WO2004048995A1

WO2004048995A1 - Position determination for a mobile device

Info

Publication number: WO2004048995A1
Application number: PCT/IB2003/005187
Authority: WO
Inventors: Andrew T. Yule; Saul R. Dooley
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-11-28
Filing date: 2003-11-14
Publication date: 2004-06-10
Also published as: GB0227749D0; AU2003278545A1

Abstract

The present invention provides for a method of determining a current position of a mobile signal receiver arranged to receive positioning signals from remote transmitters and comprising the steps of identifying a first position on the basis of positional data from the said positional signals, identifying a second position on the basis of a previously reported position and velocity data for the receiver obtained from signals from the remote transmitters, comparing the said first and second positions and identifying only the second position as the required current position responsive to a determined difference between the first and second positions.

Description

DESCRIPTION

POSITION DETERMINATION FOR A MOBILE DEVICE

The present invention relates to a method and related apparatus for accurately identifying the position of a mobile device arranged to receive signals from remote transmitters and in particular, but not exclusively, to a GPS receiver, and related method of operation, for use in vehicular navigation.

Mobile signal receiving devices arranged, for example, to be used within a vehicle, for example a GPS receiver are well known for providing current positional data either to the vehicle user, or a third party monitoring the movement of the vehicle. Such known receivers are arranged to measure the code phase of signals from a constellation of GPS satellites in order to produce so-called pseudoranges, which pseudoranges are in turn employed in the calculation of the position of the receiver, and so for example the vehicle within which the receiver is used.

In an ideal mode of operation, the GPS signals travel directly from the constellation of transmitting satellites to the GPS receiver. However, some environments within which a vehicle might be moving, and thus the GPS receiver might be found, do not allow for such ideal operation. For example, in heavily built up areas with a large number of relatively tall buildings, which can create so-called "urban canyons", the GPS signals arriving at the receiver may well have been reflected off those buildings rather than arriving directly from the constellation of satellites. The signals then arriving at the receiver have then followed one or more indirect routes involving some form of reflection. Such reflections lead to so-called multipath problems which, in turn, can lead to substantial positional errors when seeking to accurately locate the GPS receiver. In addition to producing the aforementioned pseudoranges to assist with location determination, it is also known to identify Doppler shift in the GPS signals arriving at a moving receiver, and to employ such Doppler shifts in determining the velocity of the receiver.

One known vehicle navigation system is disclosed in WO-A-97/24583 and which involves the use of GPS velocity signals in providing for vehicle navigation but which exhibits disadvantages in view of the use of the delta- pseudoranges defined therein, and due to the variety of sensors such as accelerometers and odometers in providing for the required positional information.

The present invention seeks to provide for a method, and related apparatus, for determining positional information of a mobile receiver and which exhibit advantages over known such methods and apparatus.

According to one aspect of the present invention there is provided a method of determining a current position of a mobile signal receiver arranged to receive positioning signals from remote transmitters and comprising the steps of identifying a first position on the basis of positional data from the said positional signals, identifying a second position on the basis of a previously reported position and velocity data for the receiver obtained from signals from the remote transmitters, comparing the said first and second positions and identifying only the second position as the required current position responsive to a determined difference between the first and second positions.

The invention is advantageous in effectively allowing the selective use of positional information obtained on the basis of velocity data, rather than the current positional data received from the transmitters, if it is identified that the said use of the velocity data is likely to provide more accurate results.

The present invention therefore takes advantage of the fact that the accuracy with which velocity of, for example, a moving GPS receiver can be calculated is higher than the accuracy of which a one-off position can be determined. Thus, if there is found to be a difference, perhaps to a predetermined degree, between the said first and second positions then the present invention advantageously serves to select the position determined by the inclusion of velocity data as being the likely more accurate positional reading.

The features of Claims 2, 3 and 4 represent particular advantageous features for determining the said second position, and for determining whether such said second position might offer a more accurate and reliable indication of the actual current position to be reported to the user than does the said first position.

