HK1104087B - Method and arrangements relating to satellite-based positioning - Google Patents

Description

Method and arrangement relating to satellite based positioning

Technical Field

The present invention relates generally to mobile device positioning through the use of satellites, and in particular to such positioning assisted by a terrestrial-based communication node.

Description of the related Art

In recent years, determining the geographical position of an object, device or person carrying a device has become of increasing interest in many fields of application. One solution to address positioning is to use signals transmitted from satellites to determine position. Well known examples of such systems are the Global Positioning System (GPS) (see e.g., (1)) and the GALILEO system to be used at hand. The position is given as a triangulation/trilateration based on a plurality of received satellite signals, with respect to a specified coordinate system.

A standalone GPS receiver can obtain full lock on to the GPS satellite signal without any other information about the system than the nominal carrier frequency and the rules used to modulate the data carried by the signal. Basically, the three-dimensional position and the deviation of the receiver clock from the satellite time must be determined in a position calculation step.

Assisted GPS (agps) is defined as a GPS enhancement for integrating a GPS receiver into user equipment, i.e. a mobile station, of a cellular communication system (see e.g. third generation partnership project (3GPP) specifications TS 25.331 or TS 44.031 or Open Mobile Alliance (OMA) specifications for Secure User Plane Location (SUPL)). Assisted GPS is generally intended to improve the performance of GPS receivers in many different ways, including detection sensitivity, time to obtain a position estimate, accuracy, and battery power conservation. This is achieved by transferring some functions from the GPS receiver in the mobile station to the network and thus performing only a small part of the GPS tasks in the GPS receiver itself.

There are two types of AGPS, mobile station (or user equipment) infrastructure and mobile station (or user equipment) assistance. In mobile station based AGPS, the position of the mobile station is calculated in the mobile station by using ranging signal measurements determined by the mobile station and assistance data provided by the network. In mobile station assisted AGPS (also sometimes referred to as network infrastructure AGPS), the mobile station only measures and reports the timing of received ranging signals reflecting pseudoranges to space vehicles (i.e., satellites). For both types of AGPS, the measurement timing of the ranging signal is truncated modulo 1 ms, which corresponds to a distance of 300 km. When calculating the mobile station position, either in the mobile station itself or in a network location server, the full pseudoranges need to be reconstructed using a priori information about the mobile station position and ranging signal measurements determined by the mobile station to calculate the precise mobile station position.

The inventors of the present invention have realized that a problem with AGPS is that if the accuracy of the a priori information about the mobile station position is too low, i.e. the uncertainty of the mobile station position in the a priori information is too large, the truncated timing of measuring and reporting the received ranging signals may cause ambiguity in determining the pseudoranges to the spacecraft. Thus, if an incorrect pseudorange is selected and used as a basis for determining the position of the mobile station, a significant error on the order of, for example, 100 kilometers will occur in the calculated position of the mobile station.

Summary of The Invention

The problem underlying the present invention is to provide enhanced robustness against ambiguous pseudorange reconstructions in a satellite based positioning environment with assistance data.

This problem is solved by a method according to claim 1, an apparatus according to claim 19 and a computer program embodied on a computer readable medium according to claim 33.

One advantage provided by the present invention is that in conjunction with satellite-based positioning via assistance data such as assisted gps (agps), robustness is enhanced against ambiguous pseudorange reconstructions.

Another advantage of the present invention is that enhanced robustness is achieved without reducing detection sensitivity.

Yet another advantage of the present invention is that enhanced robustness is achieved with only a slight increase in processing delay.

The invention will be described in more detail below with reference to exemplary embodiments thereof and also with reference to the accompanying drawings.

Brief Description of Drawings

Fig. 1 is a diagram illustrating an exemplary case of mobile assisted AGPS to which the present invention is applied.

Fig. 2 is a diagram illustrating C/a codes and navigation data bits in a GPS ranging signal.

FIG. 3 is a block diagram illustrating a GPS navigation data format in a GPS ranging signal.

Fig. 4 is a diagram showing time in different parts of the system shown in fig. 1.

Fig. 5 is a flow chart illustrating a basic method according to the present invention.

Fig. 6 is a schematic block diagram of a mobile station.

Fig. 7 is a flowchart illustrating a process performed by the mobile station of fig. 6.

Fig. 8 is a schematic block diagram providing a location server according to an exemplary embodiment of the arrangement of the present invention.

Fig. 9A-B are flow charts illustrating a detailed exemplary embodiment of a method according to the present invention.

Fig. 10 is a diagram showing results and intermediate results when the method of fig. 9A-9B is performed.

Fig. 11 is a diagram showing an example of a computer-readable medium.

Detailed description of the embodiments

Fig. 1 shows a non-limiting exemplary scenario in which the present invention may be applied. In this exemplary case, the basic wireless communication system SYS1 is used together with a Global Positioning System (GPS) to provide mobile station assisted AGPS. The exemplary wireless communication system SYS1 shown in fig. 1 is a Universal Mobile Telecommunications System (UMTS). The communication system SYS1 comprises a network part NET1 and User Equipments (UEs) alternatively referred to as Mobile Stations (MSs). The network part NET1 comprises a core network CN1 and a UMTS Terrestrial Radio Access Network (UTRAN) RAN 1. The core network CN1 comprises a mobile services switching centre (MSC) node MSC1 providing circuit switched services and a General Packet Radio Service (GPRS) node SGSN1, sometimes referred to as Serving GPRS Support Node (SGSN), adapted to provide packet switched type services.

Each core network node MSC1 and SGSN1 is connected to the radio access network RAN1 by a radio access network interface called the Iu interface. The radio access network RAN1 includes one or more Radio Network Controllers (RNCs). For simplicity, the radio access network RAN1 of fig. 1 is shown with only one radio network controller node RNC 1. Each radio network controller is connected to and controls a plurality of Radio Base Stations (RBSs). For example, and again for the sake of simplicity, fig. 1 only shows a first radio base station node RBS1 and a second radio base station node RBS2 connected to the radio network controller node RNC 1. The interface between the radio network controller RNC1 and the base stations RBS1 and RBS2 is called the Iub interface. A mobile station, such as the mobile station MS1 shown in fig. 1, communicates with one or more radio base stations RBS1-RBS2 over a radio or air interface called the Uu interface. Each radio interface Uu, Iu interface and Iub interface is shown as a dashed line in fig. 1.

In FIG. 1, the GPS system is represented by the space vehicles, satellites SV1-SV 4. Each spacecraft SV1-SV4 transmits a corresponding ranging signal RS1-RS 4. Note that for simplicity, only four spacecraft SV1-SV4 are shown in FIG. 1.

In determining the location of the mobile station MS1 in fig. 1 using mobile station assisted AGPS, the mobile station MS1 receives assistance data from the location server 101 and reports measurement results thereto. Based on the reported measurements and a priori information about the location where the mobile station is located, the location server calculates the location of the mobile station MS 1. Depending on the way the location server is connected to the cellular network, AGPS can be divided into two categories, namely "AGPS control plane solutions" and "AGPS user plane solutions".

In "AGPS control plane solutions", location server functions, which may be implemented in a separate location server node sometimes referred to as a Serving Mobile Location Center (SMLC) or stand-alone SMLC (sas), or integrated with other functions in other network nodes such as radio network controllers, are tightly integrated with the cellular network and communicate assistance data and measurement results using so-called control plane signaling. This solution is also characterized in that the location server will typically receive information about in which cell the mobile station is currently operating, and the location server will apply this information as the a priori location of the mobile station when calculating the location of the mobile station. Thus, the uncertainty in the a priori location information corresponds to the cell size.

