An efficient acquisition method for short burst signals from low-orbit satellites

Abstract

The present invention discloses an efficient capture method for short burst signals of low-orbit satellites, comprising: S1, storing satellite signals in a signal buffer, and waiting for the sample length of the signal buffer to exceed a CW field length; S2, reading fixed-length data from the signal buffer to perform parallel PMF-FFT frequency-related operations, and outputting detection quantities corresponding to multiple frequency points; S3, storing the detection quantities corresponding to multiple frequency points in an FFT buffer, performing extreme value search and threshold judgment on the time slot detection sequence of each frequency point, and outputting sample point information of successful frequency capture; S4, after the satellite signal cache of the next time slot is completed, returning to step S2. The judgment mode of frequency capture of the present invention is the maximum value of the over-threshold, at the cost of increasing the single-carrier frequency capture calculation amount, in exchange for a significant reduction in the bit synchronization time capture calculation amount, and the total capture calculation amount is significantly reduced, and the false alarm probability of the capture method and the constant false alarm judgment criterion are consistent.

Description

Technical Field

[0001] The present invention relates to the field of satellite navigation technology, and more specifically, to an efficient capture method suitable for short burst signals of low-orbit satellites. Background Art

[0002] At present, low-orbit satellites in various countries are in a stage of vigorous development. Low-orbit satellites broadcast short-term burst communication signals to ground users to achieve communication functions with ground users. Due to the low orbit and high speed of low-orbit satellites, the Doppler frequency shift of L-band satellite signals reaches ±50KHz, the power fluctuation reaches 15dB, and the satellite visibility time is shorter. Therefore, the short burst signals broadcast by low-orbit satellites are broadcast by pre-adding short-term continuous waves to facilitate the receiver to quickly capture the carrier frequency. Usually, the structure of low-orbit short burst satellite signals is as follows: Figure 2 As shown in the figure, the synchronization header consists of a pilot CW (continuous wave) field and a pilot PRN field. The former is used for carrier synchronization frequency capture, and the latter is used for bit synchronization time capture. The waveform diagram of the synchronization header is shown in Figure 3 As shown. Single carrier frequency capture uses PMF-FFT algorithm (partial matching filter-fast Fourier transform), and the amount of calculation is proportional to (T*f s )*log2(T*f s ), the required resources are small; while the bit synchronization time capture uses multi-correlator parallel matched filtering, the calculation amount is proportional to , the amount of resources required is large, T is the length of time to search for signals, f s is the signal sampling rate. Low-orbit satellites use beamforming technology. In order to meet the requirements of large-bandwidth transmission, ground receiving terminals sometimes need to receive short-burst satellite signals from multiple beams at the same time. The receiving terminal needs to capture multiple satellite signals at the same time, which places extremely high demands on the real-time processing capabilities of the terminal. Otherwise, it will cause reception signal blocking or frame loss problems.

[0003] The capture of short burst signals includes two different stages: frequency capture and time capture. The detection quantity output by the classic frequency capture PMF-FFT algorithm uses the constant false alarm criterion for threshold detection. If the detection quantity is higher than the threshold value, it is determined that the frequency capture is successful. After the frequency capture is successful, the satellite signal is slid T cw After receiving the data of the length of (pilot CW field duration), it enters the time acquisition phase; otherwise, it is judged as frequency acquisition failure, and the sliding T pmf After the data of length (partial matched filtering duration) is collected, the detection decision is made. However, this will cause the time synchronization search width to change with the received signal power. Figure 4 It can be seen from the figure that when the signal power is weak, the time synchronization search width is small. Figure 5It can be seen that when the signal power is strong, the time synchronization search width becomes larger. In order to ensure a certain capture sensitivity, the receiver usually needs to set a smaller capture threshold, which will result in a large amount of calculation required for bit synchronization time capture, waste a lot of computing resources and computing time, and reduce the real-time processing capability of the receiver. Therefore, how to design a reasonable frequency capture detection method so that the time synchronization search width does not change with the received signal power, always maintains a fixed and optimal search width, simplifies the design of the receiver capture module, reduces the amount of calculation of the capture module, and keeps the capture real-time processing capability stable is a technical problem that needs to be solved urgently. Summary of the invention

