Method and program for generating correction information in a satellite navigation system

Abstract

To eliminate the influence of station position fluctuations in a satellite navigation system. [Solution] In satellite navigation systems, including the US GPS and Japan's Quasi-Zenith Satellite System, when multiple reference stations are installed to form a wide-area differential correction system (WADGPS), previously, position errors caused by fluctuations in station position would remain in the position information calculated by the user receiver.However, by adding a correction amount to the correction information provided by WADGPS in advance to cancel this, the position errors caused by fluctuations in station position at the user station can be reduced without changing the calculation processing content at the WADGPS user station, thereby improving the accuracy of the position information calculated by the user station.

Description

[Technical Field]

[0001] The present invention relates to a method and a program for generating correction information in a satellite navigation system. [Background technology]

[0002] Satellite navigation systems that use satellites to measure position are collectively called GNSS (Global Navigation Satellite Systems), and a representative example is the US-based GPS (Global Positioning System). GNSS generally uses a receiver to receive positioning signals transmitted by radio from satellites called navigation satellites, and measures the distance between the navigation satellite and the receiver to calculate the receiver's position. The distance between the navigation satellite measured by the receiver is called the pseudo-distance. A receiver that uses GNSS to determine its position is called a satellite navigation receiver, GNSS receiver, or user receiver. Equipment that combines a GNSS receiver and a receiving antenna is called a satellite navigation receiving station, GNSS receiving station, user station, or simply a receiving station. The error between the determined position and the true position is called the positioning error. The accuracy with which the position is measured is called the positioning precision.

[0003] To calculate the receiver's position, it is necessary to know the position of the navigation satellite that is transmitting the positioning signal, and the orbital information required for this is transmitted by the navigation satellite itself by superimposing it on the positioning signal. Because the orbital information is created by prediction, it contains an error of several meters, and this position error becomes a factor in positioning error when calculating the receiver's position.

[0004] The timing at which a navigation satellite transmits a positioning signal is predetermined, and the navigation satellite transmits the positioning signal based on the time on its own clock. This clock uses a highly accurate atomic clock, but it contains an error equivalent to a few meters in distance, and this clock error becomes a factor in positioning error when calculating the receiver's position.

[0005] Before reaching the ground, a positioning signal passes through the ionosphere and troposphere above, and delays occur as the radio signal passes through each of these regions. These delays are called the ionospheric propagation delay and the troposphere propagation delay, respectively. Therefore, when this radio signal is used as a positioning signal, these ionospheric propagation delay and troposphere propagation delay become a cause of positioning errors. The magnitude of the ionospheric propagation delay and the troposphere propagation delay converted into distance is called the ionospheric propagation delay and the troposphere delay, respectively.

[0006] The amount of ionospheric propagation delay is proportional to the total number of free electrons in the ionospheric atmosphere along the signal propagation path (the integral of the ionospheric electron density over the signal propagation path), and has the property of being inversely proportional to the square of the signal frequency. Since the signal path is the same for the same navigation satellite and receiving station, there is a difference between pseudoranges obtained using positioning signals of multiple frequencies that is inversely proportional to the square of the signal frequency due to ionospheric propagation delay. Since there are no error factors with this property for pseudoranges other than ionospheric propagation delay, the amount of ionospheric propagation delay along the signal propagation path between the navigation satellite and the receiving station can be calculated by using positioning signals of multiple frequencies.

[0007] In a satellite navigation system, the measured distance to a navigation satellite obtained by a receiver is the sum of all measurement errors due to these error factors compared to the true distance.

[0008] A receiver is installed at a fixed reference station on the ground, and the distance measured by this is used to create correction information for distance measurement errors. This information is then provided to the user, who can then correct the distance measured at the user station based on the correction information, improving the positioning accuracy of the user station. This method is called Differential GPS (DGPS). To distinguish it from DGPS, the method of calculating the position of a user station without applying correction information is called standalone positioning.