In the "AGPS user plane solution", the integration of the location server functionality with the cellular network is less compact and the assistance data and measurement results are communicated using so-called user plane signalling, i.e. ordinary user data packets are used for transparently transporting this information to the cellular network. This solution is also characterized in that the location server may not receive information about which cell the mobile station is located in, or at least may not always be able to associate a given cell identity with a particular geographical area corresponding to the cell coverage area. Thus, for AGPS user plane solutions, the uncertainty in the mobile station a priori location information may be much larger than the cell size and may correspond to, for example, the size of the country in which the mobile station is currently operating.

In the exemplary scenario of fig. 1, an AGPS user plane solution is shown, where a location server 101 is connected to a cellular network NET1 via an Internet Protocol (IP) based packet data network 102.

The GPS spacecrafts SV1-SV4 emit ranging signals RS1-RS4 with the center of the frequency spectrum of 1575.42 MHz. Fig. 2 shows how each ranging signal RS1-RS4 includes a stream 201 of navigation data bits 202 spread by spreading codes defined by so-called coarse/acquisition (C/a) codes 203 that are unique to the spacecraft from which the signal is transmitted. The C/A code 203 has a length of 1023 chips and a chip duration of 1/1.023 × 10⁶Seconds, i.e. C/A code comprised of 1.023 x 10⁶The rate of Hz varies and repeats itself +/-1 sequence every 1 millisecond. The navigation bit 202 has a bit period of 20 milliseconds, i.e., corresponding to 20C/a code repetitions.

The navigation data comprises, among other things, a set of so-called ephemeris parameters, which allow the receiver to calculate the precise position of the satellites at the time of signal transmission. The precise time of transmission may also be read from the navigation data.

Fig. 3 shows in more detail how the navigation data is further divided into 5 sub-frames 301-305, each 6 seconds in length. Each sub-frame 301-305 is divided into 10 words, each word being 0.6 seconds long and containing 30 data bits. Time stamp-GPS time of week (TOW) is transmitted in the second word-over-word (HOW) of each sub-frame 301-305. The time shown is the transmission time at the end of the considered subframe. Thus TOW is repeated every 6 seconds.

Each ranging signal RS1-RS4 basically defines a clock measured by the mobile station MS 1. The clock indicates the time of signal transmission. If the mobile station MS1 knows the GPS system time, the clock reading can be used directly to determine the time delay and hence the distance from the spacecraft that transmitted the ranging signal to the mobile station MS 1. By measuring three distances and using knowledge about the position of the spacecraft at the time of transmission, the three-dimensional position of the mobile station MS1 can then be determined. However, the mobile station MS1 typically does not know the precise GPS system time and therefore needs to be measured again to remove the mobile station clock bias.

The sequence of fig. 4 shows the clock relationships (in milliseconds) of the different parts of the system shown in fig. 1. Each spacecraft SV1-SV4 has a precise atomic clock to maintain clock stability. However, as shown in FIG. 4, the spacecraft transmissions are not perfectly synchronized with GPS system time. In fig. 4, sequence 401 represents GPS system time, sequence 411 represents the clock of spacecraft 1, sequence 41N represents the clock of spacecraft N, sequence 402 represents the clock of mobile station MS1 of fig. 1, and sequences 421 and 42N represent the times as read in ranging signals received by mobile station MS1 from spacecraft 1 and spacecraft N, respectively. By drawing a vertical line 431 through the timing diagram, a snapshot of all clock readings as observed at various points in space can be obtained. The GPS system time 401 is defined as an overall average based on a set of ground station clocks and a subset of spacecraft clocks. As shown in fig. 4, the respective spacecraft clocks 411 and 41N and mobile station clock 402 are slightly offset from GPS system time 401 (see SV clock offsets 412 and 413 and mobile station clock offset 414, respectively). A model of the individual offsets of the spacecraft clocks is transmitted from each spacecraft as part of a navigation message. When the signals arrive at a point on the earth's surface, such as the current location of mobile station MS1, they are delayed by an amount that depends on the distance from the spacecraft in question to the point on the earth's surface. As shown by the clock readings in fig. 4, the delay is typically 60-85 milliseconds (ms).

In determining the position of the mobile station using AGPS, the mobile station measures a time position, i.e., a C/a code phase, with respect to a C/a code boundary position at a selected time point for the received ranging signal. The C/a code phase (i.e., one C/a code period) is determined modulo 1 millisecond.

Based on the measured C/a code phases of the received ranging signals and assistance data (received from the network) including spacecraft ephemeris and clock correction data and a priori information about the position of the mobile station, the mobile station implementing mobile station based AGPS calculates its position at a selected point in time.

A mobile station implementing mobile station assisted AGPS, such as mobile station MS1 in fig. 1, instead transmits a radio signal reporting the C/a code phase of the received ranging signal (in terms of complete and incomplete chip representations of the C/a code from a selected point in time until the next C/a code repetition) and the GSP system time estimate corresponding to the selected point in time. A location server, such as location server 101 in fig. 1, in a cellular network or in another network calculates the location of the mobile station based on the information reported by the mobile station and a priori information about the location of the mobile station.

The inventors of the present invention have recognized that the AGPS approach of measuring the C/a code phase modulo 1 ms and thus characterizing each ranging signal modulo 1 ms in time causes problems when the uncertainty in the a priori information of the mobile station position is too large. As shown below, when the initial position uncertainty is greater than 75 km, the so-called pseudoranges to the spacecraft cannot be unambiguously reconstructed.

The GPS receiver basically measures pseudoranges to a plurality of satellites (note, however, that in mobile assisted GPS, the GPS receiver integrated in the mobile station does not perform the complete calculations required to determine pseudoranges, but only provides the basic data required to calculate pseudoranges). Pseudo-range of

ρ_i＝c·(t_u-t_ti)(1)

Wherein, t_uIs a GPS receiver clock reading (integrated in the mobile station) at reception, and t_tiIs the signal transmission time of the ith spacecraft and c is the wave propagation velocity. The pseudoranges differ from the actual range due to a number of disturbing factors (receiver clock bias, ionospheric and tropospheric delays, spacecraft clock bias, measurement errors, etc.). For clarity, the effects of most of these error sources are ignored in the following statements. There are known techniques to compensate for many of the error sources listed above (see, e.g., (1), (2)). Furthermore, since it is also well known in the art how to deal with the effects of spacecraft motion and earth rotation, these effects are also omitted (see e.g., (1), (2)). The simplified model is then that the measured pseudoranges are obeyed

ρ_i＝/x_u-x_si/+b+e_i (2)

Here, x_u＝(x_u y_u z_u) Is a row vector containing the three-dimensional coordinates of the unknown receiver position. Similarly, x_siIs a row vector containing the coordinates of the ith spacecraft. The symbol | z | represents the norm of the vector in parentheses, which is equal to (zz)^T)^1/2. In this case it can be interpreted as the distance between the GPS receiver/mobile station and the spacecraft. And, b is the receiver clock offset (expressed as distance),

b＝c·(t_u-t_GPS)(3)

wherein, t_GPSRepresenting GPS system time. Finally, e_iIs the measurement error.

The mobile station in mobile assisted AGPS only reports pseudoranges modulo 1C/a code period, i.e. the integer number of code periods in pseudorange (1) is unknown. It must be reconstructed. Let R generally denote the distance corresponding to the truncation interval used when measuring the ranging signal timing. For example, the distance R corresponding to the truncation interval of one C/A code period (1 ms) is then

R＝c·10^-3(4)

Thus, we obtain

ρ_i＝k_iR+v_i(5)

Wherein v is_iIs that v is 0. ltoreq_i< R, and wherein the integer k_iThe tolerance value of (c) needs to be reconstructed. When the a priori initial position is large, k may be allowed_iA number of values of (a). Furthermore, the presence of the common bias term b makes it difficult to reconstruct k exactly_i. However, from a position calculation perspective, relative pseudoranges are important since any constant bias terms will cancel out in the position calculation.

