Communications: electrical – Vehicle position indication – At remote location
Patent
1996-02-08
1998-05-19
Swarthout, Brent A.
Communications: electrical
Vehicle position indication
At remote location
340988, 340992, 701214, G08G 1123
Patent
active
057541256
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to automatic vehicle location systems of the type which may be used, for example, to monitor the locations of each of a plurality of mobile units such as a fleet of vehicles.
BACKGROUND OF THE INVENTION
A wide range of different basic techniques exist for automatic vehicle location. These include satellite based location systems such as, for example, the Omega system, and also systems which use local radio beacons. A mobile unit operating in one of these systems monitors phase differences in signals received from different satellites and uses these to derive the current position of the mobile unit. Typically a positional accuracy of approximately 100 meters may be obtained for a single derivation of position.
In a system comprising a central controller and a fleet of mobile units the positional information for each unit is required at the controller so that decisions relating to a mobile unit may be made, e.g. job allocation for taxis. In existing systems the information is transmitted to the controller by polling each mobile unit in the system for its position at regular intervals. In current systems the most frequent polling interval possible is generally fixed and is often as infrequent as once every 50 seconds, i.e. each mobile unit derives its position and transmits this information to the controller every 50 seconds.
Using such a polling system introduces an error in the data available at the controller and this error will vary with time elapsed since the last polling of a mobile unit for an update of its positional data. For example, a vehicle cruising at 120 kilometers per hour will travel 1666 meters in 50 seconds. Thus the average positional error at the controller will be slightly over 800 meters. Even if a vehicle is stuck in traffic at 25 kilometers per hour it will travel 347 meters in 50 seconds thus having an average positional error over the 50 second period of 173.5 meters. This error is, of course, in addition to the error in the originally derived position of the vehicle. Systems which have errors of this magnitude are not able to make best use of mobile units when allocating them to particular jobs.
Another disadvantage of systems which poll mobile units for their positions on a periodic basis is the amount of radio data communications traffic generated. This traffic can easily occupy most of the transmission time on a single radio channel and thus, if a significant number of vehicles are to receive data from the controller, more than one radio channel is required. Using current technology a single mobile radio channel might normally support approximately 100 vehicles without any vehicle location system in operation. A further two radio channels might very well be needed to carry the positional updating information from a vehicle location system provided in each mobile units if regular updates were to be produced once every 50 seconds.
SUMMARY OF THE INVENTION
Preferred embodiments of the present invention provide a system for automatic vehicle location which greatly increases the accuracy of positional information available by significantly reducing the errors in positional information which are inherent in a conventional polled vehicle location system. At the same time the amount of radio data communications traffic is significantly reduced.
This is achieved by providing both at the controller and at each mobile unit a predictive coder responsive to a position signal derived at the vehicle and to the vehicle's velocity (speed and direction) at the time that position signal was derived to derive a predicted position for the mobile unit. Thus the controller and each mobile unit have a predicted current position signal available to them.
At each mobile unit the predicted position signal derived by the predictive coder is compared with an actual position signal derived from the vehicle location unit. If the difference between the actual position signal and the predicted position signal exceeds a predetermined limit then the latest actu
REFERENCES:
patent: 4835537 (1989-05-01), Manion
patent: 5307277 (1994-04-01), Hirano
patent: 5444444 (1995-08-01), Ross
MDSI Mobile Data Solutions (UK) Ltd.
Swarthout Brent A.
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