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Tim Cash
On Jan 8, 2023, at 11:11 AM, Keith Henson <hkeith...@gmail.com> wrote:The CalTech design has the PV on the side toward the sun and microwave
transmitters on the other side. They can provide power only when the
microwave side presents some area to the earth. Which is to say they
are intermittent like ground solar, at best providing power half the
day.
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Tim Cash
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Hi John, good to hear from you.
One of the things I’m curious about, regarding your system architecture: you write “Satellites are grouped into massive arrays--100,000 satellites for 100MW--allowing for a highly scalable energy platform.” This sounds like each “virtual SPS” comprises a very large number of individual satellites, all flying in a formation of some sort. Since you’re on this mailing list, you will have seen the recent discussion (and maybe also the past discussions) of the “thinned array curse,” whereby unless *all* the radiating elements are within something like half a wavelength of each other, you end up putting a lot of power into sidelobes instead of the main lobe. Myself, I’ve just been unable to conceive of how to avoid this problem, when using separate free-flying satellites --- to get that close to each other, they’d have to be touching. Is there anything you can say about that, vis-à-vis your system design?
- Kieran
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Physically-linked, billions of small satellites, flying in “linked” swarms that pass through LEO/MEO twice each day.
This appears to be the approach being undertaken.
The practicality and orbital safety of this approach eludes me. The expectation of absolute perfection also is evident.
The world will need 50,000 GW of space solar power.
I am reminded of a small sign on the back of the dump truck, barely readable at 20 ft, that says stay back 200 ft and that the company is not responsible for any damage from things falling off the truck.
Mike Snead
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Roger;
Very interesting points, which I don’t dispute, in principle.
You wrote:
Bottom line: it is very much possible to produce a tight power beam from a diffuse cloud of satellites, orbiting together in a clustered set of orbits.
You’re making a couple of major assumptions, in practice:
- That the individual satellites carrying each of the transmitting elements are essentially RF-transparent, so that the ones closer to Earth don’t block the path between the further-back ones and the Earth (both for the pilot beam(s) and the main transmit “beam” (which is not really a beam until you’re out of the cloud of emitters). This seems very unlikely for a constellation of individual satellites, each of which would have to carry a significant amount of equipment made of metal, to generate power, and communicate amongst themselves, and perform some amount of orbital manoeuvring.
- That the orbital dynamics of the cloud of satellites (and their attached emitters) is such that, as seen from the Earth, there is a sufficient areal density of emitters across the entire cloud, that there is rarely more than half a wavelength between emitters (as seen from Earth). One issue that comes to mind here, is collisions between the satellites, and the effects of those, and/ort the implications of designing them to keep collision from happening.
There's nothing magic about an average separation of half a wavelength in the radiating elements of a 2-D phased array. It's the knee of a performance curve. When the separation is half a wavelength, the difference in the EM field pattern between the continuous wavefront case and the pattern from a phased array of discrete elements is small. Going to more elements with smaller separation doesn't improve the beam quality very much. But going to fewer elements with a larger separation degrades it more and more rapidly as the separation grows.
Yes, in principle. Again in practice, though, I assume (as did the pioneering designers of the SPS systems back in the 1970s) it will be very important to design any SPS to keep the amount of power (actually, the power flux density at the ground) outside of the main lobe to very low levels, to keep the PFD outside the rectenna fence below civilian-safe levels (as specified by the local health and safety authority), and also to minimize the amount of contamination of the RF spectrum, which is being shared by many other users. The first of those two considerations was top-of-mind for the early SPS designers (my main reference here being the fairly detailed engineering design report that Owen Maynard at Raytheon wrote about the Sandwich Concept design), which did include a sensitivity study looking at the effect of transmit element failures on the PFD outside the main lobe. The answer was that it was essential to have the elements no farther apart than that “knee in the curve” distance, and some (but not many) failed transmit elements could be tolerated.
The individual satellites just need a couple of pilot beams for reference, to determine their relative positions within the cloud. From that they can compute the proper phases and amplitudes of the radiating elements they host.
I understand the math (and the mechanization in terms of RF and digital equipment and algorithms) for retro-direction down a single pilot beam, if the relative geometry of all the elements of the transmit array is known (my very first engineering job was analysis of a couple of types of phased-array radars in space, one type of which was close enough to the SPS Reference Design to make this clear). The idea of using a couple of pilot beams, to figure out that geometry, is new to me. Can you describe that, or point me to a description of how that works?
Thanks!
- Kieran
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