Defining reconstructed pseudoranges as

ρ_i ^＊＝k_i ^＊R+v_i (6)

It will then be seen that k is valid for all i, k_i ^*＝k_iIs not necessary. However, the relative pseudoranges must be reconstructed correctly, i.e.

k_i ^＊＝k_i+ X, X being an integer (7) for all i

This can be explained as follows. Substitution of (7) into (6) and use of (5) and (2) gives

ρ_i ^＊＝k_iR+XR+V_i＝ρ_i+XR＝/x_u-x_si/+b+XR+e_i＝/x_u-x_si/+b^＊+e_i(8)

That is, the reconstructed measurement equation (8) has the same structure as (2), the only difference being that b is replaced by b^*＝b+XR。

The reconstruction is done in the following way. Assume that the a priori position x of the mobile station is known_u0And uncertainty Δ such that

/x_u-x_u0/＜Δ (9)

First, we determine the predicted pseudoranges to spacecraft #1 (arbitrarily chosen in the spacecraft for which the ranging signals have been received/measured)

ρ₁′＝/x_s1-x_u0/(10)

And then we try to find k that satisfies the following condition₁ ^*

k₁ ^＊R+V₁＝ρ₁′(11)

In most cases, no exact solution is available, so we round the result to the nearest integer, so that

k₁ ^＊＝round((ρ₁′-V₁)/R)(12)

Thus, we obtain

ρ₁ ^＊＝round((ρ₁′-V₁)/R)R+V₁(12b)

The next step is to estimate the allowable value k for the case where i > 1_i ^*A collection of (a). Allowable pseudorange ρ_i ^*Satisfy the requirement of

ρ_i ^＊-ρ₁ ^＊＝ρ_i-ρ₁＝/x_u-x_si/-/x_u-x_s1/(13)

In (13), the measurement error has been ignored. To obtain a clear boundary, all possible receiver positions that are satisfied at (9) can be computed, i.e., at x_u0And a predicted range difference for all possible receiver positions within the initial position uncertainty region defined by Δ (i.e., the difference between the ranges to spacecraft #1 and spacecraft # i, respectively). Thus, we obtain the inequality

δρ_i-Δ_i≤ρ_i ^＊-ρ₁ ^＊≤δρ_i+Δ_i ，i＝2，....，n (14)

Where, δ ρ_iIs the average of the maximum and minimum distance differences that can be found anywhere within the initial uncertainty region. Similarly, Δ_iIs half the difference between the largest and smallest distance differences that can be found anywhere within the initial uncertainty region.

And rho₁ ^*Combined to form an allowable relative pseudorange ρ_i ^*-ρ₁ ^*All allowable pseudoranges p_i ^*Equation (14) is satisfied.

Using (6), equation (14) can be rewritten as

δρ_i-Δ_i≤k_i ^＊R+V_i-ρ₁ ^＊≤δρ_i+Δ_i (15)

Now by checking k for changes_i ^*Whether or not the inequality (15) is satisfied to find ρ_i ^*Is determined by the tolerance of (1). Note that k may be allowed_i ^*Several possible values of (a). The possible values of the reconstructed pseudoranges (6) are stepped at R meter intervals. Uncertainty region width of 2 delta_i. Therefore, as long as

2Δ_i＞R (16)

There is a risk that the reconstruction cannot be done unambiguously.

Several alternatives for determining the allowable pseudoranges are described below.

It may be difficult to determine δ ρ explicitly in (14)_iAnd Δ_i. A simpler way is to base the calculations on individual pseudoranges. By noting ρ_i-b＝|x_u-x_siL (ignoring measurement errors), δ ρ may be determined that satisfies the following requirements_indiAnd Δ_indi

δρ_indi-Δ_indi≤ρ_i-b≤δρ_indi+Δ_indi ，i＝1，...，n (17)

Wherein

δρ_indiIs the average of the maximum and minimum distances to satellite i that can be found anywhere within the initial uncertainty region, and where

Δ_indiIs half the difference between the maximum and minimum distances to satellite i that can be found anywhere within the initial uncertainty region.

By applying simple geometry, the maximum and minimum distances to satellite i, and thus also δ ρ, can be easily determined_indiAnd Δ_indi. By applying the triangle inequality, then one can obtain

|(ρ_i-b-δρ_indi)-(ρ₁-b-δρ_ind1)/≤|(ρ_i-b-δρ_indi)/+/(ρ₁-b-δρ_ind1)/

≤Δ_indi+Δ_ind1，i＝2，...，n (18)

Reformulating (18) and using the first equation in (13), we obtain

δρ_indi-δρ_ind1-Δ_indi-Δ_ind1≤k_i ^＊R+V_i-ρ₁ ^＊≤δρ_indi-δρ_ind1+Δ_indi+Δ_ind1

(19)

The possible values of the reconstructed pseudoranges (6) are stepped at R meter intervals. The width of the uncertainty region is 2(Δ)_indi+Δ_ind1). Therefore, as long as

2(Δ_indi+Δ_ind1)＞R (20)

There is a risk that the reconstruction cannot be done unambiguously.

Further simplified expressions can be obtained by using only the relation (9) and the trigonometric inequality (in the form of a-B ≦ a + B |). First note that we can write

|(ρ_i-b-ρ_i′-(ρ₁-b-ρ₁′))/≤|ρ_i-b-ρ_i′/+|ρ₁-b-ρ₁′/＝

//x_u-x_si/-/x_u0-x_si//+//x_u-x_s1/-/x_u0-x_s1//≤

/x_u-x_si-x_u0+x_si/+/x_u-x_s1-x_u0+x_s1/＝/x_u-x_u0/+/x_u-x_u0/≤2Δ(21)

Where ρ is_i' is the predicted pseudorange, ρ, to spacecraft # i_i′＝|x_si-x_u0|。

Using (13) and substituting expression (6), we finally get

-2Δ≤k_i ^＊R+V_i-ρ_i′-(ρ₁ ^＊-ρ₁′)≤2Δ(22)

The possible values of the reconstructed pseudoranges (6) are stepped at R meter intervals. The width of the uncertainty region is 4 Δ. Therefore, as long as

4Δ＞R (23)

There is a risk that the reconstruction cannot be done unambiguously. Thus, when the uncertainty Δ is greater than about 75 kilometers and the ranging signal timing measurements are performed using a truncation interval corresponding to one C/a code period, there is a risk of ambiguous pseudorange reconstruction.

By having the mobile station measure the ranging signal timing for each ranging signal without any truncation, the problem of ambiguous pseudorange reconstruction due to a combination of truncated measurements of ranging signal timing and too much uncertainty in the prior position can be solved. However, this may require decoding time of week information on each measured ranging signal, which greatly increases processing delay, and may also reduce the detection sensitivity of a GPS receiver integrated in the mobile station since it is much more difficult to decode the time of week information than to detect the C/a code boundary. For mobile assisted AGPS, it may also be required to modify the message for reporting the ranging signal measurement result in order to report not only the truncated timing of the ranging signal but also the full timing of the ranging signal.

