A method to improve the performance of Beidou precise point positioning service based on short messages

Abstract

The present invention relates to a method for improving the performance of Beidou precise point positioning service based on short messages, belonging to the technical field of navigation and positioning services, the method comprising: a service end estimates phase decimal deviation information according to observation data, broadcast ephemeris and PPP-B2b correction, and estimates the atmospheric delay correction of the grid point where the user is located according to the phase decimal deviation information and the user's approximate position; the service end adopts BDS short message communication service, uses a redefined BDS short message format, encodes the phase decimal deviation information and the atmospheric delay correction and sends them to the user end; the user end decodes to obtain the phase decimal deviation information and the atmospheric delay correction, and combines the observation data, broadcast ephemeris and PPP-B2b correction to realize precise point positioning ambiguity fixed service and / or precise point positioning real-time dynamic service. The present invention reduces the convergence time of positioning and improves the precise point positioning service performance of the Beidou satellite navigation system.

Description

Technical Field

[0001] The present invention belongs to the technical field of navigation and positioning services, and in particular relates to a method for improving the performance of Beidou precise single-point positioning services based on short messages. Background Art

[0002] The third generation Beidou navigation satellite system (BDS-3) officially started development in 2009, launched five new technology test satellites in 2015, completed the launch of all 30 networking satellites in June 2020, and was officially opened on July 31, 2020, marking the full completion of the Beidou global satellite navigation system.

[0003] BDS-3 provides global short message communication services and regional short message communication services. BDS-3 uses 14 MEO satellites distributed in three orbital planes to carry GSMCS payloads. The 14 MEO satellites then communicate with other satellites through intersatellite links to achieve global coverage. When the user uses a user terminal with a power of 10W, the maximum length of a single message of the BDS-3 global short message service is 560 bits. BDS-3 regional short message communication is achieved using 3 GEO satellites, with a maximum short message length of 14,000 bits. The regional short message communication service of the Beidou-2 Satellite Navigation System (BDS-2) has a maximum short message length of 628 bits.

[0004] BDS-3 also provides precise point positioning service (hereinafter referred to as PPP-B2b), which broadcasts precise orbit and clock correction and code deviation correction information of BDS-3 and other global navigation system satellites (currently supporting BDS-3+GPS) to users through three GEO satellites. Users can use one receiver to simultaneously receive GNSS observations, broadcast ephemeris and PPP-B2b corrections to perform real-time PPP solution, thereby improving navigation and positioning accuracy.

[0005] The nominal positioning accuracy of PPP-B2b is less than 30cm in the horizontal direction, less than 60cm in the vertical direction, and the convergence time is less than 60 minutes. Although the experimental results of some scholars show that the actual positioning performance of PPP-B2b is better than the nominal positioning accuracy, the service still has a certain gap compared with the same type of services of other satellite navigation systems, especially in the indicator of convergence time.

[0006] For example, Japan's Quasi-Zenith Satellite System (QZSS) broadcasts L6D and L6E signals to support centimeter-level enhanced positioning. The four satellites QZS-1 to QZS-4 use L6D signals to broadcast correction data in SSR (State Space Representation) format to support centimeter-level augmentation services (CLAS). The correction data broadcast by this signal is divided into global correction items and local correction items. Global correction items include satellite orbit corrections, satellite clock corrections, and pseudorange phase deviations; local correction items include ionospheric delay corrections and tropospheric delay corrections. CLAS mainly provides enhancement information for Japan and its surrounding waters. With CLAS, users can use the PPP-RTK (Precise Point Positioning Ambiguity Real-Time Kinematics) method to obtain centimeter-level positioning results within 1 minute. QZS-2 to QZS-4 use L6E signals to broadcast correction data to support the Centimeter Level Augmentation Service for Experiment (CLAS-E). CLAS-E is used to verify the expected application of CLAS in East Asia and Oceania. Its corrections only include global corrections (orbit correction, clock correction, pseudo-range phase deviation). Users within the effective coverage of the three QZSS satellites can receive correction information in real time, thereby achieving PPP-AR (precise point positioning ambiguity fixation). The Galileo system also plans to provide users with a high-precision service (High Accuracy Service, HAS) as a free service. This service is similar to the CLAS service of QZSS. It broadcasts high-precision satellite orbits, clock corrections and pseudo-range phase deviations on the E6b frequency to help users achieve high-precision positioning applications. The service is divided into two types: global service and European regional service. The two services are based on PPP-AR and PPP-RTK technologies respectively. It is expected that both can provide precise positioning services of 20cm in the horizontal direction and 40cm in the vertical direction. However, the positioning convergence time of the European regional service is shorter, and users can achieve high-precision positioning within 100s, while the convergence time of the global positioning service is 300s. Table 1 summarizes the main performance indicators of the precise point positioning services of BDS-3 and other satellite navigation systems. It is obvious that PPP-B2b has poorer performance than similar services of QZSS and Galileo, especially in terms of convergence time. This is mainly because both systems provide PPP-AR and PPP-RTK technologies that support non-difference ambiguity fixation, while the PPP-B2b service currently only supports precise point positioning services based on floating point ambiguity.Therefore, it is necessary to update technology to enable PPP-B2b to support PPP-AR and PPP-RTK technologies to reduce the convergence time of positioning and improve the precise point positioning service performance of the Beidou satellite navigation system.

