Time synchronization method based on low earth orbit satellite and beidou PPP-B2b

Abstract

The application discloses a time synchronization method based on low-orbit satellite and Beidou PPP-B2b, which comprises the following steps: 1) a central station and a client respectively acquire real-time PPP-B2b signals, pseudo distances, carrier phase observation values and broadcast ephemeris broadcast by a Beidou No. 3 satellite and low-orbit satellites through respective receivers; 2) the central station and the client respectively solve a clock difference of the central station receiver and a clock difference of the client receiver; 3) the client solves a difference value between a local time and a central station time; and 4) the time difference between the client and the central station is input into a digital frequency synthesis module of the client to adjust the clock difference of the client receiver. The application realizes real-time PPP time transmission through Beidou No. 3 precise point positioning service and low-orbit satellite positioning service, and the client can realize sub-nanosecond time synchronization without dependence on the Internet. Meanwhile, the application realizes time synchronization and communication through low-orbit satellite assistance, and enhances the time synchronization performance in interference, shielding and complex environments.

Description

Technical Field

[0001] This invention relates to the field of satellite navigation time and frequency transmission technology, and in particular to a time synchronization method based on low-orbit satellites and BeiDou PPP-B2b. Background Technology

[0002] With the development of Precise Point Positioning (PPP) technology, the real-time requirements of applications such as 5G communication, high-precision time synchronization, and landslide monitoring necessitate higher real-time performance and accuracy in orbit and clock correction. The International GNSS Service (IGS) launched the Real-time Pilot Project (RPP) in 2007 and opened the Real-time Service (RTS) in 2013. This service provides users with real-time satellite orbit and clock correction data via the NTRIP (Network Transport of RTCM over the Internet Protocol) protocol, using the RTCM (Radio Technical Commission for Maritime service) information format. This laid the foundation for real-time PPP time transmission. However, the IGS RTS service relies on communication networks; network outages prevent normal operation. Furthermore, the deployment of communication network infrastructure is costly and limited by terrain. Therefore, interplanetary augmentation services are an effective solution to these problems. By broadcasting orbit and clock correction data in real time from satellites, PPP services can be provided 24 / 7, globally.

[0003] To facilitate the application of real-time PPP services, the BeiDou-3 satellite system officially launched inter-satellite augmentation services and satellite-based precise ephemeris broadcasting services in October 2020. Relying on BeiDou's geostationary orbit satellites, satellite-based augmentation correction information is broadcast to the Asia-Pacific region and even globally at the B2b frequency. This global real-time broadcasting service provides a fundamental guarantee for BeiDou real-time PPP time transfer research. Currently, BeiDou-3 consists of 3 GEO satellites, 24 MEO satellites, and 3 IGSO satellites. In addition to retaining the BeiDou-2 B1I and B3I signals, it adds B1C, B2a, and B2b signals. The PPP-B2b signal can provide differential information for broadcasting ephemeris for BeiDou CNAV1 and GPS LNAV, mainly including satellite mask (included in information type 1), user ranging accuracy (included in information type 2), orbit correction (included in information type 2), inter-symbol offset correction (included in information type 3), and clock error correction (included in information type 4).

[0004] Because PPP-B2b signals are weak and susceptible to electromagnetic interference, they still pose significant safety risks in practical applications. Specifically, due to the low power of the PPP-B2b signal and the considerable distance between the satellite and the Earth's surface, the signal is extremely weak by the time it reaches the Earth's surface, typically around -160 dBW. Furthermore, surface structures, trees, and other obstacles can interfere with the signal reception of ground navigation receivers, easily causing them to fail to lock onto the satellite signal. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a time synchronization method based on low-orbit satellites and BeiDou PPP-B2b, so as to solve the technical problem of improving the time synchronization performance of BeiDou satellite navigation clients in interference, obstruction and complex environments without relying on the Internet.

