AU634587B2 - Position reporting system - Google Patents

Description

!3 .Ij AtJSTRA 634587 P/oo/011 Form PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: TO BE COMPLETED BY APPLICANT AUSPACE LIMITED PLESSEY BUILDING THYNNE STREET FERN HILL PARK BRUCE, AUSTRALIAN CAPITAL TERRITORY, DR. RODERICK CHARLES BRYANT PAUL F. KILDEA Patent and Trade Mark Attorney 19 LOGAN STREET NARRABUNDAH, A.C.T. 2604.

2617.

Complete Specification for the invention entitled: "POSITION REPORTING SYSTEM" The following statement is a full description of this invention, including the best method of performing it known to me:-' SNote: The description is to be typed in double spacing, pica type face, in an area not exceeding 250 mm in depth and 160 mm in width, on tough white paper of good quality and it is to be inserted inside this form.

14599/78- L Printed by C. J. THOMISON, Commonwealth Government Printer, Canberra 2 The invention relates to a position reporting system which will provide position information concerning a plurality of mobile units. The units may be stationary or moving. The unit may be a man or a vehicle or any movable object such as a shipping container. The units may be in cities, in jungles, in mountains, at sea or on other difficult terrain.

The physical elements to be used by mobile units should be small, light, portable, inexpensive, rugged and able to operate within a harsh environment. Factors that should be considered include shock, vibration, temperature and electromagnetic compatability.

At present, fleet vehicle operations such as taxis, couriers and large trucking concerns have no automated way to keep track of their operating vehicles. This creates a large cumulative loss to the operating companies. The loss may be direct, in the form of theft, user misappropriation and inefficient despatching and routing or indirect, through the loss of customer satisfaction caused by lack of response to the customer's needs. These losses may be stemmed by the implementation of an automated position reporting system that makes human interfaces unnecessary.

In the case of taxis, the present despatching system is a bid process, where an awaiting client's address is broadcast to all vacant taxis. A taxi-driver within a given radius of the address then broadcasts an acceptance of the bid. In theory, this practice provides the fastest response to a customer. In practice, it is not unusual for a driver on the other side of town to accept the call, leaving the customer waiting while the driver transits the city. A similar problem may be encountered when dealing with courier services, when the despatch office has no real-time 3 knowledge of the courier's whereabouts. Similarly, road services organisations could integrate a 'vehicle breakdown locator' with their present despatch system.

Car-rental agencies generally have no idea where their units are once they leave the distribution point. Their first problem is theft of the unit. Other problems may involve contract compliance or the prompt location of a unit that has mechanical problems or that has simply been discarded by a customer.

Trucking companies may suffer hijacking of shipments and misuse of resources by their operators. This may also be alleviated by an automated position reporting system.

The applications of such a system are not restricted to land based users. The hire-and-drive marine industry also has a need to track the whereabouts of its craft, as do the aircraft charter businesses. The transport industry may use the system to track the position of its shipping containers. Rescue services exemplify another application of the system.

In some applications a control and display unit may be provided at the mobile unit to allow its position to be displayed to a mobile user. This may involve extra processing in the mobile unit to compute its position or it may involve transmission of the position data from the base station to the mobile unit.

The position reporting system employed by the invention utilises signals emanating from the NAVSTAR Global Positioning System (GPS) satellite constellation of the U S Department of Defence which give highly accurate fixes globally.

4 This constellation will comprise at least 21 satellites positioned in a plurality of orbital paths at a predetermined distance above the Earth and arranged so that, at any time, at substantially any position on the Earth, there will be at least four satellites above the horizon. There is a satellite control station which, inter alia, controls highly accurate clocks in the satellites, synchronises the clocks, determines the orbits of the satellites and uploads orbital information to the satellites for retransmission to users. The satellites continuously transmit signals available to civilian users on an L-band frequency.

By measuring the time taken for an electromagnetic signal transmitted from a satellite to be received by a receiver on Earth, the range or distance between the satellite and receiver can be determined. Geometrically, only ranges from three satellites would be required to establish the position of the receiver.

