WO1983004451A1

WO1983004451A1 - A location monitoring system

Info

Publication number: WO1983004451A1
Application number: PCT/DK1983/000059
Authority: WO
Inventors: Svend Aggerholm; Oddvar BO^/THUN; Flemming Hansen
Original assignee: Storno A/S
Priority date: 1982-06-07
Filing date: 1983-06-07
Publication date: 1983-12-22
Also published as: DK154588B; GB2132398B; GB2132398A; SE8400632D0; EP0110945B1; DE3390030C2; DK255382A; EP0110945A1; DE3390030T1; DK154588C; SE8400632L; GB8402638D0; SE456287B

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

A location monitoring system

Technical Field

The invention concerns a system for monitoring the location of a plurality of mobile units operating on preestablished routes from a centrally located control station, said syste comprising a stationary base station covering one or more transmission channels with a transmitter and a receiver for each channel for selective radio communication with the mo¬ bile units comprised by the system, said mobile units simi¬ larly comprising a transmitter and a receiver for radio com¬ munication with said base station, and with signposts lo— cated in specific positions along the known routes, each of said signposts being provided with a short range transmitter for transmitting during the passage thereof position-spe¬ cific information to a storage means provided in a mobile unit.

Background Art

A system of the above-mentioned type is known in many embodi ments and is primarily used for monitoring e.g. the public bus service. In order most efficiently to utilize the avai¬ lable rolling equipment and to provide a reasonably good ser vice 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 schedule have justified the establishment and maintenance of a loca¬ tion 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 ca be diverted and controlled from the base station. A system o the above-mentioned type is useful not only in connection with the public transport of passengers, but has also prove its usefulness in connection with e.g. money transports or p trols of different kinds. Communications between the signposts and the individual mobil units take place wirelessly and are based on different tech¬ niques, but most of them are unsuitable in the present use. Thus, communications with infrared light have only a limited range, and the same applies to communications with ultrasound which has moreover a poor signal/noise ratio. RF-communica- tions at very low frequencies are known in connection with rail traffic, but this technique is expensive and difficult to maintain.

Radio communications in the microwave range constitute the most frequently used method owing to its reliability and ac¬ curacy, but is very expensive.

The apparatus used in the VHF/UHF range, on the other hand, is less expensive, but thenthere is a lot of traffic in the air so that it may be difficult to get the required number - of communications channels from the teleadministrations.

The Danish Patent Specification 132 475 discloses a traffic monitoring system, in which a sequence of messages is conti¬ nuously transmitted from a traffic control centre, each con- taining its address of a bus, and information is received from each bus about its position at the time of reply. The in terrogation messages are transmitted on one frequency, the in terrogation frequency, while the reply messages from the indi vidual buses are transmitted on another frequency, the reply frequency. This part of the communications between the traf¬ fic control centre and each individual bus takes place digi¬ tally. In addition to this, the known system provides a faci¬ lity 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 in¬ formation during the passage of a signpost, which coritinuousl

IJUREAT

O PI

.<&_/-__. W1PO transmits messages containing an identification code specifi of the signpost in question. Each signpost comprises a digit position generator whose output signal is converted by FSK technique into tonesused for modulating a constantly operati transmitter.

Thus, it is a considerable technical array which is involved in this known traffic monitoring system with separate trans¬ mitters and receivers for communications between the bus and the traffic control centre and between the bus and signposts, respectively, simultaneously with the occupation of five dif¬ ferent communications channels or frequencies.

Disclosure of Invention

A location monitoring system of the type stated in the openin paragraph is characterized according to the invention in that the transmitter at the base station and the transmitter in each signpost are arranged to transmit on the same frequency, and that the transmitter of the signpost comprises an antenna with a signal emission so arranged that in a narrow range a- round the signpost its level is so high above the mean level of the signal emitted from the base station that a satisfac¬ tory acceptance of the signal received from the signpost is obtained, and decreases so that the signal strength from the signpost at a suitable distance at either side of the said post has decreased to such a low level below the mean level of the signal emitted from the base station that satisfacto¬ ry acceptance of the signal received from the base station is obtained.

The features of the invention involve several advantages.

When the base station and the individual signposts transmit on the same frequency, the stationary equipment is simplified and thus made cheaper because at both points it allows the us of the same equipment as for other RF purposes. The cOmmunica tions equipment in the mobile units is also made cheaper in that at least one receiver is saved per unit. Finally, the need for communications channels or frequencies, which is a scanty resource, is reduced in that only a single channel is occupied instead of two like in the known system.

Brief Description of Drawings

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 showsthe units incorporated in the transmitter/receiv equipment of a mobile unit, fig. 3 shows, for different signal levels, the message accep¬ tance as a function of the difference in level betwee the signal strength of two signals on the same freque cy, fig. 4 shows the relative signal level from the antenna of a signpost as a function of the relative distance in th 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 con¬ taining all relevant information about the mobile units form ing part of the system, the routes to be traversed, and the points of time when these units are scheduled to pass speci- fie positions 'in the system of routes, and a transmitter/re¬ ceiver part 12 which is in radio con-tact 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 desks 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 equipmentis ordinary and does not concern the invention, and is accordingly not shown in th drawing. The same applies to the interface equipment between the central computer 11 and the transmitter/receiver part 12.

