A fast acquisition method for Beidou intersatellite link signals

Abstract

In the inter-satellite link communication measurement scenario, the present invention improves the traditional ephemeris-assisted signal capture method: the proposed secondary bit estimation method can better determine the position of data bit flipping, extend the coherent integration time, and improve the signal-to-noise ratio of the processed signal; the block accumulation method can reduce the FFT calculation amount, save calculation time, and improve the calculation efficiency; at the same time, the differential coherent integration is used instead of the traditional incoherent integration, which can effectively reduce the square loss problem caused by the incoherent integration.

Description

Technical Field

[0001] The invention belongs to the technical field of navigation inter-satellite link measurement and communication, and in particular relates to a fast capture method suitable for Beidou inter-satellite link signals. Background Art

[0002] The BeiDou-3 global satellite navigation system (BDS-3) was completed on July 31, 2020, and officially provides positioning, navigation and timing services to the world. BDS-3 satellites are equipped with Ka-band intersatellite link payloads, which can achieve precise orbit determination and time synchronization through intersatellite precision measurement and data transmission. By establishing a measurement communication link between navigation satellites, the intersatellite link can continuously correct the satellite's long-term forecast ephemeris and clock parameters without the support of the ground measurement and control system, autonomously generate navigation messages, and realize autonomous navigation functions, thereby improving orbit determination accuracy and enhancing the constellation configuration autonomous maintenance capability.

[0003] At present, the Beidou intersatellite link adopts a spread spectrum communication system to achieve stable and reliable signal capture, which is a key step for the Beidou navigation receiver to realize subsequent signal tracking, message demodulation, pseudo-range observation extraction and other functions. The capture of spread spectrum signals is a two-dimensional search process of pseudo-code and carrier. The size of the search range directly determines the speed and difficulty of signal capture. Through the analysis of the constellation topology and on-board routing strategy, it can be seen that there are several difficulties in the capture of intersatellite link signals: First, the large relative motion between satellites will cause a large range of Doppler frequency shift, which brings great difficulties to frequency alignment; second, the large range of intersatellite distance changes increases the uncertainty of radio signal transmission delay; third, the on-board computing resources and time are limited. The Beidou intersatellite link adopts a time-division and space-division access control system. Each link needs to complete the capture, tracking and demodulation of the signal within the allocated short time slot, which puts forward high requirements for the reliability and efficiency of the capture algorithm.

[0004] In view of the contradiction between the large dynamic range of the intersatellite link signal of the navigation constellation and the high capture performance requirements, the traditional signal capture method is difficult to meet the requirements. The main solution currently proposed is to use ephemeris information to assist signal capture. For the navigation constellation, each satellite stores navigation messages, including the broadcast ephemeris of the satellite and the almanacs of other satellites in the constellation. Using this information can provide the possibility of reducing the signal capture search range. The schematic diagram of the ephemeris-assisted signal capture method is shown in the figure. Figure 1 The specific steps of this method are as shown in Figure 2 As shown, the detailed description is as follows:

[0005] In the first step, satellite j (target satellite) uses the satellite broadcast ephemeris and the almanac of satellite i (source satellite) to calculate the coordinate vector at a given epoch and Here, t1 is defined as the time when satellite i transmits the signal, and t2 is the time when the signal arrives at satellite j;

[0006] The second step is to solve the transmission delay t through iterative calculation. ij ,have

[0007]

[0008] The specific process of time-delay iterative solution is as follows:

[0009] (1) Calculate the coordinate vector at time t1 from the ephemeris and almanac information and

[0010] (2) Calculate the transmission delay τ1:

[0011]

[0012] And solve the coordinate vector of satellite j at time t1+τ1 according to the broadcast ephemeris

[0013] (3) Calculate the transmission delay τ n+1 :

[0014]

[0015] Calculate the transmission delay difference τ d =τ n+1 -τ n ;

[0016] (4) Set the convergence threshold ε, when τ d When ≤ε, the transmission delay When τ d >ε, let τ n+1 =τ n , repeat (3) (4) until τ d ≤ε.

