Position measuring method with satellite

Abstract: A method of a position measurement in which information about satellite orbits and information about time are received from satellites to determine own posi­ tion. The method includes a step to calculate a direc­ tion in which an error occurs based on a position of a satellite used (100) and a step to determine own position by use of road map information in the direction of the error (202-205).

Description: BACKGROUND OF THE INENTION The present invention relates to a position measuring method for measuring own position by use of a radio wave from a satellite. A system for measuring the own position by use of a radio wave from a satellite has been described, for example, in "Guide to GPS Positioning", Canadian GPS Associates, Fredericton, Canada 1986. Such a position measuring system is referred to as a GPS system herebelow. In this system, a position in a plane, namely, an azimuth on the surface of the earth can be measured by use of three satellites. When using four satellites, not only the measurement in a plane (for the latitude and the longitude) but also the measurement of a height (above the sea level) can be effected. If repeated position measurements are performed using such a system, measurement errors are found to be distributed within an elliptical area or an ellipsoid (depending on whether two-dimensional or three-dimensional measurements are performed), the size and shape of which may be described by a covariance matrix. Measurement errors have an amount and a direction, so that they may be considered as vectors. In the GPS system, since the measurement is accomplished by use of satellites, the precision thereof is determined by positions of the satellites. When the satellites to be used are located at positions relatively near to the own position or the condition to receive the radio waves is distributed, there arises a problem that the accuracy of the measurement is lowered. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a position measuring method in the GPS system in which an error in the measurement can be reduced and the deterioration of the measuring accuracy can be prevented. According to the present invention, this is accomplished by the methods defined in the appended claims 1 and 3. When satellites are located at unfavorable locations or the condition to receive radio waves therefrom is disturbed, the direction where the error takes place is determined depending on the arrangement of satellites from which radio waves can be received. If a correction with respect to the direction of the error can be achieved by use of another information, for example, information from a map, the error can be reduced and hence the measuring accuracy is improved. In addition to the correction of the error by use of map information, the error can also be corrected by use of a combination with another satellite. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a schematic diagram showing the principle of the position measurement in the GPS system; Fig. 2 is a diagram schematically illustrating the relationships between satellite positions and directions of errors appearing in the results of the measurement; Fig. 3 is a flowchart illustrating an operation to attain a direction of the error; Fig. 4 is a schematic diagram showing a method to correct the error of the position measurement by use of map information; Fig. 5 is a flowchart depicting a method to correct the error of the position measurement by use of map information; Fig. 6 is a diagram schematically showing a method to correct the error in the position measurement by use of a combination with another satellite; Fig. 7 is a flowchart illustrating an operation to accomplish the correction of Fig. 6; and Fig. 8 is a graph showing the relationships of the direction of the fluctuation between the actual measurement and the calculated results and those between the error of the position measurement and the error indices determined by the arrangement of the satellite. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, an embodiment of the present invention will be described. Fig. 1 is a schematic diagram showing the principle of the position measurement in the GPS system including satellites 1-4 used in the GPS system and own position 5. In this system, positions of the satellites are calculated from data sent from the satellites, the data concerning orbits of the satellites; furthermore, the period of time necessary for the radio wave to travel from the satellite to the measuring position is measured for each satellite so as to attain a distance from the satellite to the measuring position, thereby solving a system of equations of position measurement to obtain the own position. It has been found in this situation that the error in the position measurement is closely related to the arrangement of the satellites. According to the calculated results, for a favorable arrangement of the satellite, the absolute error of the position measurement is about 30 m; whereas for an unfavorable arrangement of the satellites, the absolute error of the position measurement is at least 200 meters (m). However, the fluctuation of the measured results in a case of an unfavorable arrangement of the satellite, namely, the error of the position measurement is found to be attended with an orientation determined by the satellite arrangement. Fig. 2 shows an arrangement of the satellites in which reference numerals 6-8 each indicate satellite positions. In this diagram, the outer-most circle represents an angle of elevation 0 and the center of the circle is the zenith. The direction of the fluctuation ϑ can be calculated from the satellite arrangement. Fig. 3 is a flowchart showing the calculation method of the fluctuation direction ϑ. Although Fig. 2 illustrates a case where a 2-dimensional position measurement is achieved by use of three satellites, the calculation method smilarly applies to a case where a 3-dimensional position measurement is effected by use of four satellites. In computation of the direction of the fluctuation (i.e. the direction of the error) is achieved according to the flowchart of Fig. 3. First, in step 100, the positions of the satellites are calculated. The method of the position calculation has already been known. Next, a unit vector from the own position to the satellite is calculated for each satellite in step 101. It is assumed here that an x axis, a y axis, and a z axis are respectively drawn in the directions to the east, north, and zenith viewed from the own position. Step 102 obtains a matrix , which is then used in step 103 to calculate a covariance matrix . Step 104 calculates the fluctuation direction ϑ from the elements of the covariance matrix . Although Fig. 3 shows a case where four satellites are used, if a 2-dimensional position measurement is accomplished with three satellites, the matrix of the step 102 becomes to be and the fluctuation direction ϑ is calculated through the steps 103-104. Fig. 4 shows a method to correct measured results by use of the fluctuation direction and road map information, whereas Fig. 5 is a flowchart of the correction method. Assume that the result of the computation of the position measurement indicates point 14 in step 200. Step 201 judges whether or not the point 14 is on a road. Incidentally, reference numerals 9-10 represent roads in Fig. 4. If the point 14 is not on the road, step 100 is executed to attain the fluctuation direction ϑ. In Fig. 4, the figure enclosed with a dotted line 11 indicates a distribution of the measured results. In step 202, a direct line 12 having a direction ϑ is drawn to pass the point 14. When this direct line 12 has an intersection with respect to a road, the intersection is regarded as the own position. However, since the direct line 12 generally intersects a plurality of roads, a direct line 13 is drawn to pass the points 15 and 16 representing the results of the last measurement and the second last measurement, respectively. Step 204 then attains an intersection 17 between the direct lines 12 and 13. In step 205, a point 18 on the direct line 12 and nearest to the point 17 is recognized to be the own position, and then step 206 displays the point 18 as the own position. Fig. 6 shows a method in which the position measurement is achieved by use of two satellites and the measurement results are corrected depending on the combination of the satellites; whereas Fig. 7 is a flowchart illustrating the operation of the method. Step 301 selects a combination of two satellites and then step 302 effects the respective operations for the position measurement so as to attain points 19-20 as results of the point measurements. In the routine 100, the fluctuation directions ϑ₁ and ϑ₂ are attained from locations of the satellites 1-2 of the combination. Dotted lines 23-24 respectively indicate distributions of the results of the position measurements. In step 303, two direct lines 21-22 respectively having the fluctuation directions are drawn to pass the results of the position measurements. Step 304 obtains an intersection 25 of the two direct lines 21-22 as the own position. Fig. 8 is a graph showing relationships of the fluctuation direction between the measured values and the calculated values and those between the errors of the position measurements and the error indices determined by the satellite arrangement. The estimated values of the fluctuation direction ϑ substantially agree with the measured values thereof for the error index equal to or greater than ten, which enables to confirm that the fluctuation angle ϑ is appropriately specified. Incidentally, the errors of the position measurements are indicated by the values before the correction in this graph. The error index is substantially proportional to the measurement error and the error of the position measurement is at least 200 m when the error index is at least 30. When the correction method according to the present invention is applied to the results of the position measurement, the error of the position measurement after the correction can be reduced to about 50 m as indicated by a shade in Fig. 8. According to the present invention, in a case where the own position is determiend by use of the GPS system, the fluctuation directions of the results of the position measurements can be beforehand estimated from the arrangement of satellites so as to correct the results of the position measurements, which loads to an effect that the absolute error of the own position thus attained can be minimized.