[0009] There are several specific DGPS methods, but the most common method uses distances measured by a single reference station to generate distance corrections for each navigation satellite and provide them to user stations. This method, sometimes called Local Area DGPS (LADGPS), does not create correction information for each cause of positioning error. Therefore, as the distance between the user station and the reference station increases, the common component of the positioning error decreases, and positioning accuracy tends to deteriorate. It is generally recognized by those skilled in the art that DGPS can be used within a range of approximately several hundred kilometers from the reference station, or up to several tens of kilometers depending on ionospheric conditions.

[0010] Another DGPS method is Wide Area Differential GPS (WADGPS). This method uses distance measurements from multiple reference stations (called reference station groups) to generate correction information for each positioning error factor, such as the navigation satellite's clock error, the navigation satellite's position error, ionospheric propagation delay, and tropospheric propagation delay, and provides this information to the user station. Since each of these error factors manifests itself differently as distance measurement errors depending on the user station's location, the user station calculates the correction value it should use from the correction information according to its approximate location and uses it for correction.

[0011] The clock error of a navigation satellite appears as a uniform distance measurement error regardless of the location of the user station. The position error of a navigation satellite appears as a distance measurement error due to the dot product with the line of sight when the navigation satellite is viewed from the user station. The ionospheric propagation delay appears as a distance measurement error due to the delay proportional to the total number of electrons in the ionospheric atmosphere on the path from the ranging signal transmitted by the navigation satellite to the user station. The tropospheric propagation delay appears as a distance measurement error due to the delay proportional to the integral of the refractive index of the neutral atmosphere on the path from the ranging signal transmitted by the navigation satellite to the user station. In other words, except for the clock error of the navigation satellite, the distance measurement error appears differently depending on the location of the user station.

[0012] Of these, ionospheric propagation delay is inversely proportional to the square of frequency, so if user stations and reference stations measure distance using positioning signals of multiple frequencies, they can obtain distance excluding ionospheric propagation delay as a linear combination of these measurement results. Furthermore, tropospheric propagation delay can be estimated with sufficient accuracy using a simple tropospheric propagation delay model, and corrections can be made at user stations and reference stations. Therefore, by providing user stations with the clock error and position error of navigation satellites, WADGPS can provide effective correction services to user stations over a wide geographical area.

[0013] The geographical area in which WADGPS provides its services is called the service area. WADGPS correction information is generated based on the distance measured by each reference station in the group of reference stations, so the correction information is generated so that the correction effect can be obtained at the positions of all reference stations in the group of reference stations. The geographical area in which the correction information is valid is inside and around the geometric shape formed by the group of reference stations. Therefore, WADGPS reference stations must be placed so that they generally cover the service area.

[0014] As a practical example of WADGPS, SBAS (Satellite-Based Augmentation System) has been standardized for aircraft. SBAS currently in operation include Japan's MSAS, the United States' WAAS, Europe's EGNOS, India's GAGAN, and South Korea's KASS. [Prior art documents] [Non-patent literature]

[0015] [Non-Patent Document 1] Sinko, J.: A Compact Earth Tides Algorithm for WADGPS, Proceedings of ION GPS 1995, pp.35-44, 1995 [Non-patent document 2] Arata Sengoku and Tadahiro Sato: Tidal Displacements at Domestic SLR, VLBI, and GPS Observation Sites and Their Effects on Baselines, Ocean Survey Technology, No. 7, pp. 1-7, 1995 Summary of the Invention [Problem to be solved by the invention]

[0016] WADGPS uses distances measured by a group of reference stations to generate correction information for each cause of positioning error, such as clock error of the navigation satellite and position error of the navigation satellite, and provides this information to user stations.

[0017] The solid Earth (crust, mantle, and core) is constantly deforming due to tidal forces and other factors, and the resulting movement of GNSS receiving stations is called station position fluctuation. There are several factors that cause station position fluctuation, but the dominant component in each case is a daily or half-day period. In the Earth-fixed coordinate system normally used for GNSS position calculations, station position fluctuation is corrected, so the fixed point always has a constant Earth-fixed coordinate value.

