METHOD OF GPS NAVIGATION AND RECEIVER

Abstract: A method of determining the position of a GPS receiver and a GPS receiver using the method are disclosed, the method comprising the steps of (a) measuring pseudoranges from at least four orbiting GPS satellites; (b) providing a pseudorange from a virtual satellite based at the centre of the earth to a previously known position of the GPS receiver; and (c) from the pseudoranges of (a) and (b), determining the position of the GPS receiver.

Description: DESCRIPTION METHOD OF GPS NAVIGATION AND RECEIVER This invention relates to a method of determining the position of a GPS receiver and a GPS receiver for the same. At present, GPS is most notably associated with the Navigation System with Time and Ranging (NAVSTAR) GPS, an all weather, spaced based navigation system developed and operated by the US Department of Defense. However, the general principles underlying GPS are universal and not merely limited to NAVSTAR. Accordingly, GPS hereinafter refers to any global positioning system comprising a plurality of radio transmitters at different locations and a receiver which uses information relating to the time of arrival of transmissions of the radio transmitters in order to determine location. The general principles underlying GPS and methods and apparatus for its implementation are known. For example, see GPS Principles and Applications (Editor, Kaplan) ISBN 0-89006-793-7 Artech House, hereinafter "Kaplan". As is well known, in order to determine the position of a GPS receiver and the time offset between the receiver clock and the GPS system time, pseudorange measurements are required in respect of at least four orbiting satellites. In situations where signals from only three orbiting satellites can be acquired, it is not normally possible to conclusively determine the position of the GPS receiver. However, from Japanese patent application 63-134975(A) "GPS navigation arithmetic processing method and GPS navigation arithmetic processor using said method", it is known to assume a virtual satellite based at the centre of the earth and to provide a fourth pseudorange from the virtual satellite to a last known position fix. The virtual satellite pseudorange augments the three measured pseudoranges such that the position of the GPS receiver can be estimated. It is an object of the invention to provide a method and apparatus for improved GPS navigation. Accordingly, a method of determining the position of a GPS receiver is provided comprising the steps of (a) determining pseudoranges from at least four orbiting GPS satellites; (b) providing a pseudorange from a virtual satellite based at the centre of the earth to a previously known position of the GPS receiver; and (c) from the pseudoranges of (a) and (b), determining the position of the GPS receiver. Also provided is a GPS receiver using a method according to the present invention, the GPS receiver comprising a receiver means for acquiring signals from at least four orbiting GPS satellites, and processing means for providing the pseudoranges of (a) and (b) and determining the position of the GPS receiver therefrom. Although not necessary to determine the current position of a GPS receiver when signals from at least four GPS satellites have been acquired, the inventors have appreciated that use of a pseudorange from a virtual satellite based at the centre of the earth can actually improve the accuracy of the determination of position. The method of the present invention is particularly suitable for land based use of GPS receivers and especially for vehicle navigation. This is because during such use, the assumption of constant altitude since a last known position fix is reasonably safe, the land based user being in at least indirect contact with the earth's surface at least most of the time and changes in altitude take place relatively slowly. Furthermore, with respect to a GPS receiver which determines its position from NAVSTAR GPS signals having selective availability operative, the improvement in accuracy derived from the invention may be further attributed to the rate of change of the selective availability error exceeding the rate of change of the altitude of the user. A method and GPS receiver according to the present invention will now be described, by way of example only, with reference to the following figures in which: Figures 1 and 2 are graphs showing the positional error encountered using a method of determining the position of a GPS receiver according to the present invention compared to a conventional method, when between 3 and 7 signals from orbiting satellites are acquired; and Figure 3 shows, schematically, a GPS receiver implementing a method according to the present invention. In order to determine the position of a GPS receiver in three dimensions (x u , y u , z u ) and the time offset t u between the receiver clock and the GPS system time, pseudorange measurements p are made in respect of the four orbiting satellites in accordance with known methods. The measured pseudoranges can be expressed as follows: p, = Js, - u] + c u [Equation 1] wherein in a Cartesian co-ordinate system, s represents the position of a satellite, u represents position of the GPS receiver, c is the speed of light and j ranges from 1 to 4 and refers to the satellites. Equation 1 can be expanded into the following set of equations: Pi V( χ ι - χ u) 2 + (y. - y u ) 2 + (z. - Zu) 2 + ct u [equation 2] = ( x 2 -X u ) 2 +(y 2 -y ) 2 + (z 2 -z 2 + ct u [equation 3] P 3 = ( x 3-X ) 2 +(y 3 -y u ) 2 +(z3-z 2 + ct u [equation 4] P„ = ( x 4 -x u ) 2 +(y 4 -y u ) 2 +(z 4 -z u ) 2 + ct u [equation 5] where x y and z. denote the jth satellite's position in three dimensions. A further pseudorange is provided from a virtual satellite based at the centre of the earth to a previously known position of the GPS receiver. As the virtual satellite position vector is (0, 0, 0), the virtual satellite pseudorange, Pce , can be expressed as: P ce = V u' + y u 2 + z 2 , + ct u [equation 6] where u' denotes a previously known position. The virtual satellites velocity vector is (0, 0, 0) and its Doppler shift is zero. As