Patent GB-1446212-A (GB)
Title: Airborne altitude indicator device
IPC: G01S11/06
Filed: 1973-10-29
Abstract: 1446212 Radar THOMSON-CSF 29 Oct 1973 [31 Oct 1972] 50253/73 Heading H4D An airborne altitude indicator indicating when a predetermined altitude is descended to comprises electromagnetic continuous wave radiating and receiving means for obtaining the Doppler signal produced by ground reflection, detecting means which receive the Doppler signal and feed two circuits which have different transfer functions and which are connected to an amplitude comparison circuit, and utilization circuits fed by the comparison circuit to produce an indication when the ratio of the outputs of the two circuits reaches a value indicative of the predetermined altitude. The arrangement is directed to making the indication produced by the utilization circuits independent of several parameters-notably the reflection coefficient of the ground. An oscillator 1 radiates an unmodulated continuous wave via aerial 2 and the received wave causes an alternation in the load on the oscillator due to its different phase with respect to the transmitted wave. The average current in the oscillator varies accordingly and gives the Doppler signal which is passed to amplifier 3 and detected in device 4. The detected envelope is passed to circuits 5 and 6 and their outputs passed to a comparator 7. It is stated that the amplitude of the Doppler signal is dependent upon the height h above the ground, on the angle of reception #, on sensitivity S of the oscillator i.e. the ratio of the derived Doppler signal amplitude to that of the high frequency wave received, and on the coefficient of reflection RS of the ground. In the terminal stages of flight, the parameters w, S and RS may be considered constant as well as V z , the vertical component of speed. Accordingly, the signal S1 may be given as A.f(t) where A is a constant due to V z , #, S and RS and f(t) represents the height. Under different conditions, the parameters will change and S1 is then given by Al.f(t) where Al represents the new parameters. Circuits 5, 6 have different transfer functions and produce signals S2=Y1(t) and S3=Y2(t). Comparator 7 gives an output when they are in a ratio K. When the input changes to Al.f(t) then S2, S3 become (A/Al).Y1(t), (A/Al).Y2(t) and again device 7 gives an output when they are in a ratio K. This always occurs at a fixed time t 0  before touchdown, as the constants A, Al &c. cancel out, i.e. signals S2, S3 are normalized. Therefore a signal is produced from device 7 at a time t 0  regardless of variations in the parameters. As t 0  corresponds to a height ho = V z t 0  this signal will occur at a predetermined height ho which is constant if V z  is maintained constant. If V z  varies then ho will vary accordingly and the embodiment of Fig. 2 (not shown) is directed to reducing the ho variation. In the embodiment two detectors (40), (41) and filters (10), (11) are provided. The filters alter the amplitude of the signals fed to circuits (5), (6) in such a way that t 0  is reduced or increased if V z  increases or reduces respectively thus reducing ho variation. Figs. 3, 4 (not shown) illustrate detailed circuitry and responses S2, S3 for devices 5, 6. Fig. 5 (not shown) illustrates suitable frequency response curves for filters 10, 11 of Fig. 2, the filters being low pass and high pass respectively.