7.2 COMMAND PHASE FRONT CONTROL 7.2.1 PHASE ESTIMATION Sensing antenna elements are distributed within and around the receiving antenna site. The signal-to-noise ratio will be high at the sensors. Ground measurements can be used to estimate the transmit aperture phase distribution. For example, if the amplitude and phase of the beam were measured all over the earth, an inverse Fourier transform would yield the transmit aperture distribution. Since this approach is quite impractical, a reduced set of measurements in the vicinity of the main beam and possibly at the sidelobes is used. The main beam can be approximated by curve fitting the amplitude measurements. The peak of the beam reveals any linear phase gradients across the array. The beamwidth reflects any defocusing. The location of the peak and the beamwidth could be used to determine the appropriate linear and quadratic phase front corrections. An optimum system of this kind would be a Kalman filter which included all a priori information (e.g. , measured or calculated array surface distortion) and estimated the required subarray phase compensation based on pattern measurements on the ground. Array temperature and other data measured on board would be telemetered down so that the appropriate phase corrections could be telemetered up. The computer model should rapidly improve as time goes on and data are accumulated. As an example of this technique, consider a uniformly illuminated array that is 1 km in diameter and has 20m wide subarrays. The wavelength is 0. 10 meters. The rectenna array subtends an angle of milliradians at the array. The edge rectenna elements are at the dB part of the beam for a properly aligned and focused system.
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