Abstract: Information is communicated by microwave radio between transponders (9,10) carried by vehicles (7,8) travelling on a road (1) and a station (22) adjacent the road. The communications from the transponders are effected by the transponders suitably modulating their reflections of beams of microwave energy transmitted by the station from aerials (B) mounted on a gantry (11) above the road, these aerials irradiating respective communication areas (28,29,30). In order to prevent communications from different transponders overlapping and hence interfering with each other the transponders are enabled for their communications by microwave energy from further aerials (A), which energy has a higher frequency to enable it to be beamed at relatively small respective activation areas (23-27) the sizes of which are such that they can each only contain one vehicle and hence one transponder at any given time.

Abstract: A signal receiving arrangement comprising first and second groups (14, 16) of frequency down-converting stages (18 to 24 and 26 to 34), each stage including a mixer (40) and a local oscillator (42) for frequency down-converting a signal applied to the stages (18 to 24 and 26 to 34), the local oscillator frequencies of successive stages (18 to 24 and 26 to 34) being progressively higher with the frequency differences between successive stages in each group forming an arithmetic progression, wherein the frequency down-converted outputs of the stages of each group are combined and the pair of residue frequencies formed by mixing down the input signal are used to determine on the basis of the Chinese remainder theorem which stage in each group was operative on the input signal and thereby ascertain the actual frequency of the input signal.

Abstract: In, for example, a passive direction-finding radio receiver employing a multi-beam antenna which has e.g. ninety-six feeds which form a corresponding number of overlapping reception beams, the output signal amplitudes of the various feeds can be compared with a reference to yield a sequence of binary digits, e.g. as shown in FIG. 2. The position of a radio source is given by the position in the sequence of the center of the largest group of "1"s. In order to quickly determine this position the sequence is subjected repeatedly to a processing operation in which each "1" which does not have "1"s as its two immediate neighbors is changed to a "0", thereby repeatedly stripping the two end "1"s off each group of "1"s until none remain. The required position is that of the single "1" or pair of "1"s present immediately before all have become "0"s.

Abstract: A method and a system for radio direction-finding by measuring the Time of Arrival (ToA) of the leading edge of signals from a distant source at two relatively closely spaced receiving elements. In order to give a good degree of immunity to multipath, the times at which the instantaneous detected amplitudes of the received signals first exceed a minimal threshold value such that received signals can be satisfactorily distinguished from noise is measured in such a manner that the measured time is not affected by multipath which involves more than a few meters additional path length for the indirect, delayed signal. A suitable timing circuit is disclosed.By making ToA measurements on three coplanar, non-collinear receivers, directions of incidence in three dimensions can be determined.A method and a system using both ToA and phase-difference measurements can provide the accuracy of interferometry but be simpler and cheaper.