Patent GB-1268634-A (GB)
Title: Improvements in or relating to radio distance measuring equipment
Inventor: THORNE THOMAS GEORGE
IPC: G01S13/34
Filed: 1968-04-11
Abstract: 1,268,634. Radio altimeter. T. G. THORNE. 3 July, 1969 [11 April, 1968], No. 17609/68. Heading H4D. Specification describes a radio altimeter comprising, Fig. 4, a transmitter 30 radiating a signal frequency modulated at frequency P by means of oscillator 31, a receiver 33 for receiving Doppler frequency shifted reflections of the transmitted signal, a first mixer 34 arranged to combine the received signal with part of the transmitted signal, tuned amplifier means 35 for selecting from the output of mixer 34, one of the pair of component signals centred about the harmonic nP of the modulation frequency P, second and third mixers 36 and 37 for combining the selected signal from the first mixer with respective signals of frequency nP derived from the modulation oscillator 31, but having different phase shifts and means responsive to the ratio of the amplitudes of the outputs from the second and third mixers at the Doppler frequency d to produce a signal indicative of the range between the transmitter/receiver and reflecting surface. In the described embodiment n = 1 and the outputs of mixers 36 and 37 are respectively  where: f o  = carrier frequency, m = modulation index, J 1  = Bessel function of first kind and first order, T = propagation time, # 1  = delay imparted to output of oscillator 31 by delay 40, # 2  = delay imparted to output of oscillator 31 by delay 41. The delay imparted by tuned amplifier 35 is cancelled by phase shifters 38 and 39. The manner in which the ratio of their outputs, i.e. Sin(#pT + # 1 )/Sin(#pT + # 2 ), varies with the range dependent #pT, is shown in Fig. 3 for various values of # 1  and # 2 . Rather than determine the ratio directly, an AGC system 47 varies the amplification of amplifier 35 so that the amplitude of signal 37 out remains constant and the amplitude of signal 36 out thus represents said ratio, and, from the known relationship of Fig. 3 also said range. Each of the signals 36 out and 37 out are synchronously detected in respective mixer/detectors 44/45 using the other signal as reference, to produce outputs representing said amplitudes. The output of mixer/detector 45 is thus fed to the AGC system 47 and the output of mixer/ detector 44 is fed to altitude indicator 50. As seen in Fig. 5 range ambiguity occurs when (#pT+ # 2 )##. To determine that this ambiguous state is not prevalent, the modulation frequency P is itself frequency modulated by oscillator 51. This causes the characteristic curves of Fig. 5 to shift from the solid to the dotted line positions. The observed change in the ratio value is very much larger for the ambiguous curves than for the correct (first) curve, and this criterion can be detected in a fault logic circuit 53. The correct operation of the altimeter is monitored by means of an artificial signal injected into the receiver by means of a line from the transmitter via a range simulating delay line 54, with a delay T<SP>1</SP>, a Doppler frequency shift simulating oscillator 65 and a variable attenuator 56 controlled by AGC 47 such that the amplitude of the artificial signal is equal to a normal echo signal. The oscillator 55 is chosen such that the simulated Doppler shift at the outputs of mixers 36 and 37 is rejected by filters 42 and instead is selected by filters 57 and 58 to mixer/detectors 59 and 60 operating similarly to mixer/detectors 44/45. The outputs of the mixer detectors 59, 60 are connected to an amplifier 63 via resistors 61, 62 having values R 1 , R 2  such that Since the outputs of mixer/detectors 59/60 are of opposite polarities, correct working of the altimeter is indicated by zero voltage at the amplifiers, and this is achieved by feeding the amplifier output as a control signal for variable phase shifter 39. The output of mixer/detector 59 and the input of amplifier 63 are fed to logic circuit 53. To check the calibration of meter 50, oscillator 64 is substituted for oscillator 69, such that the simulated Doppler frequency shift is passed by filters 42/43 and the delay T<SP>1</SP> registers on 50. Logic circuit 53 has three flag outputs 66, 67, 68 respectively indicating, " correct operation ", " altitude indication is ambiguous ", " incorrect (failure operation) ".