[0007] Table 1 Statistics of precise point positioning service performance of various satellite navigation systems

[0008] Summary of the invention

[0009] The purpose of the present invention is to provide a method for improving the performance of Beidou precise point positioning service based on short messages, so as to solve the problem that the precise point positioning service of the Beidou satellite navigation system does not support PPP-AR and PPP-RTK technologies, thereby making the convergence time of positioning longer.

[0010] The present invention provides a method for improving the performance of Beidou precise point positioning service based on short messages to solve the above technical problems. The method comprises the following steps:

[0011] 1) The server estimates the phase fraction deviation information of the uncalibrated hardware delay based on the observation data, broadcast ephemeris and PPP-B2b correction information received from the observation station, and estimates the atmospheric delay correction information of the user's grid point based on the phase fraction deviation information of the uncalibrated hardware delay and the user's approximate position sent by the user end;

[0012] 2) The server adopts the BDS short message communication service and the redefined BDS short message format to encode the phase fraction deviation information of the uncalibrated hardware delay and the atmospheric delay correction information of the user's grid point and send them to the user end; the BDS short message format includes a message header, a data area and a CRC checksum, the message header consists of a synchronization header, a message type, an epoch time and a satellite number field, and the data area is used to place the field content corresponding to the phase fraction deviation information and the atmospheric delay correction information;

[0013] 3) The user end receives and decodes the phase decimal deviation information and the atmospheric delay correction information of the user's grid point encoded in the BDS short message format, obtains the phase decimal deviation information and the atmospheric delay correction information based on the decoding, and combines the received observation data, broadcast ephemeris, and PPP-B2b correction information to realize precise point positioning ambiguity fixed service and / or precise point positioning real-time dynamic service.

[0014] The beneficial effects of the present invention are as follows: a new BDS short message broadcasting format is predefined to broadcast the UPD (uncalibrated phase delays, UPD) product and atmospheric delay correction estimated by the server, and the user end implements PPP-AR and PPP-RTK based on the UPD product and atmospheric delay correction, thereby reducing the convergence time of positioning and improving the precise point positioning service performance of the Beidou satellite navigation system.

[0015] Furthermore, when the server sends phase fractional deviation information, the field contents of the data area in the BDS short message format are system identification, satellite prn number, wide lane fcb and narrow lane fcb; when the server sends an ionospheric correction message, the field contents of the data area in the BDS short message format are system identification, satellite prn number, STEC polynomial coefficients, and STEC correction; when the server sends a tropospheric correction message, the field contents of the data area in the BDS short message format are system identification, satellite prn number and tropospheric zenith wet delay correction of each grid point.

[0016] Furthermore, in step 1), the process of estimating the atmospheric delay correction information of the grid point where the user is located according to the phase fraction deviation information and the approximate position of the user sent by the user end is as follows:

[0017] A) The server estimates the atmospheric delay correction information based on the phase fraction deviation information;

[0018] B) The user end performs standard single-point positioning based on the observation data, broadcast ephemeris and PPP-B2b correction received from the observation station, obtains the user's approximate position, and sends it to the server end;

[0019] C) The server selects a set number of satellites based on the user's approximate location and the set altitude angle, and simultaneously determines the user's grid point and selects the atmospheric delay correction information for the user's grid point.