[0017] Step 3: Get the transmission delay After that, the velocity vector of satellite i at time t1 can be calculated based on the ephemeris and almanac information and satellite j in Velocity vector at time Then the Doppler frequency f of the received signal d for:

[0018]

[0019] Among them, f T is the signal transmission frequency, and θ is the angle between the velocity vector and the direction vector.

[0020] Step 4: Calculate the transmission delay based on the ephemeris and almanac information and Doppler frequency f d , greatly reducing the original frequency and chip search range, and reducing the transmission delay and Doppler frequency f d Serves as the starting search point for a two-dimensional search.

[0021] At this time, traditional capture methods (such as serial, code parallel, frequency parallel capture, etc.) are used to complete the signal capture. The specific principle is as follows Figure 3 shown.

[0022] The fifth step is to multiply the input intersatellite link intermediate frequency signal with the in-phase and quadrature signals output by the local carrier generator, and obtain the baseband complex signal I through a low-pass filter. L +jQ L , and perform FFT on the obtained complex signal;

[0023] Step 6: Perform FFT on the pseudo code signal output by the local pseudo code generator and take the conjugate, then multiply it with the result of step 5 and perform IFFT transformation on the product;

[0024] Step 7: modulo the IFFT result of step 6 and perform coherent integration within a data bit range (to avoid reducing the gain effect due to data inversion) to obtain coherent gain and improve the signal-to-noise ratio.

[0025] Step 8: To further improve the signal-to-noise ratio of the signal, the coherent gain result is incoherently integrated. Generally, the time length of several data bits can be selected, which will bring incoherent gain while also introducing square loss.

[0026] In the ninth step, threshold judgment is performed on the result. If a sufficiently strong peak appears, it means that signal capture is achieved. The position corresponding to the peak corresponds to the pseudo-code phase, and the frequency value of the local carrier at this time is the carrier frequency of the signal. If no sufficiently strong peak appears, the frequency of the local carrier generator is reset to the value of the next frequency well, and steps five to eight are repeated.

[0027] Although the acquisition method introduced above can greatly reduce the search range in the frequency domain and code domain with the help of the ephemeris-assisted signal acquisition method, the uncertainty range of the delay and Doppler frequency is still very large, and the traditional FFT-based code parallel acquisition algorithm still consumes a lot of computing time. At the same time, when the signal transmission quality is poor or the inter-satellite communication distance is far, the acquisition efficiency and reliability of this method are also low. Summary of the invention

[0028] In view of this, the present invention proposes a more efficient and sensitive FFT code parallel acquisition algorithm based on the ephemeris auxiliary signal acquisition strategy. On the one hand, the method uses the block accumulation method to reduce the amount of capture calculation and shorten the running time of the long-time integration acquisition algorithm; at the same time, the method uses the quadratic half-bit estimation method to solve the influence of navigation data jump on coherent integration, prolongs the coherent integration time to 3 / 4 of the data bit length, and uses differential coherent integration to offset the square loss caused by traditional incoherent integration, thereby improving capture sensitivity and reliability.

[0029] A fast acquisition method for Beidou intersatellite link signals, comprising:

[0030] In the first step, the target satellite j uses the broadcast ephemeris of the satellite and the almanac of the source satellite i to calculate the coordinate vector of the target satellite at the two given epochs respectively;

[0031] The second step is to solve the transmission delay t according to the coordinate vector of the target satellite at two given epochs. ij ;

[0032] Step 3: Get the transmission delay After that, the Doppler frequency f of the received signal can be calculated based on the ephemeris and almanac information. d ;

[0033] Step 4: Calculate the transmission delay based on the ephemeris and almanac information and Doppler frequency f d , the transmission delay and Doppler frequency f d As the starting search point for a two-dimensional search;

[0034] Step 5: Multiply the input intersatellite link intermediate frequency signal with the in-phase and quadrature signals output by the local carrier generator, and obtain the baseband complex signal through a low-pass filter;

[0035] Step 6: Perform the first half-bit block accumulation, specifically:

