Patent GB-582708-A (GB)
Title: Improvements in arrangements for obstacle detection and measurement of distances by electromagnetic waves
IPC: G01S13/34
Filed: 1942-04-03
Abstract: 582,708. Radiolocation. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. April 3, 1942, No. 4506. [Class 40 (v)] Arrangements for obstacle detection of the kind utilising electro-magnetic waves which are cyclically frequency modulated or swept between frequency limits and in which the beat frequency signal is obtained between the contemporaneous transmitted wave and the received wave after reflection from an obstacle are provided with means for converting the beat frequency into a pure constant frequency sine wave. The transmitter T radiates the mean frequency F1 modulated according to a waveform f(t). The reflected signal is received on the frequency F1[1+f(t+#<t>)] and is detected in the receiver D with the contemporaneous transmitted wave to yield a beat note the frequency kf <1>(t) of which is proportional to the rate of change of transmitted frequency. This beat note is amplified in a low-frequency amplifier LFA. A frequencychanging oscillator O generates a mean frequency F2 and this is frequency modulated by a wave whose amplitude depends upon the distance of the obstacle and whose form is the first differential coefficient with respect. to time of the transmitter frequency sweep. The actual frequency of the oscillator O may be F2+kf<1>(t). Detection between the beat wave kf<1>(t) and this oscillation in the frequency changer FC yields the constant beat frequency F2, to which the intermediate frequency amplifier IFA is tuned. The band width of this amplifier may be reduced indefinitely according to the signal to noise ratio required. In order to avoid direct excitation of the amplifier IFA by passage of the frequency of the oscillator O through F2 during the frequency cycle, the frequency changer FC may be a modulator of the balanced type in which the oscillator injection does not appear at the output. Alternatively the output from the oscillator may be injected through a frequency selective network which rejects' the frequency F2 but passes the adjacent frequency bands. Amplitude modulation is avoided by raising the signal level in the amplifier LFA. Another method is to choose a value of excursion for the oscillator O which provides an overall frequency spectrum not containing the frequency F2, for example for a frequency sweep sin wt of F1 the carrier wave F2 may be exactly cancelled by a critical frequency modulation k cos wt of F2. Obstacles at different distances may then be detected by controlling the transmitter frequency excursion F1f(t) so as always to produce the same beat frequency F2. The effect of a moving obstacle is to add or subtract the Doppler frequency d to the received signal so as to displace the beat-note kf<1>(t) to kf<1>(t)+d. Such a signal may be correctly received, either by detuning the amplifier IFA to (F2 - d) or (F2+d) or by detuning the mean frequency of the oscillator O by an amount d. The Doppler frequency may be measured to give the relative velocity between the obstacle and detecting apparatus. Jamming may be avoided by using a speech wave for modulating the transmitter.