Patent GB-996492-A (GB)
Title: Position determination by means of radio waves
IPC: G01S1/02
Filed: 1962-04-04
Abstract: 996,492. Radio position finding. SOUTH AFRICAN INVENTIONS DEVELOPMENT CORPORATION. April 4, 1962 [April 17, 1961], No. 13015/62. Heading H4D. In a system for determining the position of a radio receiver with reference to a set of equiphase lines, comprising two aerials each radiating a signal having a frequency in an upper channel and a signal having a frequency in a lower channel, wherein the upper channel signals from each aerial and the lower channel signals from each aerial are caused to differ in frequency by an equal amount and are heterodyned together in the receiver to obtain two equal beat frequency signals and wherein the said relative position of the receiver is determined by comparing the places of the beat frequency signals, the ambiguity normally found in such a system is overcome by altering the various radiated frequencies in steps through a sequence of valves to provide a plurality of equiphase curve patterns of various gradients. The system as shown (Fig. 1) comprises two transmitters A and B and a receiver P. The transmitter A comprises two generators 11 and 12, controlled by a timer 13 and feeding lower channel frequency f 1  and upper channel frequency f 3  or f 3a  to a common aerial for transmission to the receiver P and the transmitter B. The transmitter B also comprises lower and upper channel generators 14 and 15 controlled by a timer 16 and feeding frequencies f 2  or f 2a  and f 4a  or f 4a  to a common aerial. The lower and upper channel frequencies from the transmitter A are received by the transmitter B and are fed, together with the corresponding lower and upper channel frequencies from generators 14 and 15, to two respective lower and upper channel tuned heterodyne receivers 18 and 19 to give the beat frequencies of the respective channels. The two beat frequencies are fed, to test their equality, to a phase discrimination 20, any difference found therein providing a control signal to vary the frequency of the signal from generators 15. The upper channel beat frequency signal is also used to frequency modulate a lower channel generator 17 also controlled by the timer 16. The receiver P comprises two heterodyne receivers as in transmitter B, to produce the lower and upper heat frequency signals which are then compared for phase in a phase indicator 24 from which the position of the receiver, relative to the hyperbolic equiphase lines generated by the transmitters may be found. The timers 13 and 16 control the emitted frequencies in five steps as follows (Fig. 2). Step 1.-f 1  and f 3  are generated by transmitter A whilst transmitter B generates f 2  and f 4  with f 2 >f 1 , f 3 >f 4 , and (f 2  - f 1 ) = (f 3  - f 4 ). It can be shown that in this case the phase difference measurements are associated with a frequency of (f 1 +f 3 ) generating a fine pattern of hyperbolic equiphase lines whereby only the position of the receiver relative to two adjacent equiphase lines can be determined, thus giving rise to considerable ambiguity. Step 2.-f 1  and f 3  are generated by transmitter A as before, but transmitter B now generates f 2a  and f 4  with f 2a <f 1  and (f 1  - f 2a ) = (f 3  - f 4 ). A medium pattern is now generated associated with a frequency of f 3  - f 1 , giving rise to less accuracy but also less ambiguity. Step 3.-f 1  and f 3  are still generated by transmitter A, but transmitter B now generates f 2  and f 4a  with f 4a >f 3  and (f 2  - f 1 ) = (f 4a - f 2 ) A negative medium pattern is now generated associated with a frequency of - (f 3  - f 1 ), such that by subtracting the phase difference thus obtained from that obtained in step 1, a fine pattern phase difference measurement is obtained associated with a frequency of 2f 3  and having no instrument errors. Similarly combining the phase readings of steps 2 and 3 gives an error free reading associated with a frequency of 2(f 3  - f 1 ). Step 4.-The two lower channel frequencies remain as in step 3 whilst the upper channel frequencies are lowered to f 3a  and f 4b  with (f 2  - f 1 ) = (f 4b  - f 3a ) as before. This gives rise to another negative medium pattern associated with a frequency of - (f 3a  - f 1 ) and giving rise to an associated frequency of (f 3  - f 3a ) in combination with step 2. Step 5.-The two upper channel frequencies are as in steps 1 and 2 whilst transmitter B radiates a carrier of frequency f 1  frequency modulated by the frequency difference (f 3  - f 4 ). This frequency modulation is extracted at the receiver for comparison with the normally produced beat frequency (f 3  - f 4 ). This gives rise to a reading associated with a frequency of f 3 . Phase difference measurements of various accuracies can therefore be made associated with frequencies: (f 3a  - f 3 ), (f 3  - f 1 ), 2(f 3  - f 1 ), f 3 , (f 1  + f 3 ) 2f 3 , the lower the associated frequency and accuracy, the less the degree of ambiguity.