In the drawing the mobileunit (14) issymbolized by a vehicle of a type, e.g. a bus; the essential feature to the system, how¬ ever, 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 transmit¬ ter 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 aft^'-interrogation message trans mits its reply message containing the identification code for the unit in question and information about its . current position. This communication betwee the base station 10 and the mobile units 14 takes place through two channels, an inter rogation channel of the frequency Fl 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 fol¬ lowing signpost in the system. The travelled distance between two signposts is measured continuously e.g. by means of an o- dometer, and this measured value is applied to the position store likewise in a known manner, so that the position infor¬ mation transmitted with the reply message is correct within a certain margin. According to the invention, all the signposts 15 transmit on the same frequency Fl as the base station 10, which is indi¬ cated by a single arrow 18. This involves the advantage that an extra receiver is saved in each mobile unit whileϋainimizin 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 Fl to a receiver 23. The receiver se¬ lects 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 store at the given point of time in the control unit 24 to be trans mitted 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. Second ly, the emission diagram of the signpost antenna 16 must be

_O PI so narrow that its position is defined with reasonable accura cy, and so narrow with respect to the spacing between the signposts that the probability of non-acceptance of an inter¬ rogation 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-acceptanc as a function of the difference in level at the reception-lo¬ cation 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 accep- tance of 95% requires a difference in level of about 24 dB, and these values are fairly independent of the absolute sig¬ nal level. This appears from fig. 3 where this relation is plotted in a system of coordinates,with the message accep¬ tance 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 di¬ stance in metres calculated inthe direction of motion of a mobile unit forming part of the system. As expected, the dia¬ gram 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 6dB 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 ar- ranged in such a manner that the 6dB level is equal to the mea

OMPI

^• S level of the signal from the base station 10, the message ac¬ ceptance 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 a- bout 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 informa¬ tion from the signpost passed and stores it for subsequent transmission to- the base station.

Similar operating conditions can be obtained with other an¬ tenna forms, such as reflector antennas or more complex an¬ tenna 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 messag from the base station 10 is about 90%, cf. fig. 3. This would be excellent if the base station constantly transmitted mes¬ sages 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 re¬ ceive a eall from the base station is not to be too great.

In fig. 5 the coverage profile of the base station is illu¬ strated. The spacing between the signposts is designated by a, and the trapezoidal curves between them indicate the mes¬ sage acceptance of messages from the base station. The spac- ing b indicates the length of the distance opposite a sign¬ post where the acceptance is less than 90%. If a message ac¬ ceptance completely outside the operating ranges of the sign- posts of 95% is required, and if this quantity is called 1-P it can be shown that the probability P that a mobile unit doe not receive a message from the base station can be expressed by:

P=P_Λ + b/a (1-P₀) hence the minimum spacing between the signposts:

If b-= 100 , P = 0.1 and P = 0.05, a minimum spacing a . 2 km results.

The numerical examples in the foregoing must not be interpre restrictively, however, because the determination of the de¬ scribed differences in level in signal strengths e.g. greatl depend upon which message acceptance levels are found satisf tory in given situations.

Itmay occur, however, that the interrogation frequency Fl al located to a base station 10 is too low for the same frequen to be used as an operating frequency for the-signposts becau 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 eac mobile unit can be extended with an own oscillator and fre¬ quency converter, so that the mobile unit still uses the sam 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 conver¬ ter 33, which receives a signal from an own oscillator 34 os¬ cillating on the frequency Fb-Fl. The signal composed by the converter 33 is passed through a bandpass filter 35 of the centre frequency Fl 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 additio link 36. To provide for a situation where the operating fre¬ quency of the base station is too low with respect to the op¬ timum one desirable for the signposts thus just requires an additional technical array of a modest extent.

-BUREΛ

OMPI

• r^ WIPO NATlO

Claims

P A T E N T C L A I M S

1. A system for monitoring the location of aplurality of mo¬ bile units operating on preestablished routes from a centrall located control station, said system comprising a stationary base station (10) covering one or more transmission channels with a transmitter and a receiver (12) for each channel for selective radio communication with the mobile units (14) com¬ prised by the system, said mobile units similarly comprising a transmitter (25) and a receiver (23) for radio communicatio with said base station (10) , and with signposts located in specific positions along the known routes, each of said sign¬ posts being provided with a short range transmitter (15) for transmitting during the passage thereof position-specific in¬ formation to a storage means (24) provided in a mobile unit (14) _/ C h a r a c t e r i z e d in that the transmitter (12) at the base station (10) and the transmitter (15) in each in¬ dividual signpost are arranged to transmit on the same freque cy (Fl) , and that the transmitter. (15) of the signpost com¬ prises an antenna (16) with a signal emission so arranged that in a narrow range around the signpost its level is so high above the mean level of the signal emitted from the base station (10) that a satisfactory acceptance of the signal received from the signpost is obtained, and decreases so that the signal strength from the signpost (15,16) at a suitable distance at either side of the said post has decreased to such a low level below the mean level of the signal emitted from the base station (10) that satisfactory acceptance of this received signal is obtained.

- 2. A system as claimed in claim 1, in which the operating frequency .(Fl) of the base station (10) is alternatively much lower than the operating frequency (Fb) of the signposts (15,

16), c h a r a c t e r i z e d in- hat additional circuits ( 36) are coupled in each mobile unit (14) in front of the rece (23) for frequency conversion from the operating frequency (F of a signpost (15,16) to the operating frequency (Fl) of the base station (10) , while maintaining the position-specific information transmitted from a signpost to the receiver (23) of a mobile unit (14) .

3. A system as claimed in claim 2, c h a r a c t e r i z e in that said additional circuits comprise a series connectio of a dividing filter (31) for the operating frequency (Fb) o the signpost, an amplifier (32) , a frequency converter (33) , a bandpass filter (35) of the centre frequency equal' to the operating frequency (Fl) of the base station (10) and an add tion link (36) , said series connection being inserted betwee the receiver (23) and an antenna (21) , said frequency conver¬ ter (33) having coupled to it an oscillator (34) of an oscil lation frequency equal to the difference between the operatin frequencies (Fb-Fl) of the signpost and .the base station.