[0004] The purpose of the present invention is to provide a high-efficiency capture method for short burst signals of low-orbit satellites, so as to overcome the defects of the prior art.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] An efficient acquisition method for short burst signals of low-orbit satellites, comprising the following steps:

[0007] S1, storing the satellite signal into the signal buffer, waiting for the sample length of the signal buffer to exceed the length of a CW field;

[0008] S2, reading out fixed-length data from the signal buffer to perform parallel PMF-FFT frequency correlation operations, and outputting detection quantities corresponding to multiple frequency points;

[0009] S3, storing the detection quantities corresponding to the multiple frequency points into the FFT cache, performing extreme value search and threshold judgment on the time slot detection sequence of each frequency point, and outputting the sample point information of successful frequency capture;

[0010] S4. After waiting for the satellite signal buffering of the next time slot to be completed, return to step S2.

[0011] Furthermore, the signal buffer depth in step S1 is set to 4*T cw *f s , where T cw is the duration of the short burst signal pilot CW field, f s is the sampling rate, the pilot CW signal s cw The expression of (n) is: ;

[0012] Where A0 is the signal amplitude, n is the sample point index, j is the imaginary number symbol, f i is the carrier frequency of the satellite signal, is the initial carrier phase, t s=1 / f s is the sampling interval of the sample points;

[0013] The sampled satellite signal r(n) is the noise signal w(n) and s cw The expression of the sampled satellite signal r(n) is: ;

[0014] In the formula, s cw (n) is the pilot CW signal, with a duration of T cw , w(n) is a continuous noise signal.

[0015] Furthermore, the signal buffer adopts a cyclic writing mode. When the signal buffer is full of N cw =T cw *f s When a signal sample point is obtained, the starting time slot index k0 is set to 0.

[0016] Furthermore, the step S2 specifically includes the following steps:

[0017] S21, read N continuously from the signal buffer cw Sampled satellite signal points , where k0 is the starting time slot index, n is the sample point index, and N pmf is the number of samples for each segment matched filter time slot;

[0018] S22, the output signal sample point and N dop +1 frequency carrier sample points are mixed simultaneously and output N dop +1 Mixing Signal :

[0019] ;

[0020] Where n dop It is the frequency segment index, and its value range is [0,N dop ],N dop +1 is the number of frequency segments, is the sample index, N dop +1 frequency local carrier signal The expression is: ;

[0021] In the formula, t s is the sampling point interval, is the sampling frequency of the partially matched filter output, T pmf is the time slot length of the partial matched filter;

[0022] S23, the output N dop +1 mixed data divided into L cwSegment, sum the sample points in each segment to perform partial matched filtering, and get L cw The segmented integral value is used as the partial matched filtering result : ;

[0023] Where, L cw is the number of time slot segments, k is the index of the partial matched filtering result, and its value range is [0:L cw -1];

[0024] S24, to N dop +1 partial matched filter result is simultaneously transformed by FFT and the FFT result is output As the detection quantity corresponding to multiple frequency points, FFT results The expression is: ;

[0025] Where m=[0,N fft -1] is the sub-frequency point index in the frequency segment, N fft is the number of FFT points.

[0026] Furthermore, the step S3 specifically includes the following steps:

[0027] S31. Store the FFT output result obtained by the time slot index k0 into the rth row of the FFT cache. The size of the FFT cache is R rows and C columns, R=L cw +1, C = N fft *(N dop +1), L cw is the number of time slot segments, N dop is the number of frequency segments, N fft is the number of FFT points, r=mod(k,R), FFT output uses a circular write-in and read-out method, mod() is a modulo operation, and the column index c of the FFT cache is set to 0;

[0028] S32, read the FFT cache data of the cth column, if the mod(r-1,R)th data in the column is a maximum value and greater than the capture threshold value, then it is determined that the frequency capture of the frequency index c and the time slot (k0-1) is successful, and the time capture starting point is set to (k0-1+L cw -2)*N pmf , output frequency capture successful sample point information;

[0029] S33, if c = N fft *(N dop +1)-1, then go to step S4, otherwise set c=c+1 and return to step S32.