equations 2 to 6 are over determined (there being more equations than unknowns), the equations will be inconsistent in that errors in the pseudorange values will preclude any combination of location and time offset from exactly solving the equations. Instead, a conventional iterative technique based on linearisation may be employed to obtain a best estimate for the position of the GPS receiver in three dimensions (x u , y u , z υ ) and the time offset t u between the receiver clock and the GPS system time. Such techniques are well known, for example, see chapter 2.4.2 of Kaplan, ibid. The positional error encountered using a method of determining the position of the GPS receiver according to the present invention has been compared to a conventional method using both real and simulated data, and when between 3 and 7 signals from orbiting satellites have been acquired. The real data was obtained by analysing conventional GPS equipment using the method of the present invention when acquiring NAVSTAR GPS signals for which selectively availability was operative and whereby the GPS receiver was stationary. The simulated data was obtained using conventional simulation equipment and was based on a moving vehicle scenario using precise satellite ephemeris data and associated pseudoranges. Figure 1 relates to real data and shows the position error E in metres on the ordinate axis against the number of orbiting satellites acquired j on the abscissa. Curve 1 shows the position error relating to a conventional method in which a position fix is generated based solely on signals acquired from the orbiting satellites. As such, at least four orbiting satellites signals are required. Curve 2 shows the position error related to a method according to the present invention. Figure 2 is as figure 1 except based on simulated data. Curves 3 and 4 show the position error relating to a conventional method of determining the position of a GPS receiver and a method according to the present invention respectively. It can be seen from figures 1 and 2 that when pseudoranges are derived from 4 orbiting satellites, providing an additional pseudorange from a virtual satellite based at the centre of the earth results in an estimated 8% (real data) and 50% (simulated data) improvement in the accuracy of the position fix. When pseudoranges are derived from 5 orbiting satellites, there is an estimated 14% (real data), 63% (simulated data) improvement in the accuracy of the position fix. The difference in improvement between the real and simulated data is attributable inter alia to the absence of selective availability error in the simulation. Equations 1 to 5 describes a simplified model of pseudoranges for illustration purposes only. In reality, measured pseudorange errors arise due to many factors including atmospheric delay, receiver noise and resolution offset, multipath offset, receiver hardware offsets and selective availability degradation. These errors may be compensated for accordingly and are described in chapter 7.1.2 of Kaplan, ibid. Figure 3 shows, schematically, a GPS receiver implementing a method according to the present invention. The GPS receiver is of a generally known architecture, comprising receiver means in the form of an antenna 5 and a signal pre-processor 6. In use, radio frequency (RF) signals are received by the antenna and pre-processed in the pre-processor; typically by passive bandpass filtering in order to minimise out-of-band RF interference, preamplification, down converting to an intermediate frequency (IF) and analog to digital conversion. The GPS receiver further comprises processing means 7, 8, 9 in the form of several receiver channels 7, a receiver processor 8 and a navigation processor 9. The digitised IF signals are processed in each of the digital receiver channels and the satellite signals acquired and tracked in respective digital receiver channels in co-operation with the receiver processor. Such methods for acquisition and tracking are well known, for example, see chapter 4 (GPS satellite signal characteristics) & chapter 5 (GPS satellite signal acquisition and tracking), Kaplan ibid. Using the acquired navigation information including the time of arrival of the transmissions, the navigation processor 9 calculates the position of the receiver using a method according to the present invention, i.e. by providing a pseudorange from a virtual satellite based at the centre of the earth to a previously known position of the GPS receiver. The determined position is then conveyed to the user through a user interface 10, typically a visual display. The processing means 7, 8, 9 are conveniently provided in the form of either a general purpose microprocessor or a GPS application specific integrated circuit, and a method according to the present invention may be implemented using appropriate programming and configuring of such processing means. Of course, such programming and configuration is well known, and would be accomplished by a one of ordinary skill in the art of GPS without undue burden.

Claims: CLAIMS 1. A method of determining the position of a GPS receiver comprising the steps of: (a) determining pseudoranges from at least four orbiting GPS satellites; (b) providing a pseudorange from a virtual satellite based at the centre of the earth to a previously known position of the GPS receiver; and (c) from the pseudoranges of (a) and (b), determining the position of the GPS receiver. 2. A GPS receiver comprising receiver means for receiving signals from at least four orbiting GPS satellites, and processing means for (a) determining pseudoranges from the at least four orbiting GPS satellites, (b) providing a pseudorange from a virtual satellite based at the centre of the earth to a previously known position of the GPS receiver, and from the pseudoranges of (a) and (b), determining the position of the GPS receiver. 3. A GPS receiver as hereinbefore described with reference to the accompanying drawings.

Inventors: MARSDEN IAN A, YULE ANDREW T, MARSDEN, IAN, A., YULE, ANDREW, T.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: G01S5
IPC codes: G01C21, G01S1, G01S5