[0020] Furthermore, in step 3), the process of implementing the precise point positioning ambiguity fixation service by obtaining the phase fraction deviation information and the atmospheric delay correction information according to the decoding and combining the received observation data, broadcast ephemeris, and PPP-B2b correction information is as follows:

[0021] a) The user end performs PPP floating point solution calculation to obtain a floating point solution that eliminates ionospheric ambiguity;

[0022] b) Using the ionospheric-free observations of pseudorange and phase to perform MW combination, obtain the wide-lane ambiguity floating point solution, and achieve wide-lane ambiguity fixation under the correction of wide-lane fcb;

[0023] c) Calculate the narrow lane ambiguity float solution using the ionospheric ambiguity float solution and the fixed wide lane ambiguity, add the narrow lane fcb correction to the narrow lane ambiguity float solution, use the LAMBDA method to search for the integer value of the narrow lane ambiguity, and pass the fixed acceptance criterion test to ensure that the narrow lane ambiguity is correctly fixed;

[0024] d) Based on the fixed wide-lane ambiguity and narrow-lane ambiguity, the fixed solution of ionospheric ambiguity removal can be obtained, and the fixed solution of 3D position coordinates, receiver clock error and tropospheric wet delay parameters can be obtained with the fixed solution of ionospheric ambiguity removal as a constraint.

[0025] Further, after the user end decodes and obtains the ionospheric correction information in the atmospheric delay correction information, the ionospheric correction information is used to obtain the ionospheric delay at the user. The specific steps include:

[0026] 1) Calculate the STEC of each grid point using the decoded ionospheric correction information. The calculation formula of the STEC of the grid point is: in, is the STEC of satellite s at grid point k, is the multinomial coefficient of satellite s at grid point k at time t; δSTEC is the STEC correction of the satellite at grid point k;

[0027] 2) Calculate the pseudorange ionospheric delay based on the STEC of each grid point and the relationship between the pseudorange ionospheric delay and STEC. The relationship between the pseudorange ionospheric delay and STEC is: in, is the pseudo-range ionospheric delay of satellite s at grid point k at time t, f is the carrier frequency, is the STEC of satellite s at grid point k at time t;

[0028] 3) After obtaining the pseudo-range ionospheric delay, according to the grid point and the user The distance between users is interpolated using the inverse distance weighting method. The ionospheric delay at is given by: in, For users The ionospheric delay at k is the distance weighting factor, is the longitude, λ u It is the latitude.

[0029] Further, after the user terminal decodes and obtains the tropospheric wet delay correction information in the atmospheric delay correction information, the tropospheric wet delay correction information is used to obtain the tropospheric wet delay of the inclined path at the user terminal position. The specific steps include:

[0030] 1) Calculate the zenith wet delay of each grid point using the decoded tropospheric wet delay correction information. The calculation formula for the zenith wet delay of the grid point is: T k,wet =δT k,wet +δ wet0 , where T k,wet is the zenith wet delay at grid point k, δT k,wet is the tropospheric wet delay correction at grid point k, δ wet0 is the zenith wet delay variation constant;

[0031] 2) After obtaining the zenith wet delay of each grid point, use the inverse distance weighted method to interpolate the user end The zenith wet delay at is given by: Among them, T u,wet (t) is the user end at time t Zenith wet delay at T k,wet (t) is the zenith wet delay at grid point k at time t, w k is the distance weighting factor, is the longitude, λ u is the latitude;

[0032] 3) The Neil function is used to calculate the tropospheric wet delay of the inclined path at the user end position according to the satellite altitude angle.

[0033] Furthermore, the BDS short message communication service includes BDS-3 global short message communication service, BDS-3 regional short message communication service and BDS-2 regional short message communication service.

[0034] Furthermore, the epoch time is set to broadcast only the whole 30 seconds in a day.

