EP0110945B1

EP0110945B1 - A location monitoring system

Info

Publication number: EP0110945B1
Application number: EP83901813A
Authority: EP
Inventors: Svend Aggerholm; Oddvar Bothun; Flemming Hansen
Original assignee: Storno AS
Current assignee: Motorola Solutions Danmark AS
Priority date: 1982-06-07
Filing date: 1983-06-07
Publication date: 1987-01-14
Also published as: GB2132398A; WO1983004451A1; DK154588B; SE8400632L; DK154588C; SE8400632D0; DE3390030T1; SE456287B; DK255382A; GB2132398B; DE3390030C2; GB8402638D0; EP0110945A1

Abstract

In a location monitoring system for monitoring the location of a plurality of mobile units operating on preestablished routes from a centrally located control station, and where each mobile unit (14) communicates partly with a base station (10), partly with signposts (15, 16) located in specific positions along the known routes, from which the mobile units (14) receive position-specific information during passage, the transmitters (12, 15) in both the base station (10) and in each individual signpost (15, 16) are arranged to have the same operating frequency (F1). This saves an extra receiver in each of the mobile units (14) while minimizing the need for transmission channels. Moreover, the antenna (16) of the signpost has been given a signal emission in a narrow range around it and with such signal strength that the message acceptance to the signals of a signpost (15, 16) during the passage thereof is reasonably good (50%), and at the same time the probability that a message from the base station (10) is not received is very low (=0.1).

Description

The invention concerns a system for monitoring the location of a plurality of mobile units operating on routes known in advance from a central control station, said system comprising a stationary base station covering at least one transmission channel including a transmitter for transmitting speech or data communication on a first frequency and a receiver for receiving speech or data communication on a second frequency for selective radio communication with the mobile units, each of said mobile units similarly comprising transmitter means and receiver means for radio communication with said base station, and with "signposts" located in specific positions along the known routes, each of said "signposts" being provided with a short range transmitter for transferring position-indicating information to a storage and processing means provided in each mobile unit, and said control station further comprising a computer sequentially providing interrogating signals to each mobile unit, upon the receipt of which each mobile unit replies to the control station transmitting the stored and processed position information.
A system of the above-mentioned type is known in many embodiments and is primarily used for monitoring e.g. the public bus service. In order most efficiently to utilize the available rolling equipment and to provide a reasonably good service to the public, it is necessary that e.g. the scheduled arrivals and departures are maintained as close as possible. However, also other regards than the maintenance of schedules have justified the establishment and maintenance of a location monitoring system, through which it is possible at any time to get an exact survey of where in the system of routes each individual mobile unit is positioned.
For example in case of traffic jams, the individual units can be diverted and controlled from the base station. A system of the above-mentioned type is useful not only in connection with the public transport of passengers, but has also proved its usefulness in connection with e.g. money transports or patrols of different kinds.
Communications between the signposts and the individual mobile units take place by wireless and are based on different techniques, but most of them are unsuitable in the present use. Thus, communications by infrared light only have a limited range, and the same applies to communications by ultrasound, which moreover has a poor signal-to-noise ratio. RF- communications at very low frequencies are known in connection with rail traffic, but this technique is expensive and difficult to maintain.
Radio communication in the microwave range constitute the most frequently used method owing to its reliability and accuracy, but is very expensive.
The apparatus used in the VHF/UHF range, on the other hand, is less expensive, but then there is a lot of traffic in the air so that it may be difficult to get the required number of communications channels from the telecommunications authorities.
US-A-3,644,883 discloses a traffic monitoring system, in which a sequence of messages is continuously transmitted from a traffic control centre, each containing an address of an individual bus, and information is received from each bus about its position at the time of reply. The interrogation messages are transmitted on one frequency, the interrogation frequency, while the reply messages from the individual busses are transmitted on another frequency, the reply frequency. This part of the communications between the traffic control centre and each individual bus takes place digitally. In addition to this, the known system provides a facility for communicating at voice communications level between the driver and the traffic control centre, which takes place on another channel, the voice communications channel.
Moreover, each individual bus receives position-specific information during the passage of a signpost, which continuously transmits messages containing an identification code specific of the signpost in question.
Each signpost comprises a digital position generator, the output signal of which is converted by FSK technique into tones used for modulating a constantly operating transmitter.
Thus, considerable technical efforts are involved in this prior art traffic monitoring system with separate transmitters and receivers for communications between the bus and the traffic control center and between the bus and signposts, respectively, simultaneously with the occupation of five different communications channels or frequencies.
DE-B-2 143 474 describes a system for monitoring the location of a mobile unit in which the signposts are actuated from the mobile unit itself,.e.g. by an RF-signal from the mobile unit. At the same time, the transmitter on the mobile unit is activated shortly before passage of the signpost by means of a sensor. During the passage the sensitivity of the receiver of the mobile unit is decreased by about 40 db in order not to be disturbed by messages from the control center. Shortly after the passage of the signpost the transmitter in the mobile unit transmits the position information towards the control center.
The invention is characterized by the features of the claims.
According to the invention location monitoring and voice communicating system is provided wherein the receiver means in each mobile unit comprises only one receiver for receiving all communications from the central base station and from the signpost transmitters as well, and wherein the transmitter frequency of the base station and the transmitter frequency of each individual signpost both are included in the VHF-UHF range. Preferably these two frequencies are the same or substantially the same frequency.
. The communication equipment in each of the many mobile units becomes less expensive, because only one receiver has to be installed in each mobile unit. Further, the same transmitter in the mobile unit is used for transmitting both voice communication and position data, whereby the base station also only needs one receiver. Therefore, the stationary equipment is also simplified and thereby becomes less expensive.
Finally, the need for communication channels is reduced by only using a single channel instead of two channels, as shown in the prior art.
Preferably, substantially the same carrier frequency is used in the signpost transmitters and "in the control centre transmitters. To ensure adequate acceptance of signals according to the invention the ratio between the signal level from the signpost transmitters and the signal level from the centre control transmitter is selected in such a way that the signal level on the route of the mobile units right in front of the signpost is sufficiently above the mean level of the signal from the control station to achieve an adequate message acceptance of the signals from the signpost and further in that the antenna on the transmitter of the signpost has a radiation pattern in the relatively narrow main beam directed towards the route of the mobile unit. Thereby the mobile unit will be able to accept the position indicating information during the passage of the signpost. After the mobile unit has passed the signpost only the control station signals will have a level allowing them to be received by the mobile unit receiver.
If the local conditions for receiving in the traffic area of the mobile units impede the use of substantially the same frequency, it may be advantageous to use a lower or higher frequency for the signpost transmitters, preferably in the UHF-band by providing additional circuit means at the input of the ordinary receiver in each mobile unit for frequency conversion from the signal frequency of the signpost to the signal frequency of the base station, whereby the position indicating information transmitted from the signpost to the receiver of the mobile unit can be transferred unaffected by the frequency conversion. In this embodiment only some minor auxiliary circuits are required, and basically the same receiver equipment is still used both for the speech communication and for the position indicating information.
Preferably, the auxiliary circuits comprise a series connection of a band-pass filter for the signpost transmitter frequency, an amplifier a frequency converter or mixer circuit connected to a local oscillator, a band-pass filter for the control station transmitter frequency, and some injunction, said series connection being inserted between the ordinary receiver and the ordinary antenna on said receiver.
The invention is explained more fully below with reference to the drawings, in which

