Position calculation method and storage medium storing a program therefor

[0001] The present application claims priority from Japanese application JP2004-278656 filed on Sep. 27, 2004, the content of which is hereby incorporated by reference into this application.

[0002] 1. Field of the Invention

[0003] The present invention relates to a position calculation method. More particularly, it relates to a method for calculating position of a radio device on the basis of distance-ranging results using electromagnetic waves.

[0004] 2. Description of the Related Art

[0005] For example, according to Atushi Tuchiya and Hiromichi Tuji, “Foundation of GPS Measurement”, Japan Measurement Association, 1995, a GPS receiver receives signals from a plurality of GPS satellites. Moreover, these respective reception signals allow position of the GPS receiver to be detected.

[0006] Also, according to JP-A-2004-101254, a plurality of radio base stations receive signals from a radio terminal. Moreover, these respective reception signals allow position of the radio terminal to be detected.

[0007] In both of the above-described technologies, as illustrated in FIG. 1, based on measurements of reception timings of radio signals between each of a plurality of radio devices (hereinafter, referred to as “reference stations”) 101, 102, . . . , 10n whose positions P_i (i=1, 2, . . . , n) have been already known and a radio device (hereinafter, referred to as “a detection-target station”) 100 whose position P has been unknown, distances between each of the reference stations and the detection-target station are measured, letting the distances be R_i (i=1, 2, . . . , n) respectively. Furthermore, the position P is calculated based on the above-described R_i (i=1, 2, . . . , n) and P_i (i=1, 2, . . . , n).

[0008] Between the above-described two technologies, there exists a difference that the sides for measuring the reception timings differ from each other. In principle, however, the position P is acquired as the solution of the following simultaneous equations, which are designated as Expression 1.
{R_i+cT−∥P_i−P∥}=0, i=1, . . . , n  Expression 1

[0009] In Expression 1, ||x|| denotes magnitude of a vector x, c denotes speed of the radio waves, and T denotes offset of a clock selected as the reference when the distances R_i (i=1, 2, . . . , n) are measured.

[0010] In Expression 1, the unknowns are P and T. In addition, the above-described solution can be acquired in the case where n≧3 if P is a point on a two-dimensional space, and in the case where of n≧4 if P is a point on a three-dimensional space. By the way, when operating the radio position detection systems like this in environments such as indoors where reflections of the radio signals can happen easily, waveform distortions in the radio signals are likely to occur because of multiple signal reflections. On account of this, the measurements of the respective reception timings of the radio signals, i.e., the measurements of the distances between each of the reference stations and the detection-target station, turn out to contain large quantities of offsets. As a result, offset in the position calculation result P increases in quantity.

[0011] Accordingly, it is an object of the present invention to reduce the offset in the calculation position of the detection-target station in the multipath environment by predicting the offsets contained in the measurements of the distances between each of the reference stations and the detection-target station.

[0012] In order to accomplish the above-described object, a position calculation method according to the present invention includes the following steps: Calculating error degradation quantities about the respective measurement results on the distances between the respective reference stations and the detection-target station (hereinafter, referred to as “distance-ranging results”) from the respective distance-ranging results and the positions of the respective reference stations, evaluating the respective distance-ranging results based on the respective error degradation quantities, and determining weight-assigning factors about the respective distance-ranging results, checking the number of valid distance-ranging results, and calculating the position of the detection-target station based on the respective distance-ranging results, the positions of the respective reference stations, and the respective weight-assigning factors.

[0013] According to the present invention, the offsets are predicted which are contained in the measurements of the distances between the respective reference stations and the detection-target station, thereby controlling influences which the offsets exert on the position calculation. This makes it possible to reduce the offset contained in the position calculation result.

[0014] Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

[0022] First, assume that Expression 1 is overdetermined simultaneous equations (i.e., simultaneous equations where the number of the equations is larger than the number of unknowns). Namely, assume that n≧4 holds if P is a point on a two-dimensional space, and that n≧5 holds if P is a point on a three-dimensional space.

[0023] Next, quoting the notations in FIG. 1 and Expression 1 and referring to FIG. 2, the explanation will be given below concerning an embodiment of the position calculation method according to the present invention.

[0024] In FIG. 2, S11 is a step of calculating a reference error E₀ based on the respective distance-ranging results and the positions of the respective reference stations. At the present step, as indicated in the following Expression 2, the above-described reference error E₀ is given as the minimum value of an error function E (P, T) with P and T selected as the variables, or an approximate value of the minimum value.

[0025] $\begin{array}{cc}{E}_0=\underset{P,T}{\min }\left(E\left(P,T\right)\right),E\left(P,T\right)=\sum _{k=1}^n{W}_k{\left\{R_k+cT-P_k-P\right\}}^2& \mathrm{Expression}2\end{array}$

[0026] In the same expression, assume that weight-assigning factors W_k are equal to 1 all (i.e., W_k=1, k=1, . . . , n). If, however, quality of the measurements on the distance-ranging results R_k can be acquired, the weight-assigning factors W_k may be defined as being positive numbers or 0 as numerical values for indicating the quality of the measurements on the distance-ranging results R_k. In this case, a function value may be given which will increase for the quality of the measurements. Here, the minimum value is defined as being 0. Moreover, in this case, a limitation may be imposed such that total sum of W_k up to k=1, . . . , n becomes equal to a constant number (e.g., n).