[0035] Furthermore, the value range of the wide lane fcb and the narrow lane fcb is set to [-1.500, +1.500]. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the principle block diagram of the existing PPP-RTK server processing process;

[0037] Figure 2 This is a schematic diagram of the distribution of GPS wide lane fcb values;

[0038] Figure 3 This is a schematic diagram of the range of GLONASS wide lane fcb values;

[0039] Figure 4 This is a schematic diagram of the distribution of fcb values ​​for BDS wide lanes;

[0040] Figure 5 This is a schematic diagram of the range of fcb values ​​for Galileo wide lanes;

[0041] Figure 6 This is a schematic diagram of the distribution of GPS narrow lane fcb values;

[0042] Figure 7 This is a schematic diagram of the distribution of GLONASS narrow lane fcb values;

[0043] Figure 8 This is a schematic diagram of the distribution of fcb values ​​in narrow lanes of BDS;

[0044] Fig. 9 This is a schematic diagram of the distribution of fcb values ​​in Galileo narrow lanes;

[0045] Fig.10 It is a schematic diagram of the wide lane fcb and narrow lane fcb message formats of an embodiment of the present invention;

[0046] Fig.11 1 is a schematic diagram of the ionospheric correction message format according to an embodiment of the present invention;

[0047] Fig.12 4 is a schematic diagram of the tropospheric correction message format according to an embodiment of the present invention.

[0048] Fig.13 It is a flow chart of implementing PPP-AR and PPP-RTK technology based on PPP-B2B in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings.

[0050] The server processing structure of the existing conventional PPP-RTK positioning prototype system is as follows: Figure 1 As shown. Currently, PPP-B2b only supports the first and second layers. In order to improve the service performance of PPP-B2b and enable PPP-B2b to have the capabilities of PPP-AR and PPP-RTK, it is necessary to broadcast UPD products and atmospheric delay correction information through short messages, and perform third and fourth layer processing based on the broadcast orbital clock correction number. At present, the RTCM protocol or CSSR protocol specified by the Special Committee 104 of the International Maritime Radio Technical Committee is generally used when broadcasting UPD products and atmospheric delay correction information based on the network. However, due to the limited single communication capacity of short messages (the maximum single length of global short messages is 560 bits, the maximum single length of BeiDou-2 regional short message service is 628 bits, and the maximum single length of BeiDou-3 regional short message service is 14000 bits) and the communication frequency limit, the RTCM protocol or CSSR format cannot be directly used. Therefore, it is necessary to define a new data format for broadcasting UPD products and atmospheric delay corrections.

[0051] Therefore, the technical concept of the present invention is to predefine a BDS short message format, and the server uses the BDS short message format to broadcast the estimated phase decimal deviation and atmospheric delay correction information, so that the user end can use the phase decimal deviation and atmospheric delay correction information to implement PPP-AR and PPP-RTK.

[0052] 1. Definition process of BDS short message format.

[0053] In order to reasonably determine the value range of wide-lane fcb (fractional cycal bias) and narrow-lane fcb, the UPD product for seven days from January 132 to 138, 2022 released by the iGMAS Innovation Application Center was first used to analyze the distribution range of wide-lane fcb and narrow-lane fcb values. Figure 2 to Figure 5 The value ranges of wide lane fcb of the four major systems are shown, and Table 2 statistics the maximum and minimum values ​​of wide lane fcb of each system.

[0054] Table 2 Statistics of wide lane fcb value range (unit: week)

[0055] Satellite system Minimum Maximum GPS -0.453 0.916 GLONASS -0.328 0.852 BDS -0.430 0.914 Galileo -0.470 1.120

[0056] Similarly, Figure 6 to Figure 9 The distribution of narrow lane FCB values ​​of the four major systems is shown, and the maximum and minimum values ​​of the narrow lane FCB distribution of each system are statistically shown in Table 3. Based on these statistical results, the storage space required for wide lane FCB and narrow lane FCB during data transmission can be reasonably determined.

[0057] Table 3 Statistics of the range of fcb values ​​for narrow lanes (unit: week)

[0058] Satellite system Minimum Maximum GPS -0.540 0.960 GLONASS -0.710 1.400 BDS -0.420 1.430 Galileo -0.470 1.010

[0059] When defining the BDS short message broadcast format, the message format of RTCMSSR was referenced. Each message includes a 8-bit length to the synchronization header (11010011) and a 24-bit length CRC check code. The remaining space is used to broadcast data. Therefore, the BDS short message format includes a message header, a data area and a CRC check code. The message header includes a synchronization header, a message type, an epoch time and a satellite number field. The data area is used to place the field content corresponding to each message type. Table 4 lists the detailed information of each field in the message header and data area. Among them, in order to save space, the epoch time is set to only broadcast the whole 30 seconds in a day. When the user end decodes the epoch time, multiply it by 30 to get the seconds in the day. According to the above content, the value range of wide lane fcb and narrow lane fcb is set to [-1.500, +1.500].