The present invention solves the above-mentioned problems by providing a way to significantly reduce the risk of ambiguous pseudorange reconstructions caused by a priori position uncertainty of a mobile station in the context of AGPS (including mobile station based and mobile station assisted AGPS). At the same time, the present invention also avoids the need for and associated disadvantages of decoding the time of week information on each measured ranging signal.

FIG. 5 illustrates a basic method for determining a location at which ranging signals from at least three satellites are received, where the location is known a priori to be within an initial location uncertainty area, in accordance with the present invention.

The basic method includes performing at least one pseudorange selection cycle. All embodiments of the method according to the invention comprise performing an initial selection loop 511 (comprising sub-steps 501 and 504 in fig. 5). Some embodiments of the invention may optionally include one or more additional selection loops 512 (including sub-steps 506 and 508).

After performing pseudorange selection in at least one pseudorange selection cycle, the position is determined at step 505 by using the pseudoranges selected after performing the pseudorange selection cycle. Thus, step 505 may be performed immediately after the initial selection loop 511 in some embodiments, and step 505 may be performed after one or more additional selection loops 512 in other embodiments.

The initial selection loop 511 comprises sub-steps 501-504.

In sub-step501, determining a first pseudorange ρ₁ ^*The pseudorange is an allowable pseudorange associated with the first satellite relative to the initial position uncertainty area.

At step 502, all pseudoranges associated with at least two additional satellites are determined, which pseudoranges are combined with the first pseudorange to form allowable relative pseudoranges. Mathematically, the pseudorange ρ associated with satellite i when expression (14) is satisfied_i ^*At a first pseudorange p_i ^*The allowable relative pseudoranges are formed when combined.

In sub-step 503, a set of pseudorange vectors is formed representing all possible combinations of determined pseudoranges associated with the first and at least two further satellites. Each pseudorange vector thus includes a first pseudorange associated with a first satellite (e.g., SV1 in fig. 1) and a pseudorange associated with each different satellite of at least two other satellites (e.g., SV2 and SV3 in fig. 1).

At sub-step 504, a set of selected pseudorange vectors is formed by selecting at least one vector from the set of pseudorange vectors, wherein the selecting comprises evaluating each vector in the set of pseudorange vectors according to a predetermined rule for initial vector selection, at least when the set of pseudorange vectors comprises more than one vector. In some embodiments of the invention, such evaluations may be performed regardless of the number of vectors in the set of pseudorange vectors, i.e., such evaluations may also be performed when there is a single vector. In other embodiments of the invention, such an evaluation may be performed only if there is more than one vector in the set of pseudorange vectors, i.e. if there is a single vector in the set of pseudorange vectors, this single vector may be selected without performing such an evaluation. A predetermined rule for initial vector selection is selected to eliminate pseudorange vectors representing unlikely combinations of pseudoranges. Different embodiments of the invention may apply different rules to the initial vector selection. For embodiments where the at least two other satellites are exactly equal to two satellites, i.e., each vector includes pseudoranges from three different satellites, the rule for initial vector selection will generally be based on comparing the positions associated with the different vectors to an initial uncertainty region. For embodiments where the at least two further satellites comprise more than two satellites, i.e. where each vector comprises pseudoranges from four or more satellites, an alternative to the different rules for defining initial vector selection may further comprise calculating a minimum loss function value for each new pseudorange value and selecting the pseudorange vector with the lowest minimum loss function value or all pseudorange vectors with minimum loss function values below a threshold.

The optional one or more further selection cycles 512 include sub-steps 506-508.

At sub-step 506, all pseudoranges associated with at least one further satellite are determined, which pseudoranges, in combination with the first pseudorange, form an allowable relative pseudorange (i.e., satisfy expression (14)).

In sub-step 507, a new set of pseudorange vectors is formed representing all possible combinations of the set of vectors selected in the previous selection cycle and the determined pseudoranges associated with the at least one further satellite. Since each pseudorange vector in the new set of pseudorange vectors comprises a pseudorange associated with each satellite of the at least one further satellite, the dimension of the new pseudorange vector is increased compared to the dimension of the selected pseudorange vector in the previous selection cycle.

At sub-step 508, a new set of selected pseudorange vectors is formed by selecting a pseudorange vector from the new set of pseudorange vectors, wherein, at least when the new set of pseudorange vectors includes more than one vector, the selecting includes evaluating each vector of the new set of pseudorange vectors according to a predetermined rule for subsequent selection. In some embodiments of the invention, such evaluations may be performed regardless of the number of vectors in the new set of pseudoranges, i.e., such evaluations may also be performed when there is a single vector. In other embodiments of the invention, such an evaluation may be performed only if there is more than one vector in the set of pseudorange vectors, i.e. if there is a single vector in the new set of pseudorange vectors, this single vector may be selected without performing such an evaluation. A predetermined rule for subsequent vector selection is selected to eliminate pseudorange vectors representing unlikely combinations of pseudoranges. Different embodiments of the invention may apply different rules to subsequent vector selection, for example based on comparing the positions associated with different vectors with an initial uncertainty region, based on calculating a minimum loss function value for each new pseudorange value, and selecting the pseudorange vector with the lowest minimum loss function value or all pseudorange vectors with the minimum loss function value below a threshold.

Embodiments of the present invention may be implemented in a control node on the network side (e.g. for supporting mobile assisted AGPS) and in a mobile station (e.g. for supporting mobile based AGPS).

Fig. 8-10 show a first exemplary embodiment of the method and arrangement according to the present invention implemented in the location server node 101 of fig. 1. Before proceeding with a detailed description of the first exemplary embodiment of the present invention, some of the structural and processing details performed in the mobile station MS1 of fig. 1 in connection with mobile assisted AGPS are described in detail below in connection with fig. 6 and 7.

Fig. 6 is a block diagram showing the structure of the mobile station MS 1. The mobile station MS1 includes a cellular communication module 601, a positioning module 602; a GPS RF front end 603, an antenna 604 for communicating with a cellular network, and a GPS antenna 605. The positioning module 602 includes a CPU 612, memory 610, and a Digital Signal Processor (DSP) 611. The cellular communication module 601 wirelessly receives assistance data from the cellular network and wirelessly transmits measurement results to the cellular network via a base station in the cellular network. The assistance data may include ephemeris and time corrections for the visible satellites, the approximate location of the mobile station MS1, and the approximate GPS system time. Alternatively, the assistance data may comprise explicit assistance data intended only for assisting the relevant processing. The communication module 601 forwards the received assistance data to the positioning module 602 by using the interface 606, while the measurement results are provided from the positioning module 602 to the communication module 601 by using the interface 613. The communication module 601 also provides a clock reference 607 for the GPS RF front end 603 and the positioning module 602. The GPS RF front end module 603 is controlled by the positioning module 602 using the interface 608.

Fig. 7 shows the processing performed by the mobile station MS1 upon receiving a location request.

When the positioning module 602 receives a positioning request from the communication module 601, it requests the GPS RF front end 603 to provide GPS signal samples in step 701. GPS RF front end 603 receives the GPS frequency band through antenna 605, down-converts the signal to baseband, separates the signal into in-phase (I) and quadrature (Q) components, samples and converts the signal to digital format, and outputs the signals to positioning module 602 through interface 609. The location module 602 stores the received I and Q data in the memory 610.

Step 702 defines the processing performed on each individual ranging signal RS1-RS4 included in the measurement report transmitted at step 707. Note that even though fig. 7 shows sequential processing of each individual ranging signal (see step 704), it is preferable to perform the processing related to the different ranging signals in parallel.

