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

Bob kb8tq kb8tq at n1k.org
Fri Aug 31 18:21:58 EDT 2018


Hi

I think we have a little bit of confusion here. WWVB is not going to help anybody navigate. 
It’s not going to help track people with ankle bracelets or trucks stopping at bars. Car thieves 
jamming Lojack still happens. Turn iWWVB on or off, this stuff still goes on. None of this is 
a Time Nuts sort of issue. 

The only thing WWVB *might* do is provide timing. That’s very different than navigation. A 
mobile this or that driving by puts your GPSDO into holdover. It maintains time while it is in
holdover. Minutes or hours, possibly days … works the same way. The system keeps running
just like GPS was doing fine. If after a day or more, the GPS is still jammed, that single  cell tower 
shuts down. Take out one cell tower and the system keeps running. There is a lot of overlap on 
these systems. Towers go down a lot more often than you might think …..

Do all systems work identically in terms of timing? Of course not. If timing is critical to operation,
systems do use GPSDO’s. The same basic principles apply. The main question would be one
of overlap between elements of the system. 

The same jamming that takes out GPS for timing also takes it out for normal navigation. Take
it out over an entire city and everybody’s vehicle navigation system goes out. Do that even for
a couple of hours and it’s on the evening news. Do that for a day and there *is* a response. 
That’s the kind of thing needed to impact utility systems (like cell towers) in a significant way. 
It simply does not happen ….City wide is very hard to do from the ground.  From the air, you 
can get the coverage. It’s tough to keep doing it from the air for days on end. 

So, interns of “the world ends if / when WWVB turns off” … not so much.

In terms of the initial question, GPSDO’s in general are pretty good at handling the typical 
jamming they might run into. 

Bob



> On Aug 31, 2018, at 3:52 PM, Scott McGrath <scmcgrath at gmail.com> wrote:
> 
> And here is one of the schematics running around the ‘net.   This one is noise based
> 
> <image1.png>
> 
> 
> 
> 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"....
> 
> Cheers,
> Gerhard
> 
> 
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