Low Earth Orbit Satellite Signal Acquisition Method Based on PMF-FFT

Abstract

The present invention proposes a method for capturing low-earth orbit satellite signals based on PMF-FFT. First, the downlink signals of low-earth orbit satellites are received and mixed; the mixed signals are processed with an adjustable parameter window; the windowed signals are input into the PMF module to calculate the correlation with the local pseudo-code to obtain the output of the PMF module; each segmented matched filter output in the PMF module is processed with an adjustable parameter window; an N-point FFT is performed on the output results of the segmented matched filters; the N-point FFT results are normalized to obtain the normalized amplitude-frequency response; threshold judgment is performed based on the normalized amplitude-frequency response to achieve capture. The present invention not only solves the problems existing in traditional signal capture methods, such as difficult capture of wide frequency bands, long capture time, and poor anti-Doppler effect, but also overcomes the problem that in the case of low signal-to-noise ratio, the normalized power spectral density at the target Doppler frequency shift decreases due to energy loss in the traditional PMF-FFT method, and it is impossible to break through the preset capture threshold and cause missed capture, realizing the rapid capture of low-earth orbit satellite navigation signals.

Description

Technical Field

[0001] The present invention relates to the field of satellite positioning and navigation, and specifically to a method for capturing signals of low-earth orbit satellites based on PMF-FFT. Background Art

[0002] In recent years, with the accelerated deployment of low-earth orbit satellite constellations, low-earth orbit satellite technology has seen explosive development in fields such as communication and navigation integration. Different from traditional narrowband navigation signals, the new generation of low-earth orbit satellite constellations adopt a broadband signal system, which improves anti-jamming ability and positioning accuracy through large-bandwidth transmission and provides high-throughput services for the space-ground integrated network. Such broadband signals have a more complex time-frequency structure, and due to the low satellite orbit altitude (500 - 1200 kilometers) and high moving speed (7.5 - 7.8 km / s), their Doppler frequency shift dynamic range far exceeds that of traditional GNSS signals, with a high frequency offset change rate. At the same time, the path loss caused by long-distance signal transmission makes the signal-to-noise ratio (SNR) at the receiving end often lower than -25 dB. Therefore, high-dynamic, wide-frequency-offset, and low-signal-to-noise ratio triple challenges are posed to the signal capture technology of low-earth orbit satellites.

[0003] Although the traditional parallel code phase search algorithm based on FFT (Fast Fourier Transform) can achieve parallel search of Doppler frequency shift, it is vulnerable to noise interference in a low-signal-to-noise ratio environment and has insufficient capture sensitivity; while the time-domain serial search method based on a matched filter (MF) has high sensitivity, but its computational complexity increases exponentially with the expansion of the Doppler range, making it difficult to meet the high-speed dynamic capture requirements of signals of low-earth orbit satellites in the constellation.

[0004] The signal capture algorithm based on PMF-FFT (Piecewise Matched Filter - Fast Fourier Transform), through a technical architecture that combines time-domain segmented processing with frequency-domain parallel search, combines the advantages of the piecewise matched filter and the fast Fourier transform, and can improve the speed and accuracy of signal capture. However, this algorithm has an inherent signal energy accumulation limitation in engineering implementation: due to the segmented processing of the signal, the integration time of each sub-segment is shortened, and the coherent accumulation gain decreases accordingly. This decrease in processing gain is specifically manifested as the scalloping loss phenomenon and the main lobe energy attenuation effect in frequency-domain detection. Under low-signal-to-noise ratio conditions, the above energy loss will lead to a decrease in the normalized power spectral density at the target Doppler frequency shift, making the detection statistic unable to break through the preset capture threshold, and ultimately resulting in the problem of missed capture of signals of low-earth orbit satellites in the constellation. Summary of the Invention

[0005] To solve the problems existing in the prior art, the present invention proposes a method for capturing low-earth orbit satellite signals based on PMF-FFT. On the basis of the traditional PMF-FFT method, improvements are made respectively for the main lobe attenuation of the PMF part and the scallop loss of the FFT part by adding an adjustable window, which not only solves the technical problems of difficult capture of wide frequency bands, long capture time, and poor anti-Doppler effect existing in the traditional signal capture method, but also overcomes the problem that in the case of low signal-to-noise ratio, due to energy loss, the normalized power spectral density at the target Doppler frequency shift drops and the preset capture threshold cannot be broken through, resulting in missed capture in the traditional PMF-FFT method, and realizes the rapid capture of the navigation signals of the new generation of satellite network low-earth orbit satellites.