The ranging signal y received by the mobile station MS1 from any spacecraft SV1-SV4 as a function of time t can be written in a simplified manner as:

y(t)＝a·c(t-τ)·d(t-τ)·exp{i·(ω₀t+ω_at+φ)}+e(t)

here, a is the amplitude of the received signal, C (t) is the C/a code of the spacecraft, and d (t) is the navigation data bit stream (see fig. 2). The term τ is the unknown delay of the signal as a function of the distance from the spacecraft to the position of the mobile station MS1, ω₀Is the GPS carrier frequency, ω_dIs the Doppler frequency of the signal, phi is the unknown phase, and e (t) is noise.

The digital signal processor 611 in the positioning module 602 determines the C/a code boundary of the ranging signal using a correlation that tests all possible code phases and doppler shifts for the ranging signal at step 702. Once step 702 is initially completed for a ranging signal, DSP 611 maintains synchronization with the ranging signal by tracking changes in the C/A code edge definition of the ranging signal.

If more ranging signals need to be acquired ("yes" is selected at step 704), then step 702 is repeated for the next ranging signal (as already discussed, the processing of step 702 is preferably performed in parallel for several ranging signals, rather than in the order shown in FIG. 7). The determination in step 704 as to whether more ranging signals should be acquired may be based on the number of ranging signals acquired so far (at least 3 or preferably 4 ranging signals should be acquired, but acquiring more ranging signals increases the accuracy of the computed position) and timing requirements (the response time for providing measurement report signals may be configured by parameters, for example, within 16 seconds of receiving a positioning request).

If sufficient ranging signals are acquired ("no" selection at step 704), then the GPS time of week (TOW) at the selected time point is estimated at step 705. Note that step 702 is preferably performed for the first ranging signal, and step 705 is performed in parallel with acquiring additional ranging signals based on the ranging signal.

There are several alternatives for how step 705 may be performed. Generally, the TOW estimation is based on determining the TOW transmitted in a so-called Hand Over Word (HOW) of a ranging signal, preferably the first acquired ranging signal (see fig. 3), and then compensating for the propagation delay from signal transmission by the spacecraft until signal reception by the mobile station MS 1.

The determination of the transmitted TOW may be performed by directly decoding the transmitted TOW. This alternative implies demodulating the data at a rate of 20 milliseconds and typically requires determining the subframe boundaries and then decoding the transport word from which the TOW, i.e. the transmission time t, can be derived_ti. Each subframe is 6 seconds in length, and therefore, this process may require approximately 8 seconds of navigation data to be collected. The TOW demodulation works up to about-172 dBW assuming a 0dB antenna and is in fact the limiting factor for detection sensitivity.

Alternatively, the transmitted TOW may be determined by reconstruction using correlation techniques. This process also requires the generation of demodulated data bits, but rather than direct decoding, correlation with known transmitted navigation data bits (e.g., the content of so-called telemetry words and HOW words that may be sent to the mobile station as part of the assistance data). This requires that GPS time is known a priori to be within a few seconds. This process uses a slightly lower signal level than direct TOW decoding, but it is highly likely that performance is limited by the tracking loop, which may release lock at such a low signal level. Typically, a phase locked loop or an automatic frequency control loop is used for this purpose. But it is expected that this will work up to about-179 dBW.

Propagation delay compensation can be performed by applying an expected average propagation delay of 77 milliseconds. Alternatively, Ari Kangas and Janos according to the inventorsThe principles detailed in co-pending U.S. patent application filed on 29/9/2004, may result in more accurate propagation delay compensation from assistance data received by mobile station MS1 from the cellular network.

In step 706, the time position relative to the selected time point is measured for the next C/A code boundary after the selected time point. More specifically, for each acquired ranging signal, the position of the next C/a code boundary after the selected time point is measured by recording the number of complete and incomplete chips from the selected time point until the next C/a code boundary. Finally, in step 707, a measurement report signal reporting the measurement result is wirelessly transmitted by the mobile station MS1 to the cellular network NETI. The measurement report signal is in this exemplary case transmitted as normal user data in the user plane addressed to the location server 101. Thus, the measurement report signal is transparently routed via the IP-based network 102 to the location server 101 through the cellular network NET 1.

Fig. 8 schematically shows the structure of a location server 101 according to a first exemplary embodiment of the arrangement of the present invention. The location server includes a communication module 801 and a positioning module 803. The communication module 801 receives the measurement report and forwards the measurement data to the positioning module 803. The positioning module comprises one or more processors CP1 designed to determine the position of the mobile station MS1 using the provided measurement data (including measurement timing information for each reported ranging signal) and a priori information about the position of the mobile station. The a priori information may be derived, for example, from a Public Land Mobile Network (PLMN) identity included in the signal from the mobile station MS1 and indicating the network in which the mobile station MS1 is operating. The PLMN identity may be included, for example, as part of the cell identity of the cell in which the mobile station is currently operating. Using the provided PLMN identity, the positioning module 803 may derive the a priori location information, e.g. by retrieving from a table the coordinates of the center of the country in which the mobile station MS1 is operating and the radius corresponding to the maximum distance from the center up to the border of the country. The hierarchical nature of cell identities can also be used to identify a particular region within a country in which a mobile station operates, particularly for large countries. Maintaining a table of center/radius information for different countries or regions within the country is much less burdensome than attempting to maintain a global database with information about the geographic coordinates of each cell.

Before discussing in more detail the processing performed by the location server 101 according to an exemplary embodiment of the method of the present invention, some basic calculations for determining the location of a mobile station are described below.

The measurement equation (2) can be expressed in vectorized form as

ρ＝/1_n·x_u-X_s/+b1_n+e (24)

Where ρ is a column vector of length n containing pseudoranges, 1_nIs a column vector of length n containing only 1. X_sIs a matrix in which the ith row contains the coordinates x of the ith satellite (spacecraft)_si. Let it be assumed here that the norm | Z | is computed for each row of the matrix Z in brackets.

Around an unknown parameter x_uThe Taylor series expansion of the initial estimate of b can be expressed as

ρ＝/1_n·x_u-X_s/+(b-b₀)1_n+e＝/1_n·x_u0-X_s/+G·((x_u-x_u0)(b-b₀))^T+v(25)

Wherein G is a geometric matrix containing pseudoranges with respect to a parameter x_uAnd b, v is the sum of the measurement error term e and a higher order taylor series term. Suppose that

r_i＝/x_u0-x_si/(26)

G is then the matrix with row i equal to

G_i＝[(x_u0-x_si)/r_i (y_u0-y_si)/r_i (z_u0-z_si)/r_i1](27)

(25) Is equal to

(x_u b)^T＝(x_u0 b₀)^T+(G^TG)^-1G^T(ρ-/1_n·x_u0-X_s/-b₀1_n)(29)

Least squares minimum loss function value equal to

V＝(ρ-/1_n·x_u0-X_s/-b₀1_n)^T(I-G(G^TG)^-1G^T)(ρ-/1_n·x_u0-X_s/-b₀1_n)(30)

The minimum loss function value is a measure of how well the predicted pseudoranges match the measured pseudoranges based on the updated parameter estimates.

Fig. 9A-B illustrate the processing performed by the location server 101, and more specifically by the positioning module 803, according to a first exemplary embodiment of the method of the present invention.

The process begins at step 901 by computing a first pseudorange ρ for any first satellite (spacecraft) according to equation (12b)₁ ^*. In addition, all pseudoranges ρ₂ ^*Sum (of the second satellite) (. rho)₃ ^*(of the third satellite) is determined using any of expressions (15), (19) or (22) with respect to the first pseudorange ρ₁ ^*The combination forms an allowable relative pseudorange. Suppose n is found for the second and third satellites, respectively₂And n₃Allowable pseudoranges representing determined pseudoranges ρ associated with the first, second and third satellites, respectively₁ ^*、ρ₂ ^*And ρ₃ ^*All possible combinations of (2) by n₂·n₃A set of candidate pseudorange vectors.

