Next message: Red: "Re: Astronomy and Celestial Navigation"
Alex-
<It is much harder to jam a signal coming
from the sky on very short waves.
(Roughly speaking, you have to send the jamming signal from
the same direction, but this is clearly impossible).>
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.
<Destroying a US military satellite will probably mean
a full scale war with the US, which I think is unlikely.>
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.
