SRC-451

Microwave Power Transmission System Studies, Volumes I to IV, December 1975, NASA-CR-134886, Raytheon ER75-4368

Commissioned by NASA Lewis Research Center, contract NAS 3-17835. Who paid for a study is recorded because it bears on how its conclusions should be read, not because it makes them wrong.

Published claims resting on this source

• CLM-0954 The 1975 designers worked out how much their kilometre-wide antenna would bend, precisely so they could decide how good the beam-steering correction had to be. The biggest bend came from building it slightly wrong in the first place - roughly three quarters of a metre to a metre and a half at the edge, depending on how the stated angle is read - and that does not move. Heat bending could be held to a few centimetres if the top and bottom of the truss were kept within four degrees of each other.
• CLM-0959 The original 1975 calculation found the satellite's radio noise would be fine for ordinary and troposcatter radio links and a problem only for radio astronomy. The 1978 review that redrew its chart swapped the two "fine" limit lines for stricter ones and then listed troposcatter as a problem. The disagreement is over which limit to apply, not over the arithmetic.
• CLM-0960 The 1975 study found the satellite's harmonics - the multiples of its transmit frequency - would trouble radio astronomy at the second and third harmonic, would trouble ordinary radio only at the klystron's second harmonic unless a filter was added, and would be clear at the fourth. A later reading of a redrawn chart says all three harmonics exceed everything. The original does not say that.
• CLM-0961 Everything in the 1975 interference analysis is for one satellite. The study itself says that a hundred of them would raise the noise and harmonics a hundredfold - 20 decibels - and that nobody had looked at what that would mean.
• CLM-0965 In 1973 an astronaut on Skylab put together two long poles from short sections in just over two hours, about six minutes per join. A 1975 antenna study used that as its only real-world data point for how fast people can build things in space.
• CLM-0966 In 1975 the antenna alone was expected to take one to two years to assemble in orbit with about fifty people, and robot arms were judged cheaper than astronauts doing it by hand.
• CLM-0968 The 1975 antenna study worked out how hot the middle of the transmitter gets depending on how efficient the tubes are and how tightly they are packed. With the tubes at 70 per cent efficiency and an aluminium frame, it found they had to be spread almost evenly across the whole kilometre to stay within what the frame could take.
• CLM-0969 If the tubes are only 70 per cent efficient, the transmitter's own face is not big enough to get rid of the heat they make, even spread perfectly evenly. At 85 per cent it is comfortable. The tipping point is almost exactly the efficiency the 1975 study said its tubes achieved.
• CLM-0972 If you take a 1975 estimate of how fast people could build structure in space, checked against one Skylab task, and apply it to the 1978 design's framework, you get a crew of a few hundred to build it in six months, which is roughly what the design said. For the framework, the 1978 numbers add up.
• CLM-0973 Apply that same measured pace to the whole satellite rather than just its framework, and the 1978 design would have needed several thousand people, not a few hundred, to finish in six months. The design assumed something made people two to fourteen times more productive and never said what.
• CLM-0989 The same figure, minus 160, appears in two 1970s papers meaning two different things: in one it is the noise reaching the ground, in the other it is the noise of a single tube compared with its own signal, which after adding up ninety thousand tubes and the distance to the ground comes out 27 decibels lower. Anyone quoting "minus 160" has to say which.
• CLM-1086 The number 2.45 GHz has a birthplace: a 1975 study for NASA that swept frequency from 1 to 5 GHz against cost, found a broad flat region near 2 GHz, and picked the industrial band sitting in it. The atmosphere was checked separately and found nearly transparent below 3 GHz.
• CLM-1315 The 1975 estimate of the satellite's radio noise, taken at its own frequency, is about a hundred million times above today's threshold for spoiling radio astronomy. The noise in the astronomy band itself would be lower, by an amount nobody has yet read from the 1975 study.
• CLM-1433 In the 1975 study, at 300 dollars a kilogram to launch and assemble, a 1 GW station cost about 1.7 to 3.5 times as much per kilowatt as a 20 GW one. The large stations were cheapest at about 1.25 to 2.25 GHz, but their curves are shallow: about 1,900 to 2,300 dollars per kilowatt across the whole band for the largest.
• CLM-1434 With launch and assembly at a third of the price and a cheaper power source, the 1975 study's costs per kilowatt fall at every size, by about 40 per cent for 1 GW and about two thirds for 20 GW. The largest stations still sit at their cheapest between about 1.5 and 3 GHz, around 610 dollars per kilowatt for 20 GW.

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