Value chain · CLM-0981
In plain terms
Smaller patches tolerate more bending: halving the patch from 10 m to 5 m doubles the allowed tilt for the same half-per-cent loss. A separate team found that a metre of bending is close to the limit of what a pointing sensor can tell apart, and that a pointing sensor can use at most three patches while keeping within an arc-minute of error. The tilt limit was set by the Soviet exposure standard at the first grating lobe.
The claim as recorded
Two 1978-1980 studies treat subarray size as the dial that buys tolerance to structural tilt. Boeing: "The baseline 1 arcmin. random tilt, combined with the above systematic tilt, will combine to give a 0.5% efficiency loss for a 10 m x 10 m subarray. Going to a 5 m x 5 m subarray would allow the tilt requirement to be relaxed to 2 arcmin. systematic and 2 arcmin. random for the same scanning loss of 0.5%." Tennessee: a 1 m structural displacement across a 1010 m aperture makes the tilt error about 0.89 of a subarray beamwidth, and a budget of one arc-minute each for noise, vibration and mechanical effects allows at most three subarrays in the sensing array (the source's "M<=3"), and at M = 3 the vibration allowance of 8.3 per cent of the beamwidth is not exceeded even under 1 m of mode-2 vibration. The 1 arc-minute systematic tilt requirement itself "is derived from these, to meet the Soviet microwave level standard at the first grating lobe".
Evidence behind this: T3, two contractor reports by independent teams using different methods reaching the same direction.
Scope and limits
TWO DIFFERENT MECHANISMS, NOT ONE RESULT: Boeing's loss is scanning loss from tilted subarrays under phase control; Tennessee's is angle-sensing error in a monopulse-style sensor. Neither states a structural amplitude spectrum; Tennessee's 1 m is an assumed vibration amplitude. MED because the 0.5 per cent and the M<=3 come from figures (Boeing's grating-lobe chart, Tennessee's Figures 12 and 13) not re-derived here.
What it assumes
Boeing's figures are for the 1978 slotted-waveguide baseline with per-subarray phase control; Tennessee's are for a gimballed antenna with an RF angle sensor, a different steering approach.
Where it comes from
p.112 from the render: "The design requirement of 1 arcmin. of systematic tilt are derived from these, to meet the Soviet microwave level standard at the first grating lobe. The random tilts have only second order effect on grating lobe levels and affect primarily the array scanning loss. The baseline 1 arcmin. random tilt, combined with the above systematic tilt, will combine to give a 0.5% efficiency loss for a 10 m x 10 m subarray." "Going to a 5 m x 5 m subarray would allow the tilt requirement to be relaxed to 2 arcmin. systematic and 2 arcmin. random for the same scanning loss of 0.5%." SRC-463 p.7: "a mechanical pointing control accuracy of three (3) arc-minutes and an electronic phased array pointing accuracy of three (3) arc-seconds"; p.28, letterspaced in the layer: "if delta-z = 1 m, D = 1010 m, X = 11.64 m, and lambda = 0.1224 m, then |theta_xn| is about 0.89 theta_sub, so tilt errors may be nearly as large as the subarray's beamwidth"; p.82: "The mechanical pointing accuracy, with the various anomalies present, was found to be well within 1 arc-minute... we require M<=3 subarrays in the linear array [the page prints a less-than-or-equal sign; the text layer renders it "M~3"]. The allowed vibration error is therefore 1 min/12 min = 0.083 or 8.3% of theta_B maximum. From Figure 12 this allowance is not exceeded for M=3 even for mode 2 (k=2) vibration (delta-z = 1 m)."
Basis: arc-minutes of tilt; subarray side in metres; number of subarrays in a sensing array
Related records
- Answers question
- QST-013 not published in this release
Cite this
energyaperture.com/c/clm-0981
CLM-0981, Energy Aperture, as of 1980.
Connected records
Other published records linked to this one. A list, not a judgement.
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