At step 902, a least squares estimation according to (29) is performed for each candidate pseudorange vector, but since only three measurements are used, only a two-dimensional position fix may be made. Since the a priori uncertainty in the vertical dimension is generally smaller than the horizontal uncertainty, it is natural to estimate the x (latitude) and y (longitude) coordinates. Estimating a reduced parameter vector (x) for all candidate pseudorange vectors_u y_u b)^T。

In step 903, the position associated with the candidate pseudorange vector is compared to the a priori position x of the mobile station by considering the size of the update step in the horizontal dimension_u0And comparing the pseudo range vector with an initial position uncertainty area defined by the uncertainty delta to complete the selection of the most possible pseudo range vector candidate. Thus, for all candidate vectors ((x) is determined_u-x_u0)²+(y_u-y_u0)²)^1/2. Due to ((x)_u-x_u0)²+(y_u-y_u0)²)^1/2< Δ for the actual parameter vector x_u、y_uAre known a priori and therefore those candidates that generate an update level greater than a defined threshold chosen to be greater than delta are excluded from further calculations. Since the least squares equation (29) is only an approximation of the original non-linear problem (24), it may be preferable to choose a threshold value greater than Δ, with the fact that the least squares solution must typically be performed iterativelyMargin (margin).

Note that in this case the residual V in (30) cannot be used, since in most cases it will be equal to zero at three measurements, three equations.

In a next step 904 it is checked whether all pseudoranges have been reconstructed, i.e. whether all measured satellite ranging signals have been considered.

If all pseudoranges have been reconstructed (yes at step 904), processing continues at step 913 where the mobile station position is calculated/estimated.

Otherwise ("no" is selected at step 904), processing continues at step 905 where any expression (15), (19) or (22) is used to determine an association with an additional (fourth) satellite and with the first pseudorange ρ₁ ^*All pseudoranges p combined to form an allowable relative pseudorange₄ ^*. By combining the set of pseudorange vectors selected in step 903 with all determined pseudoranges ρ associated with further (fourth) satellites₄ ^*A new set of all possible candidate pseudorange vectors is formed.

In step 906 loss function values (30) are calculated for all candidate pseudorange vectors, assuming that in this case also only a two-dimensional position vector is estimated. Note that in this step, no explicit solution (29) needs to be computed.

At step 907, candidate pseudorange vectors are selected that minimize the loss function (30).

In a next step 908 it is checked whether all pseudoranges have been reconstructed, i.e. whether all measured satellite ranging signals have been considered.

If all pseudoranges have been reconstructed (yes at step 908), processing continues at step 913 where the mobile station position is calculated.

Otherwise ("no" option at step 908), in the following steps, sufficient ranging signal measurements are available to implement the three-dimensional solution. Thus, at step 909 for ρ_k ^*The process of pseudorange reconstruction is repeated, where k is 5, i.e. by using any of expressions (15), (19) or (22), the first pseudorange ρ is determined in association with a further (fifth) satellite₁ ^*All pseudoranges p combined to form an allowable relative pseudorange₅ ^*And by combining the set of candidate pseudorange vectors selected in the preceding selection step with all determined pseudoranges ρ associated with a further (fifth) satellite₅ ^*A new set of all possible candidate pseudorange vectors is formed.

At step 910, loss function values are computed for all candidate pseudorange vectors in the new set using (30). At step 911, the least probable candidate pseudorange vector is excluded from further consideration based on the loss function values, i.e., a new set of selected candidate pseudorange vectors is formed by excluding the unlikely candidate pseudorange vectors formed at step 909. In this embodiment, candidate pseudorange vectors are selected that minimize the loss function.

In step 912 it is checked whether all pseudoranges have been reconstructed, i.e. whether all measured ranging signals have been considered. If all pseudoranges have been reconstructed ("yes" alternative at step 912), processing continues at step 913 where the mobile station position is calculated.

Otherwise ("no" option at step 912), the process of pseudorange reconstruction and exclusion of impossible pseudorange vectors is repeated from step 909 to determine pseudoranges ρ associated with additional satellites k_k ^*Wherein k is increased before each new iteration of steps 909-911.

When all pseudoranges have been reconstructed, i.e. all acquired satellite ranging signals have been considered, the mobile station position x is calculated/estimated by determining a least squares solution (29), possibly using an iterative scheme, in step 913_u。

The pseudorange selections performed in the process shown in fig. 9 are organized in initial pseudorange selection loops comprising steps 901-.

Fig. 10 shows an exemplary case of results and intermediate results when performing the method according to fig. 9A-B. In this exemplary case, it is assumed that ranging signal measurements associated with the four satellites of fig. 1 are available, which means that only an initial selection cycle (steps 901-.

FIG. 10 illustrates first pseudoranges ρ associated with a second satellite SV2 and a third satellite SV3, respectively₁ ^*(r11 in FIG. 10), pseudo range ρ₂ ^*(r21, r22 in FIG. 10) and ρ₃ ^*(r 31, r32 in fig. 10), in this exemplary case, these pseudoranges are determined at step 901. Based on the determined pseudoranges, a set of pseudorange vectors 1001 representing all possible combinations of said pseudoranges is also formed at step 901.

From the set of pseudoranges 1001 formed at step 901, a set of selected pseudorange vectors 1002 is derived at step 902-. As shown in fig. 10, in this particular exemplary case, two pseudorange vectors are selected at step 902-903.

FIG. 10 also shows pseudoranges ρ determined at step 905 associated with additional fourth satellite SV4₄ ^*(r41, r42 in FIG. 10). Determining a pseudorange p based on a position associated with a fourth satellite SV4₄ ^*Also formed at step 905 is a representation of pseudorange ρ₄ ^*And a new set 1003 of pseudorange vectors for all possible combinations from the previous selection cycle (i.e., the initial selection cycle in this exemplary case) selected set 1002 of pseudorange vectors.

From the new set of pseudoranges 1003 formed at step 905, a new set of selected pseudorange vectors 1004 is derived at step 906 and 907 by selecting the pseudorange vector that minimizes the minimum loss function (30).

Finally, according to this exemplary scenario, the elements of a single pseudorange vector in the new set of pseudorange vectors 1004 are selected for use in step 913 in determining the location at which ranging signals from four satellites SV1-SV4 are received (represented in FIG. 10 by vector 1005).

In addition to the exemplary first embodiment of the invention disclosed above, there are several ways of providing rearrangements, modifications and substitutions of the first embodiment to create further embodiments of the invention.

In a first exemplary embodiment of the present invention, the processing steps shown in FIGS. 9A-B are performed by digital data processing circuitry in the form of one or more conventional programmable processors. However, any digital data processing circuit capable of performing the described processing may be used, such as state machines, ASICs, discrete logic circuitry, etc. In a first exemplary embodiment of the present invention, as in other embodiments of the present invention using programmable devices, the controlling computer program (software) is embodied as machine-readable instructions stored on some computer-readable medium such as RAM, a hard disk drive, an electronic read-only memory, an optical storage device (e.g., a CD-ROM shown schematically in fig. 11), etc. Programmable devices performing processing according to the present invention may be dedicated to this task or may also be used for processing relating to other tasks.

