A method for reducing grid product step of real-time ionospheric delay

Abstract

The application discloses a method for reducing the grid product step of real-time ionospheric delay, and relates to the field of satellite positioning. The method specifically comprises the following steps: selecting reference stations in a set area and satellites corresponding to the reference stations; performing PPP ambiguity fixing for each reference station and extracting ionospheric delay for each satellite; selecting predicted ionospheric delay or checked ionospheric delay according to the extraction result of the ionospheric delay; unifying the ionospheric delay reference of different reference stations under the same reference satellite and constructing a modeling reference station set {Site}; constructing carrier phase double difference for checking for short baseline reference stations and eliminating coarse error satellites; updating the modeling reference station set {Site} and including satellites that are commonly observed by more than 5 reference stations into a modeling satellite collection {Sat}; checking station-satellite pairs in a current epoch by using station-satellite pairs in a history epoch for modeling, and eliminating coarse error station-satellite pairs; performing polynomial fitting for each satellite, generating grid residual, and completing modeling. The application can stably generate high-precision ionospheric delay grid products.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of satellite positioning, and in particular to a method for reducing the step of real-time ionospheric delay grid products. BACKGROUND

[0002] Precise atmospheric correction information is the key to realize PPP-RTK (Precise Point Positioning-Real-Time Kinematic) fast ambiguity fixing. Whether the user can be stably provided with high-precision ionospheric delay grid products is one of the key factors for the user to obtain real-time centimeter-level positioning.

[0003] Affected by natural factors such as extreme weather, solar activity, geomagnetic activity, and non-natural factors such as equipment failure, power failure, and network failure, the ionospheric delay extracted by the regional reference station through ambiguity fixing inevitably has phenomena such as jump and discontinuity, which affects the accuracy of ionospheric delay grid modeling. In addition, due to the limitation of the number and distribution of reference stations, the internal reference stations are difficult to be evenly distributed. If the reference stations around the reference station with incorrect ionospheric delay are sparse, the regional fitting surface determined by the polynomial will be abnormally distorted due to the lack of constraints from other reference stations, resulting in a step in the generated ionospheric delay grid product. For the problem of ionospheric delay product step, the existing technical solution mainly increases the number of regional reference stations and timely eliminates abnormal reference stations by real-time monitoring of reference station observation values. This scheme can reduce the ionospheric delay product step and improve the product availability, but it also has the following shortcomings: first, this scheme requires additional construction of reference stations, increasing the cost of station construction and maintenance; second, this scheme requires accurate identification and elimination of reference stations with abnormal observation quality, increasing the development and operation cost. SUMMARY

[0004] Therefore, the present application provides a method for reducing the step of real-time ionospheric delay grid products, which can stably generate high-precision ionospheric delay grid products without increasing the number of reference stations, thereby reducing the cost of station construction and operation.

[0005] The application provides a method for reducing the grid product step of real-time ionospheric delay, which comprises the following steps: selecting reference stations in a set area and satellites corresponding to the reference stations; performing PPP ambiguity fixing for each reference station and extracting ionospheric delay for each satellite; selecting predicted ionospheric delay or checking ionospheric delay according to the extraction result of the ionospheric delay; unifying the ionospheric delay reference of different reference stations under the same reference satellite and constructing a modeling reference station set {Site}; constructing carrier phase double difference for short baseline reference stations to check and eliminate coarse error satellites; updating the modeling reference station set {Site} and including satellites that are observed by more than 5 reference stations into a modeling satellite set {Sat}; checking station-satellite pairs in the current epoch by using station-satellite pairs in the historical epoch to participate in modeling, and eliminating coarse error station-satellite pairs; and performing polynomial fitting and grid residual generation for each satellite to complete modeling.

[0006] Further, the time difference between the historical epoch and the current epoch is less than a set value ;

[0007] The method further comprises the following steps: if the ionospheric delay extraction fails, the ionospheric delay extracted from the historical epoch is used for prediction; if the ionospheric delay extraction succeeds, the ionospheric delay extracted from the PPP floating point solution is used for checking the ionospheric delay extracted from the PPP ambiguity, and the coarse error satellites are eliminated.

