Patent GB-1108115-A (GB)
Title: Improvements in or relating to radio distance measuring systems
IPC: G01S13/34
Filed: 1965-09-22
Abstract: 1,108,115. Radio altimeters. MARCONI CO. Ltd. 22 Sept., 1965 [24 Sept., 1964], No. 40300/65. Heading H4D. The invention relates to radio altimeters or the like using sinusoidally frequency modulated transmissions, wherein range is determined by varying the modulation index of the transmission and measuring the corresponding variation in the amplitude of a chosen side band component in the resultant beat frequency signal, said component having a known Bessel function variation with the modulation index. In the altimeter of Fig. 2 (which is for use in a landing phase and assumes the presence of a doppler frequency shift fd), with switches 14, 15 and 16 initially in the positions shown, the transmitter transmits an unmodulated wave fc to the ground. The echo is received, doppler frequency shifted and passed via attenuator 4 to a mixer 5. Attenuator 4 reduces the echo to 77% of its initial value. The mixer is also fed with the transmitter output and so produces a beat frequency output having a formula Since however the transmission is unmodulated, #fc, the frequency swing is zero. Equation (1) can be expanded by Fourier analysis into a plurality of side band pairs, separated by 2fd and centred about frequencies o, fm, 2fm . . . nfm, the sideband pair centred about frequency nfm having an amplitude given by the value of the nth order Bessel function Jn for the modulation index used. From the plot of Bessel functions shown in Fig. 1, it is seen that the amplitude of the doppler frequency component of the beat signal, i.e. Jo is 77% for amodulation index of 1, when the amplitude with no modulation is taken as 100%. The effective modulation index is, from equation (1) equal to 2. or since equal to with #fc in Mc/s and h in ft. Thus in Fig. 2, the doppler frequency component is selected from the mixer output by filter 6, and its amplitude detected by integrator 8 and passed to amplitude comparator 10. Switches 14, 15 and 16 are now switched to their second position whence transmitter 1 is frequency modulated by modulator 11 with a deviation #fc controlled by the output of comparator 10. The echo signal is no longer attenuated by 4, and comparator 10 continues to feed a control signal to modulator 11 until the amplitude of the doppler frequency component determined by integrator 9 is equal to the 77% value given by integrator 8. When this occurs the modulation index is 1 such that and a meter 12 measuring the frequency deviation #fc, can indicate the height h. From Fig. 1, it is seen that at a modulation index of 2.4, the amplitude of the doppler frequency component is a minimum zero. In the arrangement of Fig. 3 (not shown), the transmitter 1 is continuously frequency modulated with a modulation index which varies linearly. The control signal of the modulator 11 is accordingly produced by a ramp generator 18 which produces a rising voltage until stopped by a pulse from a means detecting the minimum zero of the doppler frequency component amplitude. The above described embodiments are only suitable for low altitude work, since the noise spectrum extends over the doppler frequency range and at high altitudes the resultant weak dopple frequency component may be lost in the noise. For use at high altitudes the embodiment of Fig. 2, is modified by the addition of a frequency modulator continuously frequency modulating the transmitter with a very high modulation frequency, fms, such that the side band at 3fms, in the mixer output is well outside the noise bandwidth and may be treated as the unmodulated carrier of Fig. 2. Filter 6 now has its pass band centred on 3fms. The frequency deviation caused by the additional modulator may be varied independence on the altitude found, to track the peak in the J3 Bessel junction. This last described embodiment may be incorporated in a doppler speed and altitude aircraft navigation system Fig. 5 (not shown). The invention can also be applied to pulse transmissions, wherein the pulse carrier is coherent and frequency modulated, or to interrupted continuous waves. When used in an airborne altimeter, the radiated frequencies are such that the ground reflection is specular.