Patent GB-661769-A (GB)
Title: Improvements in frequency modulation radio altimeters
Inventor: BUCKS KARL EDUARD
IPC: G01S13/34
Filed: 1949-03-17
Abstract: 661,769. Radiolocation. MINISTER OF SUPPLY. March 17, 1950 [March 17, 1949], No. 7375/49. Class 40 (vii). In a radio altimeter employing linearly frequencymodulated transmissions, the echoes from which are heterodyned with the transmission to obtain beatfrequency signals, the beat frequency is measured to determine altitude within one range of heights, while within another range the beat frequency is maintained constant and the corresponding rate of change of frequency of transmission measured to determine altitude. In Fig. 1, a modulating waveform generated at MOD controls the frequency variation of the signal transmitted from transmitter TX. The heterodyne output from receiver RX is applied to a counter circuit which delivers a steady-voltage output proportional to the frequency of the input. The counter output drives a servomotor which rotates the shaft of a potentiometer POT and a pointer P measuring altitude. The potentiometer has two outputs 1 and 2 which are fed back to the servomotor and modulator respectively. As shown in Fig. 2, the potentiometer comprises a contact segment c, a resistance segment r and two diametrically opposed wiper arms I and II providing outputs 1 and 2 respectively. It will be seen that, in the height range 0-400 ft., the servomotor rotates the potentiometer shaft until the output 1 balances the height-dependent counter output. In the height range 400-4000 ft., output 1 is constant; however, output 2 decreases with increase of height and thereby controls the slope of the modulating waveform generated at MOD so as to maintain the counter output constant. Fig. 5 shows an alternative servo and potentiometer system. The counter output is applied to the grid of valve V<SP>1</SP> which is biassed by a potential tapped off from a potentiometer LR pot and connected thereto via-switch S. The potential at the anode of valve V<SP>1</SP> controls the direction of rotation of motor M<SP>1</SP> which consequently drives the wiper of LR pot to a point where the bias therefrom balances the output from the counter. Simultaneously, motor M<SP>1</SP> drives the rotor of potentiometer HR pot. However, by virtue of gearing 33 and 34, the rotor of HR pot only reaches the edge of contact segment c<SP>2</SP> while the rotor of LR pot makes one complete revolution. After one complete revolution of the rotor of LR pot, the wiper of HR pot contacts the resistance segment of HR pot and also actuates a member a to cause switch S to move to the position shown in the Figure. The position of the rotor of HR pot controls the speed of motor M<SP>2</SP> which is used to derive the modulating waveform. Thus, when the height exceeds a certain value the modulating waveform is controlled to maintain the beatfrequency constant. The position of the rotor of HR pot may alternatively control the amplitude instead of the frequency of the modulating waveform. In addition to driving the potentiometer rotors, motor M<SP>1</SP> also actuates a dial d showing 0-100 feet and a counter e showing hundreds and thousands of feet.