[ntp:questions] Sure gps looses all sattelite fixes

unruh unruh at invalid.ca
Thu Mar 15 15:55:00 UTC 2012

On 2012-03-14, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Wed, Mar 14, 2012 at 1:32 PM, unruh <unruh at invalid.ca> wrote:
>> On 2012-02-24, unruh <unruh at invalid.ca> wrote:
>>> 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
>>> dropout.
>>> 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
>>> ftp://theory.physics.ubc.ca/outgoing/drop{out,in}.pdf
>>> 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
>>> emission?
>> Just wanted to report here. I switched the antennae on the two Sure GPS
>> I have. Both worked fine for about 4 days, and suddenly the antenna that
>> had failed before failed again-- no sattelites found.
>> So, I am sure it is the antenna that is failing.
>> Sure has said that they will send me a new one, but it has not arrived
>> yet.
>> _______________________________________________
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> Is it an "active antenna"  These run on 5 volts that is supplied over
> the antenna lead.   The antenna can appear to have failed if either
> the 5V is not supplied or is blocked somehow.   You might want to
> verify that 5 volts DC is reaching your antenna.

Sure supplies an active antenna which claims to require 3-5V. My
measurement shows that 4V is supplied. But antenna A fails when
connected to either Sure board 1 or 2, while antenna B works when
connected to either board. That suggests it is the antenna not the board
that is the problem. And the failure is sporadic, which would require a
sporadic 5V again something that is belied by the difference between the
two . There might be a loose connection somewhere in the wire to the
antenna or inside, but both I count as antenna failure. .

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