Patent GB-648810-A (GB)
Title: Improvements in or relating to means for determining distance and velocity by electromagnetic waves
IPC: G01S13/58
Filed: 1945-01-08
Abstract: 648,810. Radiolocation. PATELHOLD PATENTVERWERTUNGS- & ELEKTROHOLDING AKT.-GES. Jan. 8, 1945, No. 673. Convention date, Oct. 27, 1943. [Class 40 (vii)] In a frequency-modulated radar system means are provided for the simultaneous determination of the distance and radial velocity of the reflecting object. In the system shown in Fig. 1, the transmitter Sv radiates a constant frequency flv and the components received directly and indirectly after reflection produce a beat-frequency voltage ev of frequency Fv in the receiver Ev due to the Doppler effect when there is relative velocity between the system and the reflecting object. The transmitter Ss radiates a frequency f1s with a saw-tooth variation, Fig. 2, which produces a beat-frequency voltage es in the receiver Es. When the distance to the reflecting surface is constant, this beat frequency Fd is directly proportional to the distance but when the distance is increasing the frequency of the reflected signal is reduced by an amount Fv so that the beat frequency Fs is equal to (Fd + Fv) as shown in Fig. 2 ; similarly for a decreasing distance F<SP>1</SP>s=(Fd - Fv). The beat frequency outputs from receivers Ev and Es are fed to discriminators Dv and Ds which produce voltages Uv and Us proportional to the input frequencies, the voltage Uv being fed to an indicator Zv calibrated in velocity and the sum or difference of Us and Uv being fed to the distance indicator Zd. The transmitted frequencies f1v and f1s may be in the same or different frequency ranges since in the former case any unwanted beat frequency produced by the two reflected signals beating together in the receiver is higher than Fd or Fv except during the small interval of time when f1v is sweeping through the range (f1v+Fd) and in the latter case the unwanted beat frequency is always higher than Fd and so may be suppressed by low-pass filters. In a modification, Fig. 3 (not shown), the sum and difference voltages are obtained by applying the beatfrequency voltages ev and es to a modulator whose output is fed to the distance indicator through a discriminator. The constant frequency transmitter may be omitted if the variable frequency characteristic includes a constant frequency portion as shown in Figs. 4 or 5 in which the beat frequencies F<SP>1</SP>s, F<SP>111</SP>s and F<SP>11</SP>s produced in the periods of increasing, constant and decreasing frequencies respectively are equal to (Fd+Fv), Fv and (Fd - Fv) when the distance to the reflecting object is increasing ; the values of Fv and Fd can thus be obtained if a simultaneous indication of the beat frequencies is produced by applying them to a resonant form of frequency meter, e.g. tuned reed or multiple resonant circuit feeding a cathode-ray tube indicator. Alternatively, with the variation shown in Fig. 4, the successively occurring beat frequencies F<SP>1</SP>s and F<SP>111</SP>s are made coincident by applying the receiver output through parallel retardation devices and the difference frequency Fd=(F<SP>1</SP>s - F<SP>111</SP>s) is then obtained, Fig. 8 (not shown) ; the frequency F<SP>111</SP>s=Fv may be isolated by applying the receiver output through a low-pass filter. With the frequency variation shown in Fig. 6, the two beat frequencies F<SP>1</SP>s=(Fd+Fv) and F<SP>11</SP>s=(Fd - Fv) may be brought into coincidence in a similar manner and applied to a modulator to give sum and difference frequencies 2Fd and 2Fv which are fed to distance and velocity indicators, Fig. 9 (not shown). Alternatively the two beat frequencies F<SP>1</SP>s and Flls are modulated at M, Fig. 10, by a voltage eg of variable frequency Fg produced by the oscillator Ov and the modulation products are retarded by unequal amounts in the networks N1, N2 so that the