[0006] The time synchronization method based on low-Earth orbit satellites and BeiDou PPP-B2b of this invention includes the following steps:

[0007] 1) The central station and the client respectively acquire real-time PPP-B2b signals, pseudorange, carrier phase observations and broadcast ephemeris broadcast by BDS-3 satellite, and pseudorange, carrier phase observations and broadcast ephemeris broadcast by low-orbit satellite through their respective receivers;

[0008] 2) The central station and the client respectively calculate the clock bias of the central station receiver and the clock bias of the client receiver through the following steps:

[0009] ① Satellite orbit correction:

[0010] The satellite position vector X is calculated using the broadcast ephemeris received by the receiver. brdc ,

[0011] The received PPP-B2b signal is decoded, and the orbit information contained in the PPP-B2b signal is matched with the broadcast ephemeris to calculate the satellite position correction vector ΔX; the formula for calculating ΔX is:

[0012]

[0013] In the formula, r, These represent the satellite's position vector and velocity vector in the broadcast ephemeris, respectively; e R e C e A These are the unit vectors corresponding to the radial, tangential, and normal directions, respectively; Δ orbit This is the orbital correction vector;

[0014] Then calculate the corrected satellite position using the following formula:

[0015] X orbit =Xbrdc -ΔX (2)

[0016] In the formula, X orbit Indicates the corrected satellite position;

[0017] ② Satellite clock error correction:

[0018] After the orbit information contained in the PPP-B2b signal is successfully matched with the broadcast ephemeris, the version number in the satellite clock correction information is then matched with the version number in the orbit correction message, as follows:

[0019]

[0020] In the formula, t s-c t represents the corrected satellite clock bias correction; brdc C0 represents the satellite clock correction value calculated from the broadcast ephemeris; C0 represents the clock correction parameter provided by PPP-B2b.

[0021] ③ Satellite inter-symbol bias correction:

[0022] By referring to the differential code offset correction types in PPP-B2b, satellite inter-code offsets are corrected in real time to achieve synchronization processing of various signals. The correction algorithm is as follows:

[0023] P corr =P ini -P sig (4)

[0024] In the formula, P corr P represents the corrected observation; ini P represents the original observation value; sig This indicates the inter-symbol deviation of the corresponding signal;

[0025] ④ Using pseudorange and carrier observations from BDS-3 and low-Earth orbit satellites, a mathematical model for joint precise point positioning using BDS and low-Earth orbit satellites is derived:

[0026] Both BDS and low-Earth orbit satellites transmit dual-frequency signals. By combining the two frequencies, the first-order term of ionospheric error is eliminated. The mathematical model of the combined observations is expressed by the following equation:

[0027]

[0028] In the formula, f1 and f2 are carrier frequencies; P1, P2, Φ1, and Φ2 are the pseudorange and carrier observation values ​​corresponding to f1 and f2, respectively; P IF Φ IF These are pseudorange and carrier observations, respectively, for the ionospheric combination.

[0029] The ionospheric pseudorange and carrier phase observations of client r and satellite s are expressed by the following formula:

[0030]

[0031] In this context, the superscript B indicates a BDS satellite, the upper L indicates a low-Earth orbit satellite, the subscript IF indicates an ionospheric-free combination, the subscript r indicates a client, P indicates pseudorange, and Φ indicates carrier observation. dt represents the actual distance from the client r to the satellite s; c represents the speed of light; dt r dt represents the receiver clock bias. s Indicates satellite clock bias; d trop For tropospheric delay; d orb λ1 is the satellite orbit error; N is the integer ambiguity of the carrier frequency f1; ε(P) is the observation noise error of the pseudorange; and ε(Φ) is the observation noise error of the carrier.

[0032] ⑤ Using the BeiDou satellite broadcast ephemeris orbit, satellite clock bias, and satellite code offset corrected in steps ①, ②, and ③, as well as the low-Earth orbit satellite orbit, low-Earth orbit satellite clock bias, pseudorange, and carrier phase observations, the precise single-point positioning mathematical model described in step ④ is solved to obtain the clock bias of the central station receiver and the clock bias of the client receiver.

