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Improving Navigation
Dave Michelson wrote:
wrote: Actually, this one is very close. If a 70 meter dish failed today, I'd bet they'd do it. A new 70 meter class dish costs $100M, and an array of smaller dishes has lots of advantages. It can be quite a bit cheaper, depending on the details. It can be split to point at many targets, it has fewer points of failure, it can be maintained a little at a time, the size can be increased in smaller increments, it can be spread out geographically to add weather and disaster diversity, and so on.... The days of big dishes are numbered. I agree with all of the above, but am curious about the cost. It would take at least eight or ten 25-metre dishes to equal the collecting area of a single 70-metre dish. (25-metres being a fairly standard diameter reflector for use in radio astronomy arrays :-) It appears the arrays are potentially quite a bit cheaper. This is a fight between two factors - first, collecting area is cheaper for small dishes, per m^2. (or expressed another way, the cost of big dishes scales faster than diameter^2). On the other hand, a single big dish needs only one feed/pre-amp, but an N element array needs N of them. As a point of reference, recent DSN 35 meter antenna are about $45 million. The 70 meter dishes have not been built recently, but would clearly be a lot more. These tradeoffs can be made in many ways. For example, the ATA (Allen Telescope Array) uses 6m dishes, the largest that can currently be stamped in one step with satellite dish technology. You'd need at least 136 of these to equal a 70m collecting area. However, you really need more, since you can't afford 136 super-duper helium cooled masers, and really high feed efficiency is hard in a small dish. So instead you go with a liquid nitrogen cooled receiver, and a simpler feed. The net result is double the noise temperature, so you need twice the collecting area, or maybe 270 dishes. The Allen Telescope project thinks that these will cost perhaps $50K each, in pseudo-mass production, so that's only $13m for the signal collection. Then you need to add the signals up, but that's not so expensive nowadays. But the ATA dishes and feeds only go to 10-12 GHz or so, and JPL needs 32 GHz. See http://www.skatelescope.org/skaberke...tion%202.9.pdf for a somewhat dated (2001) JPL analysis using small dishes like those of the ATA. Nowadays, JPL is tending towards a design with 12m dishes and better receivers. This will win if you can find a way to stamp out the 12m dishes at low cost. See http://www.skatelescope.org/PDF/SKA2...aceNetwork.pdf The pattern of a widely spaced array of large antennas would be a lot more complicated than that of a single large antenna. In particular, the grating lobes would be very noticeable. (Almost all conventional arrays are closely spaced arrays of small antennas.) Having said, that, this has been done before with the VLA and suitable use of feedback control and RF-over-fibre links would solve lots of immediate problems. This should not be much of a problem, since a spacecraft will look like a point source, mostly against a dark sky. If you simply add the beams with the right phase, you get the same central sensitivity as the big dish, just crazier sidelobes. This may not be a problem for spacecraft, but it is for astronomy, so to help with this, the ATA is basically placing the antennas at random on their site (actually, they try to get a good mix of short-long baselines, good U-V coverage, etc, but it *looks* random) to reduce the grating lobes. The only case I can see this being an issue with the DSN is when they try to track spacecraft very close to a bright source such as the Sun. The good news, of course, is that much technical experience and expertise has developed over the past decade (in particular) from work on multi-antenna radio astronomy projects ranging from the VLA to the SKA (Square Kilometre Array). (Yes, there are significance differences between aperture synthesis and conventional arrays, but by and large the people involved could likely handle both or either as required.) The communication job is much easier than the astronomy job, since you are not forming images, just optimum response at one point. The people skills are similar, as you say, but the astronomy work requires huge correlators, and often high instantaneous bandwidths, where the communications just needs tunable delay lines. Lou Scheffer |
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