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
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