However, accurate range information would be obtained only if highly accurate clocks were provided at the receiver. In the position reporting system of this invention, the clock provided at the receiver is not of similar stability to the satellite clocks.

Because of inherent clock error at the receiver, "pseudo" ranges are measured rather than precise ranges. Accordingly, it is necessary to measure pseudoranges from four satellites to enable clock correction to be computed. This allows a lower cost mobile unit to be designed and accurate position fixes to be obtained.

To this end, a pseudoranger on a mobile unit receives NAVSTAR-GPS satellite signals and measures pseudoranges to four satellites.

It then transmits the pseudoranges to a base station which fixes the position of the mobile unit by processing the pseudoranges.

__fl 5 It is an object of the invention to provide a system which will provide real-time position information concerning a plurality of mobile units to a base station.

It is another object of the invention to provide a system which will enable a prompt determination of the position of a mobile unit.

It is also an object of the invention to provide an economic position information system employing low cost mobile units and using low cost industry standard computing facilities at the base station.

According to the invention, a system for reporting the position of one or more mobile units comprises a pseudoranger at each mobile unit adapted to receive NAVSTAR-GPS satellite signals and measure pseudoranges to four satellites, means at each mobile unit to transmit information concerning the pseudoranges to a base station, means at the base station to receive information from each mobile unit and means at the base station to feed the information to a processor for computation of a position fix for one or more of the mobile units.

S The operation of the system is illustrated in Fig.l. The pseudoranger on mobile unit 5 receives NAVSTAR-GPS satellite signals from four "above the horizon" satellites 1, 2, 3 and 4 and measures pseudoranges to the four satellites and transmits them together with any other relevant information such as mobile unit identification to processor 7 at the base station via a narrow bandwidth telemetry link. The same signals from satellites 1, 2, 3 and 4 are also received by receiver 6 at the base station and _I I 6 fed to processor 7 to allow correction of any&ys-tmeffla errors.

The position of the pseudoranger is fixed at the base station by processing the pseudoranges to obtain 3-dimensional position coordinates and the pseudoranger clock error. The position coordinates may be displayed in a geodetic framework, e.g. a street map appropriate to the user on a display such as 8.

In Figs. 2 and 3, flow diagrams illustrate a preferred division of functions between the base station and a mobile unit. Of course other functions may be allocated to the base station or to the mobile units. For example, in addition to extracting pseudoranges, other parameters may also be extracted. As another C' its example, S-"4 mobile unit could solve for position and transmit "raw" position information to the base station. The base station would then make differential corrections to these raw positions rather than to the pseudoranges.

The mobile specific hardware may consist of an omnidirectional antenna (O dB circularly polarised L-band), a R F front end and a processing unit. The R F front end may provide a R F amplifier operating at L-band, and one or more downconversion stages.

The processing unit may provide some analogue signal processing in hardware and some Digital Signal Processing (DSP) in software.

By implementing most of the pseudoranger functions by DSP in software, hardware complexity is minimised leading to a compact, light and power efficient pseudoranger. Most importantly, a DSP oriented approach will provide a high degree of flexibility allowing the pseudoranger to be adapted to a variety of user 7 requirements without the need to modify the hardware and to be easily upgraded as user requirements change.

The mobile unit may extract pseudoranges and other parameters from the signal using correlation-based search and tracking algorithms.

The base station hardware may consist essentially of processing elements. Where possible, off-the-shelf hardware and commonality with the mobile units is preferred.

For the purpose of accuracy, it is preferred to compute the positioning solution by means of a differential algorithm. This has the advantage that, whereas point positioning gives 100m accuracy, differential positioning will provide accuracy of the order of The position information on the mobile units may be assessed by an operator, or there may be a graphics output at the base station. Because the base station processor may compute position fixes for a large number of mobile units, it has deployment information available which is useful to a base station operator.

This deployment information may also be used by a command and control system at, for instance, the base station or at a despatch centre or at a theft prevention system based at, for instance, Police Headquarters.