[0018] The primary factor in station position fluctuations is solid earth tides. Tidal forces due to the gravitational pull of the moon and sun cause tides in the oceans, but they also act on the solid earth. The resulting deformation of the solid earth is called solid earth tide, and is modeled as a function of the time in local time and the latitude of the observation point. The magnitude is said to be up to about 30 centimeters in the vertical direction and 7 centimeters in the horizontal direction, and an accurate model is based on spherical harmonics, but a simple model has been proposed for GNSS; for example, Non-Patent Document 1 can be used.

[0019] The second factor in station position fluctuations is ocean loading. When ocean mass shifts due to tides, its weight causes deformation of the solid Earth. This effect is called ocean loading, and although it depends on the shape of the land area, it is said that the maximum amount of fluctuation in coastal areas can reach about 10 centimeters. According to Non-Patent Document 2, it has been reported that in Japan it is up to about 7 centimeters.

[0020] Other known factors that cause station position fluctuations include deformation of the solid Earth due to various loads, but the magnitude of these fluctuations is thought to be less than about 2.5 centimeters. The magnitude and direction of station position fluctuations are a function of the station location, but because the cause is deformation of the solid Earth, they are nearly identical between observation stations that are, for example, less than about 100 kilometers apart. As the distance between observation stations increases, differences in station position fluctuations appear, but the station position fluctuations do not change suddenly with changes in distance.

[0021] The reference stations used by WADGPS also undergo fluctuations in their respective positions. Navigation satellites do not experience fluctuations in their positions, so when calculating the position error of a navigation satellite, it is necessary to remove the components due to fluctuations in their positions. A simple calculation model for solid earth tides is given in Non-Patent Document 1.

[0022] The position error of the navigation satellite provided by WADGPS is calculated after removing the component due to the station position fluctuation, so the calculated position of the user station does not include the station position fluctuation. Not considered However, in reality, the user station's position will fluctuate, and this difference will appear as a position error at the user station.

[0023] That is, the following relationship exists between the coordinate value X without correction for station position fluctuation and the coordinate value Y with correction for station position fluctuation, where the station position fluctuation is ΔX.

[0024] (Number 1) Y+ΔX(X)=X

[0025] Since there is no station position fluctuation in the navigation satellite, the position of the navigation satellite is not corrected for station position fluctuation, and the calculated position of the user station shows X. To display this in the Earth-fixed coordinate system normally used for GNSS position calculations, the station position fluctuation must be corrected, so Y must be calculated using the following formula.

[0026] (Number 2) Y=X-ΔX(X)

[0027] Since such calculations are not defined in SBAS, for example, a position error equivalent to ΔX remains in the position information calculated by the user receiver. In other words, the position error generated at a WADGPS user station includes a component caused by station position fluctuations.

[0028] One way to eliminate this and output position information based on an Earth-fixed coordinate system that has been corrected for station position fluctuations to the user receiver is to calculate and correct the station position fluctuations within the user station. However, this method requires changes to the calculation process of the user station.

[0029] The object of the present invention is to reduce position errors that occur in a user station due to fluctuations in the station position without changing the calculation processing content in the WADGPS user station. [Means for solving the problem]

[0030] The position information calculated by the user receiver contains a residual position error equivalent to the station position variation ΔX. One method of removing this error and outputting position information based on an Earth-fixed coordinate system that has been corrected for the station position variation to the user receiver is to calculate and correct the station position variation within the user station, but this method requires changes to the calculation process of the user station.

[0031] Another method is to add a correction amount to the correction information provided by WADGPS in advance to correct for station position fluctuations, which does not require any changes to the user station's calculations.

[0032] If the coordinates of reference station k are expressed as a three-dimensional vector Xk, the station position fluctuation at reference station k can be written as a three-dimensional vector ΔX(Xk). Due to the influence of the station position fluctuation, the pseudorange measured at reference station k changes by the following ΔP: e(i,k) is the unit vector from reference station k to navigation satellite i, and the symbol "'" represents transposition.

