[time-nuts] GPS jamming susceptibility

[Magnus Danielson]

On 11/23/2010 12:19 AM, Bob Camp wrote:
> Hi
>
> There is *very* little signal hitting the ground from a normal GPS bird. Even a few mili watts close at hand is going to be an enormous overload. The typical GPS does not use a lot of bits in the front end A/D.
>
> I suspect that if you tuned your little gizmo down to the FM broadcast band, it would take out your favorite FM station quite nicely. Same would be true of your cell phone if you tuned it there. Jamming from close by isn't all that hard to figure out, or to implement. There are switching power supplies that make wonderful jammers for low frequency signals. If it's RF, it can be jammed. The real question is can you jam it from a reasonable distance?

There are a few reports and articles going into the susceptibility of 
civilian receivers to jammers. Some public texts have also been written, 
so the field is not completely covered only on green paper.

A CW jammer will basically grab the AGC and as it gains down the CW the 
GPS reception is gained down with it. In particular 1-bit receivers is 
susceptable to this effect. 1,5-bit receivers with separate AGC 
detection was developed and was able to combat the CW jammer situation.
The relative time when the code can control the bits quickly becomes 
just a fraction since a sine spends long times in the extremes far away 
from detection limits.

Next thing to attack is lack of supression in the C/A code, and list of 
offset-frequencies which is more susceptible can be found.

Noise jammers is also possible.

Things like these alongside the weak signal makes civilian receivers 
quite sensitive, so quite a bit of line-of-sight distance can be jammed 
with a fairly low output.

Cheers,
Magnus