The exemplary embodiment of the invention for use in the context of mobile station based AGPS can be derived from the first embodiment of the invention shown by substantially replacing step 707 of fig. 7 in the positioning module 602 of the mobile station MS1 with a process according to fig. 9A-9B. Thus, the calculations performed by the positioning module 903 of the location server 101 in the first exemplary embodiment are transferred to be performed by the positioning module 602 of the mobile station MS 1. A priori estimates of the mobile station's position, as well as satellite ephemeris data and clock corrections, may be provided by the network as assistance data for use in position calculations. The mobile station MS1 will thus in this case be an exemplary embodiment of the arrangement according to the invention.

The invention can of course be applied in the context of AGPS control plane and user plane solutions. With respect to AGPS control plane solutions, the invention may be of great interest for application in the context of extended range cells (in GSM, extended range cells may have a radius of up to 100 km) or when cell identity positioning is not implemented in the network (which is typically used as a basis for determining a priori location information).

An alternative to performing the pseudorange selection loop 909-911 until all measured ranging signals have been considered would be to exit the loop of the iterative selection loop after the selection loop that resulted in a single pseudorange vector selection. A priori position x of the mobile station may then be determined based on the selected single pseudorange vector_u0And an updated position uncertainty area within the initial position uncertainty area defined by the uncertainty delta. The updated position uncertainty region may then be used to determine the unknown position of the mobile station, i.e., the position from which the satellite ranging signals were received. Since the updated position uncertainty area may be much smaller than the initial position uncertainty area, there is virtually no risk of ambiguous pseudorange reconstructions when using the updated position uncertainty area to determine an unknown position. The true position calculation can be performed as a normal GPS positioning.

In case it is desired to handle even larger a priori position uncertainties in combination with AGPS, the present invention may be combined with the teaching of international patent application PCT/IB2004/052040 by performing measurements modulo the navigation data bit length on the received ranging signals as specified in said international patent application and then determining the position at which said ranging signals are received as disclosed in the present patent application.

To provide enhanced robustness against errors such as estimated GPS time at signal arrival, position calculations may be performed according to co-pending U.S. patent application 60/545194 to Ari Kangas.

There are several alternative rules that may be applied by different embodiments of the invention when evaluating and selecting pseudorange vectors in further pseudorange selection cycles. In addition to applying the minimum loss function (30), a weighted minimum loss function may be applied

V＝(ρ-/1_n·x_u-X_s/-b₀1_n)^TQ^-1(ρ-/1_n·x_u-X_s/-b₀1_n)(31)

Where Q is the covariance matrix of the range signal measurement error.

Instead of selecting the pseudorange vectors that minimize the loss function as in steps 907 and 909 in fig. 9A-9B, all pseudorange vectors that produce a loss function less than a defined threshold may be selected. Furthermore, the applied evaluation rules may represent different evaluation criteria in different iterations of the selection loop. One such example would be to select the pseudorange vector that minimizes the loss function in the last iteration, but to select all pseudorange vectors that yield the smallest loss function value below a defined threshold in all iterations except the last iteration. Another example of different evaluation criteria in different cycles would be to apply different thresholds (typically applying smaller and smaller thresholds) to different iterations. Evaluation of the pseudorange vectors may also be performed by computing a position associated with each pseudorange vector and comparing the computed position to an initial position uncertainty region (i.e., similar to the comparison performed in the initial pseudorange selection loop). It is also possible to evaluate the pseudorange vector by a combined criterion based on both the minimum loss function value (according to, for example, equation (30) or (31)) and the comparison of the associated position with the initial uncertainty region.

In the first exemplary embodiment of the present invention, truncated timing measurements may be used for all ranging signals RS1-RS 4. In other embodiments of the present invention, one or more ranging signals may be measured without truncation. However, as long as truncated ranging signal measurements are performed for at least one satellite, i.e. the available measurements comprise truncated timing measurements of at least one satellite, there is a risk of ambiguous pseudorange reconstructions when the area of prior position uncertainty is large, and therefore the invention can be used to provide enhanced robustness against ambiguous pseudorange reconstructions.

Although the invention has been applied in an assisted GPS environment in the first exemplary embodiment of the invention, the invention can of course also be applied in connection with other satellite based positioning systems, such as GALILEO or GLONASS, to handle situations where there is a risk that truncated ranging signal measurements combined with large a priori position uncertainty areas imply a risk of ambiguous pseudorange reconstruction.

Reference to the literature

(1) "Navstar GPS space segment/Navigation user interface" (Navstar GPS space segment/Navigation user Interfaces, ICD-GPS-200, Vision IRN-200C-003, 11October 1999).

(2) "global positioning system: theory and Applications "(Parkinson, Spilker Global positioning System: the Theory and Applications, Volnme 1, AIAA, 1996)

Claims (32)

1. A method for determining a position at which ranging signals (RS1-RS4) from at least three satellites (SV1-SV4) are received, wherein results of measurements performed on said ranging signals (RS1-RS4) comprise truncated timing measurements of at least one of said satellites (SV1-SV4), and wherein said position is a priori known to be located within an initial position uncertainty area, said method comprising the steps of:

performing at least one pseudorange selection cycle (511, 512);

determining (505) the position using pseudoranges (1004) selected after performing the at least one pseudorange selection cycle (511, 512),

wherein the at least one pseudorange selection loop comprises an initial selection loop (511) comprising the sub-steps of:

determining (501) a first pseudorange (r11) that is an allowable pseudorange associated with a first satellite (SV1) relative to the initial position uncertainty area;

determining (502) all pseudoranges (r21, r22, r31, r32) associated with at least two further satellites (SV2, SV3), which pseudoranges in combination with the first pseudorange (r11) form an allowed relative pseudorange;

forming (503) a set of pseudorange vectors (1001) representative of all possible combinations of said determined pseudoranges (r11, r21, r22, r31, r32) associated with said first (SV1) and at least two further satellites (SV2, SV 3);

forming (504) a set of selected pseudorange vectors (1002) by selecting at least one vector from the set of pseudorange vectors (1001),

wherein said selecting comprises evaluating each vector in said set of pseudorange vectors (1001) according to a predetermined rule for initial vector selection, at least when said set of pseudorange vectors (1001) comprises more than one vector.

2. The method according to claim 1, wherein said evaluation of each vector according to said predetermined rule for initial vector selection is performed only if said set (1001) of pseudorange vectors comprises more than one vector.

3. The method according to claim 1, wherein said evaluation of each vector according to said predetermined rule for initial vector selection is further performed when said set (1001) of pseudorange vectors comprises a single vector.

4. A method according to any of claims 1-3, wherein evaluation according to said predetermined rule for initial vector selection comprises comparing positions associated with said vectors in said set (1001) of pseudorange vectors with said initial position uncertainty area.

5. A method according to any of claims 1-3, wherein, when said at least two further satellites comprise more than two further satellites, the evaluation according to said predetermined rule for initial vector selection comprises calculating a minimum loss function value for each vector in said set (1001) of pseudorange vectors.

6. The method of claim 5, wherein a vector associated with a minimum loss function value below a predetermined threshold is selected for inclusion in the selected set of pseudorange vectors (1002).

7. The method of claim 5, wherein the vector associated with the lowest minimum loss function value is selected for inclusion in the selected set of pseudorange vectors (1002).

8. A method according to any of claims 1-3, wherein said at least one pseudorange selection cycle (511, 512) comprises at least one further selection cycle (512), said at least one further selection cycle comprising the sub-steps of:

determining (506) all pseudoranges (r41, r42) associated with at least one further satellite (SV4), said pseudoranges being combined with said first pseudorange (r11) to form an allowed relative pseudorange;

forming (507) a new set (1003) of pseudorange vectors representing all possible combinations of pseudorange vectors selected in a previous selection cycle (511, 512) and said determined pseudoranges (r41, r42) associated with said at least one further satellite (SV 4);

forming (508) a new set of selected pseudorange vectors (1004) by selecting at least one vector from said new set of pseudorange vectors (1003), wherein, at least when said new set of pseudorange vectors (1003) comprises more than one vector, said selecting comprises evaluating each vector of said new set of pseudorange vectors (1003) according to a predetermined rule for subsequent vector selection.

