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When light squared was shut down for messing with gps frequencies in their satellite network, the military did not say that light^2 was actually broadcasting ou
by josh2600 6y ago
When light squared was shut down for messing with gps frequencies in their satellite network, the military did not say that light^2 was actually broadcasting outside of the expected frequency band. The deal is that the military just doesn’t use filters on most of their broadcast and reception equipment so their operating frequency is actually way wider than they say publicly.
This is because filters cost money and reduce the effectiveness of the broadcast system. If you could get away with operating over a much wider spectrum for any service, you probably would.
This is why when the military says “this interferes with gps even though it’s outside of the gps band were supposed to be using” what they mean is that they are using a much wider band of spectrum than what’s in the specs.
Ultimately, the fcc doesn’t have regulatory authority over the dod.
- michaelt 6y ago> The deal is that the military just doesn’t use filters Do you have a citation for that? I would have thought the issue is not that GPS receivers don't use filters, but rather than no filter has a perfectly sharp cut-off and GPS signals are exceptionally faint. For example, if the GPS signal is 0.1 femtowatts at 1575MHz, even if you've got a filter that can remove 99.99999% of a noise signal at 1600MHz you can still overwhelm the GPS signal with 1 nanowatt of power.
- javajosh 6y agoOh man, I bet there's a nice market of GPS jammers out there - especially useful for the home team in any action, since they don't need help with directions. I wonder if that counts as a weapon?
- wl 6y agoSome truckers use them for manipulating the trackers on their vehicles. Some of them are stupid enough to have them active when they are anywhere near an airport, which makes it far more likely they'll get caught.
- duskwuff 6y agoPrecisely -- no RF filter has a perfect "brick wall" response, and no transmitter will have a perfectly pure output either. GPS receivers already have to use some pretty crazy tricks to pull a signal out of the noise; it doesn't take a lot of interference to make that impossible.
- gorkish 6y agoThere is also the insertion loss and physical size of the filter to consider. It's a four variable problem where you can only optimize for any two.
- wl 6y agoThat's not accurate. Every GPS receiver in existence uses filtering. The question is, how sharp does that filter have to be? The GPS frequencies were chosen to be away from frequencies used by transmitters that might be near GPS receivers. Lightsquared proposed repurposing satellite downlink frequencies (which they bought at a discount because this restriction limited their use) for the purposes of terrestrial networking, breaking that assumption. In order to maintain similar functionality, GPS receivers would require significantly sharper filters, increasing bulk, cost, and complexity.