[0033] 2) The client calculates the difference between its local time and the central station time:

[0034] The calculated clock difference of the central station receiver is sent to the low-orbit satellite through the low-orbit satellite communication terminal. The client obtains the clock difference of the central station receiver sent by the central station through the low-orbit satellite communication terminal. The time difference between the two locations is obtained by subtracting the clock difference of the central station receiver from the clock difference of the client receiver.

[0035] 3) The time difference between the client and the central station is input into the client's digital frequency synthesis module. The clock error of the client receiver is adjusted through the digital frequency synthesis module. The digital frequency synthesis module includes a frequency multiplier connected to the client receiver's oven-controlled crystal oscillator, a phase accumulator connected to the frequency multiplier and the frequency control word module, an adder connected to the phase accumulator and the phase correction value module, a phase amplitude lookup table connected to the adder, a digital-to-analog converter connected to the phase amplitude lookup table, and a bandpass filter connected to the digital-to-analog converter. The time difference between the client and the central station is input into the frequency control word module and the phase correction value module, respectively. The signal output by the oven-controlled crystal oscillator is multiplied by the frequency multiplier and the output signal is the working clock of the client receiver.

[0036] Furthermore, adjusting the operating clock of the client receiver in step 3) includes the following steps:

[0037] (1) Adjust the frequency of the operating clock output of the frequency multiplier. The adjustment algorithm is as follows:

[0038] F(n)=F(n-1)+α0×clkdrift (8)

[0039] FCW = F(n) × 2 48 / Fsysclk (9)

[0040] In the formula, α0 is the loop system of the first-order frequency-locked loop. The selection principle is that the frequency adjustment amount should not exceed the pulling range of the receiver baseband loop. If it is too large, it will easily lead to the receiver losing lock. clkdrift is the clock drift value. Fsysclk is the working clock. F(n) is the calculated frequency value. FCW is the frequency control word.

[0041] (2) When the clkdrift digital value approaches the frequency division, phase adjustment is initiated. Phase correction is used to gradually correct the receiver's clock error to near 0. The time adjustment accuracy is Tres, Tres = 0.1us × 2. -14 =6ps, with an adjustment range of (-100, +100)ns;

[0042] (3) When the clock error gradually approaches Tres, the phase-locked loop (PLL) stage is entered. The PLL is used to track clkdrift and clock error, and to tame and converge the receiver clock error to close to 0.

[0043] The beneficial effects of this invention are:

[0044] This invention relates to a time synchronization method based on low-Earth orbit (LEO) satellites and the BeiDou PPP-B2b system. It utilizes an external standard time source at the central station and an external local time source at the client. The central station and client establish a communication link via LEO satellites, and real-time PPP time transfer is achieved through BeiDou-3 precise point positioning service and LEO satellite positioning service. The client can achieve sub-nanosecond time synchronization without relying on the internet. Furthermore, this invention enhances time synchronization performance under interference, obstruction, and complex environments by using LEO satellites for time synchronization and communication assistance. Attached Figure Description

[0045] Figure 1 This is a flowchart illustrating the time synchronization method based on low-Earth orbit satellites and BeiDou PPP-B2b.

[0046] Figure 2 Basic schematic diagram for crystal oscillator discipline. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0048] This embodiment of a time synchronization method based on low-Earth orbit satellites and BeiDou PPP-B2b includes the following steps:

[0049] 1) The central station and the client respectively acquire real-time PPP-B2b signals, pseudorange, carrier phase observations and broadcast ephemeris broadcast by BDS-3 satellite, and pseudorange, carrier phase observations and broadcast ephemeris broadcast by low-orbit satellite through their respective receivers.

[0050] 2) The central station and the client respectively calculate the clock bias of the central station receiver and the clock bias of the client receiver through the following steps:

[0051] ① Satellite orbit correction:

[0052] The satellite position vector X is calculated using the broadcast ephemeris received by the receiver. brdc ,

[0053] The received PPP-B2b signal is decoded, and the orbit information contained in the PPP-B2b signal is matched with the broadcast ephemeris to calculate the satellite position correction vector ΔX; the formula for calculating ΔX is:

[0054]

[0055] In the formula, r, These represent the satellite's position vector and velocity vector in the broadcast ephemeris, respectively; e R e C e A These are the unit vectors corresponding to the radial, tangential, and normal directions, respectively; Δ orbit This is the orbital correction vector;

[0056] Then calculate the corrected satellite position using the following formula:

[0057] X orbit =X brdc -ΔX (2)

[0058] In the formula, X orbit This indicates the corrected satellite position.