[0036] For the baseband complex signal after filtering in the fifth step, select signals with at least five consecutive data bit lengths in the signal, the number of which is set to n, divide each bit data into the first half and the second half to obtain two half-bit data, regard each half-bit data as multiple data units with 1ms as a unit, and accumulate the sampling points at the same position of all data units, thereby compressing the half-bit data block into a 1ms data block, that is, realizing the block accumulation operation;

[0037] Step 7: Perform FFT transformation on the 2n 1ms data blocks compressed in step 6 respectively, and perform FFT and conjugate the pseudo code signal output by the local pseudo code generator, and then multiply it with each FFT-processed data block and perform IFFT transformation, so as to obtain 2n groups of IFFT transformation results;

[0038] Step 8: modulo the n groups of IFFT transformation results corresponding to the first and second half bits and perform non-coherent accumulation to obtain two groups of accumulation results; respectively take the maximum values ​​of the two groups of results, compare the two maximum values, and the half-bit group corresponding to the smaller maximum value has a navigation data jump;

[0039] The ninth step is to determine the half-bit position where the navigation data jumps, and then perform a second half-bit estimation and block accumulation on the half-bit data where the data jump occurs. Specifically, the method of the sixth step is used to divide each half-bit data where the data jump occurs into the first half and the second half to obtain two quarter-bit data, and then continue to perform the block accumulation operation according to the method of the sixth step, and then according to the methods of the seventh and eighth steps, the position of the data jump is accurately calculated to 1 / 4 of the data bit length; the tenth step is to determine that the range without flipping within a bit data is the remaining three quarters of the bit data excluding the quarter-bit data where the data jump occurs, and the IFFT transformation results of the bit data within the range without the jump are coherently accumulated;

[0040] In the eleventh step, the results of the coherent accumulation are subjected to differential coherent accumulation, specifically:

[0041] For the n groups of coherent accumulation results obtained in the tenth step, two adjacent groups are defined as a data pair, and n-1 data pairs are obtained. The previous group of coherent accumulation results in each data pair is conjugated and multiplied with the next group of coherent accumulation results to obtain the multiplication result of the data pair; the multiplication results of the n-1 data pairs are accumulated to obtain the differential coherent accumulation result;

[0042] In the twelfth step, a threshold decision is made on the differential coherent accumulation result. The position corresponding to the peak corresponds to the pseudo-code phase, and the frequency value of the local carrier at this time is the carrier frequency of the signal. If no peak appears, the frequency of the local carrier generator is reset to the value of the next frequency well, and steps five to eleven are repeated.

[0043] Preferably, in the first step, the coordinate vectors at a given epoch are expressed as and Where t1 is the time when satellite i transmits the signal, and t2 is the time when the signal arrives at satellite j;

[0044] In the second step, solve the transmission delay t ij The formula is:

[0045]

[0046] Preferably, in the second step, the transmission delay t is solved by iterative calculation ij The specific process includes:

[0047] (1) Calculate the coordinate vector at time t1 from the ephemeris and almanac information and

[0048] (2) Calculate the transmission delay τ1:

[0049]

[0050] And solve the coordinate vector of satellite j at time t1+τ1 according to the broadcast ephemeris

[0051] (3) Calculate the transmission delay τ n+1 :

[0052]

[0053] Calculate the transmission delay difference τ d =τ n+1 -τ n ;

[0054] (4) Set the convergence threshold ε, when τ d When ≤ε, the transmission delay When τ d >ε, let τ n+1 =τ n , repeat (3) and (4) until τ d ≤ε.

[0055] Preferably, in the third step, the Doppler frequency f of the received signal d The calculation methods include:

[0056] Get transmission delay Then, the velocity vector of satellite i at time t1 is calculated based on the ephemeris and almanac information. and satellite j in Velocity vector at time The Doppler frequency f of the received signal d for:

[0057]

[0058] where f T is the signal transmission frequency, and θ is the angle between the velocity vector and the direction vector.