[0030] Furthermore, step S4 is to wait for the signal buffer to be full again. pmf When a signal sample point is obtained, the starting time slot index k0 is set to k0+1, the frequency capture of the next time slot is performed, and the process returns to step S2.

[0031] Compared with the prior art, the advantages of the present invention are: the present invention improves the frequency capture judgment method to a maximum value of the over-threshold, at the cost of increasing the single-carrier frequency capture calculation amount in exchange for a significant reduction in the bit synchronization time capture calculation amount, thereby greatly reducing the total capture calculation amount, and at the same time, the false alarm probability of the capture method and the constant false alarm judgment criterion remain consistent. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1 The present invention is a flow chart of a highly efficient capture method for short burst signals of low-orbit satellites.

[0034] Figure 2 This is a schematic diagram of the frame structure of a short burst signal from a low-orbit satellite.

[0035] Figure 3 This is a schematic diagram of the synchronization header waveform of the short burst signal of a low-orbit satellite.

[0036] Figure 4 It is a schematic diagram of the traditional constant false alarm detection results when the satellite signal power is weak.

[0037] Figure 5 This is a schematic diagram of the traditional constant false alarm detection results when the satellite signal power is strong.

[0038] Figure 6 It is a schematic diagram of the detection result of the maximum value method of detecting the threshold in the present invention.

[0039] Figure 7 It is a schematic diagram of data sliding in step S4 of the present invention.

[0040] Figure 8 It is a structural diagram corresponding to step S2 in the present invention.

[0041] Fig. 9 It is a schematic diagram of the maximum value of the time slot sequence of a certain frequency point in step S3 of the present invention. DETAILED DESCRIPTION

[0042] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0043] In this embodiment, it is assumed that the pilot CW field duration is T cw =1ms, pilot PRN field duration T prn =1ms, partial matching filter time slot length T pmf =0.05ms, partial matched filter sampling rate F pmf =20KHz, number of partial matched filter segments L cw =20, FFT points N fft =32, signal sampling rate f s =1MHz, signal sampling interval t s =1us, the sampling length of the pilot CW field is N cw =1000, the sample length of each time slot of partial matched filtering is N pmf =50, the satellite signal Doppler range is ±50KHz, the satellite signal power range is -120dBm~-105dBm, then the number of frequency segments required is N dop =4.

[0044] See also Figure 1 As shown, this embodiment discloses an efficient capture method for short burst signals of low-orbit satellites, comprising the following steps:

[0045] Step S1: Store the satellite signal into a signal buffer, and wait until the sample length of the signal buffer exceeds a CW field length.

[0046] Step S2: Read out data of a fixed length from the signal buffer to perform parallel PMF-FFT frequency correlation operations, and output detection quantities corresponding to multiple frequency points.

[0047] Step S3: store the detection quantities corresponding to the multiple frequency points into the FFT buffer, perform extreme value search and threshold judgment on the time slot detection sequence of each frequency point, and output the sample point information of successful frequency capture.

[0048] Step S4: After waiting for the satellite signal buffering of the next time slot to be completed, return to step S2.

[0049] In this embodiment, the signal buffer depth in step S1 is set to 4000, T cw =1ms is the duration of the short burst signal pilot CW field, f s =1MHz is the sampling rate, the pilot CW signal s cw The expression of (n) is: ;

[0050] Where A0 is the signal amplitude (power fluctuation is between -120dBm and -105dBm), n is the sample point index, j is the imaginary number symbol, and f i is the carrier frequency of the satellite signal (the carrier frequency fluctuates within ±50KHz), is the initial carrier phase, t s =1us is the sampling interval of the sample points.

[0051] The sampled satellite signal r(n) is the noise signal w(n) and s cw The expression of the sampled satellite signal r(n) is: ;

[0052] In the formula, s cw (n) is the pilot CW signal, with a duration of T cw , w(n) is a continuous noise signal.

