NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Astronomy and Celestial Navigation
From: hellos
Date: 2006 Jun 4, 19:38 -0400
From: hellos
Date: 2006 Jun 4, 19:38 -0400
Alex-GPS uses the FM capture effect, that is, the units lock on to the strongest signals with no knowledge of the source. This is very different from the AM HF broadcasts, since AM receivers don't lock in to a single signal, they receive "all" signals at once and you are left to sort through them by other means. Also, with shortwaves you can get some directionality from the antenna placement to say "I only want signals from this bearing" but with GPS, you "must" look for signals in a full 180-degree hemisphere with a 360-degree bearing. You can't use directionality to block out signals, and still look at the satellites scattered across the sky moving through it. There were plans posted for a short-range GPS jammer online a couple of years ago. Junk box parts, easily lofted by a helium balloon, short range and easily found and targeted by active weapons. But the jamming is simple, you just need lots of them to cover more square miles. If a country wanted to use our own GPS system to direct an attack on us, we probably wouldn't know enough to disable it until that had happened. The military has been so generous with the civilian signal because they now can locally distort the signal, i.e. they can tell the satellites over a given area "lie and say you are over there instead" so that civilians and enemies get a false signal, while the correct encrypted signal is still sent over the military channels. But if someone was engaged in hostilities with us, was already actively in a limited state of war, and wanted to cripple *our* navigation? So what, they shoot down some satellites and what will happen in retaliation? We won't go nuclear and we won't be able to target resources at them as quickly or accurately. It makes good logic to expect the system to be targeted--if someone is already willing to have a shooting war, especially if they are not targeting the US mainland but we are instead attacking them somewhere else. Still, the risks of hitting a floater, a submerged container, or being pirated off Yemen or Malaysia, are probably higher, than the risk of losing the GPS system.