[0059] ② Satellite clock error correction:

[0060] After the orbit information contained in the PPP-B2b signal is successfully matched with the broadcast ephemeris, the version number in the satellite clock correction information is then matched with the version number in the orbit correction message, as follows:

[0061]

[0062] In the formula, t s-c t represents the corrected satellite clock bias correction; brdc This represents the satellite clock correction number calculated from the broadcast ephemeris; C0 represents the clock correction parameter provided by PPP-B2b.

[0063] ③ Satellite inter-symbol bias correction:

[0064] By comparing the differential code bias (DCB) correction types in PPP-B2b, real-time correction of satellite inter-symbol bias is performed to achieve synchronization processing of various signals. The correction algorithm is as follows:

[0065] P corr =P ini -P sig (4)

[0066] In the formula, P corr P represents the corrected observation; ini P represents the original observation value; sig This indicates the inter-symbol deviation of the corresponding signal.

[0067] ④ Using pseudorange and carrier observations from BDS-3 and low-Earth orbit satellites, a mathematical model for joint precise point positioning using BDS and low-Earth orbit satellites is derived:

[0068] Both BDS and low-Earth orbit satellites transmit dual-frequency signals. By combining the two frequencies, the first-order term of ionospheric error is eliminated. The mathematical model of the combined observations is expressed by the following equation:

[0069]

[0070] In the formula, f1 and f2 are carrier frequencies; P1, P2, Φ1, and Φ2 are the pseudorange and carrier observation values ​​corresponding to f1 and f2, respectively; P IF Φ IF These are pseudorange and carrier observations, respectively, for the ionospheric combination.

[0071] The ionospheric pseudorange and carrier phase observations of client r and satellite s are expressed by the following formula:

[0072]

[0073] In this context, the superscript B indicates a BDS satellite, the upper L indicates a low-Earth orbit satellite, the subscript IF indicates an ionospheric-free combination, the subscript r indicates a client, P indicates pseudorange, and Φ indicates carrier observation. dt represents the actual distance from the client r to the satellite s; c represents the speed of light; dt r dt represents the receiver clock bias. s Indicates satellite clock bias; d trop For tropospheric delay; d orb λ is the satellite orbit error; λ1 is the wavelength corresponding to the carrier frequency f1; N is the integer ambiguity of the carrier; ε(P) is the observation noise error of the pseudorange; and ε(Φ) is the observation noise error of the carrier.

[0074] ⑤ Using the BeiDou satellite broadcast ephemeris orbit, satellite clock bias, and satellite inter-code bias corrected in steps ①, ②, and ③, as well as the low-Earth orbit satellite orbit, low-Earth orbit satellite clock bias, pseudorange, and carrier phase observations, the precise single-point positioning mathematical model described in step ④ is solved to obtain the clock bias of the central station receiver and the clock bias of the client receiver.

[0075] 2) The client calculates the difference between its local time and the central station time:

[0076] The calculated clock difference of the central station receiver is sent to the low-Earth orbit satellite via the low-Earth orbit satellite communication terminal. The client obtains the clock difference of the central station receiver sent by the central station through the low-Earth orbit satellite communication terminal. The time difference between the two locations is obtained by subtracting the clock difference of the central station receiver from the client's receiver clock difference.

[0077] 3) The time difference between the client and the central station is input into the client's digital frequency synthesis module. The clock error of the client receiver is adjusted through the digital frequency synthesis module. The digital frequency synthesis module includes a frequency multiplier connected to the client receiver's oven-controlled crystal oscillator, a phase accumulator connected to the frequency multiplier and the frequency control word module, an adder connected to the phase accumulator and the phase correction value module, a phase amplitude lookup table connected to the adder, a digital-to-analog converter connected to the phase amplitude lookup table, and a bandpass filter connected to the digital-to-analog converter. The time difference between the client and the central station is input into the frequency control word module and the phase correction value module, respectively. The signal output by the oven-controlled crystal oscillator is multiplied by the frequency multiplier and the output signal is the working clock of the client receiver.