Fig. 1 is a schematic view of the individual main components of a location monitoring system,
Fig. 2 shows the units incorporated in the transmitter/receiver equipment of a mobile unit,
Fig. 3 shows, for different signal levels, the message acceptance as a function of the difference in level between the signal strength of two signals on the same frequency,
Fig. 4 shows the relative signal level frmm the antenna of a signpost as a function of the relative distance in the direction of motion,
Fig. 5 is a sketch to illustrate the coverage profile of a base station, and
Fig. 6 is a diagram of a modified transmitter/ receiver in a mobile unit.

Best mode of carrying out the invention

In Fig. 1 a stationary base station is generally designated by 10. In principle it comprises a central computer 11 containing all relevant information about the mobile units forming part of the system, the routes to be traversed, and the points of time when these units are scheduled to pass specific positions in the system of routes, and a transmitter/receiver part 12 which is in radio contact with a mobile unit 14 via an antenna 13. Also, the base station comprises the usual equipment for such a station for the monitoring of the system of routes, such as control desk with display and printer units and radio telephony equipment for the setting up of voice communications connections with any of the mobile units in the system but such equipment is ordinary and does not concern the invention, and is accordingly not shown in the drawing. The same applies to the interface equipment between the central computer 11 and the transmitter/ receiver part 12.
In the drawing the mobile unit (14) is symbolized by a vehicle of a type, e.g. a bus; the essential feature to the system, however, is that each mobile unit 14 follows a known route and passes certain fixed points on the route. Such a fixed point is marked by a signpost 15, which is in principle a transmitter which, through an antenna 16, constantly emits a signal of a suitable strength and containing information about the position of the signpost in question in the system of routes.
A sequence of interrogation messages is emitted from the base station 10, each containing its individual address to a mobile unit 14 which upon reception of an interrogation message transmits its reply message containing the identification code for the unit in question and information about its current position. This communication between the base station 10 and the mobile units 14 takes place through two channels, an interrogation channel of the frequency F1 and a reply channel of the frequency F2 as marked by a double arrow 17.
The position of the signpost 15 is transmitted to the mobile unit 14 during the passage thereof, and this information is stored in a known manner in a store until passage of the following signpost in the system. The travelled distance between two signposts is measured continuously e.g. by means of an odometer, and this measured value is applied to the position store likewise in a known manner, so that the position information transmitted with the reply message is correct within a certain margin.
According to the invention, all the signposts 15 transmit on the same frequency F1 as the base station 10, which is indicated by a single arrow 18. This involves the advantage that an extra receiver is saved in each mobile unit while minimizing the need for transmission channels. It is not unknown that it is difficult, if not impossible, to find free channels in the VHF/UHF band.
Fig. 2 shows the most important units of the invention which are incorporated in the transmitter/receiver equipment of a mobile unit 14. Via an antenna 21 the mobile unit receives a message from the base station 10 or from a signpost 15, and this message is passed on through a bandpass filter 22 of the centre frequency F1 to a receiver 23. The receiver selects the received messages so that only the message from the base station 10 which has the correct address, or a message from a signpost 15 causes a response. This consists in the storage of the position code of a signpost in a control unit 24, which also receives information from an odometer (not shown) about the travelled distance since the last passage of a signpost, or the response is that an accepted interrogation message causes the information on the position which is stored at the given point of time in the control unit 24 to be transmitted to a transmitter 25, which transmits this information together with the identification code of the mobile unit through another bandpass filter 26 of the centre frequency F2.
In order for the communications between the base station and the mobile unit and between the signpost and the mobile unit, respectively, to take place expediently, it is necessary that certain criteria are met. Firstly, it is necessary that the message acceptance is reasonably high both on reception of interrogation messages from the base station and on reception of position messages during the passage of a signpost. Secondly, the emission diagram of the signpost antenna 16 must be so narrow that its position is defined with reasonable accuracy, and so narrow with respect to the spacing between the signposts that the probability of non-acceptance of an interrogation message is reasonably low, but not so narrow that the message acceptance is too poor during the passage of a signpost at normal average speed. This calls for a certain minimum spacing between the individual signposts.
