Patent GB-586116-A (GB)
Title: Improvements in or relating to detection of obstacles by electromagnetic waves
IPC: G01S7/282
Filed: 1941-03-11
Abstract: 586,116. Radiolocation. STANDARD TELEPHONES & CABLES, Ltd.,.and EARP, C. W. March 11, 1941, No. 3297. [Class 40 (v)] In an arrangement for the detection of obstacles by means of radio pulses comprising short trains of high-frequency waves, pulses of random phase and timing are transmitted and the received impulses after reflection at the obstacle are corrected for the random phase displacement by means of an automatic phase correcting circuit in the receiver, and then integrated. Pulses are generated in a transmitter T, Fig. 1, and comprise short trains of waves at radio frequency f. These are radiated from the aerial A1 and also applied to a modulator M, to which a frequency FŒf generated in an oscillator O1 is also applied. The output from the modulator M is a train of pulses comprising waves of frequency f. A frequency f generated in an oscillator 02 is applied with this pulse train of frequency f to a balanced detector D. The output from this is a train of D.C. pulses, the sense and magnitude of which depend on the relative H.F. phases of the wave comprising the pulses and waves of the same frequency from the steady reference oscillator 02. This output controls the frequency or phase of the oscillator O1 through a frequency control tube C. The output of the oscillator O1 is applied to a freqency changer FC in the receiver circuit. After a time equal to that taken by the transmitted wave to reach and return from the obstacle, a pulse is received at the receiver antenna R1 and its frequency is changed to f in the changer FC. This phase-corrected train of pulses is then integrated, for example as described in Specification 586,115 and shown in Fig. 2. The phase stable pulses P, Fig. 2, are fed over two paths to a modulator M2, one or both paths including a further modulator M1 for combining the phase stable pulses with a carrier wave from an oscillator O of constant frequency. A delay network D produces a predetermined difference in transmission delay through the two paths, this delay being equal tb the repetition period of the transmitted pulses. A maximum signal to noise ratio is thus obtained. The difference in frequency between the output and the oscillator O 1 gives an indication of the relative velocity between the transmitter-receiver and the obstacle. Another arrangement for the integration of the pulses at the output of the frequency-changer FC is to apply the coherent pulse train to a selective filter either before or after beating down the frequency. In another arrangement, the coherent pulse train is applied to a balanced detector with a locally generated pulse train of the same high frequency, the output being selected as a direct current. The coherent pulse train from the output of FC may be detected together with a similar pulse train of a different mean high frequency and the output from this detector may be tuned to select a continuous-wave indication at the difference frequency. A "signal time-gate" " arrangement may be provided to render the receiver effective only for the echoes of the pulses transmitted after reflection from an obstacle at a predetermined distance. This " gate " may be in the form of a train of pulses of the same frequency as the transmitted pulses and the time of occurrence of the "gate" train of pulses may be set manually or automatically to coincide with the echoes received from a particular distance. An oscillation of the required' pulse frequency may be used to produce the original transmitter pulses, or may be synchronized by them. A manual or automatic phase control of this oscillation is then connected at the input to a limiting device, producing a square wave-form of the pulse frequency. A high-pass filter is arranged to produce alternate positive and negative pulses of D.C. at times corresponding to the vertical sides of the square wave, and a halfwave rectifier is used to remove the negative pulses. Such D.C. pulses may be converted to H.F. pulses by applying them to a negatively biassed amplifier fed from a high-frequency source. A controllable D.C. pulse train may be used to switch on the receiver at moments corresponding to the arrival of reflections from a particular obstacle by application to the bias of an over-biassed amplifier. Alternatively, the frequency-changer FC may be over-biassed and set into operation by the locally-produced pulse train, or the injection from the oscillator O1 to the frequency changer FC may take place through a similarly pulsed amplifier. By these means all signals except the desired one are eliminated and the position of the manual or automatic control indicates the range of the obstacle. In another arrangement, the output from the receiver R is combined with a manually timed pulse train at a different frequency in a modulator. A filter in the output circuit at the difference frequency selects a particular obstacle and integration is effected in the same process.