summation frequency F2=(F<SP>11</SP>s+Fg) and the difference frequency F3=(F<SP>1</SP>s - Fg) occur simultaneously and are applied to the detector G to produce the difference frequency Fr which is indicated by the meter Z0. The frequency of oscillator Ov is varied until Fr is zero when Fg=Fv and F2=F3=Fd ; hence the velocity is given by the calibration of oscillator Or and the distance is indicated by the frequency meter Zd connected to the output of modulator M. In Fig. 12 (not shown), the alternately produced beat frequencies obtained with the transmitting frequency variation shown in Fig. 6 are first modulated with a fixed frequency Fh and the upper side band (Fs+Fh) is then modulated with a variable frequency F<SP>1</SP>d, the lower side band (Fs+Fh-F<SP>1</SP>d) of the resultant product being further modulated by a variable frequency F<SP>1</SP>v. When the distance to the reflecting object is increasing, the lower side band of the final product is selected during the period of increasing transmitter frequency giving an output F<SP>1</SP>3=(Fh+Fd - F1d+Fv-F<SP>1</SP>v) since Fs=(Fd+Fv) and during the period of decreasing transmitter frequency the upper side band is selected giving an output F"3= (Fh+Fd-F<SP>1</SP>d-Fv+F<SP>1</SP>v) since in this case Fs=(Fd-Fv). The frequency F<SP>1</SP>v is first adjusted until F<SP>1</SP>3=F<SP>11</SP>3 which obtains when Fv=F<SP>1</SP>v, thus enabling Fv to be determined and the frequency F<SP>1</SP>d is then adjusted until F<SP>1</SP>3 and F"3 are equal to Fn which obtains when Fd=F<SP>1</SP>d, thus enabling Fd to be determined. The latter adjustment is facilitated by providing a second frequency meter capable of indicating small deviation from Fn and the former adjustment is facilitated by bringing F<SP>1</SP>3 and F"3 into coincidence by delay devices and deriving the difference frequency which is applied to an indicator. Fig. 13 shows another system operating with the frequency variation shown in Fig. 6 in which the beat frequency output Fs from the receiver E is modulated at M4 by a fixed frequency Fh from oscillator Oh and the upper side band (Fs+Fh) is selected and applied alternately to the modulators M5 and M6 during the increasing and decreasing portions respectively of the transmitter frequency variation. Variable frequencies F<SP>1</SP>s and F<SP>11</SP>s are applied to modulators M5 and M6 from the oscillators O<SP>1</SP>s and O<SP>11</SP>s and the difference frequencies F<SP>1</SP>5= (Fh+Fd+Fv-F<SP>1</SP>s) and F<SP>11</SP>6=(Fh+Fd-Fv- F<SP>11</SP>s) are applied to frequency indicators K5 and K6. The frequencies F<SP>1</SP>s and F<SP>11</SP>s are adjusted until F<SP>1</SP>5=F<SP>11</SP>6 which obtains when Fv=¢(F<SP>1</SP>s-F<SP>11</SP>s), thus enabling Fv to be determined and the frequencies F<SP>1</SP>s and F<SP>11</SP>s are then simultaneously adjusted until F<SP>1</SP>5= F<SP>11</SP>6=Fk, where Fk is some predetermined frequency, which obtains when Fd=Fk - Fh+ ¢(F<SP>1</SP>s+F<SP>11</SP>s), thus enabling Fd to be determined. The values of Fv and Fd may be conveniently displayed respectively on two scales which are geared to the frequency controls of oscillators O<SP>1</SP>s and O<SP>11</SP>s such that the scales indicate respectively the sum and difference of the frequency control settings. The simultaneous adjustment of frequencies F<SP>1</SP>s and F<SP>11</SP>s to equality with Fk may be facilitated by the provision of discriminators D5 and D6 and meters Z5 and Z6 to indicate small deviations from Fk or the outputs from the discriminators may be applied to control the oscillators O<SP>1</SP>s and O<SP>11</SP>s automatically. The calibration may be checked by connecting a standard delay line L between the transmitter S and receiver E and the distance range of the indicators in the embodiments shown in Figs. 12 and 13 may be changed by altering the fixed frequency Fh. Specifications 581,120, 584,496 and 587,357 are referred to.