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From: David Spain on 20 Dec 2009 16:13
Geoffrey A. Landis wrote:
> Unfortunately, if you put up multiple satellites with small apertures,
> the ground footprint you get is based on the size of the *small*
> apertures, not the total area, and not the effective size of the
> phased array. This is the "Thinned Array Curse"-- I wrote an article
> for Wikipedia on it once http://en.wikipedia.org/wiki/Thinned-array_curse
>

Hi Geoff,

Thanks for stopping by. Question for you, have you studied LEO powersats
at all? And if so, what kind of antenna would likely be employed. Would
it be something mechanical or could some kind of switched phased array
work?

Presumably with a LEO satellite the antenna could be made much smaller?

Any references you could provide would be greatly appreciated, thanks!

Dave
From: Pat Flannery on 20 Dec 2009 20:47
Geoffrey A. Landis wrote:
> On Dec 19, 1:01 pm, Pat Flannery <flan...(a)daktel.com> wrote:
>> ...
>> Sylvia's multiple SPS constellation with the individual satellites
>> widely spaced certainly has simplicity on its side.
>
> I'm coming in late to this discussion, but it's worth pointing out
> that the reason that solar-power satellite concepts are large is that
> you need a large transmitting array to make a small spot on the
> ground. (the canonical numbers are that a 1 kilometer transmitter in
> GEO puts a 11 kilometer spot on the ground, assuming 2.45 GHz). This
> drives the sytem to large sizes.

I think Sylvia's plan was to build a couple of big ones, so that one
would be in sunlight while the other was eclipsed.

>
> Unfortunately, if you put up multiple satellites with small apertures,
> the ground footprint you get is based on the size of the *small*
> apertures, not the total area, and not the effective size of the
> phased array. This is the "Thinned Array Curse"-- I wrote an article
> for Wikipedia on it once http://en.wikipedia.org/wiki/Thinned-array_curse
>
> This is worth emphasizing: you can't use phased array techniques
> ("synthetic aperture") to make multiple small transmitters widely
> separated look like one big transmitter-- this works for receivers,
> but not for transmitters. More specifically, for a transmitter you
> can beam to a smaller spot using a synthetic array of widely-separated
> transmitters, but you don't put high power density in that spot: if
> you synthesize an array to put down a spot that's, say, 1/100th the
> area of the spot from one of the individual transmitters, you will
> only put 1/100th of the transmitted power into that spot.

Would the inflatable bowl sunlight reflectors/microwave transmitter dishes?

>
>
>> ...
>>> and some interesting comments on how to best make SPS pay by Geoffrey Landis
>>> at Glenn which also talks about the alternative of placing a SPS at L2 instead
>>> of GEO to supplement ground-based solar:
>>> http://www.nss.org/settlement/ssp/library/2004-NASA-ReinventingTheSolarPowerSatellite.pdf
>
> It's also here: http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2004/TM-2004-212743.html
>
> I had an earlier paper just on the L2 satellite concept; it's in the
> old Montreal conference proceedings: G. Landis, "A Supersynchronous
> Solar Power Satellite," SPS-97: Space and Electric Power for Humanity,
> Aug. 24-28, 1997, Montreal, Canada, pp. 327-328. May be a little hard
> to find these days.
>
> For what it's worth, the L2 idea doesn't work economically as an entry-
> level solar power satellite-- the economic case might work after the
> "easy" markets have been saturated.
>
>> At GEO it's already a long way to the rectenna on the ground; sticking
>> it clean out in a L2 halo orbit beyond lunar distance is going to make
>> the focusing of microwave beam onto a reasonable sized rectenna even
>> more difficult.
>
> Yep, exactly. It's only economic when it's scaled to huge size. Not
> an entry-level system!
>
>> The idea of the relay satellites is interesting, but
>> then you run up power loses unless you make them into some sort of
>> simple reflectors rather than microwave receiver/transmitters.
>
> Yes, actually, every analysis I've seen that tries to do reasonable
> engineering on power relay satellites seems to show that they are very
> nearly as expensive as solar power satellites.

Yeah, the weight of the hypothetical reflector system is going to be
around the same as a solar array, and the cost of getting the parts to
GEO drives the overall system price far more than the materials you use
to build it.
This could lead to some interesting discussions versus using high
efficiency solar cells that weigh more per cell, but would mean a
smaller array size, versus lower efficiency cells that are low in
weight, but mean a larger array.
One thing that does need looking at is the fact that a huge solar array
of very lightweight construction is effectively a solar sail, and the
sunlight hitting it is going to accelerate it during half of its orbit
and slow it down during the other half.
If it weren't for the amount of time it would take, by cleverly angling
the array either face-on or edge-on to the Sun as it orbits, you might
be able to move the SPS from HEO to GEO via the solar sail effect alone.

Pat
From: Sylvia Else on 20 Dec 2009 19:13
Pat Flannery wrote:
> Geoffrey A. Landis wrote:
>> On Dec 19, 1:01 pm, Pat Flannery <flan...(a)daktel.com> wrote:
>>> ...
>>> Sylvia's multiple SPS constellation with the individual satellites
>>> widely spaced certainly has simplicity on its side.
>>
>> I'm coming in late to this discussion, but it's worth pointing out
>> that the reason that solar-power satellite concepts are large is that
>> you need a large transmitting array to make a small spot on the
>> ground. (the canonical numbers are that a 1 kilometer transmitter in
>> GEO puts a 11 kilometer spot on the ground, assuming 2.45 GHz). This
>> drives the sytem to large sizes.
>
> I think Sylvia's plan was to build a couple of big ones, so that one
> would be in sunlight while the other was eclipsed.

With one being sufficient for the lowish load around midnight, but both
being available during the day to support the higher load.

Sylvia.
From: Pat Flannery on 20 Dec 2009 23:15
Pat Flannery wrote:

Whoops.
> Would the inflatable bowl sunlight reflectors/microwave transmitter dishes?
....have the same focusing problem?

Pat
From: Sylvia Else on 20 Dec 2009 21:35
Pat Flannery wrote:
> Pat Flannery wrote:
>
> Whoops.
>> Would the inflatable bowl sunlight reflectors/microwave transmitter
>> dishes?
> ...have the same focusing problem?
>
> Pat

Not as far as I can see - because they are not thinned arrays.

If I understand the issue correctly, it's not saying we can't build this
thing using a phased array - just that we can't expect to thin the array
for the purpose of achieving a smaller receiving antenna. Our array has
to be a filled array of whatever size is required.

Sylvia.
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