sorry to post again, but it just dawned on me that I meant to post some
info concerning M12+, M12, GT, UT+, and probably even VP receivers. This
issue is one of the reasons the M12M is late coming out of the chute. If
I have posted this before I apologize. Just too many things rattling
around in my brain.

CRITICAL: Whenever you run a Self-Test on any of these receivers, MAKE
ABSOLUTELY SURE that you wait until you have gotten the response message
before you issue any additional commands. If you get REALLY lucky and
send your command at EXACTLY the wrong time while the UART is in limbo
you can get stuck in a lovely "do...while" loop that can only be exited
by cycling power to the receiver. A clue that this has occurred is that
the 1PPS also stops. The Self-Test code in the M12M has been changed to
keep this from happening.

Looking back, this explains a lot of the unexplained random failures in
cell towers reported over the years by a couple of the carriers.
Absolutely unrepeatable. Almost impossible to troubleshoot. We just
found it by accident.

Now, before any of you wise guys try to repeat this problem, bear in
mind that statistically it seems to happen maybe once per 200,000 Self
Tests.

Randy




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