[ntp:questions] Sure gps looses all sattelite fixes

Chris Albertson albertson.chris at gmail.com
Wed Mar 14 23:02:28 UTC 2012

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.
> _______________________________________________
> questions mailing list
> questions at lists.ntp.org
> http://lists.ntp.org/listinfo/questions

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.

Chris Albertson
Redondo Beach, California

More information about the questions mailing list