[0008] Further, the checking by using the ionospheric delay extracted from the PPP floating point solution comprises the following steps: checking according to the difference between the ionospheric delay extracted from the PPP floating point solution and the ionospheric delay extracted from the PPP ambiguity, regarding the satellites whose difference exceeds a set threshold value as coarse error satellites and eliminating the coarse error satellites.

[0009] Further, the ionospheric delay reference of different reference stations under the same reference satellite is unified to construct a modeling reference station set {Site}, which comprises the following steps: eliminating reference stations with failed ionospheric delay extraction or reference stations that cannot obtain historical epoch ionospheric delay; unifying the ionospheric delay of different reference stations under the same reference satellite according to a set reference to form a modeling reference station set {Site}.

[0010] Further, the short baseline reference station refers to two reference stations with a distance less than a set distance threshold value .

[0011] Further, when the carrier phase double difference is constructed for the short baseline reference station to perform checking, the expression of the carrier phase double difference is as follows:

[0012] (1)

[0013] (1) wherein, is a double difference operator, and the subscripts represent different frequencies; L represents a carrier phase observation value, ρ represents a satellite-terrestrial distance, T represents a troposphere delay, I represents an ionosphere delay, N represents a carrier phase ambiguity, M represents a multipath error, ε represents an observation noise.

[0014] Further, according to the formula (1), the formula (2) can be obtained, and the formula (2) is as follows:

[0015] (2)

[0016] According to the formula (2), a double difference ambiguity and a double difference ambiguity are obtained, and , are subtracted, and a satellite exceeding a set threshold is regarded as a gross error satellite and is eliminated.

[0017] Further, the station-satellite pair of the historical epoch participating in modeling is used to check the station-satellite pair of the current epoch participating in modeling, and the gross error station-satellite pair is eliminated, including: if the station-satellite pair has been used for modeling of the historical epoch, then the ionosphere delay of the station-satellite pair of the current epoch is directly compared with it, and if it is greater than a set threshold , then the station-satellite pair is directly eliminated; if the station-satellite pair has not been used for modeling of the historical epoch, then it is directly used for modeling of the ionosphere delay of the current epoch.

[0018] Further, the expression of the polynomial fitting is as follows:

[0019] (3)

[0020] In the formula (3), I is an ionosphere delay, lat, lat 0 are the longitude and latitude of the modeling reference station, respectively, lon, lon 0 are the longitude and latitude of the center point of the region, respectively, C 00 、C 01 、C 10 、C 11 is a model coefficient.

[0021] Further, the grid residual generation includes: according to the positions of the reference stations and the grid points, the inverse distance weighted method is used to calculate to each grid point to obtain the ionosphere residual value.

[0022] Compared with the prior art, the present application has the following beneficial technical effects:

[0023] The method for reducing the step of real-time ionospheric delay grid product provided by the present application fully utilizes the characteristics that ionospheric delay has a change trend that does not change in a short time, and predicts the ionospheric delay of the current epoch by the ionospheric delay extracted in the previous epoch and the ionospheric delay change amount, which can be used to check the accuracy of the ionospheric delay extracted in the current epoch, suppress the interference of gross error values, replace the missing ionospheric delay, and ensure the continuity of ionospheric delay grid modeling; the advantages of known regional reference station coordinates are fully utilized, carrier phase double difference observations are introduced to check gross error satellites, and the gross error station-satellite pairs are eliminated to avoid the step of ionospheric delay grid product; the information of reference station-satellite pairs participating in ionospheric delay grid modeling in the history is fully utilized to check the reference station-satellite pairs of ionospheric delay grid modeling in the current epoch, so as to avoid the decline of the availability of ionospheric delay grid product; the method is simple to implement, does not need to increase monitoring reference stations, has low station construction and operation cost, and is suitable for commercial operation. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The flowchart of the method for reducing the step of real-time ionospheric delay grid product provided by the present application is provided. DETAILED DESCRIPTION

[0025] The specific embodiments of the present application are further described in detail below in combination with the drawings and examples. The following examples are used to illustrate the present application, but are not used to limit the scope of the present application.

