[time-nuts] "The GPS navigation is the weakest point,"

[Jim Lux]

On 12/15/11 2:24 PM, Azelio Boriani wrote:
> There are GPS simulators for lab use (never seen live or in a picture), I
> suppose they have one connector to feed the GPS receiver antenna.
> Generating in one equipment all the signals you don't need many but only
> one precise timing source.
>
>

Not quite (there's a discussion of this on the list about a year or so 
ago)...

It's harder than you think to generate realistic fake signals for a 
moving target.

At work (JPL) we have a fancy Spirent GPS simulator.  And sure enough, 
it can generate all the signals your receiver would see given a 
particular path you expect your receiver to follow.

But, in order to use that to provide a spoofing signal, you'd need to 
know (fairly precisely)

a) the position and velocity of the victim
b) the position and velocity(zero) of the jamming station

You calculate what the expected time,code phase, and doppler of the GPS 
signals would be at the victim.  Then, you subtract out the time from 
jammer to victim and the doppler from jammer to victim, and use that 
generate your spoofing signal.

Then, the trajectory of the spoofed position has to be something that is 
internally consistent (i.e. the acceleration, velocity, and position all 
have to agree in the Kalman filter), and you have to continously update 
your jamming signal with continuously updated position and velocity of 
the victim.


Spoofing GPS is very hard.It was designed to be so, both for its 
original military purposes and because you want internal consistency 
checks to make sure you aren't displaying false information to a user.

Jamming GPS to deny it is relatively easy. A high power swept tone does 
it very nicely on inexpensive receivers. There are more sophisticated 
approaches.  You can buy them for $20 on the internet that plug into a 
car cigarette lighter.  You get one of these jammers, put it on an 
airplane with a big power amplifier and fly above your sovereign 
territory and you can deny GPS to pretty much everyone underneath you. 
There are receiver designs that can tolerate tone or swept or barrage 
jammers, but they are more expensive, heavier, etc, and I suspect they 
wouldn't bother on a UAV.