Patent GB-638167-A (GB)
Title: Improvements in or relating to radio drift and speed indicators for use on aircraft
IPC: G01S13/60
Filed: 1947-05-28
Abstract: 638,167. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 28, 1947, No. 14228..Convention date, Aug. 16, 1940. [Class 40 (vii)] [Also in Group XL (b)] A radio speed or drift indicator for use on an aircraft comprises means for transmitting radio waves along differently directed paths, means for receiving reflected waves transmitted over the two paths and means for beating the reflected waves together, so that, due to the Doppler effect, there is obtained a beat frequency signal having a frequency which is a measure either of the ground speed or of the drift of the aircraft. To measure ground speed the two beams are directed in downward divergent paths fore and aft of the craft, while to measure drift the beams are directed in downward divergent paths on opposite sides of the craft. An oscillator 1, Fig. 3, is coupled to a pulse generator 3 and thence to a transmitter 5 which supplies pulse signals to the half-rhombic aerial 33. With the switch 35 in the position shown, downwardly directed beams 39 and 41 are radiated fore and aft of the craft. (When the switch 35 is closed so that the aerial is terminated in its characteristic impedance, a single forwardly directed beam only is radiated from the aerial.) Reflected waves are amplified and detected in receiver 25 and applied to the cathode-ray tube to produce vertical deflections in the type-A display thereof. A slit 45 and photo-cell 43 are provided so that echo signals from a selected range and hence from a selected angle of depression may be analysed. The output of the photo-cell is filtered at 47, amplified at 49 and applied to frequency meter 51 from the indication of which the ground speed of the craft may be determined. If the slit 45 is fixed, the angle of depression of the analysed signals will vary with the altitude of the craft and since the Doppler effect depends upon this angle, the meter 51 cannot be directly calibrated in terms of speed. To overcome this the scale of the range scan of the cathode-ray tube may be made adjustable and the echo signal from the ground directly beneath the craft aligned with a mark on the tube. The angle of depression of the analysed signal is then fixed and the meter 51 may be calibrated to read in miles per hour. Fig. 1 illustrates the invention applied to measure drift. The transmitter output is applied to the aerial arrays 7 and 9, the signal fed to the array 9 being attenuated and fed over a path a halfwavelength longer than that to the array 7. The aircraft's wings are used as reflectors with the result that two downwardly divergent beams are directed on opposite sides of the aircraft. In a further embodiment, Fig. 4 (not shown), for measuring drift, the cathoderay tube display employs a circular trace, Fig. 6, and echo signals are applied to a radiallydeflecting electrode of the tube. A variabletiming-stroke pulse is generated from the master oscillator and applied to the grid of the cathode-ray tube to produce a spot 111 of increased intensity. The strobe pulse is also used to gate the receiver output which is then passed through filter and amplifier circuits to a frequency meter. From the beat frequency and the strobe and vertically received ground echo timings the drift of the aircraft may be determined.