The subject matter of Claim 5 takes advantage of the recursive nature of the determination of the said second position such that the position reported to the user can be dependent on more than one consecutive velocity reading.

On this basis, the feature of Claim 6 can advantageously be employed for determining the current reported position.

The features of Claims 7 and 8 provide particular advantageous steps in defining criteria for use in determining whether the said first position, or the said second position is reported as an accurate representation of the current position.

The features of Claims 8, 9 and 10 relate to aspects of a particular advantageous algorithm for use in estimating variance in interval position and velocity fixes. Advantageously, the core calculation of position variance α is based on the maximum range residual of all the SVs involved in the position fix (rr_maχ) and the number of separate SVs used (N):

If N > 5, 0² = ---!-™!-. ^pι N

Otherwise, σj = 4242 whilst the velocity variance σ² _v, is half the maximum Doppler residual

(drmax):

_2 _ "'max ^vι 2 According to another aspect of the present invention there is provided a mobile signal receiver including means for determining the current position thereof, the receiver comprising means for identifying a first position on the basis of positional data from signals received from a remote transmitter, for identifying a second position on the basis of a previous reported position and velocity data derived from the said received signals, comparing means for comparing the said first and second identified position, and control means arranged to identify only the said second position as the current position responsive to a difference between the said first and second determined position.

Advantageously, the mobile signal receiver is arranged to function in accordance with any one or more of the methods defined above.

As will therefore be appreciated, the present invention can take advantage of a realisation that the acknowledgment that GPS velocity measurements are inherently more accurate than positional measurements. This is generally because it is easier, and more accurate, to resolve rate of change of carrier phase, than it is to measure absolute code phase which is required for mere positional measurements. The present invention can therefore provide for the positional determination on the basis of velocity determination which, for example, offers greater resistance to multipath effects and so can lead to accurate navigation requirements in environments such as "urban canyons".

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a graphical representation of the reported position of a receiver driven round a known locality and employing a method and apparatus currently known in the art:

Fig. 2 is a schematic block diagram of a mobile receiver according to an embodiment of the present invention: and

Fig. 3 is a graphical representation correspond to that of the locality illustrated in Fig. 1 but in which the reported positions of the mobile receiver are derived in accordance with an embodiment of the present invention. Turning first to Fig. 1 , there is illustrated the reported positions from a Trimble receiver driven around the centre of Singapore and which offers positional information, and does vehicular navigation, on the basis of code phase measurements of GPS stabilities signals as currently known in the art. As will be appreciated, during particular passages of the route taken, particularly those corresponding to relatively built up regions, the accuracy of the identified position is generally poor.

Turning now to Fig. 2, there is illustrated, in schematic form, a mobile receiver 10 for use, for example, in a vehicle in which navigation data is to be provided by means of signals received from a constellation of GPS satellite.

The signals from the constellation of GPS satellites (not shown) are received at the receiver 10 and, via receiver interface 12, are delivered to a GPS position determining processor 14, and also a receiver velocity determining processor 16. The processor 16 serves to determine the current velocity of movement of the receiver 10, and thus the vehicle within which it is located, through resolution of the rate of change of carrier phase of the signals received from the satellites. The GPS positional processor 14 however serves to provide for a location identification on the basis of absolute code phase of the incoming signal.

The receiver 10 is arranged to provide a reported current position data signal 18 which can serve to drive a display within the vehicle so as to provide a user interface by which location and navigation information can be presented to, for example, the driver of the vehicle. Alternatively, or in addition, the positional data signal 18 can be provided to a monitoring device remote from the vehicle and by which third parties can monitor the location of the vehicle.

The positional data signal 18 to be reported as the current position is derived from a comparison arrangement 20 and the last reported data signal is delivered via a temporary storage means 22 to a velocity integration processor 24.

As will be appreciated, the output from the velocity determination processor 16 is delivered to the velocity integrated processor 24 where it is combined with data from the temporary storage means 22 and identifying the last reported position.