[0059] The present invention has the following beneficial effects:

[0060] In the inter-satellite link communication measurement scenario, the present invention improves the traditional ephemeris-assisted signal capture method: the proposed secondary bit estimation method can better determine the position of data bit flipping, extend the coherent integration time, and improve the signal-to-noise ratio of the processed signal; the block accumulation method can reduce the FFT calculation amount, save calculation time, and improve the calculation efficiency; at the same time, the differential coherent integration is used instead of the traditional incoherent integration, which can effectively reduce the square loss problem caused by the incoherent integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a schematic diagram of the existing ephemeris auxiliary signal acquisition method;

[0062] Figure 2 The present invention is a flow chart of the existing ephemeris auxiliary signal acquisition method;

[0063] Figure 3 The schematic diagram for the existing FFT-based parallel capture;

[0064] Figure 4 A flow chart of the improved algorithm for capturing the ephemeris-aided signal provided by the present invention;

[0065] Figure 5 It is the principle diagram of FFT code parallel capture of the present invention;

[0066] Figure 6 This is a schematic diagram of the first half-bit estimation in an embodiment of the present invention;

[0067] Figure 7 Schematic diagram of the second half-bit estimation in an embodiment of the present invention. DETAILED DESCRIPTION

[0068] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0069] The present invention provides a fast capture method applicable to Beidou intersatellite link signals, which can improve the sensitivity and reliability of intersatellite link signal capture and reduce the amount of calculation.

[0070] The algorithm is described in detail below. First, assume that the center frequency of the signal is 23 GHz, the spreading code rate is 10.23 MHz, the spreading code period is 1 ms, and the inter-satellite data transmission rate is 50 bps, that is, the length of a data bit is 20 ms, corresponding to 20 spreading code period lengths. Figure 4 The specific steps of the algorithm proposed by the present invention are shown as follows: Figure 5 To improve the principle diagram of FFT code parallel acquisition, the method is mainly divided into the following steps (the first four steps are consistent with the traditional method, i.e., the steps of ephemeris-assisted capture range reduction):

[0071] In the first step, satellite j (target satellite) uses the satellite broadcast ephemeris and the almanac of satellite i (source satellite) to calculate the coordinate vector at a given epoch and Here, t1 is defined as the time when satellite i transmits the signal, and t2 is the time when the signal arrives at satellite j;

[0072] The second step is to solve the transmission delay t through iterative calculation. ij ,have

[0073]

[0074] The specific process of time-delay iterative solution is as follows:

[0075] (1) Calculate the coordinate vector at time t1 from the ephemeris and almanac information and

[0076] (2) Calculate the transmission delay τ1:

[0077]

[0078] And solve the coordinate vector of satellite j at time t1+τ1 according to the broadcast ephemeris

[0079] (3) Calculate the transmission delay τ n+1 :

[0080]

[0081] Calculate the transmission delay difference τ d =τ n+1 -τ n ;(4) Set the convergence threshold ε, when τ d When ≤ε, the transmission delay When τ d >ε, let τ n+1 =τ n , repeat (3) (4) until τ d ≤ε.

[0082] Step 3: Get the transmission delay After that, the velocity vector of satellite i at time t1 can be calculated based on the ephemeris and almanac information and satellite j in Velocity vector at time Then the Doppler frequency f of the received signal d for:

[0083]

[0084] where f Tis the signal transmission frequency, and θ is the angle between the velocity vector and the direction vector.

[0085] Step 4: Calculate the transmission delay based on the ephemeris and almanac information and Doppler frequency f d , reducing the original frequency and chip search range, and delaying the transmission and Doppler frequency f d As the starting search point for a two-dimensional search;

[0086] The fifth step is to multiply the input intersatellite link intermediate frequency signal with the in-phase and quadrature signals output by the local carrier generator, and obtain the baseband complex signal I through a low-pass filter. L +jQ L ;

[0087] Step 6: Perform the first half-bit block accumulation.

[0088] In order to avoid the situation where adjacent data bits are not inverted, for the baseband complex signal after the fifth step of filtering, the present invention selects a signal with at least five consecutive data bits in the signal for analysis. In this embodiment, 5 bits are selected. Figure 6 As shown, each bit of data is divided into the first half and the second half to obtain two half-bit data. In this embodiment, each data bit is divided into two parts, the first 10ms and the second 10ms. The first half of each data bit is named F1, F2, F3, F4, F5, and the second half of each data bit is named L1, L2, L3, L4, L5. Since the pseudocode signal is a periodic signal with a period of 1ms, the 10ms length signal of each half-bit data in this embodiment can be regarded as 10 data units with 1ms as a unit. By accumulating the sampling points at the same position of these 10 data units, each data block with a length of 10ms can be compressed into a data block of 1ms, that is, the block accumulation operation is realized.