[0026] In the description of the present application, it should be noted that the orientation or positional relationship indicated in the description is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the indicated parts or elements must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0027] In order to provide better positioning services, a new generation of high-precision satellite positioning method PPP-RTK is proposed. PPP-RTK combines the advantages of precise point positioning (PPP) and network real-time kinematic (NRTK) positioning methods. The idea is to use a grid of ground reference stations to obtain satellite phase bias, ionospheric delay, and tropospheric delay information. Users can obtain enhanced correction information to achieve rapid ambiguity fixing, thereby achieving instantaneous high-precision positioning. Since ionospheric delay is easily affected by extreme weather such as typhoons, tsunamis, solar activity, and geomagnetic activity, and the activity of the ionosphere varies at different latitudes, how to obtain high-precision and high-stability ionospheric delay products in real time is the key to stable realization of user high-precision real-time positioning.

[0028] Due to the rapid temporal and spatial variation of ionospheric delay, in order to save broadcast resources and facilitate user use, and considering the strong temporal and spatial correlation of ionospheric delay within a certain regional range, the ionospheric delay is modeled by grid according to regional division. In order to obtain regional ionospheric grid products, first, the PPP ambiguity of each reference station is fixed and the ionospheric delay error is extracted. On this basis, a polynomial related to the latitude, longitude and regional center point is used for overall surface fitting. Users can obtain polynomial fitting values and grid residual values according to the approximate position of the reference station, and then obtain ionospheric delay correction products as prior constraints to achieve rapid ambiguity fixing at the user end.

[0029] As the basis of regional ionospheric grid products, regional reference stations stably provide high-quality and high-precision GNSS observation values, which is the key to maintaining the stable generation of high-precision ionospheric delay grid products. However, regional reference stations are not only easily affected by natural factors such as extreme weather, solar activity and geomagnetic activity, but also affected by non-natural factors such as receiver equipment failure, power failure and network failure. Due to this influence, the ionospheric delay extracted after the PPP ambiguity fixing of some reference stations will jump and be discontinuous, which will greatly interfere with the subsequent ionospheric delay grid modeling. Especially in areas with sparse distribution of reference stations, if the ionospheric delay of a reference station is extracted incorrectly, due to the lack of constraints from other reference stations around, the fitting surface determined by the polynomial will be abnormally twisted and deformed, which will result in a step change in the ionospheric delay product at the user level, reducing the availability of the product.

[0030] Please refer to Figure 1 , in view of the above-mentioned shortcomings, the present application provides a method for reducing the step of real-time ionospheric delay grid product, which can reduce the step of ionospheric delay product and improve the availability of the product.

[0031] The application provides a method for reducing the product step of real-time ionospheric delay grid, and specifically comprises the following steps:

[0032] S10, selecting a reference station in a preset area and a satellite corresponding to the reference station;

[0033] A certain area is selected, and a plurality of reference stations are arranged in the area, each reference station corresponds to one or more satellites, and real-time data streams of the reference stations in the area are input, the data streams are obtained by observation of a Beidou system or other systems.

[0034] S11, fixing PPP ambiguity for each reference station and extracting ionospheric delay for each satellite;

[0035] Taking a certain area and a certain epoch (time) as an example, precise orbit clock difference, DCB (Differential Code Bias, GNSS differential code bias) and UPD (Uncalibrated Phase Delay, uncalibrated phase delay) products are used to fix the PPP ambiguity of each reference station, and the ionospheric delay of each satellite corresponding to each reference station is extracted.