In this manner, the comparison means 20 is arranged to receive first position data from the GPS positional processor 14 representing the calculated position of the receiver 10 on the basis of the standard GPS positional signals employing absolute code phase measurements, and also, for comparative purposes, a signal from the velocity integrating processor 24 which represents a second value of the calculated position, which itself is calculated on the basis of the previous reported position and also the current velocity of the receiver 10 determined at the velocity determination processor 16.

The operation of the elements illustrated in Fig. 2 is described in further detail below but it will be appreciated that the comparison means 20 is arranged to compare the data relating to the first and second calculated positions. If, for example the positions are the same, or coincide within a determined range, then both readings are considered to be accurate and, in one example, the GPS positional data derived from the processor 14 is delivered as the current reported positional data signal 18.

However, if the comparison means 20 identifies that the readings for the first and second positions differ perhaps by a predetermined amount, then the data relating to the first position derived from the processor 14 is ignored in favour of that derived from the velocity determining processor 16 since, in view of the inherent more accurate velocity measurement obtainable at the receiver, this reading corresponding to the second position is determined to represent the more accurate reading which is then delivered as the current reported positional data signal 18.

As will therefore be appreciated, the essence of the invention is that the GPS receiver regularly (e.g. once per second) determines instantaneous position and velocity fixes as usual in a vehicular system. However before reporting the position to the user, a test is made to evaluate whether the last reported position offset by the integrated velocity is likely to give a more accurate result. Reference is made as an example to a GPS receiver that calculates a series of conventional position (p₀, Pi, P2,.-.p_n) and velocity (v₀, v^ v₂, ...v_n) fixes starting at a given time (t) with a given period (δt). If the position and velocity information is consistent, it could be expected that for an arbitrary fix (the i^th at time t + iδt), the result of the position fix (p,) would be similar to the previous position fix offset by the integrated velocity (p + P' vdt ). However, given the manner in which the velocity can be determined i.e. using a count of carrier cycles over the last time period, this calculation can be simplified to the previous position fix offset by the velocity times the time period (p +v,δt). If the position fix (p,) and the value calculated from the last one and the current velocity (p +v,δt) are the same, it doesn't matter which one of the readings is reported. However, if it is determined that there might be an error in the most recent measurements, then there is likely to be a difference between the two values. Since the calculation of velocity to be inherently more accurate and less prone to multipath errors, it is therefore reasonable to choose the provision determined by integrating velocity (p + v,δt) over the current conventional position fix (p,).

A distinction can therefore be made between the current position fix (p,) and the value reported to the user (P,), which can be defined thus: P, = p, or

P, = P,-ι + v,δt Clearly, the first reported position (P₀) must be the first calculated position fix (p₀), but, through the recursive nature of the above definition, it is possible for the reported position (P,) to be dependent on more than one consecutive velocity fix. Indeed, the above relationship can be recast in terms of the number of velocity fixes (m) the reported position (Pi) depends on thus:

P, = p,__m + δt ∑_Vj,m > 0 j=ι+1-m

It is of course important to define the criteria for determining whether to use the calculated position fix (pi) or the velocity integration (p + v,δt). In this embodiment it is proposed to keep metrics for the quality of both these calculations based on the estimated variance of position/velocity fixes.

In particular, on estimating the variances (σ^ and σ )of given position and velocity fix (pi and Vj) it is then possible to define the variance of the two 5 options as: σ% = σj or σ2_Pi = σ __P2_M + . σ _2_vio Jj_tt2

The result giving the smallest variance is then readily determined and chosen. 10 The advantage this arrangement is that, for a very small increase in processing requirement, a more accurate position fix can be obtained and reported.

It is also important to employ an appropriate algorithm for estimating the variance in individual position and velocity fixes. A variety of possible 15 measures could be used, although a particularly advantageous arrangement employs range and Doppler residuals.

