Patent GB-1054455-A (GB)
IPC: G01S13/60
Abstract: 1,054,455. Radio altimeter; frequency detection. LABORATORY FOR ELECTRONICS Inc. April 10, 1964 [April 12, 1963; Feb. 25, 1964], No. 15012/64. Headings H3A and H4D. The Specification broadly describes a terrain clearance indicator for aircraft wherein interrupted continuous radio waves are beamed towards the earth, the echoes of these waves are received from the earth, and the phase relation between the envelopes of the transmitted and received waves is measured in a phase detector to provide a signal which varies the P.R.F. of the waves until the signal is zero, whereby the P.R.F. provides an indication of terrain clearance. The Specification describes in detail the application of the indicator to the 50% duty cycle pulse Doppler navigation system described in Specification 875,369, wherein the indicator replaces the altimeter 23 in Fig. 6 of the said Specification and supplies the system with a self-contained altitude measuring capability. In Fig. 1, the indicator is shown inside the dotted line and the Doppler navigation system outside the dotted line. An I.F. signal shown at Fig. 2, (A 1 ) (not shown), containing an initial leakage signal portion at a frequency of 42 Mc/s. and a received signal portion at a frequency of 42 Mc/s. + fd, where fd is the Doppler frequency, is taken from the output of the force receiver/ mixer 31 and passed to a broad band limiting amplifier 54 to produce the limited output shown at Fig 2, ( ( AA 1 ), and having a frequency spectrum (AA) comprising discrete frequency components of equal amplitude separated by the pulse repetition period and including a component of 42 Mc/s. The output is then mixed in mixer 55 with a signal at a frequency of 1 kc/s. plus the pulse repetition frequency, from generator 51 which also controls the pulse repetition frequency of the transmitted waves. The output of the mixer has a frequency spectrum shown at (C) being the same as that shown at (AA) with each component now having side band components respectively 1 kc/s. above and below. A narrow band filter and detector 56 first of all only passes the component at 42 mc/s. together with its sidebands and then detects them to produce a signal at a frequency of 1 kc/s. having a phase representing the phase of the envelope of the received signal. A reference signal at a frequency of 1 kc/s. is produced by feeding a signal from generator 51 and part of the transmitted signal from modulator 13 to a demodulator 58. The reference signal is fed to one input of a phase detector 57 via a 90- degrees, phase shifter 59 and a variable phase shifter 61, and the output of detector 56 is fed to the other input of the phase detector 57 via a limiting amplifier 60. At altitude above 2,000 feet the variable phase shifter does not produce any phase shift. The output of the phase detector 57 is fed, together with a correcting signal from circuit 67, to control a servo amplifier 62, which in turn controls a servomotor 63 which varies the frequency of the output of oscillator 52, which in turn controls the frequency of the output of generator 51, until the two inputs to the phase detector are in phase. The output of the generator 51 is fed to a pulse repetition frequency detector 64 to give an altitude indication at 65a by means of read-out computation means 66 and a pitch corrector 65 fed with signals from a vertical gyro 25. Below 2,000 feet the servomotor 63 controls the variable phase shift 61 instead of oscillator 52 so that the pulse repetition frequency (P.R.F.) of the transmitted waves no longer varies, and the received echo waves start to overlap the transmitted waves by an amount (TT -) as shown in Fig. 3. Due to the gating action of gated amplifier 33 however, only the unshaded part of the received wave is passed to the Doppler navigation system and to amplifier 54. The phase detector 57 therefore senses a phase difference # with reference to the middle of the gated portion of the received wave, where #= #-#/2 Since # = 2# (P.R.F.)t, where t is the go and return line of the wave, the height h of the aircraft above the ground can be given by h= K(1/PRF).(1 - 2#/#), where K is a constant, and this function is analogued in read-out computation means 66 which is supplied with a D.C. signal from detector 64 representing the P.R.F., and is controlled by servomotor 63. P.R.F. detector 64 is shown in Fig. 4 (not shown), and comprises a reference pulse generator 70 synchronized to produce pulses of constant duration at a frequency equal to the P.R.F., by means of signals from generator S1. These pulses gate through a D.C. signal V to an  integrating circuit 76. The output of the circuit 76 is then compared in differencing circuit 78 with a constant signal, and the difference output is amplified to produce the D.C. signal V # 1/PRF   The read-out computer 66 is shown in Fig. 5 (not shown), and comprises two potentiometers 82 and 84 having their wipers controlled by servomotor 63. Potentiometer 82 is fed with the output of P.R.F. detector 64, and is cornposed of two parts-a portion AB, traversed by its wiper when the aircraft is above 2,000 feet, and of such low resistance that the wiper signal is substantially equal to the output of detector 64, and a linear portion BC, traversed by the wiper when the aircraftis below 2,000 feet, and of such a resistance that the wiper signal is an analogue of the term (1 - 2#/#). The potentiometer 84 is fed with a constant voltage and comprises a single linear portion, wholly traversed by its wiper when the aircraft is above 2,000 feet, such that the wiper signal which is used to control the variable frequency oscillator 52 is proportional to the aircraft's altitude. The wiper output of potentiometer 82 is fed to altitude indicating meter 65, via a potentiometer 88 having its wiper positioned in dependence on the output of the vertical gyro 25.