[0036] S12, selecting predicted ionospheric delay or checked ionospheric delay according to the extraction result of the ionospheric delay;

[0037] If the PPP ambiguity fixing of a certain reference station fails to extract the ionospheric delay, or the current epoch observation data of the reference station is missing to extract the ionospheric delay, the ionospheric delay extracted at a historical epoch and the ionospheric delay change amount are used for prediction, and the time difference between the historical epoch and the current epoch is less than a set value . Since the ionospheric delay has the characteristic that the change trend is unchanged in a short time, the ionospheric delay extracted at the previous epoch and the ionospheric delay change amount can be used to predict the ionospheric delay at the current epoch, which can be used for checking the accuracy of the ionospheric delay extracted at the current epoch on one hand, and suppressing the interference of gross error values on the other hand. When the reference station fails to extract the ionospheric delay due to PPP ambiguity fixing failure, observation value missing and other factors, the predicted value can be used for replacement to ensure the continuity of the ionospheric delay grid modeling. In order to distinguish the real extracted ionospheric delay from the predicted ionospheric delay, the two are respectively denoted as I real 、 I predict . The real extracted ionospheric delay I real , that is, the successfully extracted ionospheric delay I real , is extracted by the PPP float solution of the regional reference station I real_pppReal extraction of ionospheric delay I real Check, the difference exceeds the set threshold The satellite is regarded as a gross error satellite, and is eliminated to avoid the grid product from appearing a step.

[0038] S13, the ionospheric delay reference of different reference stations under the same reference satellite is unified, and a modeling reference station set {Site} is constructed;

[0039] The reference stations that fail to extract ionospheric delay or cannot obtain historical epoch ionospheric delay are eliminated, and then the ionospheric delays of different reference stations under the same reference satellite in the remaining reference stations are unified according to the set reference to form a modeling reference station set {Site}.

[0040] S14, the carrier phase double difference is constructed for the short baseline reference station to check and eliminate the gross error satellite;

[0041] The short baseline reference station refers to the reference station with a distance less than a set distance threshold Two reference stations with a distance less than a set distance threshold are selected to construct the carrier phase double difference, and the expression of the carrier phase double difference is as follows:

[0042] (1)

[0043] (1) In the formula, is a double difference operator, and the subscript indicates different frequencies; L represents a carrier phase observation value, ρ represents a satellite-geodetic distance, T represents a tropospheric delay, I represents an ionospheric delay, N represents a carrier phase ambiguity, M represents a multipath error, ε represents an observation noise.

[0044] The double difference observation can eliminate the errors of the satellite end and the receiver end and most of the atmospheric delay errors. The remaining main errors are satellite-geodetic distance, residual atmospheric error, multipath, and observation noise. Since the satellite-geodetic distance can be calculated using the known precise orbit and the coordinates of the regional reference station, the tropospheric delay can be substituted into the estimated value after the PPP ambiguity is fixed. Again, since the multipath of the carrier phase observation value and the observation noise are small in magnitude and can be ignored, the double difference ionospheric delay of the short baseline reference station tends to zero. At this time, the carrier phase double difference is only the double difference ambiguity, which can check the integer ambiguity of the reference station after the PPP ambiguity is fixed, eliminate the gross error satellite, and avoid the grid product from appearing a step. Therefore, formula (1) can be expressed as formula (2):

[0045] (2)

[0046] According to the formula (2), double-difference ambiguity can be obtained At the same time, the inter-satellite single-difference ambiguity of the two reference stations is replaced by the inter-station single-difference, and another set of double-difference ambiguities can be obtained , , The difference is obtained, and the satellite whose difference value exceeds the set threshold is regarded as a gross error satellite and is eliminated to avoid the step of the grid product.

[0047] S15, update the set of modeling reference stations {Site}, and include the satellite observed by more than 5 reference stations into the set of modeling satellites {Sat};

[0048] After completing the check of the carrier phase double-difference for the short baseline reference station, the set of modeling reference stations {Site} is updated, and then the satellite observed by more than 5 reference stations (at the same time) is included into the set of modeling satellites {Sat}, and the corresponding reference station list is recorded.

