Patent GB-1486319-A (GB)
Title: Distance measurement
IPC: G01S5/12
Filed: 1974-10-22
Abstract: 1486319 Radio position finding SOC D'ETUDES RECHERCHES ET CONSTRUCTIONS ELECTRONIQUES 22 Oct 1974 [23 Oct 1973] 45670/74 Heading H4D In a method of measuring distance a first transmitter sends out a first wave train, phase inversion modulated according to a first repetitive chain of a pseudo-random sequence, which wave train is received by a first receiver, whereat its modulation is correlated with a locallygenerated progressively delayed copy of the pseudo-random sequence until the two chains coincide. A second wave train is then sent out by a second transmitter associated with the first receiver, phase inversion modulated according to the delayed (synchronized) copy of the pseudo-random sequence for reception by a second receiver whereat its modulation is correlated with a locally-generated progressively delayed copy of the pseudo-random sequence until the two chains coincide. Finally, the time displacement between a reference copy of the pseudo-random sequence and the latter mentioned delayed copy is determined. In a first embodiment, the second receiver is associated with the first transmitter in an interrogator, the reference copy being the first repetitive chain. The measured time displacement between it and the latter mentioned delayed copy is thus indicative of twice the distance between the interrogator and the responder. More than one responder may be provided, with operation of the transmitters on a time-multiplex basis (Figs. 4, 5, not shown). A mobile interrogator may obtain an unambiguous position determination from three responders in known fixed positions (Fig. 15, not shown). In a second embodiment, the first transmitter is at a known fixed point. The first receiver and the associated second transmitter are at another known fixed point. The second receiver is mobile, and comprises an additional channel, for receiving directly from the first transmitter. The pseudo-random sequence so received is also correlated with a locally-generated pseudo-random sequence. The time displacement between the two locallygenerated sequences is then proportional to the difference in the distances between the second receiver and the first transmitter and between the second receiver and the second transmitter. By providing another receiver-transmitter at a third known fixed point, and a third channel in the mobile second receiver, the latter may obtain an unambiguous position determination (Fig. 16, not shown). A responder, as used in the first embodiment, is shown in Fig. 6, and an interrogator, which is able to co-operate with two responders, is shown in Fig. 7. A single side band correlator, e.g., 112, Fig. 6, operates a control 113 which adjusts the frequency of a supply from a source 103 to the local pseudorandom-sequence generator 105. The frequency of transmission is within the range 420 to 450 MHz, which is reduced to an I.F. of 38 MHz on reception. The pseudo-random code sequence has 127 (r) bits each of duration of 0À5 Ásec. (T1), which corresponds to a frequency of 1-9 MHz. Approximately 37 successive sequences are transmitted in each pulse, to give a pulse duration of 2À5 msec. (T2), (Fig. 8, not shown). The units 112, 113 of Fig. 6 are shown in more detail in Fig. 11 (not shown).