Patent GB-584496-A (GB)
Title: Improvements in or relating to radio measurement of the distance of an object
IPC: G01S13/34
Filed: 1944-04-07
Abstract: 584,496. Radiolocation. SPERRY GYROSCOPE CO., Inc. , April 7, 1944, No. 6578. Convention date, April 10, 1943. [Class 40 (v)] In a radio distance measuring apparatus frequency modulation is employed and signals having amplitudes that depend in distinctive ways upon distance are produced when the direct and reflected waves are mixed, the range being indicated by the amplitude ratio of two of the signals. The output of a detector fed with both the transmitted and reflected frequency modulated waves contains lowfrequency waves corresponding to the fundamental and harmonics of the modulating signal. The peak amplitudes of the low-frequency waves are a function of the time delay of the reflected waves and all the harmonics achieve or pass through a maximum value when the path of the reflected wave is altered by a quarter-wavelength of the average carrier frequency. Substantially constant variation in ground clearance of the aircraft causes the peak amplitudes of the low-frequency components to be maximized or minimized at a rate proportional to the rate of change of ground clearance, with the production of side-bands centring on the low-frquencies which can be detected to indicate rate of change of altitude. The maximum peak amplitudes of the lowfrequency components are Bessel functions of the altitude of the aircraft and the ratio of the maximum peak amplitudes of one harmonic to the next lowest harmonic is substantially proportional to the ground clearance of the aircraft and is employed as a measure of this quantity. The maximum peak amplitude of the first harmonic or fundamental low-frequency wave is also approximately proportional to the amplitude of the reflected wave and is employed as an approximate indication of the reflectivity and nature of the ground surface. In the radio altimeter of Fig. 2 an ultra-high-frequency oscillator modulator 14 is frequency modulated via amplifier 12 by the audio oscillator 11, the frequency deviation being adjusted by the amplifier gain control 13. The frequency modulated wave transmitted from radiator 15 is received directly, and after reflection from the ground, on antenna 16 and is fed to detector 17, the low-frequency output being amplified at 18. The low-frequency output is fed to band-pass filters. 21, 22 which pass frequency bands grouped around the first and second harmonics respectively of the audio oscillator 11, to rectifiers 23, 23<1>. Rectifiers 23, 23<1> may be of the full wave type shown in Fig. 3, comprising two rectifiers 26, 27 with a load circuit 28, 29 of extremely long time constant so that the output voltage on lead 33 is substantially proportional to the maximum peak amplitude of the input audio wave. Curves J1(Z), J2(Z), Fig. 1, show the relation between amplitude f(Z) of the first and second harmonic audio waves and the ground clearance which is proportional to Z. The curve J2(Z) / J1(Z) is substantially a straight line and a ratio meter 44 fed through switches 38, 39 with the outputs from rectifiers 23, 23<1> is thus calibrated with a plurality of ground clearance scales, one for each adjustment of the frequency modulation deviation. The meter 45 indicates the amplitude of the first harmonic only. As the amplitude of the reflected signals is inversely proportional to the ground clearance, the curve J1(Z) / Z illustrates the amplitude of the first harmonic wave which is substantially independent of ground clearance for small values of Z, so that meter 45 may be calibrated in terms of ground reflectivity. In another mode of operation, switches 38, 39 and 41 are thrown to the left and negative potential from source 43 applied over lead 30 as bias to rectifier 23 prevents rectification of the first harmonic below a predetermined amplitude. The rectified voltage is fed back over the A.V.C. line 20 to reduce the gain of amplifier 18 which is set by gain control 19 to maximum. The audio output supplied to rectifier 23 is thus maintained substantially equal to the potential of source 43 and the amplification of amplifier 18 is closely equal to the amplitude of source 43 divided by the amplitude of the first audio harmonic. The second harmonic wave is thus subjected to this same amplification and meter 42 is supplied with an output proportional to the ratio of the first and second harmonics and indicates aircraft ground clearance. When the ground clearance is altering appreciably, the long time constant of the rectifier load circuits may cause a lag in the meter readings. This may be overcome by placing switch 56 in the lower middle position when battery 46 energizes relay 36 in the rectifier circuits causing resistance 34 to be connected in shunt to resistance 29 and the rectifier load circuit time constants to be reduced to a low value. True maximum peak ratios continue to be measured due to the changing altitude of the aircraft. Alternatively, switches 54, 55, 56 may be placed in the upper middle position and although the time constants of the rectifier circuits are maintained short, the correct peak amplitudes are determined irrespective of changing altitude due to the fact that energy is fed from the transmitter to the receiver through an ultra-high-frequency phase shifter 51 controlled by a sweep generator 52, so that the low frequencies fed to the rectifiers are continually swept through their maximum peak amplitudes. The phase shifter 51 may comprise an electron beam velocity modulation amplifier having input and output cavity resonators coupled by an electron beam. The beam accelerating voltage is controlled by the sweep generator 52 and varies the transit time of the electron stream and thus the phase displacement between input and output oscillations. To determine the gradient or rate of change of altitude, switches 54, 55 and 56 are placed in the upper position and the side-bands of the low-frequency waves are mixed in detector 57 to produce a lower frequency which is proportional to the rate of maximizing of the low frequencies and thus indicates the gradient. This lower frequency is separated by a low-pass filter. 58 and applied to a frequency-measuring circuit 59 and meter 61 calibrated in rate of change of altitude. The output from the frequency measuring circuit 59 is also employed to operate relays 36 in the rectifier circuits when the gradient is sufficient to ensure correct operation of meters 42 or 44 with the short time constant load circuits, the long time constant load circuits only being employed when the gradient is substantially zero so that under no condition can there be a lag in the ground clearance indication.