DOPPLER MICROWAVE LANDING SYSTEMS

摘要

1406439 Position finding PLESSEY CO Ltd 2 Jan 1973 [4 Jan 1972] 335/72 Heading H4D A microwave Doppler landing system includes an aerial array and a reference aerial next to one end of the array, a transmitter providing the reference aerial and array with reference and scanned signals, having frequencies differing by a predetermined offset frequency, a commutator for applying the scanned signal to the elements of the array so that the point of transmission is moved along the array and back and means to change the scanned signal frequency by twice the offset frequency when the element closest the reference aerial is energized and when the rate of change of amplitude of the beat signal between the scanned and reference signals is zero. A reference generator supplies the reference aerial 16 and offset generator 17 with the reference frequency signal (e.g. 5000 MHz). The offset generator 17 adds or subtracts 50 KHz the offset being positive or negative in accordance with the direction of scanning (i.e. up and down respectively). This scanned signal passes to commutator 19 feeding a vertical array of aerial elements. When the lower element 20 is energized gate 25 is enabled and when phase discriminator 23 and phase detector 24 detect in-phase or antiphase signals a pulse is passed to control 26 which reverses the offset. Since the offset is reversed at the upper end of the array also, the offset remains the same in the upward scanning direction and the changeover is effected smoothly. Digitizer 18 is used to add small phase changes at the offset changeover between the scanned signals at the upper element in order to reduce the effects of the reverse in scanning direction. The system may also be used for azimuth D.F. In the aircraft receiver (Fig. 2, not shown) the signals are detected, the Doppler signal is extracted and passed to a spectrum analyser or zero-crossing detector. The Doppler frequency is proportional to elevation angle from the array, for small angles. For larger angles, e.g. for use with helicopters or VTOL aircraft suitable correction is applied.