[time-nuts] WWVB and Free Democracies Survival

[Jim Lux]

On 7/15/12 6:25 PM, Chris Albertson wrote:
> On Sun, Jul 15, 2012 at 5:24 PM, Magnus Danielson
> <magnus at 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.
>