[0033] (Number 3) ΔP(i,k)=-e(i,k)'·ΔX(Xk)

[0034] To add a correction amount to the correction information provided by WADGPS in advance to remove the position error equivalent to the station position fluctuation ΔX, we consider adding a correction amount ΔS(i) to the satellite position correction information for navigation satellite i. To correct the change in (Equation 3), we simply determine the correction amount ΔS(i) so that the following equation is satisfied.

[0035] (Number 4) e(i,k)' ΔS(i)=ΔP(i,k)

[0036] If (Equation 4) is solved simultaneously for all reference stations visible to navigation satellite i, ΔS(i) can be determined by the least squares method. Since the unknown vector ΔS(i) is a three-dimensional vector, if three or more reference stations are visible, ΔS(i) can be determined by a simultaneous equation based on (Equation 4).

[0037] Considering only a certain reference station (in reality, (Equation 4) cannot be solved at a single reference station, but the nature of the solution that can be inferred from the form of the formula is interpreted for a specific reference station), ΔS(i) is determined so that adding ΔS(i) to the satellite position causes the satellite position to move in the opposite direction of ΔX. Since the same effect applies to all navigation satellites, the position calculated by the user receiver will move in the opposite direction of ΔX, just like the navigation satellite. This achieves the correction of (Equation 2).

[0038] In reality, ΔS(i) is determined using all reference stations visible to navigation satellite i, but because the difference in station position fluctuations does not become so large even when the distance between reference stations is large, ΔS(i) is determined to have the effect of correcting station position fluctuations not only for a specific reference station but for all reference stations used in the calculation. Since the satellite position correction information for navigation satellite i is generated using reference stations visible to navigation satellite i among the WADGPS reference stations, its effective range is the geographical range within the service area within which navigation satellite i is visible, and ΔS(i) can be expected to have the effect of correcting station position fluctuations for user stations within the service area that are visible to navigation satellite i, in the same way as for reference stations.

[0039] In other words, by calculating ΔS(i) for all navigation satellites and adding this to the satellite position correction information, the position error corresponding to the station position fluctuation ΔX is removed from the position information calculated by the user receiver, and the user receiver outputs position information that has been corrected for the station position fluctuation.

[0040] As a factor in station position fluctuation, it is sufficient to consider the solid earth tides, but ocean loading may also be taken into account.

[0041] Non-patent document 1 provides a simple formula for calculating solid earth tides, but does not describe a method for removing position errors corresponding to station position fluctuations from the position information calculated by the user receiver.

[0042] Non-patent document 2 reports on ocean loads in Japan, but does not describe a method for removing position errors corresponding to station position fluctuations from the position information calculated by the user receiver.

[0043] The invention of claim 1 is a method for generating correction information in a satellite navigation system that performs wide-area differential correction, comprising: a plurality of navigation satellites that transmit positioning signals; a user station that receives the positioning signals transmitted by the plurality of navigation satellites and measures the distance between them; a reference station that receives the positioning signals transmitted by the plurality of navigation satellites using a receiver fixed on the ground and measures the distance between them; and a master station that uses the distances measured by the reference station to generate correction information for each cause of positioning error, such as clock error and position error of the navigation satellite, and provides this correction information to the user station, wherein when generating correction information for the position error of the navigation satellite, the master station adds in advance a correction amount that cancels out fluctuations in the position of the user station due to fluctuations in the station position.

[0044] The invention of claim 2 is a method for generating correction information in the satellite navigation system of claim 1, characterized in that fluctuations in the station position are accounted for due to solid earth tides.

[0045] The invention of claim 3 is a method for generating correction information in the satellite navigation system of claim 1, characterized in that fluctuations in the station position are accounted for due to solid earth tides and ocean loads.