Various changes and modifications may be made to cater for specific user requirements. For example, it may be possible to use an existing communications link between the mobile units and the base station rather than provide one integral to the system.

8 Similarly, use may be made of an existing graphics output at the base station.

Again, various common functions may be carried out at the base station or at a mobile unit. Thus, instead of making the "best four" satellite selection at the base station, such selection could be performed by a mobile unit. However, for cost reasons, it is usually preferred to perform common functions at the base station. Other common functions include the computation of the position solution from the pseudoranges, the collection or storage of almanac data for use in the "best set" computations and the collection of ephemeris data for use in position solutions.

Generally, a mobile unit that does not require human interfaces is preferred, although a mobile user interface may be provided if required. Further, a mobile unit may be provided with a control and display unit. The operating radius of the system is dependant on user requirements and may dictate features of system design and/or communications link.

Sometimes, the terrain may be such that a mobile unit and perhaps the base station may not have line-of-sight to some of the NAVSTAR satellites and hence will not receive the L-band signals from those satellites. In such a case, degraded modes of operation can be built into the receiver if required by the user but with a lower accuracy of positioning information.

The cost of the mobile equipment may be significantly reduced compared to stand alone GPS receivers because the base station 9 assumes responsibility for a number of functions normally performed by a stand alone GPS receiver. Since some of these functions are not specific to a particular receiver, they need only be performed once for all the pseudorangers each time they are required.

Other changes and modifications may be made within the broad concepts described and the scope of the invention is only restricted by the subsequent Claims.

Claims (16)

1. A system for reporting the position of one or more mobile units comprising a pseudoranger at each mobile unit adapted to receive NAVSTAR-GPS satellite signals and measure pseudoranges to four satellites, means at each mobile unit to transmit informat- ion concerning the pseudoranges to a base station, means at the base station to receive information from each mobile unit and means at the base station to feed the information to a processor for computation of a position fix for one or more of the mobile units.

2. A system according to Claim 1, wherein the information transmitted from a mobile unit includes the pseudoranges.

3. A system according to Claim 1 or 2, characterised in that the information transmitted by a mobile unit includes identifi- cation of that unit.

4. A system according to Claim 1, 2 or 3, characterised in that the processor at the base station computes 3-Dimensional position co-ordinates of a mobile unit and the pseudoranger clock error.

A system according to Claim 1, 2, 3 or 4, characterised in that means are provided at the base station and/or the mobile units to extract other parameters from the signals in addition to the pseudorange.,

6. A system according to any preceding Claim, characterised in that the functions common to the base station and the mobile units are carried out at the base station. -Rj4 i 11

7. A system according to any preceding Claim, characterised in that the position information in respect of the mobile units may be assessed by an operator at the base station.

8. A system according to Claim 7, characterised in that a graphic display is located at the base station on which position information in respect of the mobile unit may be displayed.

9. A system according to any preceding Claim, characterised in that a control and display unit is provided at a mobile unit adapted to display the position of that mobile unit.

A system according to Claim 9, characterised in that pro- cessing means are provided at the mobile unit for computation of a position fix for that unit.

11. A system according to Claim 9, characterised in that means are provided at the base station to transmit position data to the mobile unit.

12. A system according to Claim 8 or 9 or 1C or 11, characte- rised in that the position of a mobile unit may be displayed in a geode-.tic framework.

13, A system according to any preceding Claim, characteriser in that the position solution is computed by means of a differtntial algorithm.

14. A system according to any preceding Claim, characterised in that a receiver is provided at the base station adapted to receive the same signalsleA the pseudorangers and means are ~II~ 12 provided to feed those signals to said processor to allow correction of any, syszt-at i errors.

A system according tojacn preceding Claim, characterised in that means are provided at a mobile unit to process the ItS pseudoranges to solve for position and means to transfer this information to the base station fcr subsequent processing.

16. A system for reporting the position of one or more mobile units substantially as herein described and illustrated in the accompanying drawings. DATED this llth. day of OCTOBER 1990. AUSPACE LIMITED By Its Patent Attorney, PAUL F. KILDEA. ii Ohl-