[0060] Table 4 Field information table

[0061]

[0062] According to Table 4, there are three types of messages: wide-lane FCB and narrow-lane FCB messages, ionospheric correction messages, and tropospheric correction messages. The formats of the three messages are as follows: Figures 10-12 As shown. The formats of the three messages all include a message header, a data area and a CRC checksum. The message headers of the three message formats all include a synchronization header, a type ID, an epoch time and a number of satellites. However, the field contents of the data area of ​​the three message formats are different. For wide-lane fcb and narrow-lane fcb messages, the field contents of the data area of ​​the BDS short message format are system identification, satellite prn (pseudo random noise code) number, wide-lane fcb and narrow-lane fcb; for ionospheric correction messages, the field contents of the data area of ​​the BDS short message format are system identification, satellite prn number, STEC (Slant Total Electron Content) polynomial coefficients, and STEC corrections; for tropospheric correction messages, the field contents of the data area of ​​the BDS short message format are system identification, satellite prn number and tropospheric zenith wet delay corrections for each grid point.

[0063] 2. A method for improving the performance of Beidou precise point positioning service based on the BDS short message format defined above.

[0064] The method of this embodiment includes the following steps:

[0065] 1) The server estimates the phase decimal deviation information based on the observation data, broadcast ephemeris and PPP-B2b correction information received from the observation station, and estimates the atmospheric delay correction information of the user's grid point based on the phase decimal deviation information and the user's approximate position sent by the user end;

[0066] 2) The server uses the BDS short message communication service and the predefined BDS short message format to encode the phase fractional deviation information and the selected atmospheric delay correction information and send them to the user end;

[0067] 3) The user end receives and decodes the phase decimal deviation information and the selected atmospheric delay correction information encoded in the BDS short message format, obtains the phase decimal deviation information and the atmospheric delay correction information based on the decoding, and combines the received observation data, broadcast ephemeris, and PPP-B2b correction information to realize precise point positioning ambiguity fixed service or precise point positioning real-time dynamic service.

[0068] Each step is described in detail below.

[0069] In order to fully save storage space, only the satellites visible to the user end are broadcast to the user end. This requires the user end to first obtain its own approximate position using the standard single-point positioning method, and then send its own approximate position to the server end through short message communication. The server end selects a set number of satellites according to the set altitude cutoff angle based on the approximate position sent by the user end, and sends the correction data of these satellites to the user end. Among them, the server end also uses the user end's approximate position to determine the grid where the user end is located, and broadcasts the ionospheric correction and tropospheric correction of the corresponding grid point.

[0070] like Fig.13 As shown, based on the above idea, the specific process in step 1) is:

[0071] A) The server estimates the phase decimal deviation information based on the observation data, broadcast ephemeris and PPP-B2b correction information received from the observation station. The server estimates the atmospheric delay correction information based on the phase decimal deviation information.

[0072] Further, the phase fractional deviation information here includes wide lane fcb and narrow lane fcb. The atmospheric delay correction information here includes ionosphere correction information and tropospheric wet delay correction information.

[0073] B) The user end performs standard single-point positioning based on the observation data, broadcast ephemeris and PPP-B2b correction received from the observation station, obtains the user's approximate position, and sends it to the server end;

[0074] C) The server selects a set number of satellites based on the user's approximate location and the set altitude angle, and simultaneously determines the user's grid point and selects the atmospheric delay correction information for the user's grid point.

[0075] Regarding step 2), at present, BDS-3 provides global short message communication services and regional short message communication services, and continues to provide regional short message communication services of BDS-2 (The second generation Beidou navigation satellite system, BDS-2). The service capabilities of these types of short message communication services are different. Table 5 summarizes the main performance indicators of these service types. According to the different performance of each type of service, the above format is flexibly used to broadcast UPD products and atmospheric corrections.

[0076] Table 5 Main performance indicators of different types of short message communication services

[0077] Maximum single message length (bits) Service frequency BDS-3 Global Short Message Communication Service 560 Once a minute BDS-3 Regional Short Message Communication Service 14000 Once every 30 seconds BDS-2 Regional Short Message Communication Service 628 Once a minute

[0078] The maximum single message length of BDS-3 global short message communication service and BDS-2 regional short message communication service is short, and one message is broadcast each time. For UPD products, one message can broadcast 10-15 satellite data. Tropospheric correction messages can also broadcast 10-15 satellite data in one message. Ionospheric correction messages occupy a large control, and the corresponding satellite data can be broadcast in two times.

