[time-nuts] Effects of Simple GPS jamming on GPSDO's ?

Scott McGrath scmcgrath at gmail.com
Fri Aug 31 15:52:46 EDT 2018

And here is one of the schematics running around the ‘net.   This one is noise based

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On Aug 31, 2018, at 3:41 PM, Gerhard Hoffmann <dk4xp at arcor.de> wrote:

> Am 31.08.2018 um 19:39 schrieb jimlux:
>> On 8/31/18 10:15 AM, Bob kb8tq wrote:
>> Hi
>> Having spent a lot of my life designing GPSDO’s it’s a “that depends” sort of thing.
>> For a simple noise jammer, yes, they pretty much all will go into holdover. When the
>> jammer goes away, they come out of holdover. There are a few older units that may not
>> do quite as well with various sorts of broadband jamming.  With a spoofing jammer that is flying
>> around overhead and simulating an entire constellation … you could see any of them do odd
>> things. An airborne jammer flying over this or that city likely gets you into a “act of war” sort of issue.
>> It’s something you build if you are a nation state.
>> The performance with noise jammers is not a guess. It’s based on field experience and
>> all those never ending meetings I keep referring to …..
IIRC, there was a truck driver who successfully jammed all those airworthy GPS systems
at SF airport trying to hide his private detours, just by passing on the highway with
El Cheapo hardware.

> In effect, a broadband jammer (or, probably, a tone jammer that overwhelms the 1 bit ADC receiver) is the same as a "loss of signal" - the receiver probably doesn't know the difference - it just drops sync and tries to unsuccessfully reacquire.

I think that Holmes wrote somewhere that the easiest way to jam was a carrier quite close
to the frequency where the suppressed carrier of the BPSK would be. It could be weak because
it would have some processing gain, even if not completely sync to the rest of the signals.
The typical 1 or 2 Bit ADC has no chance to see it separated from the rest.

> So you can test your hold over behavior with aluminum foil (or your hand) over your antenna<grin>
OMG, I first read "with aluminium foil hat over your head"....


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