[0049] S16, check the station-satellite pair of the current epoch with the station-satellite pair of the historical epoch, and eliminate the gross error station-satellite pair;

[0050] Each satellite in the region is included in the set of modeling satellites {Sat}, and the reference station list of each satellite is traversed. If the station-satellite pair has been used for modeling in the historical epoch, the ionospheric delay of the station-satellite pair of the current epoch is directly compared with it. If it is greater than the set threshold , the station-satellite pair is directly eliminated; if the station-satellite pair has not been used for modeling in the historical epoch, it is directly used for ionospheric delay modeling in the current epoch, which can avoid the decrease in the availability of the ionospheric delay grid product.

[0051] S17, polynomial fitting and grid residual generation are performed for each satellite, and modeling is completed.

[0052] For each satellite in the set of satellites {Sat}, a polynomial related to the latitude, longitude and regional center point of the reference station is used for overall surface fitting, and the ionospheric residual part is calculated to each grid point according to the positions of the reference station and the grid point by using the inverse distance weighting method, to complete the generation of the ionospheric delay grid product. The expression of the polynomial fitting is:

[0053] (3)

[0054] In the formula (3), I is the ionospheric delay, lat, lat 0 are the latitude and longitude of the modeling reference station, respectively, lon, lon 0 are the latitude and longitude of the regional center point, respectively, C00 、C 01 、C 10 、C 11 is a model coefficient, according to the model coefficient, the modeling is completed. The user end can obtain the polynomial fitting value and the grid residual value according to the reference station sketch position, then obtains the ionospheric delay correction product and serves as the prior constraint, and realizes the user end fast ambiguity fixing.

[0055] S18, output stable high-precision ionospheric delay grid product.

[0056] From the above description, it can be known that the method for reducing the step of real-time ionospheric delay grid product provided by the application makes full use of the characteristics that ionospheric delay has a change trend unchanged in a short time, and the ionospheric delay and the ionospheric delay change amount extracted in the previous epoch are used to predict the ionospheric delay in the current epoch, which can be used to check the accuracy of the ionospheric delay extracted in the current epoch, suppress the interference of gross error values, replace the missing ionospheric delay, and ensure the continuity of ionospheric delay grid modeling; the advantages of known regional reference station coordinates are fully utilized, the carrier phase double difference observation is introduced to check the gross error satellite, and the gross error station-satellite pair is removed to avoid the step of the ionospheric delay grid product; the information of the reference station-satellite pair participating in the ionospheric delay grid modeling in the history is fully utilized to check the reference station-satellite pair of the ionospheric delay grid modeling in the current epoch, and the availability of the ionospheric delay grid product is avoided to decline; the method is simple to realize, does not need to increase the monitoring reference station, has low station operation and maintenance cost, and is suitable for commercial operation.

[0057] The above is only a preferred embodiment of the application, and does not limit the application in any form. Although the application has been disclosed as above, it is not intended to limit the application. Any person skilled in the art can make some changes or modifications to the above disclosed technical content to obtain equivalent embodiments with equivalent changes, without departing from the technical solution of the application. Any simple modification, equivalent change and modification of the above embodiments based on the technical essence of the application are still within the scope of the technical solution of the application.

Claims

1. A method of reducing grid product step for real-time ionospheric delay, characterized by: The method comprises the following steps: selecting reference stations in a set area and satellites corresponding to the reference stations; fixing PPP ambiguity for each reference station and extracting ionospheric delay for each satellite; selecting predicted ionospheric delay or checking ionospheric delay according to the extraction result of the ionospheric delay; unifying the ionospheric delay references of different reference stations under the same reference satellite and constructing a set of modeling reference stations {Site}; constructing carrier phase double difference for checking for short baseline reference stations and eliminating coarse difference satellites; updating the set of modeling reference stations {Site} and including satellites that are observed by more than 5 reference stations in a set of modeling satellites {Sat}; checking station-satellite pairs in a current epoch by using station-satellite pairs in a history epoch involved in modeling, and eliminating coarse station-satellite pairs; performing polynomial fitting and grid residual generation for each satellite to complete modeling.