[0078] The basic structure of the BeiDou disciplined clock is actually a low-frequency phase-locked loop with a reference frequency containing a certain amount of noise. Its working principle is as follows: Figure 2 As shown, the original 10MHz output of the oven-controlled crystal oscillator is multiplied. According to the Nyquist sampling theorem, the operating clock should be greater than 20MHz. Considering the sampling points and quantization noise, this embodiment selects a 4x multiplication, i.e., the operating clock is 40MHz.

[0079] In this embodiment, adjusting the operating clock of the client receiver in step 3) includes the following steps:

[0080] (1) Adjust the frequency of the operating clock output of the frequency multiplier. The adjustment algorithm is as follows:

[0081] F(n)=F(n-1)+α0×clkdrift (8)

[0082] FCW = F(n) × 2 48 / Fsysclk (9)

[0083] In the formula, α0 is the loop system of the first-order frequency-locked loop. The selection principle is that the frequency adjustment amount should not exceed the pulling range of the receiver baseband loop. If it is too large, it will easily lead to the receiver losing lock. clkdrift is the clock drift value. Fsysclk is the working clock. F(n) is the calculated frequency value. FCW is the frequency control word.

[0084] (2) When the clkdrift digital value approaches the frequency division, phase adjustment is initiated. Phase correction is used to gradually correct the receiver's clock error to near 0. The time adjustment accuracy is Tres, Tres = 0.1us × 2. -14 =6ps, with an adjustment range of (-100, +100)ns;

[0085] (3) When the clock error gradually approaches Tres, the phase-locked loop (PLL) stage is entered. The PLL is used to track clkdrift and clock error, and to tame and converge the receiver clock error to close to 0.

