Patent GB-753721-A (GB)
Title: Improvements in or relating to radio-echo distance measuring systems
IPC: G01S1/02
Filed: 1953-08-17
Abstract: 753,721. Frequency-modulated radar; valve oscillators. RAYTHEON MANUFACTURING CO. Aug. 17, 1953 [Aug. 15, 1952], No. 22684/53. Classes 40 (6) and 40 (7). In a frequency-modulated radar altimeter in which the altitude is displayed by the deflection of a pointer over a scale, means are provided for deflecting the pointer off the scale, e.g. behind a mask, when the echo signal is not reliable. In a first embodiment, Fig. 1, the normal triangular frequency-modulating waveform is amplitude modulated by an auxiliary sinusoidal signal of lower frequency, e.g. 10 c/s, and a normal display is produced only when the transmitted and received auxiliary signals are co-phasal. In a second embodiment, Fig. 2 (not shown), the normal display is produced only when the amplitude of the beat signal produced by heterodyning the transmitted and received signals is greater than a critical value. The first embodiment is described as applied to a system in which the beat signal is applied to a frequency discriminator whose output is applied to so control the amplitude of the triangular modulating waveform that the beat frequency is maintained constant; the amount of control required is dependent on the altitude and is transmitted via selsyns to the indicator. In the second embodiment the amplitude of the triangular modulating waveform is constant and the output from the frequency discriminator is proportional to altitude, the output being converted to a pointer rotation by means of a senc system. As shown in Fig. 1, the transmitter oscillator 10 is frequency modulated by a triangular waveform generated in modulator 12 which is amplitude modulated by the sinusoidal auxiliary signal generated by the R.C. oscillator 13. The received signal is heterodyned in detector 36 with an output from transmitter 10 and the resultant beat signal applied to a frequency discriminator 37 giving a D.C. altitude-dependent voltage which is amplitude modulated by an A.C. component of frequency equal to that of the auxiliary signal. This A.C. component is applied via capacitor 38 and amplifier 52 to a phase discriminator 40 to which are also applied push-pull auxiliary signals from a phase splitter 26 which is coupled to oscillator 13. When the A.C. component is cophasal with the auxiliary signal the output from discriminator 40 renders valve 60 conductive, thereby energizing relay 67 to close contacts 76, 77 and 78 and the D.C. output from discriminator 37 is applied via relay contact 78 and amplifier 80 to a servomotor 83 which actuates the variable resistor 102 controlling the amplitude of the triangular waveform generated in modulator 12. The system is such that motor 83 runs until the output from discriminator 37 is equal to a datum value and the angular position of the motor shaft is then proportional to altitude, this angle being transmitted to the range pointer 97a by a selsyn transmitter 84, 90-92 and receiver 93-96. When the A.C. component of the output from discriminator 37 is not in phase with the auxiliary signal, relay 67 is de-energized and a D.C. voltage from the junction of resistors 81 and 82 is applied via contact 75 to the input of servo amplifier 80 in place of the output from discriminator 37; this D.C. voltage causes servomotor 83 and the rotor 84 of the selsyn transmitter to rotate continuously. Deenergization of relay 67 also breaks the connections between stators 93, 95 of the selsyn receiver and stators 90, 92 of the selsyn transmitter and connects stator 93 to a tapping 100 on the secondary 85 of the transformer 86 so that the rotor 94 rotates the pointer 97a into a position 97 behind a mask 101. In the second embodiment, Fig. 2 (not shown), a similar servo-selsyn arrangement is employed but the amplitude of the triangular modulating waveform is constant. The output from the frequency discriminator is backed off by a D.C. voltage whose amplitude is controlled by the servomotor and the beat signal from the detector 36 is rectified and applied to control a valve having in its anode circuit a relay corresponding to relay 67 of Fig. 1 so that the relay is energized only when the amplitude of the beat signal exceeds a critical value. Valve circuits. The anode circuit of the R.C. oscillator 13 is coupled to a phase splitter 26 whose grid-cathode circuit is included in the oscillator feed-back path, push-pull outputs being derived from the anode and cathode of valve 26.