[time-nuts] Re: GPS failed

[Andy Talbot]

I have had one, and possibly two, low cost active antennas  go unstable and
start oscillating.  These are the  magnetic car mount type with a ceramic
patch antenna.   The ceramic patch has quite a high Q and determines the
frequency the unstable RF front end takes off at - which is obviously at
1575.42 plus / minus a few tens of kHz.   Needless to say, all local GPS
receivers are jammed

I also heard a case of a GPS antenna going unstable, oscillating and taking
out most of the boats in a marina.   The Radio Communications Agency (as
our enforcement body was then, before it became Ofcom) had to be called
out to identify the problem.

Andy
www.g4jnt.com



On Mon, 11 Jul 2022 at 14:37, Lux, Jim via time-nuts <
time-nuts at lists.febo.com> wrote:

> On 7/10/22 4:19 PM, skipp Isaham via time-nuts wrote:
> > Hello to the Group,
> >
> > I'd like to get some opinions and war stories regarding GPS reliability
> at
> > high RF level and elevation locations.
> >
> > Background:  Three different hill-top GPS receivers, all different
> types, using
> > different antennas mounted on an outside fixiture, plain view of the
> open sky,
> > all stopped working.
> >
> > Test antennas were brought in and placed on a fixture well away from the
> > original antennas, the recevers went back in to capture and lock.
> >
> >  From what I understand, the original antennas are what I would call
> straight
> > preamp with no pre-selection / filtering.
> >
> > The ordered and now inbound replacements are said to contain a SAW filter
> > system. It is the intent of the client to just place these "improved
> antennas" in
> > to service and get on with life.
> >
> > I would suspect a GPS antenna (and receiver) could be subject to RF
> overload
> > or blocking, however, we're assuming nothing major has changed at the
> site, nor
> > any nearby location.  One might think there are more GPS receivers being
> pushed
> > out of reliable operation by the world around them, I'm just not hearing
> those stories
> > from a lot of people using them (GPS receivers).
>
> yes, this happens.  We used to have a Pendulum timing receiver with a
> typical "small white cone" type amplified antenna - if someone was on
> the roof with a cellphone, it lost lock, presumably from the (way out of
> band) emissions.
>
>
> As to where the interfering source is - it doesn't take much, and it
> could be some distance away.  After all, this was the big deal with
> LightSquared - it was moderately high powered terrestrial broadcast
> transmitters in the satellite downlink band next to GNSS.
>
>
> >
> > Any new install GPS receiver antenna ordered will/should contain some
> pre-selection
> > to potentially avoid a problem, even some years down the road? Seems
> like that's
> > where things are going... no more off the shelf, wide band, (hot)
> preamplified GPS antennas
> > in busy locations?
> >
> > Thank you in advance for any related comments and/or opions ...
> >
> > cheers,
> >
> > skipp
> >
> > skipp025 at jah who dot calm
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