On 11 Oct, 2011, at 05:59 , John Hasler wrote: >> I presume, to ensure that the military can continue to function (at >> some level) in the absence of usable GPS. > > They coud do that by turning off their receivers. More likely they are > testing anti-jamming techniques (and jamming techniques as well).
Yes, they aren't necessarily jamming their own receivers, I suspect they are practicing jamming other people's receivers while keeping their own working. Military GPS receivers (owned by those friendly with the US military owners of GPS, at least) use an additional service (PPS, for "Precise Positioning Service") at a second frequency which is much harder to jam than the GPS L1 band service (the C/A service, originally for "Coarse Acquisition") that the rest of us are limited to. This isn't just a European thing, either. A friend who regularly flies himself from California to New Mexico gets frequent FAA notifications concerning GPS jamming at White Sands Missile Range. And, I guess, what they are practicing might not just be jamming of (i.e. denying service to) GPS receivers. I read somewhere that a decade or so ago a Chinese GPS-guided missile test fired near Taiwan went in a dramatically different direction than was expected, suggesting an ability to not only deny civilian GPS service but also to actively spoof it (something which probably prompted China's current enthusiasm for launching their own satellite navigation system). That would be a far more complicated trick which might, in fact, require considerable practice to get good at. All this might also hint at the reason that many countries have rules prohibiting critical services, like telecommunications infrastructure, from depending on GPS for anything important despite the technical advantages this might otherwise provide. That's too bad. Dennis Ferguson _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