[0046] The invention of claim 4 is a program for generating correction information in a satellite navigation system that performs wide-area differential correction, comprising: a plurality of navigation satellites that transmit positioning signals; a user station that receives the positioning signals transmitted by the plurality of navigation satellites and measures the distance between them; a reference station that receives the positioning signals transmitted by the plurality of navigation satellites using a receiver fixed on the ground and measures the distance between them; and a master station that uses the distances measured by the reference station to generate correction information for each cause of positioning error, such as clock error and position error of the navigation satellite, and provides this correction information to the user station, wherein, when generating correction information for the position error of the navigation satellite, the program operates in the master station and adds in advance a correction amount that offsets fluctuations in the position of the user station due to fluctuations in the station position.

[0047] The invention of claim 5 is a program for generating correction information in the satellite navigation system of claim 4, characterized in that fluctuations in the station position are accounted for due to solid earth tides.

[0048] The invention of claim 6 is a program for generating correction information in the satellite navigation system of claim 4, characterized in that fluctuations in the station position are accounted for due to solid earth tides and ocean loads. [Effects of the Invention]

[0049] The inventions of claims 1 to 6 are configured as described above, and by adding a correction amount to correct station position fluctuations in advance to the correction information provided by WADGPS, it is possible to reduce position errors that occur in user stations due to station position fluctuations without changing the calculation processing content in the WADGPS user station. [Brief explanation of the drawings]

[0050] [Figure 1] 1 is a schematic diagram illustrating an embodiment of the present invention, illustrating a method and program for generating correction information in a satellite navigation system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0051] Specific embodiments of the present invention will be described in detail below with reference to the drawings. [Example]

[0052] An embodiment of the present invention will be described in detail with reference to FIG.

[0053] Each of the navigation satellites 1 (1a, 1b, etc.) transmits a positioning signal.

[0054] The reference stations 2 (2a, 2b, etc.) receive the positioning signals transmitted by the navigation satellites 1 (1a, 1b, etc.) and measure the distance from each navigation satellite.

[0055] The position of reference station 2 (2a, 2b...) actually fluctuates due to station position fluctuation 3 (3a, 3b...). Coordinate value 4 (4a, 4b...) corrected for station position always shows a constant value, but coordinate value 2 (2a, 2b...) to which station position fluctuation 3 (3a, 3b...) has been added is not a constant value because it has not been corrected for the station position fluctuation.

[0056] The master station 5 calculates the clock error and position error of the navigation satellite by using the residual obtained by subtracting the distance between the navigation satellite and the reference station, calculated from the orbit information of the navigation satellite 1 (1a, 1b...), the amount of tropospheric propagation delay, and the corresponding amount of ionospheric propagation delay from the distance measured by the reference station 2 (2a, 2b...).Since the coordinate values ​​of the navigation satellite 1 (1a, 1b...) do not change with the station position, the correction information at this stage does not include any components related to the station position change.

[0057] The master station 5 calculates the station position fluctuation 3 (3a, 3b...) at each reference station using a station position fluctuation model, and constructs the simultaneous equations of (Equation 4). By solving this, correction amounts 7 (7a, 7b...) are obtained for each navigation satellite to cancel out the fluctuation in the user station's position caused by the station position fluctuation, and this is added to the satellite position correction information. When correction amounts 7 (7a, 7b...) are added to the satellite position correction information, the position of the navigation satellite becomes 8 (8a, 8b...). The master station 5 stores this new correction information 6 in a specified transmission format and provides it to the user station 9.

[0058] In this case, the station position change model may take into account changes due to the solid earth tides.

[0059] Alternatively, the station position variation model may include both variations due to solid earth tides and variations due to ocean loading.

[0060] Next, the operation will be explained.

[0061] The user station 9 receives the positioning signals transmitted by the navigation satellites 1 (1a, 1b, etc.) and measures the distance from each navigation satellite. After applying the correction information provided by WADGPS to the measured distance, the position of the receiving station is calculated.