[0089] Step 7: Perform FFT transformation on the 10 1ms data blocks compressed in step 6 respectively, and perform FFT and conjugate the pseudo code signal output by the local pseudo code generator, and then multiply it with each FFT-processed data block and perform IFFT transformation, so as to obtain 10 groups of IFFT transformation results;

[0090] Step 8: Modulo the 5 groups of IFFT transformation results corresponding to the first and second half bits and perform non-coherent accumulation to obtain two groups of accumulation results. Take the maximum values ​​of the two groups of results respectively, compare the two maximum values, and the half-bit group corresponding to the smaller maximum value has a navigation data jump; in this embodiment, it is assumed that the navigation data jump exists within the first 10ms of the data bit;

[0091] In the ninth step, after determining the half-bit position where the navigation data jumps, in order to further determine the position of the bit flip, extend the duration of the coherent integration, and improve the signal-to-noise ratio of the signal, a second half-bit estimation and block accumulation are performed on the half-bit data where the data jump occurs. In this embodiment, the jump occurs in the first 10ms of the data bit, so a second half-bit estimation and block accumulation are performed on this part, such as Figure 7 As shown in the figure, it is a schematic diagram of the second half-bit estimation. As in the first half-bit estimation step, the method of step 6 is used to divide each half-bit data with data jump into the first half and the second half to obtain two quarter-bit data, and then the block accumulation operation is continued according to the method of step 6, and then the position of the data jump is accurately calculated to 1 / 4 of the data bit length according to the methods of steps 7 and 8. In this embodiment, it is assumed that the position of the data jump is within the first half of each data bit (i.e., the first 5ms of 10m), that is, Figure 7 The intervals corresponding to f1, f2, f3, f4, and f5;

[0092] In the tenth step, by two half-bit estimations, it can be determined that the range of a bit data without flipping is the remaining three quarters of the bit data excluding the quarter-bit data where data jump occurs (the length without jump is 15ms in this embodiment), and the IFFT transformation results of the bit data within the range without jump (calculated in steps eight and nine) are coherently accumulated to obtain 5 groups of coherent accumulation results with a length of 1ms;

[0093] In the eleventh step, the results of the coherent accumulation are subjected to differential coherent accumulation, specifically:

[0094] For the 5 groups of coherent accumulation results obtained in the previous step, two adjacent groups are defined as a data pair, so 4 data pairs are obtained. In each data pair, the previous group of coherent accumulation results is conjugated and multiplied with the next group of coherent accumulation results to obtain the multiplication result of the data pair; the multiplication results of the four data pairs are accumulated to obtain the differential coherent accumulation result;

[0095] In the twelfth step, a threshold decision is made on the differential coherent accumulation result. If a sufficiently strong peak appears, it indicates that signal capture is achieved. The position corresponding to the peak corresponds to the pseudo-code phase, and the frequency value of the local carrier at this time is the carrier frequency of the signal. If no sufficiently strong peak appears, the frequency of the local carrier generator is reset to the value of the next frequency well, and steps five to eleven are repeated.