[0079] The maximum single message length of the BDS-3 regional short message communication service is relatively large, so three messages can be broadcast at a time, each containing data from 10-15 satellites.

[0080] For step 3), the process of obtaining the phase fraction deviation information and atmospheric delay correction information through decoding and combining the received observation data, broadcast ephemeris, and PPP-B2b correction information to realize the precise point positioning ambiguity fixation service is as follows:

[0081] a) The user end performs PPP (precise point positioning) floating-point solution calculation to obtain a floating-point solution that eliminates ionospheric ambiguity;

[0082] b) MW using pseudorange and phase deionospheric observations Combination, obtain the floating point solution of wide lane ambiguity, and realize wide lane ambiguity fixation under the correction of wide lane fcb;

[0083] c) Calculate the narrow lane ambiguity float solution using the ionospheric ambiguity float solution and the fixed wide lane ambiguity, add the narrow lane fcb correction to the narrow lane ambiguity float solution, use the LAMBDA (Least-square Ambiguity Decorrelation Adjustment) method to search for the integer value of the narrow lane ambiguity, and pass the fixed acceptance criterion test to ensure that the narrow lane ambiguity is correctly fixed;

[0084] The process of passing the fixed acceptance criteria test is:

[0085] When the wide lane ambiguity is fixed successfully, it can be substituted into the ionospheric combined ambiguity to obtain the narrow lane ambiguity. In view of the correlation of PPP ambiguity (especially short-term observations) and the short wavelength of narrow lane ambiguity, which is easily affected by other unmodeled errors, the LAMBDA search method is used to fix the narrow lane ambiguity, that is, to solve the following integer least squares problem:

[0086]

[0087] In the formula, is a real ambiguity vector, and N is an integer ambiguity vector. The solution process of the above formula is a discrete search process. From the integer ambiguity combinations searched out, the group that minimizes the objective function is selected as the final ambiguity fixed solution, and the likelihood ratio test, i.e., ratio test, is performed on the fixed narrow lane ambiguity:

[0088]

[0089] In the formula, k is the threshold value, which is a constant given by experience. and are the optimal solution and suboptimal solution satisfying the integer least squares criterion of formula (1), respectively. If the fixed ambiguity satisfies the test condition of formula (2), the ambiguity fixed solution is accepted, otherwise the ambiguity floating point solution is adopted.

[0090] d) Based on the fixed wide-lane ambiguity and narrow-lane ambiguity, the fixed solution of ionospheric ambiguity removal can be obtained, and the fixed solution of 3D position coordinates, receiver clock error and tropospheric wet delay parameters can be obtained with the fixed solution of ionospheric ambiguity removal as a constraint.

[0091] After the user end decodes and obtains the ionospheric correction information in the atmospheric delay correction information, the ionospheric correction information is used to obtain the ionospheric delay at the user to support the PPP-RTK technology. The specific steps include:

[0092] 1) Calculate the STEC of each grid point using the decoded ionospheric correction information. The calculation formula of the STEC of the grid point is: in, is the STEC of satellite s at grid point k, is the multinomial coefficient of satellite s at grid point k at time t; δSTEC is the STEC correction of the satellite at grid point k;

[0093] 2) Calculate the pseudorange ionospheric delay based on the STEC of each grid point and the relationship between the pseudorange ionospheric delay and STEC. The relationship between the pseudorange ionospheric delay and STEC is: in, is the pseudo-range ionospheric delay of satellite s at grid point k at time t, f is the carrier frequency, is the STEC of satellite s at grid point k at time t.

[0094] 3) After obtaining the pseudo-range ionospheric delay, according to the grid point and the user The distance between users is interpolated using the inverse distance weighting method. The ionospheric delay at is given by: in, For users The ionospheric delay at k is the distance weighting factor, is the longitude, λ u It is the latitude.