[0053] In this embodiment, the signal cache adopts a cyclic writing method. When the signal cache is full of N cw =1000 signal samples, set the starting time slot index k0=0 and go to step S2.

[0054] See also Figure 8 As shown, step S2 specifically includes the following steps:

[0055] Step S21: Continuously read N from the signal buffer cw = 1000 satellite signal samples , where k0 is the starting time slot index, n is the sample point index, and N pmf =50 is the number of samples for each segment matched filter time slot.

[0056] Step S22: compare the output signal sample points with N dop +1 carrier sample point is mixed simultaneously, and 5 mixed signals are output :

[0057] ;

[0058] Where n dop =[0,4] is the frequency segment index, 5 frequency segments, is the sample point index, the local carrier signal of 5 frequencies The expression is: Where, t s =1us is the sampling point interval, =20KHz is the sampling frequency of the partial matched filter output, T pmf =0.05ms is the time slot length of the partial matched filter.

[0059] Step S23: Perform partial matched filtering on the five mixed frequency data output simultaneously, and output the partial matched filtering result. : ;

[0060] Where, L cw =20 is the number of time slot segments, and k=[0,19] is the index of the partial matched filtering result.

[0061] Step S24: Perform FFT transformation on the five partial matched filtering results simultaneously and output the FFT results. As the detection quantity corresponding to multiple frequency points, FFT results The expression is: ;

[0062] Where m = [0,31] is the sub-frequency point index in the frequency segment, N fft =32 is the number of FFT points.

[0063] In this embodiment, the step S3 specifically includes the following steps: Step S31, storing the FFT output result obtained by the time slot index k0 into the rth row of the FFT cache, the size of the FFT cache is R rows and C columns, R=L cw +1=21, C=N fft *(N dop +1)=160,L cw is the number of time slot segments, N dop is the number of frequency segments, N fft is the number of FFT points, r=mod(k,R), FFT output adopts cyclic writing and reading mode, mod() is the modulus operation, and the structure of FFT cache is as follows Figure 8 As shown, the cache column index c=0 is set.

[0064] Step S32, read the FFT buffer data of the cth column, if the mod(r-1,R)th data in the column is a maximum value and greater than the capture threshold value, such as Fig. 9 As shown, it is determined that the frequency index c and the frequency capture of the time slot (k0-1) are successful, and the time capture starting point is set to (k0-1+L cw -2)*N pmf ,like Figure 6 As shown, this judgment method will fix the search data length captured by the bit synchronization time to 2*N. pmf The sample points captured successfully are output.

[0065] Step S33: if c=N fft *(N dop +1)-1=159, then go to step S4, otherwise set c=c+1 and return to step S32.

[0066] In this embodiment, step S4 is to wait for the signal buffer to be full again. pmf =50 signal samples, set the starting time slot index k0=k0+1, and perform frequency capture for the next time slot, such as Figure 7 As shown, and return to step S2.

[0067] The present invention improves the decision mode of single carrier frequency capture to detect the maximum value of the over-threshold, at the cost of increasing the single carrier frequency capture calculation amount in exchange for a significant reduction in the bit synchronization time capture calculation amount, so that the total capture calculation amount is greatly reduced, and at the same time, the false alarm probability of the capture method and the constant false alarm decision criterion are kept consistent.

[0068] Although the embodiments of the present invention are described in conjunction with the accompanying drawings, the owner of this application may make various variations or modifications within the scope of the appended claims. As long as they do not exceed the protection scope described in the claims of the present invention, they should be within the protection scope of the present invention.

Claims

1. An efficient capture method for short burst signals of low-orbit satellites, characterized in that: The following steps are involved: S1. Store the satellite signal into a signal buffer. The satellite signal includes a pilot CW field and a pilot PRN field. The pilot CW field is used for carrier synchronization frequency capture, and the pilot PRN field is used for bit synchronization time capture. Wait for the sample point length of the signal buffer to exceed the length of a CW field. S2, reading out fixed-length data from the signal buffer to perform parallel PMF-FFT frequency correlation operations, and outputting detection quantities corresponding to multiple frequency points; S3, storing the detection quantities corresponding to the multiple frequency points into the FFT cache, performing extreme value search and threshold judgment on the time slot detection sequence of each frequency point, and outputting the sample point information of successful frequency capture; S4. After waiting for the satellite signal buffering of the next time slot to be completed, return to step S2.

