[ntp:questions] Sure gps looses all sattelite fixes

unruh unruh at invalid.ca
Fri Feb 24 18:08:08 UTC 2012

I have a computer with a Sure GPS. Over the past week the unit has lost
all sattelite fixes a number of times. Recently I started saving the
GPGSV sentences, and caught at least one of those losses. The unit would
have 12 sattelites in view, but the S/N ratio dropped suddenly over
about 10 sec from about 51 dB or so ( which seems to the max) to
"untracked" (lets call it 0) In this case, after 10 min, the sattelites
came back. over about a minute to the levels they had before the
These dropouts seem to happen about 1 a day, and as I said extend from
minutes to half a day. 

The graphs of the signal strengths at the edges of these dropouts is in

The horizontal axis is seconds after some epoch. The vertical is S/N
ratio in dB as reported by the GPGSV sentences. the colours are
different sattelites ( colours repeat since they were assigned
atomatically based on sattelite number, there  are fewer colours than
sattelites-- 40 or so with 12 to 13 of them visible at any one time)

Note that there is also evidence of much shorter dropouts in other parts
of the data ( sometimes of only one sattelite). 

Is this a flaw in the Sure unit (NOte that the sure unit drops its PPS
signal as soon as it does not a fix. Some units-- Garmin?-- keep sending
out PPS based on the freerunning onboard clock for quite a while, as I understand it.
Teh Sure does not, which may be an advantage or a disadvantage.
(ntpd/chrony grab one of the other servers which produces
discontinuities in time when the pps comes back)

Is it evidence of some outside noise source (jamming?) , microwave tower

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