[time-nuts] Jamming GPS

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Mon Nov 16 15:22:54 UTC 2009




On 11/16/09 7:17 AM, "John Green" <wpxs472 at gmail.com> wrote:

> All the discussion regarding jamming GPS has been very interesting. Because
> it is spread spectrum, I always assumed it would be difficult to jam GPS.
> The Monterey Bay incident indicates that it is extremely easy to jam GPS.
> So, as soon as another project is out of the way, I intend to do some
> experiments, more of a practical than scientific nature. I have a Z3801 and
> Tbolt to start with. I would also like to experiment with a hand held and a
> GPS enabled cellphone. To prevent unintended interference, signal levels
> will have to be carefully controlled and transmission time will have to be
> kept short. I want to try CW as well as modulations of varying types. Any
> suggestions as to the most effective modulation type to try?
> 


Do you want to jam during acquisition or during track?

Your cellphone probably has what's called an "assisted GPS" receiver, where
the cell system tells the receiver in the phone initial estimates of code
phase as well as which satellites are in view.  That is, a lot of the
initial acquisition steps are already done.  (it reduces nanojoules/fix
dramatically, and that's important in a cellphone.. And it makes the
receiver cheaper and that's important too)

Most inexpensive receivers use a single bit quantizer, so they're quite
vulnerable to a CW signal at the right frequency, because that will dominate
over the received spread signal, especially before code acquisition.





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