If an experiment is made with two different transmitters transmitting on the same frequency but with different levels, with a view to determining the order of the message acceptance as a function of the difference in level at the reception location between the signal strength of the two transmitters, it will be found that the message acceptance is 50% already at a difference in level of only 6 dB, while a message acceptance of 95% requires a difference in level of about 24 dB, and these values are fairly independent of the absolute signal level. This appears from Fig. 3 where this relation is plotted in a system of coordinates with the message acceptance in % plotted along the ordinate and the difference in level in the signal strength of the transmitters measured in dB along the abscissa.
Fig. 4 shows an emission diagram for a two- element halfwave end-fire antenna suitable as an antenna 16 for a signpost 15. The ordinate of the diagram indicates the mean value of the relative signal level in dB, and its abscissa the relative distance in metres calculated in the direction of motion of a mobile unit forming part of the system. As expected, the diagram is relatively symmetrical about the axis of the antenna and exhibits a narrow opening angle. About 10 m at either side of the signpost the signal level has already decreased 6 dB with respect to maximum signal strength in the main direction of the antenna, and just under 50 m to the side the signal strength of the antenna has decreased by a further 20 dB. If the transmitter strength of the individual signpost 15 is arranged in such a manner that the 6 dB level is equal to the mean level of the signal from the base station 10, the message acceptance of a message transmitted from a signpost is about 50%, cf. the curves in Fig. 3, and if a mobile unit passes a signpost at an average speed of 10 m/s, the unit receives about 5 to 10 messages from the signpost, the duration of a message including interval being about 100 ms. Thus, it is ensured that the mobile unit receives the position information from the signpost passed and stores it for subsequent transmission to the base station.
Similar operating conditions can be obtained with other antenna forms, such as reflector antennas or more complex antenna systems.
It moreover appears from Fig. 4 that the signal strength from the signpost about 50 m to its side has decreased to a level of about 20 dB below the mean level of signals from the base station, and at this point the message acceptance of a message from the base station 10 is about 90%, cf. Fig. 3. This would be excellent if the base station constantly transmitted messages of the same address, but since this is not the case, it is necessary to have a certain minimum spacing between the signposts if the probability that a mobile unit does not receive a call from the base station is not to be too great.
In Fig. 5 the coverage profile of the base station is illustrated. The spacing between the signposts is designated by a, and the trapezoidal curves between them indicate the message acceptance of messages from the base station. The spacing b indicates the length of the distance opposite a signpost where the acceptance is less than 90%. If a message acceptance completely outside the operating ranges of the signposts of 95% is required, and if this quantity is called 1-P_o, it can be shown that the probability P that a mobile unit does not receive a message from the base station can be expressed by:
hence the minimum spacing between the signposts:
If b=100 m, P=0.1 and P_o=0.05, a minimum spacing a,,,In=2 km results.
The numerical examples in the foregoing must not be interpreted restrictively, however, because the determination of the described differences in level in signal strengths e.g. greatly depend upon which message acceptance. levels are found satisfactory in given situations.
It may occur, however, that the interrogation frequency F1 allocated to a base station 10 is too low for the same frequency to be used as an operating frequency for the signposts because the emission angle of the antennas 16 then cannot be kept as narrow as desired according to the foregoing. In that case, according to the invention, the transmitter equipment in each mobile unit can be extended with an own oscillator and frequency converter, so that the mobile unit still uses the same receiver as before. This extension is shown in Fig. 6. With respect to the diagram in Fig. 2, the extension consists in a dividing network 31 for the operating frequency Fb of the signpost, said dividing network 31 passing on the signal from the antenna 21 through an amplifier 32 to a frequency converter 33, which receives a signal from an own oscillator 34 oscillating on the frequency Fb-F1. The signal composed by the converter 33 is passed through a bandpass filter 35 of the centre frequency F1 and now contains the information which was transmitted to the antenna 21 on the operating frequency Fb. The signal is conveyed to the receiver through an addition link 36. To provide for a situation where the operating frequency of the base station is too low with respect to the optimum one desirable for the signposts thus just requires an additional technical array of a modest extent.