[0086] The frequency division of the clock generated by the digital frequency synthesis module depends on the number of bits in the phase accumulator. In this embodiment, a 48-bit accumulator is used, so the frequency division is 40MHz × 2. -48 =0.142uHz, with an adjustment precision of 0.0142×10⁻¹⁰ relative to 10MHz. -6 In addition to frequency correction, the digital frequency synthesis module also performs phase correction. In this embodiment, the phase correction amount is 14 bits, which is directly added to the high 14 bits of the phase accumulator. For a 10MHz clock, the adjustable time resolution is Tres = 0.1µs × 2. -14 =6ps, the accuracy has exceeded that of clock error calculation.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A time synchronization method based on low-Earth orbit satellites and BeiDou PPP-B2b, characterized in that: Includes the following steps: 1) The central station and the client respectively acquire real-time PPP-B2b signals, pseudorange, carrier phase observations and broadcast ephemeris broadcast by BDS-3 satellite, and pseudorange, carrier phase observations and broadcast ephemeris broadcast by low-orbit satellite through their respective receivers; 2) The central station and the client respectively calculate the clock bias of the central station receiver and the clock bias of the client receiver through the following steps: Satellite orbit correction: The satellite position vector is calculated using the broadcast ephemeris received by the receiver. , The received PPP-B2b signal is decoded, and the orbit information contained in the PPP-B2b signal is matched with the broadcast ephemeris to calculate the corresponding BeiDou satellite position correction vector. ; The calculation formula is: （1） In the formula, , These represent the satellite's position vector and velocity vector in the broadcast ephemeris, respectively; , , These correspond to the unit vectors in the radial, tangential, and normal directions, respectively. This is the orbital correction vector; Then calculate the corrected satellite position using the following formula: （2） In the formula, Indicates the corrected satellite position; Satellite clock correction: After the orbit information contained in the PPP-B2b signal is successfully matched with the broadcast ephemeris, the version number in the satellite clock correction information is then matched with the version number in the orbit correction message, as follows: （3） In the formula, This indicates the corrected satellite clock bias correction. This represents the satellite clock error correction calculated from the broadcast ephemeris; This indicates the clock correction parameters provided by PPP-B2b; Satellite inter-symbol bias correction: By referring to the differential code offset correction types in PPP-B2b, satellite inter-code offsets are corrected in real time to achieve synchronization processing of various signals. The correction algorithm is as follows: （4） In the formula, Indicates the corrected observation value; Represents the original observation values; This indicates the inter-symbol deviation of the corresponding signal; Using pseudorange and carrier observations from BDS-3 and low-Earth orbit satellites, a mathematical model for joint precise point positioning using BDS and low-Earth orbit satellites is derived: Both BDS and low-Earth orbit satellites transmit dual-frequency signals. By combining the two frequencies, the first-order term of ionospheric error is eliminated. The mathematical model of the combined observations is expressed by the following equation: (5) In the formula, , For carrier frequency; , , , They are respectively , The corresponding pseudorange and carrier observations; , These are pseudorange and carrier observations, respectively, for the ionospheric combination. The ionospheric pseudorange and carrier phase observations of client r and satellite s are expressed by the following formula: （6） In this context, the superscript B indicates a BDS satellite, the upper L indicates a low-Earth orbit satellite, the subscript IF indicates an ionospheric-free combination, and the subscript r indicates a client. Indicates pseudorange, Represents carrier observations; The distance from client r to satellite s is denoted by r; c is the speed of light. Indicates receiver clock bias. Indicates satellite clock bias; For tropospheric delay; For satellite orbital error; carrier frequency The corresponding wavelength; For integer ambiguity of the carrier wave; The observation noise error is due to the pseudorange. This refers to the observation noise error of the carrier wave. Utilization steps , and The corrected BeiDou satellite broadcast ephemeris orbit, satellite clock bias, and inter-symbol bias, as well as the low-Earth orbit satellite orbit, low-Earth orbit satellite clock bias, pseudorange, and carrier phase observations, are used for the steps. The precise single-point positioning mathematical model described in the paper is used to solve the clock error of the central station receiver and the clock error of the client receiver. 3) The client calculates the difference between its local time and the central station time: The calculated clock difference of the central station receiver is sent to the low-orbit satellite through the low-orbit satellite communication terminal. The client obtains the clock difference of the central station receiver sent by the central station through the low-orbit satellite communication terminal. The time difference between the two locations is obtained by subtracting the clock difference of the central station receiver from the clock difference of the client receiver. 4) Input the time difference between the client and the central station into the client's digital frequency synthesis module. Adjust the clock error of the client receiver through the digital frequency synthesis module. The digital frequency synthesis module includes a frequency multiplier connected to the client receiver's oven-controlled crystal oscillator, a phase accumulator connected to the frequency multiplier and the frequency control word module, an adder connected to the phase accumulator and the phase correction value module, a phase amplitude lookup table connected to the adder, a digital-to-analog converter connected to the phase amplitude lookup table, and a bandpass filter connected to the digital-to-analog converter. The time difference between the client and the central station is input into the frequency control word module and the phase correction value module, respectively. The signal output by the oven-controlled crystal oscillator is multiplied by the frequency multiplier and the output signal is the working clock of the client receiver. Adjusting the operating clock of the client receiver includes the following steps: (1) Adjust the frequency of the operating clock output of the frequency multiplier. The adjustment algorithm is as follows: （8） （9） In the formula, For a first-order frequency-locked loop system, the selection principle is that the frequency adjustment amount should not exceed the pulling range of the receiver baseband loop, as excessive adjustment can easily lead to receiver loss of lock. This is the clock drift value; For working clock; It is the calculated frequency value. It is a frequency control word; (2) When the clkdrift digital value approaches the frequency division, phase adjustment is switched to gradually correct the receiver's clock error to near 0 using phase correction. The accuracy of the time adjustment is Tres, Tres = 0.1us × 2 -14 =6ps, adjustable range is (-100, +100)ns; (3) When the clock error gradually approaches Tres, the phase-locked loop (PLL) stage is entered. The PLL is used to track clkdrift and clock error, and to tame and converge the receiver clock error to close to 0.

Images