I wrote: > 1) Better front-end selectivity > 2) Better front-end dynamic range
David Woolley writes: > I don't really see how these help against all but the crudest jammers, > which transmit pure CW carriers or are off frequency. Better dynamic range means that the jammer must be closer or more powerful. Both it and better selectivity help against out-out-band interference, which is one of the concerns (probably a more serious one than jamming) (and most jammers *are* going to be of the crudest sort). I wrote: > 3) Directional antennas for fixed sites such as cell towers David Woolley writes: > That might help a bit, if you rejected signals near the horizon (which are > good for navigation, but may be bad for time). Signals near the horizon are weak and have gone through a lot of atmosphere. You don't really want them for any purpose if you can find better (and you can unless you are in the [ant]arctic). > However, to get full benefit from directionality, you'd need to run > large phased arrays and steer their beams to track the individual > satellites. As most military users want the navigation data, they > would be better off with steerable beams. 99.9% of jammers and 99.99% of interference sources are going to be on the ground. A well-designed "fence" (shielding, really) with a cutoff at, say, 30 degrees above the horizon can easily knock them down 100db or more. Due to the use of spread-spectrum jamming GPS has no effect as long as it stays within the dynamic range of the receiver (spoofing is a different issue entirely, and *much* more difficult to achieve). My point is that this "weakness" is more of a design weakness in commercial receivers than a fundamental weakness in GPS. In any case designers of things like cell towers should no more assume that GPS is always "just there" than they should assume that electric power is always "just there". -- John Hasler [email protected] Dancing Horse Hill Elmwood, WI USA _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