9. The method according to claim 8, wherein said evaluation of each vector according to said predetermined rule for subsequent vector selection is performed only if said new set of pseudorange vectors (1003) comprises more than one vector.

10. The method of claim 8, wherein said evaluating each vector according to said predetermined rule for subsequent vector selection is further performed when said new set of pseudorange vectors (1003) comprises a single vector.

11. The method of claim 8, wherein evaluating according to the predetermined rule for subsequent vector selection comprises comparing positions associated with the vectors in the new set (1003) of pseudorange vectors with the initial position uncertainty area.

12. The method of claim 8, wherein evaluating according to the predetermined rule for initial vector selection comprises calculating a minimum loss function value for each vector in the new set of pseudorange vectors (1003).

13. A method according to claim 12, wherein the vector associated with the smallest loss function value below a predetermined threshold is selected for inclusion in said new set of selected pseudorange vectors.

14. A method according to claim 12, wherein the vector associated with the lowest minimum loss function value is selected for inclusion in said new set of selected pseudorange vectors.

15. A method according to any of claims 1-3, wherein the satellite is part of a global positioning system.

16. A method according to any of claims 1-3, wherein after a pseudorange selection cycle resulting in a single pseudorange vector selection, said single pseudorange vector is used to determine an updated position uncertainty area within said initial position uncertainty area, and said step of determining said position is performed using said updated position uncertainty area.

17. A method as claimed in any one of claims 1 to 3, wherein the allowable relative pseudoranges are satisfied

δρ_i-Δ_i≤ρ_i ^＊-ρ₁ ^＊≤δρ_i+Δ_iWherein

ρ_i ^*-ρ₁ ^*Is determined by the pseudorange ρ associated with satellite i_i ^*And the first pseudorange p associated with the first satellite₁ ^*The relative pseudo-range formed is,

δρ_iis the average of the maximum and minimum differences between the range to satellite i and the range to the first satellite found at any point within the initial position uncertainty region,

Δ_iis half the difference between the maximum and minimum differences between the range to satellite i and the range to the first satellite found at any point within the initial position uncertainty region.

18. The method of any of claims 1-3, wherein the first allowable pseudorange associated with the first satellite satisfies

ρ₁ ^＊＝round(ρ₁′-v₁)/R)R+v₁Wherein

ρ₁ ^*Is the first pseudorange associated with the first satellite,

ρ₁is to the secondThe predicted pseudoranges for a satellite are determined,

r is a truncation interval, expressed as a distance, applied to measurements on ranging signals from the first satellite, an

v₁Is a measured truncated pseudorange to the first satellite.

19. An apparatus (101) for determining a position at which ranging signals (RS1-RS4) from at least three satellites (SV1-SV4) are received, wherein results from timing measurements performed on said ranging signals (RS1-RS4) comprise truncated timing measurements of at least one of said satellites (SV1-SV4), and wherein said position is a priori known to be located within an initial position uncertainty area, said apparatus comprising digital data processing circuitry (CP1) adapted to:

performing at least one pseudorange selection cycle (511, 512);

determining the position using the pseudoranges selected (1004) after the at least one pseudorange selection cycle,

wherein the at least one pseudorange selection cycle comprises an initial selection cycle (511),

and wherein the digital processing circuit (CP1) is adapted to perform the initial selection loop (511) by:

determining a first pseudorange (r11) that is an allowable pseudorange associated with a first satellite (SV1) relative to the initial position uncertainty area;

determining all pseudoranges (r21, r22, r31, r32) associated with at least two further satellites (SV2, SV3), which pseudoranges in combination with the first pseudorange (r11) form an allowed relative pseudorange;

forming a set of pseudorange vectors (1001) representative of all possible combinations of said determined pseudoranges (r11, r21, r22, r31, r32) associated with said first (SV1) and at least two further satellites (SV2, SV 3);

forming a set of selected pseudorange vectors (1002) by selecting at least one vector from said set of pseudorange vectors (1001),

wherein said selecting comprises evaluating each vector in said set of pseudorange vectors (1001) according to a predetermined rule for initial vector selection, at least when said set of pseudorange vectors (1001) comprises more than one vector.

20. The device according to claim 19, wherein said data processing circuit is adapted to evaluate each vector according to said predetermined rule for initial vector selection only if said set (1001) of pseudorange vectors comprises more than one vector.

21. The apparatus according to claim 19, wherein said data processing circuit is adapted to evaluate each vector according to said predetermined rule for initial vector selection also when said set of pseudorange vectors comprises a single vector.

22. An apparatus according to any one of claims 19-21, wherein evaluation according to said predetermined rule for initial vector selection comprises comparing a position associated with said vector in said set of pseudorange vectors to said initial position uncertainty area.

23. An apparatus according to any one of claims 19-21, wherein, when said at least two further satellites comprise more than two further satellites, the evaluation according to said predetermined rule for initial vector selection comprises calculating a minimum loss function value for each vector of said set of pseudorange vectors.

24. The apparatus of claim 23, wherein a vector associated with a minimum loss function value below a predetermined threshold is selected for inclusion in the selected set of pseudorange vectors.

25. The apparatus of claim 23, wherein the vector associated with the lowest minimum loss function value is selected for inclusion in the selected set of pseudorange vectors.

26. An apparatus according to any of claims 19-21, wherein said at least one pseudorange selection cycle (511, 512) comprises at least one further selection cycle (512), and wherein said digital data processing circuitry is adapted to perform said at least one further selection cycle by:

determining all pseudoranges (r41, r42) associated with at least one further satellite (SV4), which pseudoranges in combination with the first pseudorange (r11) form an allowed relative pseudorange;

forming a new set (1003) of pseudorange vectors representing all possible combinations of pseudorange vectors selected in a previous selection cycle (511, 512) and said determined pseudoranges (r41, r42) associated with said at least one further satellite (SV 4);

forming a new set of selected pseudorange vectors (1004) by selecting at least one vector from said new set of pseudorange vectors (1003),

wherein, at least when said new set of pseudorange vectors (1003) comprises more than one vector, said selecting comprises evaluating each vector of said new set of pseudorange vectors (1003) according to a predetermined rule for subsequent vector selection.

27. An apparatus according to claim 26, wherein said data processing circuit is adapted to evaluate each vector according to said predetermined rule for subsequent vector selection only if said new set of pseudorange vectors (1003) comprises more than one vector.

28. An apparatus according to claim 26, wherein said data processing circuit is adapted to evaluate each vector according to said predetermined rule for subsequent vector selection also when said new set of pseudorange vectors comprises a single vector.

29. The apparatus of claim 26, wherein evaluation according to the predetermined rule for subsequent vector selection comprises comparing a position associated with the vector in the new set of pseudorange vectors to the initial position uncertainty area.

30. The apparatus of claim 26, wherein the evaluation according to the predetermined rule for initial vector selection comprises calculating a minimum loss function value for each vector in the new set of pseudorange vectors (1003).

31. The apparatus of claim 30, wherein a vector associated with a minimum loss function value below a predetermined threshold is selected for inclusion in the new set of selected pseudorange vectors.

32. An apparatus according to claim 30, wherein the vector associated with the lowest minimum loss function value is selected for inclusion in said new set of selected pseudorange vectors.