[0095] After the user end decodes and obtains the tropospheric wet delay correction information in the atmospheric delay correction information, the tropospheric wet delay correction information is used to obtain the tropospheric wet delay of the inclined path at the user end position to support the PPP-RTK technology. The specific steps include:

[0096] 1) Calculate the zenith wet delay of each grid point using the decoded tropospheric wet delay correction information. The calculation formula for the zenith wet delay of the grid point is: T k,wet =δT k,wet +δ wet0 , where T k,wet is the zenith wet delay at grid point k, δT k,wet is the tropospheric wet delay correction at grid point k, δ wet0 is the zenith wet delay variation constant.

[0097] 2) After obtaining the zenith wet delay of each grid point, use the inverse distance weighted method to interpolate the user end The zenith wet delay at is given by: Among them, T u,wet (t) is the user end at time t Zenith wet delay at T k,wet (t) is the zenith wet delay at grid point k at time t, w k is the distance weighting factor, is the longitude, λ u It is the latitude.

[0098] 3) The Neil function is used to calculate the tropospheric wet delay of the inclined path at the user end position according to the satellite's altitude angle. Usually, the electromagnetic wave propagation path is not along the zenith direction, so it is necessary to map the zenith direction delay to a certain inclined propagation direction, which requires a mapping function (MF). The tropospheric delay in the inclined direction is the sum of the product of the dry and wet mapping functions and the zenith dry and wet components. The relationship is:

[0099] z(e)=z h ×mf h (e)+z w ×mf w (e)

[0100] Where z(e) is the total delay, z h 、z w are the zenith dry and wet delays, mf h (e), mf w(e) are the dry and wet mapping functions respectively, and e is the altitude angle. The mapping function mf is usually expressed in the form of a continued fraction:

[0101]

[0102] In the formula, the parameters a, b, and c are constants far less than 1, and the dry and wet mapping functions use different parameters (a h , b h , c h ) and (a w , b w , c w ).

[0103] There are many kinds of mapping functions, such as Neil function, VMF1 and GMF function, etc. The mapping function in this embodiment is preferably the Neil function.

[0104] The Neil function is a global atmospheric delay projection function established using data from 26 sounding balloon stations distributed around the world. Its interference mapping coefficient a h The calculation is as follows:

[0105]

[0106] In the formula, is the latitude of the station, t is the annual cumulative day, and parameter a avg and a amp The latitude value closest to the station latitude provided by the Neil coefficient table is linearly interpolated, and the parameter b h 、c h and (a w , b w , c w ) is also obtained by linear interpolation from the parameter table provided by Neil. The Neil model uses the temperature values ​​and relative humidity profiles of some northern latitude stations in the US standard atmospheric model, and assumes that the northern and southern hemispheres are symmetrical, so its accuracy is poor in the southern hemisphere.

