On 7/15/12 6:25 PM, Chris Albertson wrote:
On Sun, Jul 15, 2012 at 5:24 PM, Magnus Danielson
<mag...@rubidium.dyndns.org> wrote:
The benefit of WAAS and EGNOS is that they have a fixed location in the sky.
so you could use a highly directional antenna, like a parabolic antenna,
which would provide suppression of most jamming signal unless they are
overhead.
I've seen a military GPS antenna that was a large phased array. I
guess the idea was to use the almanac to track the moving GPS sats.
The goal was to reject jammers by using a dozen or so synthesized
pencil beams. (like a radar in a fighter jet) You could not put this
in a hand held device as it was as BIG. That is the main problem
with directional antenna they need to be large with respect to the
wavelength. I don't know if this ever was used in a real deployed
system.
Doesn't need to be all that big... lambda at 1.5 GHz is 20cm.. an array
that's, say, 50x50 cm would have significant interference rejection
capability (i.e. you don't have to synthesize a pencil beam, you just
need to put a null on the interference.. and you can null N-1 sources
when you have N elements, so a 9 element (3x3) array would do nicely..
(and give you your attitude as a side effect)
I think this antenna type was also the design proposed for a
distributed low orbit comms system too. The current geo-sync
comm-sats make for simple antenna but all of the proposed tactical
"launch on an hour notice" comm-sats would be in LEO (low earth orbit)
and launched with something like Pegasus or a re-purposed ICBM.
The problem is that having a few dozen low power sats in LEO seriously
complicates the portable ground stations, hence experiments with
flat-plate phased array.
Not really.. it depends on the frequency and the number of sources you
need to track.
THis is a very active research area. The very last payload deployed
by the Space Shuttle was "pico-sat" a 5x5x10 inch satellite. It was
built at the place I worked at as a test of a new pico-sized bus.
This example had some sensors but really the test was if the thing
could be commanded from the ground and do anything usfull at all.
Look at the photo in the link. The sat is the box inside the bigger
box. One of the recent innovations was to cut out patterns in
sheets metal and stack many sheets to create a 3D tank and plumbing
system with integrated rocket nozzles. The goal was to reduce costs
by having a design that can be manufactured by robots.
http://www.space.com/12354-final-space-shuttle-satellite-deployment-picosat.html
So YES "everyone" knows these big satellites are targets. In a major
war with a sophisticated enemy they would be gone soon. So the plan
is to design systems that can be built stored and launched on VERY
short notice in very large numbers. This kind of research has maybe
a 25 year horizon maybe longer as you need to build up a new "eco
system" of space qualified parts and engineers familiar with them and
do many launches and tests.
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