Patent GB-1046870-A (GB)
Title: Improvements in and relating to radio-location systems
IPC: G01S3/48
Filed: 1963-10-08
Abstract: 1,046,870. Radio location systems. COMPAGNIE FRANCAISE THOMSON-HOUSTON. Oct. 8, 1963 [Oct. 8, 1962], No. 39617/63. Heading H4D. An object in space emitting an ultra-high frequency carrier wave, modulated by a series of much lower frequency signals in fixed and known phase relationship with each other, is located by apparatus comprising two receiving stations at the ends of a base line adapted to receive the modulated signal, and comprising phase comparison means operating on each of the modulations as received at the two receiving stations in turn, until an unambiguous determination of the hyperbolic surface on which the object is located is made. In the embodiment shown three receiving stations 1, 2 and 3 are used with the base lines 1, 2 and 1, 3 perpendicular such that the location of the object may be further defined as on the line of intersection of the hyperbolic surfaces resulting from phase comparisons between receivers 1 and 2, and 1 and 3. The modulation is at four harmonically related frequencies. A reference frequency of 25 kc/s. is produced by control station 4 and is fed to three receiving stations 1, 2 and 3, and a transmitting station 1<SP>1</SP>. The receiving stations, and the transmitting station, each comprise a frequency multiplying circuit, Fig. 2 (not shown), which comprises a buffer amplifier receiving the 25 kc/s. signal and having a tuned mode circuit tuned to 25 kc/s., followed by a grounded cathode frequency multiplier having a tuned anode circuit tuned to 50 kc/s. The succession of amplifiers and multipliers continues until frequencies of 200 kc/s., 1À6 Mc/s., 12À8 Mc/s. and 102À4 Mc/s. are available. An automatic phase control circuit can be incorporated in the multiplying circuit wherein the phase of the 25 kc/s. signal is compared with that of the 200 kc/s. signal via a variable phase shifter to produce an error signal which corrects the phase of the 200 kc/s. signal by means of the phase shifter. The four modulation frequencies at transmitter 1 are modulated on to a 3 cm. carrier and transmitted to the object O. The object O retransmits the modulated carrier, after a known delay, and with a slightly different carrier frequency. The modulated carrier from the object is received by the three receiving stations and the modulators are detected and compared in phase with the corresponding frequencies from the frequency multiplier circuits. The results of the phase comparisons are converted to digital form and transmitted to the control station 4, whereby the direction and distance of the object are calculated. Normally the phase measurements are made at 102À4 Mc/s., measurements at the lower modulations being resorted to to overcome ambiguities. When the initial position of the object is known, the phase measurements of 102À4 Mc/s. supply the incremental changes in position and are sufficient. In order that the distances between the receiving stations and the control station shall be known with sufficient accuracy, the reference frequencies are initially passed back to the control station and the resulting phase difference determined. The receiving station 1 and transmitting station 1<SP>1</SP> may be at the same location with the same aerial. The line joining the stations may be replaced by radio links.