2. The efficient capture method for short burst signals of low-orbit satellites according to claim 1, characterized in that: The signal buffer depth in step S1 is set to 4*T cw *f s , where T cw is the duration of the short burst signal pilot CW field, f s is the sampling rate, the pilot CW signal s cw The expression of (n) is: ； Where A0 is the signal amplitude, n is the sample point index, j is the imaginary number symbol, f i is the carrier frequency of the satellite signal, is the initial carrier phase, t s =1 / f s is the sampling interval of the sample points; The sampled satellite signal r(n) is the noise signal w(n) and s cw The expression of the sampled satellite signal r(n) is: ； In the formula, s cw (n) is the pilot CW signal, with a duration of T cw , w(n) is a continuous noise signal.

3. The efficient capture method for short burst signals of low-orbit satellites according to claim 2, characterized in that: The signal buffer adopts a cyclic writing mode. When the signal buffer is full of N cw =T cw *f s When a signal sample point is obtained, the starting time slot index k0 is set to 0.

4. The efficient capture method for short burst signals of low-orbit satellites according to claim 3, characterized in that: The step S2 specifically includes the following steps: S21, read N continuously from the signal buffer cw Sampled satellite signal points , where k0 is the starting time slot index, n is the sample point index, and N pmf is the number of samples for each segment matched filter time slot; S22, the output signal sample point and N dop +1 frequency carrier sample points are mixed simultaneously and output N dop +1 Mixing Signal : ； Where n dop It is the frequency segment index, and its value range is [0,N dop ],N dop +1 is the number of frequency segments, is the sample index, N dop +1 frequency local carrier signal The expression is: ; In the formula, t s is the sampling point interval, is the sampling frequency of the partially matched filter output, T pmf is the time slot length of the partial matched filter; S23, the output N dop +1 mixed data divided into L cw Segment, sum the sample points in each segment to perform partial matched filtering, and get L cw The segmented integral value is used as the partial matched filtering result : ； Where, L cw is the number of time slot segments, k is the index of the partial matched filtering result, and its value range is [0:L cw -1]; S24, for N dop +1 partial matched filter result is simultaneously transformed by FFT and the FFT result is output As the detection quantity corresponding to multiple frequency points, FFT results The expression is: ； Where m=[0,N fft -1] is the sub-frequency point index in the frequency segment, N fft is the number of FFT points.

5. The efficient capture method for short burst signals of low-orbit satellites according to claim 4, characterized in that: The step S3 specifically comprises the following steps: S31. Store the FFT output result obtained by the time slot index k0 into the rth row of the FFT cache. The size of the FFT cache is R rows and C columns, R=L cw +1, C = N fft *(N dop +1), L cw is the number of time slot segments, N dop is the number of frequency segments, N fft is the number of FFT points, r=mod(k,R), FFT output uses a circular write-in and read-out method, mod() is a modulo operation, and the column index c of the FFT cache is set to 0; S32, read the FFT cache data of the cth column, if the mod(r-1,R)th data in the column is a maximum value and greater than the capture threshold value, then it is determined that the frequency capture of the frequency index c and the time slot (k0-1) is successful, and the time capture starting point is set to (k0-1+L cw -2)*N pmf , output frequency capture successful sample point information; S33, if c = N fft *(N dop +1)-1, then go to step S4, otherwise set c=c+1 and return to step S32.

6. The efficient capture method for short burst signals of low-orbit satellites according to claim 5, characterized in that: Step S4 is to wait for the signal buffer to be full again. pmf When a signal sample point is obtained, the starting time slot index k0 is set to k0+1, the frequency capture of the next time slot is performed, and the process returns to step S2.

Images