[0027] Incidentally, a concrete method for determining the reference error E₀ is equivalent to the method for solving the function minimization problem. Accordingly, this concrete method has been publicly known, and thus will be omitted.

[0028] Also, when determining the reference error E₀, if P_prv and T_prv satisfying the following Expression 3 had been acquired in the past attempt, the reference error E₀=E(P_prv, T_prv) may be definable.

[0029] $\begin{array}{cc}\underset{P,T}{\min }\left(E\left(P,T\right)\right)\approx E\left(P_{\mathrm{prv}},T_{\mathrm{prv}}\right)& \mathrm{Expression}3\end{array}$

[0030] S12 is a step of calculating error degradation quantities about the respective distance-ranging results. At the present step, the error degradation quantities ΔE_i about the respective distance-ranging results R_i are given by the following Expression 4.
ΔE_i=E₀−E_i0, i=1, . . . , n  Expression 4
Here, as indicated in the following Expression 5 and Expression 6, E_io is given as the minimum value of an error function E_i (P, T) with P and T selected as the variables, or an approximate value of the minimum value.

[0031] $\begin{array}{cc}{E}_i\left(P,T\right)=\sum _{\underset{k\ne i}{k=1}}^n{W}_k{\left\{R_k+cT-P_k-P\right\}}^2,& \mathrm{Expression}5\\ i=1,\dots ,n& \\ E_{\mathrm{i0}}=\underset{P,T}{\min }\left(E_i\left(P,T\right)\right),i=1,\dots ,n& \mathrm{Expression}6\end{array}$

[0032] Also, for simplifying the calculation, E_io may be given by Expression 5 and the following Expression 7. Here, P and T yielding the reference error E₀ are set as being P₀ and T₀, respectively.
E_i0=E_i(P₀, T₀) E(P₀, T₀)=E₀, i=1, . . . , n  Expression 7

[0033] It can be interpreted that the error degradation quantities ΔE_i indicate quantities of errors by which the distance-ranging results R_i influence the reference error E₀.

[0034] S13 is a step of evaluating the respective distance-ranging results based on the respective error degradation quantities calculated at the preceding step, and updating the weight-assigning factors about the respective distance-ranging results.

[0035] FIG. 3 illustrates an example of the details of carrying-out of the above-described step. In the same drawing, S1311 to S1315 indicate the details of S13 in FIG. 2. These steps make comparisons between a threshold value Th3 and the error degradation quantities ΔE_i for i=1, . . . , n. Then, the steps update, to 0, the weight-assigning factors W_i about the error degradation quantities ΔE_i which have exceeded the threshold value Th3.

[0036] Here, updating the weight-assigning factors W_i to 0 means to eliminate the distance-ranging results R_i in the position calculation later. The reason for this elimination is that, based on the error degradation quantities ΔE_i, the error contained in the distance-ranging results R_i has been evaluated as being large up to the degree of having exceeded the limit. Incidentally, at the present step, the threshold value Th3 may be determined in advance. Otherwise, the threshold value Th3 may be determined based on the number n of the distance-ranging results. Also, values of the weight-assigning factors W_i may be updated such that, for example, the values become functions which will decrease with respect to increases in ΔE_i (however, its minimum value is made equal to 0).

[0037] S14 is a step of checking the valid distance-ranging-results number. The valid distance-ranging-results number refers to the number of the distance-ranging results which remain intact without being eliminated at the preceding step.

[0038] FIG. 4 illustrates an example of the details of carrying-out of the above-described step. At first, the number of W_i (i=1, . . . , n) which have turned out to become non-zero at the preceding step, i.e., the valid distance-ranging-results number Nvr, is determined (S1411 to S1415). Moreover, a comparison is made between a threshold value Th4 determined in advance and the valid distance-ranging-results number Nvr (S1416). Then, if the valid distance-ranging-results number Nvr has been found to be larger than the predetermined value (i.e., Nvr≧Th4), the processing proceeds to the next step S15.

[0039] Otherwise, the processing may be aborted, judging that the number of the valid distance-ranging results has been not prepared in the necessary number. In this case, the failure of the position calculation is reported to a higher-order function layer which is controlling the present step.

[0040] S15 is a step of calculating the position of the detection-target station based on the respective valid distance-ranging results and the positions of the respective reference stations. At the present step, using the weight-assigning factors W_k (k=1, . . . , n) updated at S13, P and T are determined which will yield the minimum value of the error function E (P, T) indicated in Expression 2. Then, the determined P is outputted as calculation result of the position of the detection-target station.

[0041] Next, quoting the notations in FIG. 1 and Expression 1 and referring to FIG. 5, the explanation will be given below concerning a second embodiment of the position calculation method according to the present invention.