Claims: A method of determining the position of an object on a road map comprising the steps of: a) receiving at the object orbit data from a plurality of satellites and measuring at said object elapse times between transmitting a signal from each of said satellites and receiving said signal at said object; b) determining from said orbit data the respective positions of said satellites and then determining the position of the object using the determined positions of said satellites and said elapse times; characterized by further comprising the steps of: c) calculating that direction in which the largest error in said determined object position occurs; d) correcting said determined object position by choosing an appropriate position of said object on said map along a first straight line passing through said determined position and extending in said calculated direction. A method according to claim 1, characterized by further comprising the steps of: determining a second straight line passing through the last and second last determined positions of said object; finding the point of intersection between said first and second straight lines; correcting said determined object position by choosing as the corrected position of said object on said map that point on a road which lies on said first straight line and which is closest to said intersection between said first and second straight lines. A method of determining the position of an object comprising the steps of: a) receiving at the object orbit data from a first combination of satellites from a set of satellites and measuring at said object elapse times between transmitting a signal from each of said satellites in said first combination and receiving said signal at said object; b) determining from said orbit data the respective positions of said satellites and then determining the position of the object using the determined positions of said satellites in said first combination and said elapse times; characterized by further comprising the steps of: c) calculating that direction in which the largest error in said determined object position occurs; d) performing steps a) to c) for a second combination of satellites from said set of satellites; e) choosing as the correct position of the object the point of intersection of a first straight line and a second straight line, said first straight line passing through the position of the object as first calculated and extending in the calculated direction and said second straight line passing through the position of the object as calculated in step d) and extending in the calculated direction.

Inventors: KURIHARA NOBUO, KURODA HIROSHI, SAKAMOTO SHINICHI, KURIHARA, NOBUO, KURODA, HIROSHI, SAKAMOTO, SHINICHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1987
Main classification: G01S5
IPC codes: G01C21, G01S19