2. The method of reducing grid product step for real-time ionospheric delay according to claim 1, characterized in that: The time difference between the history epoch and the current epoch is less than a set value Δt; The step of selecting predicted ionospheric delay or checking ionospheric delay according to the extraction result of the ionospheric delay comprises: if the extraction of the ionospheric delay fails, using the ionospheric delay extracted in the history epoch for prediction; if the extraction of the ionospheric delay succeeds, using the ionospheric delay extracted by the PPP float solution to check the ionospheric delay extracted by the PPP ambiguity and eliminating coarse satellites.

3. The method of reducing grid product step for real-time ionospheric delay according to claim 2, wherein: The step of using the ionospheric delay extracted by the PPP float solution to check and eliminating coarse satellites comprises: The difference between the ionospheric delay extracted from the PPP float solution and the ionospheric delay extracted from the PPP ambiguity is checked, and if the difference exceeds a set threshold ΔI ppp_inv Satellites whose residuals are considered as outliers are rejected.

4. The method of reducing grid product step for real-time ionospheric delay according to claim 1, wherein: The step of unifying the ionospheric delay references of different reference stations under the same reference satellite and constructing a set of modeling reference stations {Site} comprises: eliminating reference stations for which the extraction of the ionospheric delay fails or for which the ionospheric delay in the history epoch cannot be obtained; unifying the ionospheric delays of different reference stations under the same reference satellite according to a set reference to form a set of modeling reference stations {Site}.

5. The method of reducing grid product step for real-time ionospheric delay according to claim 1, wherein: The short baseline reference station refers to two reference stations with a distance less than a set distance threshold ΔDist.

6. The method of reducing grid product step for real-time ionospheric delay according to claim 1, wherein: When the carrier phase double difference is constructed for checking for the short baseline reference stations, the expression of the carrier phase double difference is as follows: (1) where, is the double-difference operator, the subscripts indicate different frequencies; L represents the carrier phase observation value, p represents the satellite-to-user range, T represents the tropospheric delay, I represents the ionospheric delay, N represents the carrier phase ambiguity, M represents the multipath error, and ε represents the observation noise.

7. The method of reducing grid product step for real-time ionospheric delay according to claim 6, wherein: According to equation (1), equation (2) can be obtained, and equation (2) is as follows: The double-difference ambiguities can be obtained from the equation (2) The inter-station single-difference ambiguity of the two reference stations is replaced by the inter-station single-difference, and another set of double-difference ambiguities can be obtained The double-difference equation (2) is subtracted from the equation (1) The satellites whose residuals exceeding the threshold value ΔAmb are regarded as rough error satellites and are eliminated.

8. The method of reducing grid product step for real-time ionospheric delay according to claim 1, wherein: The step of checking station-satellite pairs in a current epoch by using station-satellite pairs in a history epoch involved in modeling and eliminating coarse station-satellite pairs comprises: If the history epoch has been modeled with this station pair, directly compare the current epoch ionospheric delay of this station pair with it, if greater than a set threshold ΔI pre_cur , then directly reject this station pair; if the station-satellite pair is not used for modeling in the history epoch, the station-satellite pair is directly used for modeling of the ionospheric delay in the current epoch.

9. The method of reducing grid product step for real-time ionospheric delay according to claim 1, wherein: The expression of the polynomial fitting is as follows: I = C 00 + C 01 (lat - lat0) + C 10 (lon - lon0) + C 11 (lat - lat0)(lon - lon0) (3) (3) where I is ionospheric delay, lat and lat0 are the latitude of the modeling reference station and the latitude of the regional center point, respectively, lon and lon0 are the longitude of the modeling reference station and the longitude of the regional center point, respectively, C 00 , C 01 , C 10 , C 11 are model coefficients.

10. The method of reducing grid product step for real-time ionospheric delay according to claim 1, wherein: The grid residual generation comprises: according to the positions of the reference stations and the grid points, the inverse distance weighting method is used to calculate to each grid point to obtain the ionospheric residual value.

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