[0042] In FIG. 5, respective steps from S21 to S24 are the same as the respective steps from S11 to S14 in FIG. 2, respectively. The details of these steps have been already explained in the first embodiment, and thus will be omitted.

[0043] S25 is a step of calculating the position of the detection-target station based on the respective valid distance-ranging results and the positions of the respective reference stations. At the present step, using the weight-assigning factors W_k (k=1, . . . , n) updated at S23, P and T are determined which will yield the minimum value of the error function E (P, T) indicated in Expression 2. Then, the processing proceeds to the next step.

[0044] S26 is a step of calculating Dilution of Precision (DOP) based on the position P of the detection-target station determined at the preceding step and the positions of the respective reference stations. In the calculation of DOP, however, the positions P_k (k=1, 2, . . . , n) of the reference stations will be eliminated for which the weight-assigning factors W_k (k=1, 2, . . . , n) updated at S23 become equal to 0. Incidentally, as DOP, HDOP (Horizontal Dilution of Precision) will be calculated if P is a point on a two-dimensional space, and PDOP (Position Dilution of Precision) will be calculated if P is a point on a three-dimensional space. The calculation method for HDOP and PDOP has been publicly known, and thus will be omitted.

[0045] S26 is a step of checking the Dilution of Precision (DOP) determined at the preceding step. At the present step, a comparison is made between a threshold value Th5 determined in advance and the Dilution of Precision (DOP) determined at the preceding step. Then, if the Dilution of Precision (DOP) has been found to be smaller than the predetermined value (i.e., DOP≦Th5), it is judged that the DOP has passed the above-described checking. Accordingly, the determined P is outputted as calculation result of the position of the detection-target station.

[0046] Otherwise, the processing may be aborted, judging that the DOP has failed in the above-described checking. In this case, the failure of the position calculation is reported to a higher-order function layer which is controlling the present step.

[0047] FIG. 6 is a configuration diagram of the position detection system relating to the present invention. In the same drawing, respective reference stations receive radio signals from a detection-target station. Then, distance-ranging units of the respective reference stations measure physical quantities for indicating distances up to the detection-target station. Here, the physical quantities for indicating the distances include RSS (Received Signal Strength) of a radio signal from the detection-target station, or TDOA (Time Difference of Arrival) of the radio signal from the detection-target station.

[0048] An input unit 611 includes a configuration component such as, e.g., NIC (Network Interface Card). The input unit 611 receives, via a network, the above-described respective physical quantities for indicating the distances from the respective reference stations. A processing unit 612 includes a configuration component such as, e.g., a CPU (Central Processing Unit) for executing programs. The processing unit 612 acquires the above-described measured physical quantities from the input unit 611, then making the conversion to the respective distances R_i (i=1, 2, . . . , n) as required. In addition thereto, the processing unit 612 acquires the positions P_i (i=1, 2, . . . , n) of the reference stations from a memory unit 613 so as to execute the position calculation program illustrated in FIG. 2 or FIG. 5, then outputting the position calculation result to the memory unit 613. The memory unit 613, which includes a configuration component such as, e.g., a hard disc or memory, stores therein the positions P_i (i=1, 2, . . . , n) of the reference stations and the position calculation result P. An output unit 614 includes a configuration component, such as, e.g., NIC (Network Interface Card) or an external output port to a display or printer. The output unit 614 outputs the position calculation result stored in the memory unit 613.

[0049] FIG. 7 is another configuration diagram of the position detection system relating to the present invention. In the same drawing, a detection-target station receives radio signals from respective reference stations, respectively. Then, a distance-ranging unit of the detection-target station measures physical quantities for indicating distances up to the respective reference stations. Here, the physical quantities for indicating the distances include RSS (Received Signal Strength) of a radio signal from a reference station, or TDOA (Time Difference of Arrival) of the radio signal from the reference station.

[0050] An input unit 711 includes a configuration component such as, e.g., a serial communications module. The input unit 711 receives the above-described respective physical quantities for indicating the distances from the distance-ranging unit. A processing unit 712 includes a configuration component such as, e.g., a CPU (Central Processing Unit) for executing programs. The processing unit 712 acquires the above-described physical quantities from the input unit 711, then making the conversion to the respective distances R_i (i=1, 2, . . . , n) as required. In addition thereto, the processing unit 712 acquires the positions P_i (i=1, 2, . . . , n) of the reference stations from a memory unit 713 so as to execute the position calculation program illustrated in FIG. 2 or FIG. 5, then outputting the position calculation result to the memory unit 713. The memory unit 713, which includes a configuration component such as, e.g., a hard disc or memory, stores therein the positions P_i (i=1, 2, . . . , n) of the reference stations and the position calculation result P. An output unit 714 includes a configuration component, such as, e.g., NIC (Network Interface Card) or an external output port to a display or printer. The output unit 714 outputs the position calculation result stored in the memory unit 713.

[0051] The position calculation method according to the present invention, in a multipath environment in particular, is effective in reducing an error of position calculation of a radio device based on distance-ranging results using electromagnetic waves. Consequently, the position calculation method, in an indoors environment, serves satisfactorily to enhance a precision of position detection of a radio terminal.

[0052] It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.