Claims

1. A method for improving Beidou precise point positioning service performance based on short messages, characterized in that: The method comprises the following steps: 1) The server estimates the phase decimal deviation information of the uncalibrated hardware delay based on the observation data, broadcast ephemeris and PPP-B2b correction information received from the observation station, and estimates the atmospheric delay correction information based on the phase decimal deviation information; the user performs standard single-point positioning based on the observation data, broadcast ephemeris and PPP-B2b correction received from the observation station, obtains the user's approximate position, and sends it to the server; the server selects a set number of satellites based on the set altitude angle according to the user's approximate position, and determines the grid point where the user is located, and selects the atmospheric delay correction information of the grid point where the user is located; 2) The server adopts the BDS short message communication service and the redefined BDS short message format to encode the phase fraction deviation information of the uncalibrated hardware delay and the atmospheric delay correction information of the grid point where the user is located and send them to the user end; the BDS short message format includes a message header, a data area and a CRC check code, the message header consists of a synchronization header, a message type, an epoch time and a satellite number field, and the data area is used to place the field content corresponding to the phase fraction deviation information and the atmospheric delay correction information; when the server sends the phase fraction deviation information, the field content of the data area of ​​the BDS short message format is the system identifier, the satellite prn number, the wide lane fcb and the narrow lane fcb; when the server sends the ionospheric correction message, the field content of the data area of ​​the BDS short message format is the system identifier, the satellite prn number, the STEC polynomial coefficient, and the STEC correction; when the server sends the tropospheric correction message, the field content of the data area of ​​the BDS short message format is the system identifier, the satellite prn number and the tropospheric wet delay correction of each grid point; 3) The user end receives and decodes the phase decimal deviation information and the atmospheric delay correction information of the user's grid point encoded in the BDS short message format, obtains the phase decimal deviation information and the atmospheric delay correction information according to the decoding, and implements the precise point positioning ambiguity fixed service and / or precise point positioning real-time dynamic service in combination with the received observation data, broadcast ephemeris, and PPP-B2b correction information; after the user end decodes and obtains the ionospheric correction information in the atmospheric delay correction information, the ionospheric correction information is used to obtain the ionospheric delay at the user. The specific steps include: A) Calculate the STEC of each grid point using the decoded ionospheric correction information. The calculation formula of the STEC of the grid point is: in, is the STEC of satellite s at grid point k, is the multinomial coefficient of satellite s at grid point k at time t; δSTEC is the STEC correction of the satellite at grid point k; B) Calculate the pseudorange ionospheric delay based on the STEC of each grid point and the relationship between the pseudorange ionospheric delay and STEC. The relationship between the pseudorange ionospheric delay and STEC is: in, is the pseudo-range ionospheric delay of satellite s at grid point k at time t, f is the carrier frequency, is the STEC of satellite s at grid point k at time t; C) After obtaining the pseudo-range ionospheric delay, according to the grid point and the user The distance between users is interpolated using the inverse distance weighting method. The ionospheric delay at is given by: in, For users The ionospheric delay at k is the distance weighting factor, is the longitude, λ u is the latitude; After the user terminal decodes and obtains the tropospheric wet delay correction information in the atmospheric delay correction information, the tropospheric wet delay correction information is used to obtain the tropospheric wet delay of the inclined path at the user terminal position. The specific steps include: Ⅰ) Calculate the zenith wet delay of each grid point using the decoded tropospheric wet delay correction information. The calculation formula for the zenith wet delay of the grid point is: T k,wet =δT k,wet +δ wet0 , where T k,wet is the zenith wet delay at grid point k, δT k,wet is the tropospheric wet delay correction at grid point k, δ wet0 is the zenith wet delay variation constant; Ⅱ) After obtaining the zenith wet delay of each grid point, use the inverse distance weighted method to interpolate the user end The zenith wet delay at is given by: Among them, T u,wet (t) is the user end at time t Zenith wet delay at T k,wet (t) is the zenith wet delay at grid point k at time t, w k is the distance weighting factor, is the longitude, λ u is the latitude; III) The tropospheric wet delay of the inclined path at the user's terminal position is calculated based on the satellite's altitude angle using the Neil function.

2. The method for improving Beidou precise point positioning service performance based on short messages according to claim 1, characterized in that: In step 3), the process of obtaining the phase fraction deviation information and atmospheric delay correction information through decoding, combining the received observation data, broadcast ephemeris, and PPP-B2b correction information to realize the precise point positioning ambiguity fixation service is as follows: a) The user end performs PPP floating point solution calculation to obtain a floating point solution that eliminates ionospheric ambiguity; b) Using the ionospheric-free observations of pseudorange and phase to perform MW combination, obtain the wide-lane ambiguity floating point solution, and achieve wide-lane ambiguity fixation under the correction of wide-lane fcb; c) Calculate the narrow lane ambiguity float solution using the ionospheric ambiguity float solution and the fixed wide lane ambiguity, add the narrow lane fcb correction to the narrow lane ambiguity float solution, use the LAMBDA method to search for the integer value of the narrow lane ambiguity, and pass the fixed acceptance criterion test to ensure that the narrow lane ambiguity is correctly fixed; d) Based on the fixed wide-lane ambiguity and narrow-lane ambiguity, the fixed solution of ionospheric ambiguity removal can be obtained, and the fixed solution of 3D position coordinates, receiver clock error and tropospheric wet delay parameters can be obtained with the fixed solution of ionospheric ambiguity removal as a constraint.

3. The method for improving Beidou precise point positioning service performance based on short messages according to claim 1, characterized in that: The BDS short message communication service includes BDS-3 global short message communication service, BDS-3 regional short message communication service and BDS-2 regional short message communication service.

4. The method for improving Beidou precise point positioning service performance based on short messages according to any one of claims 1 to 3, characterized in that: The epoch time is set to broadcast only the whole 30 seconds in a day.

5. The method for improving Beidou precise point positioning service performance based on short messages according to claim 1, characterized in that: The value range of the wide lane fcb and the narrow lane fcb is set to [-1.500, +1.500].

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