Location of the Position of a Mobile ObjectThis invention relates to the location of the position of a mobile object, which is to be used in conjunction with a base station.
The invention provides location apparatus for a mobile object which is used in conjunction with a base station, which comprises processing means for deducing information about the position of the mobile by comparison of signal information transmitted from a radio transmitter of a known fixed location to the mobile or the base station along a direct path, with signal information transmitted to the mobile or the base station along an indirect path involving retransmission of the signal information from the base station or the mobile, respectively.
A knowledge of the delay in signal propagation along the indirect path over the direct path provides information about the position of the mobile relative to the fixed transmitter and the base station.
The invention also provides a method of location of a mobile object which is used in conjunction with a base station, which comprises deducing information about the position of the mobile by comparing signal information transmitted from a radio transmitter of known fixed location to the mobile or the base station along a direct path, with signal information transmitted to the mobile or the base station along an indirect path involving retransmission of the signal information from the base station or the mobile, respectively.
For the avoidance of doubt, the term direct path" as used herein is not restricted to line of sight propagation, since in some cases the signal will be reflected from obstacles e.g. buildings in a city centre.
A plurality of mobiles may be used in conjunction with a base station as part of a radio communication apparatus. Communication between the base station and the mobiles may be one way in either direction or two way.
The apparatus may form part of a mobile radio telephone system.
The apparatus may include receiving means coupled to transmission means to retransmit the signal information.
A frequency converter may be provided so that the retransmission is on a different frequency to avoid signal interference. A reverse conversion will need to be made to enable the comparison to be made. Preferably the transmitting means and frequency converter are provided in the mobile, and the processing means in the base station, but the reverse is also possible.
Advantageously, the phase of the direct and indirect signals are compared.
The radio transmitter may be any broadcast radio transmitter e.g. a commercial or public radio station, TV station, radio telephone base station or any other fixed station, radio telephone base station or any other fixed existing transmitter of radio waves i.e. electromagnetic SO waves thè frequencies of which lie between w kHz and ltHz (l,OOOGHz).
Usually, the measurements from a single transmitter will be insufficient to locate the position of the mobile, and the position will be determined more completely by measuring signals received from at least two transmitters.
Two transmitters would suffice in conjunction with a compass bearing measured at the mobile, but usually it will be necessary to calculate the position from measurements of three transmitters.
The mobile may be in a vehicle, but could be a pack to be attached to a person, or could be attached to a water craft. In the case of a vehicle, the invention enables an operator of a fleet of vehicles e.g. delivery vehicles, taxis, cabs, police cars, ambulances, buses etc.
to be able to pinpoint the position of each vehicle on a map of the territory covered: at present such information can only be obtained by individual drivers phoning their locations in from time to time.
Apparatus for and a method of locating the position of a mobile object in accordance with the invention, will now be described, by way of example, with reference to' the accompanying drawings, in which:Figure 1 is a schematic view o f a mobile and transmitters;Figure 2 shows phase comparison of direct and indirect signals and;Figure 3 shows the locus of points on which the mobil lies.
Referring to Figure 1, a radio communication system comprises several base stations, one of which is shown 1, and a plurality of mobiles, one of which is shown in a vehicle 2. The vehicle and base station are in the vicinity of three transmitters 3 to S, whose location is known. These transmitters are broadcast radio transmitters eg. radio or television transmitters.
The vehicle 2 is provided with a receiver 6 having an omnidirectional antenna, which can receive transmissions from the transmitters 3 to 5. The base station 1 is provided with a similar receiver 7.
The receiver 6 is coupled to transmitter 8 via a frequency converter (not shown). The transmitter 8 and receiver 6 on the vehicle and the receiver 7 on the base station may (but do not have to) form a part of the two-way radio communication system which exists between the mobiles and the base station.
Each receiver 6 is equipped with means for measurin signal strengths of transmissions received, and selecting the three strongest.
Each of these signals is then separately converted to a different frequency via the frequency converter and re-transmitted to the base station 1. A converter at the base station reconverts each signal to its original frequency.
Processing means (not shown) at the base station compares for each transmitter signal, the signal received from the vehicle with the signal it has itself received from the same transmitter. The latter signal has travelled directly from the transmitter to the base station, whereas the former has travelled indirectly, via the mobile. The processing means may compare eg. carrier waves (Figure Z) and there will be a phase difference between them to reflect the longer delay from the transmitter associated with the indirect path. The-base station carries information on the location of the various transmitters it can pick up and, knowing the phase difference and also the frequency of the carrier, the corresponding additional propagation distance for the indirect path can be calculated. One can then deduce that the mobile in question must lie somewhere on an ellipse, the foci of which are the base station ånd the transmitter (Figure 3).
The same calculation can then be performed for the other two transmitters, in order to pin point the positions of the mobile.
Of course, there may'bye a delay in the frequency converters and transmitter 8, but this can be compensated for.
Instead of comparing the phase difference of the carrier, comparison may be made between other characteristics of the signal eg. the modulation impressed on the carrier may be compared,Of course, it is not necessary for the mobile to retransmit the entire signal to the base station: it is sufficient for the mobile to transmit the zero - crossing points of the signals for the comparison to be made.
It will be appreciated that the processing means could be positioned in the mobile instead of in the base station i.e. the transmitter 8 could be associated with th base station rather than with the mobile.
As an alternative to the system described above, the mobile may compare the information received from pairs of transmitters 3 to 5 (especially if they broadcast the same signal eg. the same programme), and re-transmit the comparison to the base station 1, where the information can be compared with similar comparisons between the signals from pairs of the transmitters 3 to 5 received directly at the base station.
Indeed, in the case where the mobile and the base station are synchronised with each other (by means such as time slots), as will normally be the case in a radio communication system, it will be sufficient to measure the position within a time slot at which the signal from any particular transmitter (i.e. a characteristic feature of such a signal) is received (a) at the mobile and (b) at the base station. In this case, it will not be necessary in theory for the signal to be re-transmitted to the station having the processing means in order to calculate the difference in distance from the transmitter of the base station and the mobile (erg. the outputs of the receivers at the mobile and at the base station could be recorded, and recordings could be compared later at a different location), although information on the signal will usually be re-transmitted in practice in order to enable the comparison to be made. Again, measurements would be made for all three transmitters. Also, comparisons may be made at the mobile and at the base station of signals from different transmitters in pairs, especially if such transmitters are broadcasting the same signal i.e. the same programme.