In further detail, for a given SV position (P_sv), measured pseudorange (p_sv) and calculated position fix (p) with clock error (τ), we define the range residual (rr_sv) to be the difference between the pseudorange corrected by the 20 clock error and the distance between SV and calculated user position ^rr_Sv = (Ps_V -cτ) -|p_sv -p| Similarly a Doppler residual (dr_sv) can be defined in terms of the measured carrier offset of the SV signal (f_sv), the velocity of the SV(v_sv) and the calculated user velocity (v) and clock drift ( _k): ς dr - (_. ^{cς> u}'sv ^— V'sv

The core calculation of position variance σ² is based on the maximum range residual of all the SVs involved in the position fix (rr_maχ) and the number of separate SVs used (N):

If N ≥ 5, σ² = ^

Otherwise, σ² = 4242 whilst the velocity variance σ² _Vι is half the maximum Doppler residual (dr_maχ):

_2 _ "^rmax ^σv, - — g-

It should also be appreciated that the velocity fixes can be determined as suitable or unsuitable. For example, a check can be made to determine if the velocity is written as acceptable expected range. For motor vehicles a check is made for reasonable motor vehicle dynamic limits such as whether speed is less than 50m/s. If, for example, both position and velocity fix are unsuitable for use, the most recent acceptable velocity fix or position can be used.

Turning now to Fig. 3, the effects of applying the above-mentioned algorithm to GPS data recorded at the same time as that illustrate in Fig. 1 is shown and which a substantial improvement in the determination of location of the vehicle can be quite clearly seen.

Claims

1. A method of determining a current position of a mobile signal receiver (10) arranged to receive positioning signals from remote transmitters and comprising the steps of identifying a first position on the basis of positional data from the said positional signals, identifying a second position on the basis of a previously reported position and velocity data for the receiver obtained from signals from the remote transmitters, comparing the said first and second positions and identifying only the second position as the required current position responsive to a determined difference between the first and second positions.

2. A method as claimed in Claim 1 wherein the said second position is determined on the basis of the last reported position of the receiver and by an integrated velocity reading.

3. A method as claimed in Claim 1 or 2, wherein the velocity of the receiver is determined by means of a count of carrier cycles over a determined time period.

4. A method as claimed in Claim 3, wherein the said second position is derived from the previous position fix offset by the product of the velocity and the time period.

5. A method as claimed in any one or more of Claims 1-4, and including determining the position to be reported on the basis of a plurality of consecutive velocity fixes for the receiver.

6. A method as claimed in Claim 5, wherein the reported position l=ι

P- p^ +δt ∑_Vj,m > 0. j=i+1-m

7. A method as claimed in any one or more of Claims 1-6 and including the step of maintaining metrics relating to the accuracy of both the calculated position effects and the velocity integration fix so as to provide an estimated variance of the two said fixes.

5

8. A method as claimed in Claim 7 wherein the variance of the two fixes ( α²,,) is σ² or σ² + σ²δt² .

9. A method as claimed in any one or more of Claims 5, 6, 7 or 8, 10 and including the steps of identifying range and Doppler residuals so as to determine which of the individual and velocity fixes to report as the current position.

10. A method as claimed in Claim 9, wherein the range residual (rr_sv) 15 can be represented as: ^rr _sv = (P_sv -cτ) -|p_sv -p| .

11. A method as claimed in Claim 9 or 10, wherein the Doppler residual (dr_sv) can be represented as:

12. A mobile signal receiver (10) including means for determining the current position thereof, the receiver comprising means (14) for identifying a 25 first position on the basis of positional data from signals received from a remote transmitter, means (16) for identifying a second position on the basis of a previous reported position and velocity data derived from the said received signals, comparing means (20) for comparing the said first and second identified position, and control means arranged to identify only the said second position as the current position responsive to a difference between the said first and second determined position.

13. A mobile signal receiver as claimed in Claim 12, and arranged with means for operation in accordance with the method of any one or more of Claims 2 to 11.