Patent GB-876748-A (GB)
Title: Improvements relating to radio indication systems
IPC: G01S13/42
Filed: 1958-10-08
Abstract: 876,748. Radio navigation. COMPAGNIE FRANCAISE THOMSON-HOUSTON. Oct. 8, 1958 [Oct. 8, 1957], No. 32138/58. Class 40(7) A system for determining the change in the range of a mobile craft from a station by integrating the craft's radial velocity comprises means at the station arranged to transmit two signals of different frequency, means at the craft to receive the two signals, drive a difference signal therefrom, and to frequency modulate a transmitted carrier with the difference signal. The signal from the craft is received by the station and the craft's radial velocity determined by evaluation of the Doppler shift. The station receiver may be associated with an aerial system having a plurality of primary elements providing a like number of radiation patterns deviating from, and overlapping on, the aerial system axis so that the direction of the deviation of angular position of the craft from the said axis can be determined. In one embodiment the station SF includes a transmitter 1 and aerial 2 radiating signals of frequencies F and F+#F. At the craft the signals are received (their frequencies having the appropriate Doppler shifts) and the difference frequency between them, which is #F+#Fd, where #Fd is the Doppler term, used to frequency modulate a carrier of frequency f is transmitter 4. The signals radiated by aerial 5 include ones with frequencies f and  #+#F+#Fd. When these are received at the station and the difference frequency taken the resulting signal has a frequency of #F+2#Fd (a small term being neglected). As shown the receiving aerial system 6 includes two primary elements 7 and 8 having patterns overlapping on the axis and the sum of the signals received by the two elements is fed over channel S to receiver 9 where the difference frequency above-mentioned is derived. A phase discriminator 11 with an associated amplifier, motor, and modulator, loop 12, 13 compares the difference frequency signal from receiver 9 with a signal of frequency #F from transmitter 1 to give a shaft rotation proportional to the radial velocity of the craft. This is applied to revolution counter 15 to give an indication of distance variation. The receiver 9 has a second input fed from channel D with the difference of the signals from elements 7 and 8. The output from this part of the receiver is the same in frequency as that from the part fed by channel S. Determination of the phase of the "D" output relative to the "S" output indicates to which side of the aerial axis in the plane defined by elements 7 and 8 the craft lies. As it is preferred to compare the phase of fixed frequency signals the two outputs are mixed with a local signal of frequency #F+2#Fd+#m in mixers 17 and 18 to produce signals of frequency fm for comparison in phase discriminator 19. The local signal is produced in circuit 20. If four primary elements arranged in square formation are used in aerial system 6 and a third channel included in receiver 9 it is possible to determine the direction of the craft's deviation from the aerial axis in two mutually perpendicular directions.