It is not necessary to interfere with GPS in a wider area to mask your own position. The GPS signal is very low power (below the noise floor). It does not take much effort to block it. For this reason, the in car maskers (aka jammers) will block signals over a wide area unless they are precisely engineered to stay localized. There are a lot of poorly engineered maskers out there. I study these maskers and how they affect WAAS for the FAA.
