[time-nuts] WAAS.....
SAIDJACK at aol.com
SAIDJACK at aol.com
Thu Jan 9 21:07:48 UTC 2014
The latest receivers are surprisingly resilient to GPS jamming.
We tried jamming effects on all sorts of different GPS units ourselves,
and the M12's go out right away for example, while the uBlox units are tough
to jam. The new generation 7 ublox with Glonass etc should be even harder
to jam if programmed properly.
Attached is a sample plot showing GPS number of sats over a 1173 hours time
frame (49 days) of a FireFly-IIA unit sitting in our lab with an older
(and more jamming-sensitive) uBlox-5 in it. The antenna is simply a small
cheap magnetic puck sitting on top of the two-story roof, and the roof is
facing a highway. There was not a single instance of complete GPS Sat loss of
lock during that time frame, even though the antenna sits only a couple 100
feet away from from Highway 17 which has jammers on it that we can see pass
by on other GPS units.
Please note that this version of GPSCon was updated to be able to show 16
sats in the SatCount by dividing the indicated number of sats by 2 rather
than just showing a total of only 8 sats, but the indicator still says 8 sats
max. So the variations in sats tracked are actually going from about 11 to
16 during the test. We had requested this change to GPSCon so it could
support our 16 channel status output.
It may be worth a try to get an eval kit of the latest uBlox chipset
available to see how that handles typical known nuisance jamming scenarios.
Bye,
Said
In a message dated 1/9/2014 11:00:20 Pacific Standard Time, brian at lloyd.com
writes:
On Thu, Jan 9, 2014 at 12:21 PM, Hal Murray <hmurray at megapathdsl.net>
wrote:
>
> brian at lloyd.com said:
> > navigation system that is going up. For that matter, is anyone running
> one
> > of the new multi-system receivers? I notice that Garmin is selling them
> as a
> > matter of course now. The prevalence of jamming might be the reason
why.
>
> Aren't the alternatives using frequencies that are very close? Close
> enough
> so one the same receiver can pick up all the satellites. How much wider
is
> the total bandwidth? Does the filter on a typical L1 antenna reject, or
> maybe just weaken, any of the other systems?
>
GLONASS works on 1602.0 MHz (+/- ~4MHz). GPS works on 1575.42 MHz. There is
only about 20 MHz difference at 1.6GHz so it is entirely possible that a
wideband (noise-based) jammer would take out both, but be quite limited in
range. A narrow-band jammer would probably take out GPS but GLONASS uses
FDMA and separates each satellite in frequency by 0.5625 MHz. That means
that a narrow-band jammer might get one, two, or three birds but probably
not all of them.
It does seem to me that a combined GPS/GLONASS receiver is going to be more
resistant to jamming than a GPS-only receiver.
And I make no claims to being an expert. I am just mostly thinking aloud
here.
--
Brian Lloyd, WB6RQN/J79BPL
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.com
+1.916.877.5067
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