Claims

1. A system for monitoring the location of a plurality of mobile units (14) operating on routes known in advance from a central control station, said system comprising a stationary base station (10) covering at least one transmission channel including a transmitter for transmitting speech or data communication on a first frequency (F,) and a receiver (12) for receiving speech or data communication on a second frequency (F₂) for selective radio communication with the mobile units (14),

each of said mobile units similarly comprising transmitter means (25) and receiver means (23) for radio communication with said base station (10) and with signposts located in specific positions along the known routes, each of said signposts being provided with a short range transmitter (15) for transferring position-indicating information to a storage and processing means (24) provided in each mobile unit (14), and said control station further comprising a computer sequentially providing interrogating signals to each mobile unit, upon the receipt of which each mobile unit replies to the control station transmitting the stored and processed position information, characterized in that the receiver means (23) in each mobile unit comprises only one receiver for receiving all communications from the central base station and from the signpost transmitters as well, and that the transmitter frequency (F,) of the base station (10) and the transmitter frequency (F_b) of each individual signpost both are included in the VHF-UHF range.

2. A system as claimed in claim 1, characterized in that the transmitter frequency (F,) of the base station (10) and the transmitter frequency (F_b) of each individual signpost are the same or substantially the same frequency.

3. A system as claimed in claim 1 or 2, characterized in that the ratio between the signal level from the signpost transmitters and signal level from the central control transmitter is selected in such a way that the signal level on the routes of the mobile units right in front of the signposts is sufficiently above the mean level of the signal from the control station to achieve an adequate message acceptance of the signals from the signpost and further in that the antenna on the transmitter of the signpost has a radiation pattern with a relatively narrow main beam directed towards the route of the mobile unit.

4. A system as claimed in claim 1, in which the signal frequency (F,) of the base station (10) is much lower or higher than the signal frequency (F_b) of the signposts (15, 16), characterized in that additional circuits (31-36) are provided at the input of the receiver (23) in each mobile unit (14) for frequency conversion from the signal frequency (F_b) of a signpost (15, 16) to the signal frequency (F,) of the base station (10), whereby position-indicating information transmitted from a signpost to the receiver (23) of a mobile unit 14) is unaffected by the frequency conversion.