[0062] The position of user station 9 actually fluctuates due to station position fluctuation 10. Coordinate value 11 corrected for station position always shows a constant value, but coordinate value 9 to which station position fluctuation 10 has been added is not corrected for the station position fluctuation, and therefore will not be a constant value even if the user station is fixed to the ground.

[0063] The satellite position correction information provided by the master station includes correction amounts 7 (7a, 7b, etc.) that cancel out fluctuations in the position of the user station 9 due to station position fluctuations 10, so when this is applied at the user station 9, the position error due to station position fluctuations 10 is removed from the position information calculated by the user receiver, and the user receiver outputs position information 11 that has been corrected for the station position fluctuations. Because this has been corrected for the station position fluctuations, it is consistent with coordinate values ​​based on the Earth-fixed coordinate system that is normally used in GNSS position calculations, and the position error that occurs at the WADGPS user station due to station position fluctuations is reduced. [Industrial Applicability]

[0064] According to the method and program for generating correction information in the satellite navigation system of the present invention, in WADGPS, previously, position errors caused by fluctuations in station position would remain in the position information calculated by the user receiver.However, by adding a correction amount to the correction information provided by WADGPS in advance to cancel this, it is possible to reduce the position errors that occur in the user station due to fluctuations in station position without changing the calculation processing content at the WADGPS user station, thereby improving the accuracy of the position information calculated by the user station. [Explanation of symbols]

[0065] 1(1a,1b···) Navigation satellite 2(2a, 2b...) Coordinate values ​​of the reference station and its position before correction 3(3a,3b···) Station position fluctuation at the reference station 4(4a, 4b...) Coordinate values ​​of the reference station corrected for station position fluctuations 5 Master station 6 Correction information 7(7a,7b···) Satellite position correction amount to cancel station position fluctuation 8(8a,8b...) Satellite position with correction amount added 9 User station and its coordinate values ​​before correction for station position fluctuations 10 Station position fluctuations in user stations 11 User station coordinates corrected for station position fluctuations

Claims

1. a plurality of navigation satellites that transmit positioning signals; a user station that receives positioning signals transmitted from the plurality of navigation satellites and measures the distances therebetween; a reference station that receives positioning signals transmitted from the plurality of navigation satellites using a receiver fixed on the ground and measures the distances between them; In a satellite navigation system that performs wide-area differential correction, a master station generates correction information for each cause of positioning error, such as clock error and position error, of a navigation satellite using the distance measured by the reference station, and provides this correction information to the user station, The master station When generating correction information related to the position error of the navigation satellite, a correction amount that cancels out fluctuations in the position of the user station caused by fluctuations in the station position is added in advance. A method for generating correction information in a satellite navigation system, comprising:

2. Regarding the station position fluctuation, Include variations due to solid earth tides 2. A method for generating correction information in a satellite navigation system according to claim 1.

3. Regarding the station position fluctuation, Include variations due to solid earth tides and ocean loading 2. A method for generating correction information in a satellite navigation system according to claim 1.

4. a plurality of navigation satellites that transmit positioning signals; a user station that receives positioning signals transmitted from the plurality of navigation satellites and measures the distances therebetween; a reference station that receives positioning signals transmitted from the plurality of navigation satellites using a receiver fixed on the ground and measures the distances between them; In a satellite navigation system that performs wide-area differential correction, a master station generates correction information for each cause of positioning error, such as clock error and position error, of a navigation satellite using the distance measured by the reference station, and provides this correction information to the user station, Operated in the master station, When generating correction information related to the position error of the navigation satellite, a correction amount that cancels out fluctuations in the position of the user station caused by fluctuations in the station position is added in advance. A program for generating correction information in a satellite navigation system.

5. Regarding the station position fluctuation, Include variations due to solid earth tides 5. A program for generating correction information in a satellite navigation system according to claim 4.

6. Regarding the station position fluctuation, Include variations due to solid earth tides and ocean loading 5. A program for generating correction information in a satellite navigation system according to claim 4.

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