[0096] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for quickly capturing Beidou intersatellite link signals, characterized in that: include: In the first step, the target satellite j uses the broadcast ephemeris of the satellite and the almanac of the source satellite i to calculate the coordinate vector of the target satellite in the receiving epoch and the coordinate vector of the source satellite in the transmitting epoch respectively; The second step is to solve the approximate transmission delay based on the coordinate vector of the target satellite at the receiving epoch and the coordinate vector of the source satellite at the transmitting epoch. The third step is to obtain the approximate transmission delay Then, the Doppler frequency f of the received signal is calculated based on the ephemeris and almanac information. d ; Step 4: Calculate the approximate transmission delay based on the ephemeris and almanac information and Doppler frequency f d , will approximate the transmission delay and Doppler frequency f d As the starting search point for a two-dimensional search; Step 5: Multiply the input intersatellite link intermediate frequency signal with the in-phase and quadrature signals output by the local carrier generator, and obtain the baseband complex signal through a low-pass filter; Step 6: Perform the first half-bit block accumulation, specifically: For the baseband complex signal after filtering in the fifth step, select signals with at least five consecutive data bit lengths in the signal, the number of which is set to n, divide each bit data into the first half and the second half to obtain two half-bit data, regard each half-bit data as multiple data units with 1ms as a unit, and accumulate the sampling points at the same position of all data units, thereby compressing the half-bit data block into a 1ms data block, that is, realizing the block accumulation operation; Step 7: Perform FFT transformation on the 2n 1ms data blocks compressed in step 6 respectively, and perform FFT and conjugate the pseudo code signal output by the local pseudo code generator, and then multiply it with each FFT-processed data block and perform IFFT transformation, so as to obtain 2n groups of IFFT transformation results; Step 8: modulo the n groups of IFFT transformation results corresponding to the first and second half bits and perform non-coherent accumulation to obtain two groups of accumulation results; respectively take the maximum values ​​of the two groups of results, compare the two maximum values, and the half-bit group corresponding to the smaller maximum value has a navigation data jump; Step 9, after determining the half-bit position where the navigation data jump exists, perform a second half-bit estimation and block accumulation on the half-bit data where the data jump occurs, specifically: using the method of step 6, divide each half-bit data where the data jump occurs into the first half and the second half to obtain two quarter-bit data, and then continue to perform the block accumulation operation according to the method of step 6, and then use the methods of step 7 and step 8 to accurately estimate the position of the data jump to 1 / 4 of the data bit length; Step 10: determine that the range of a bit data without flipping is the remaining three quarters of the bit data excluding the one quarter bit data with data jump, and perform coherent accumulation on the IFFT transformation results of the bit data within the range without jump; In the eleventh step, the results of the coherent accumulation are subjected to differential coherent accumulation, specifically: For the n groups of coherent accumulation results obtained in the tenth step, two adjacent groups are defined as a data pair, and n-1 data pairs are obtained. The previous group of coherent accumulation results in each data pair is conjugated and multiplied with the next group of coherent accumulation results to obtain the multiplication result of the data pair; the multiplication results of the n-1 data pairs are accumulated to obtain the differential coherent accumulation result; In the twelfth step, a threshold decision is made on the differential coherent accumulation result. The position corresponding to the peak corresponds to the pseudo-code phase, and the frequency value of the local carrier at this time is the carrier frequency of the signal. If no peak appears, the frequency of the local carrier generator is reset to the value of the next frequency well, and steps five to eleven are repeated.

2. A method for quickly capturing Beidou intersatellite link signals as claimed in claim 1, characterized in that: In the first step, the coordinate vectors at a given epoch are expressed as and Where t1 is the time when satellite i transmits the signal, and t2 is the time when the signal arrives at satellite j; In the second step, solve the actual transmission delay t ij The formula is: Solving the approximate transmission delay through iterative calculation As the actual transmission delay t ij The specific process includes: (1) Calculate the coordinate vector at time t1 from the ephemeris and almanac information and (2) Calculate the first transmission delay τ1: And solve the coordinate vector of satellite j at time t1+τ1 according to the broadcast ephemeris (3) Calculate the n+1th transmission delay τ n+1 : Calculate the transmission delay difference τ d =τ n+1 -τ n ; (4) Set the convergence threshold ε, when τ d When ≤ε, the approximate transmission delay When τ d >ε, let τ n+1 =τ n , repeat (3) and (4) until τ d ≤ε.

3. A method for quickly capturing Beidou intersatellite link signals as claimed in claim 1, characterized in that: In the third step, the Doppler frequency f of the received signal d The calculation methods include: Get transmission delay Then, the velocity vector of satellite i at time t1 is calculated based on the ephemeris and almanac information. and satellite j in Velocity vector at time The Doppler frequency f of the received signal d for: where f T is the signal transmission frequency, and θ is the angle between the velocity vector and the direction vector.

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