Patent GB-1348813-A (GB)
Title: Adaptive arrays
IPC: H04B10/118
Filed: 1971-11-24
Abstract: 1348813 Adaptive arrays HUGHES AIRCRAFT CO 24 Nov 1971 [24 Feb 1971] 54574/71 Heading H4D [Also in Division H1] In an adaptive array for transmitting and receiving electromagnetic energy to and from a reflector, the beam of radiation from each radiator is characteristically modulated (tagged) and the phase of the radiation adjusted in response to the respective modulation detected in the reflected, received energy such that the radiation from all the radiators are brought into phase at the reflector. The invention is stated to overcome phase errors due to refractive index inhomogeneities. The embodiments described with reference to Figs. 1, 2, and 3 assume that the reflector 34 is near to the array such that propagation delays are negligible. The embodiment of Fig. 4 takes such delays into account. Fig. 1 illustrates part of an array system comprising a plurality of modulated laser beams, one of which is shown at 30, and a single unmodulated reference beam 32. Each beam 30 is to be brought in phase at reflector 34 with beam 32. The beam from the laser 12 is modulated by an electronically controlled phase shifter 24. The phase shift imparted by 24 is (c+mSin#m) where c is a component from phase detector 50 designed to cancel out the phase error a produced by the atmosphere and #mSin#m is an oscillatory component from oscillator 52. The net phase error of the two beams of the reflector, or the amplitude of the beat signal field varies with reflector distance as shown in Fig. 2. It is intended to bring the phase error for each beam 30 and the reference beam 32 to the maximum field amplitude point where #a=#c. The oscillatory component #mSin#mt causes an inphase amplitude change 58 when the phase error is in one sense, an antiphase amplitude change 60 when the phase error is in the opposite sense and a 2#m amplitude variation (i.e. the #m change disappears) when the phase error is zero. The reflected beams are both received back at 16 and after beating in mixer 40 with the coherent output of laser 42 are envelope detected at 46 and the amplitude change at #m selected by filter 48 and passed to phase detector 50. The phase of the amplitude change with respect to the output of oscillator 52 gives the phase correction #c. In the plural beam adaptation of the embodiment of Fig. 1 shown in Fig. 3 no unmodulated reference beam is provided, each beam being compared as to its phase at the reflector with the average field set up by all the other beams. The same average field concept is used in the reflector distance independent embodiment of Fig. 4. The phase variation of the reflected wave is substituted for the oscillator output in determining the relative phase of the amplitude variations in the reflected wave, both variations having undergone the same propagation delay. A phase locked loop 86, 88, 90 converts the intermittent output of filter 82 into a continuous output for the phase detections. If a plurality of spaced receiving points 70a- 70d, Fig. 5 (not shown), are used, standard phase comparison and a controlled phase adjustment can convert the outputs of the receiving points to the same phase from summation to a single signal. The beam splitting arrangement of Fig. 3 may be replaced by a large aperture lens system 71, 73, Fig. 6, spreading the coherent laser light out so that it can be used by the plurality of modulators.