>To decrease the jamming power required (this -is- spread spectrum,
>after all), it's helpful to have your jammer hop the same way your
>receiver will be hopping. This is pretty easy to do, since your
>jammer can trivially figure out the hops by observing the satellites
>you can see. Note also that any outfit that makes GPS's typically
GPS uses direct sequence spread spectrum, not frequency
hopping. Accurate timing is an inherent feature of direct sequence,
but usually not of frequency hopping.
The GPS C/A chipping sequences (known as Gold Codes) are openly
published, so you can generate them yourself with just a few shift
registers and some combinatorial logic. There are 32 different
sequences, each 1023 chips long, one for each satellite in the
constellation. No need to observe the satellites you're jamming.
You'll also need to impress the 50bps navigation message on the
chipping sequence, but again this is all openly documented (except for
the reserved fields that are apparently carrying encrypted data).
In many ways, a GPS spoofer is a much simpler device than a GPS
receiver.
Phil