[0006] The technical solution of the present invention is as follows:

[0007] The method for capturing low-earth orbit satellite signals based on PMF-FFT includes the following steps:

[0008] Step 1: The satellite receiver receives the downlink signal of the satellite network low-earth orbit satellite and mixes it with the local carrier to obtain the mixed signal , denotes the th sampling data;

[0009] Step 2: Perform adjustable window processing on the mixed signal to obtain the windowed signal :

[0010]

[0011] is an adjustable window:

[0012]

[0013] where is an adjustable window function parameter, is the data point serial number in the window function, is the length of the window function, which is the same as the length of the segmented matched filter in the PMF module;

[0014] Step 3: Input the windowed signal into the PMF module, perform correlation calculation with the local pseudo-code in the PMF module to obtain the output of the PMF module; the PMF module is composed of segmented matched filters, and the output of the th segmented matched filter is ;

[0015] Step 4: Perform windowing with adjustable parameters on the outputs of each segment matching filter in the PMF module to obtain the windowed output results; among which, the output of the th segment matching filter after windowing with adjustable parameters is the output result :

[0016]

[0017] Step 5: Perform N-point FFT on the output results of the P segment matching filters after windowing with adjustable parameters to obtain the point FFT result , is the FFT result serial number, and its value is , and ; perform normalization processing on to obtain the normalized amplitude-frequency response as:

[0018]

[0019] Step 6: Use the normalized amplitude-frequency response as the vertical axis of the coordinate system and the Doppler shift as the horizontal axis of the coordinate system to obtain the normalized amplitude-frequency response curve in the coordinate system. Determine whether the peak value of the normalized amplitude-frequency response curve exceeds the set threshold. If it exceeds, it is determined that the capture is successful, the current pseudo-code phase is obtained, and the horizontal axis coordinate corresponding to the peak value is the Doppler shift of the low-earth orbit satellite downlink signal; among which is:

[0020]

[0021] where is the rate of the pseudo-code, is the sampling rate of the pseudo-code.

[0022] Furthermore, in Step 1, let the expression of the intermediate-frequency signal received by the satellite receiver from the low-earth orbit satellite in the satellite network be:

[0023]

[0024] where is the amplitude of the intermediate-frequency signal, is the pseudo-code at moment, represents the th sampling data, is the pseudo-code chip duration, is the pseudo-code phase offset, is the frequency of the intermediate-frequency signal, is the Doppler shift of the intermediate-frequency signal, is the initial phase of the intermediate frequency signal, is the noise term.

[0025] Furthermore, in step 1, the intermediate frequency signal is sampled, and the sampled data slides backward with a sliding step of half the length of a pseudo-code chip. After each slide, it is mixed with the local carrier to remove the carrier frequency, and the resulting mixed signal The expression is:

[0026]

[0027] where is the imaginary symbol.

[0028] Furthermore, in step 3, let the length of the used pseudo-code chip be , the PMF module includes segmented matched filters, and the length of each segmented matched filter is X. The relationship among the three is .

[0029] Furthermore, in step 3, the output of the th segmented matched filter is :

[0030]

[0031] In the formula, is the pseudo-code at time.

[0032] In addition, the present invention also proposes an electronic device and a readable storage medium:

[0033] An electronic device includes a processor and a memory, and the memory is used to store one or more programs;

[0034] When the one or more programs are executed by the processor, the above method is implemented.

[0035] A readable storage medium stores a computer program, and when the computer program is executed by a processor, the above method is implemented.

[0036] Beneficial effects:

[0037] The present invention is based on the PMF-FFT method, which combines the advantages of the segmented matched filter and the fast Fourier transform, significantly improving the speed and accuracy of signal acquisition. In response to the main lobe attenuation of the PMF part and the scallop loss of the FFT part, improvements are made by adding adjustable parameter windows respectively. This not only solves the technical problems of traditional signal acquisition methods, such as difficulty in capturing wide frequency bands, long capture time, and poor anti-Doppler effect, but also overcomes the problem that in the case of low signal-to-noise ratio, the normalized power spectral density at the target Doppler frequency shift decreases due to energy loss in the traditional PMF-FFT method, resulting in missed capture because the preset capture threshold cannot be broken through. It realizes the fast capture of the new generation of low-orbit satellite navigation signals in the star network. The efficient computing power and good adaptability of this method not only greatly shorten the signal capture time, but also ensure stable operation under various conditions, providing solid technical support for the continuous realization of low-orbit satellite navigation and laying a foundation for the further development of low-orbit satellite navigation equipment.

[0038] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. Brief Description of the Drawings

[0039] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, where:

[0040] Figure 1 is the schematic diagram of PMF-FFT capture in the embodiment;

[0041] Figure 2 is the amplitude-frequency response diagram of PMF before and after improvement in the embodiment;

[0042] Figure 3 is the amplitude-frequency response diagram of FFT before and after improvement in the embodiment;

[0043] Figure 4 is the normalized gain diagram of PMF-FFT before improvement in the embodiment;

[0044] Figure 5 is the normalized gain diagram of PMF-FFT before and after improvement in the embodiment;

[0045] Figure 6 is the signal capture result diagram in the embodiment. Detailed Description of the Specific Embodiment

[0046] The embodiments of the present invention will be described in detail below. The embodiments are exemplary and are intended to explain the present invention, but should not be construed as a limitation to the present invention.

[0047] At present, aiming at the high-speed dynamic acquisition requirements of star network low-orbit satellite signals, traditional signal acquisition methods have technical problems such as difficulty in capturing wide frequency bands, long acquisition time, and poor anti-Doppler effect. For the typical PMF-FFT (Piecewise Matched Filtering - Fast Fourier Transform) method, although it combines the segmented coherent accumulation of the piecewise matched filter with the FFT frequency domain parallel processing, it can reduce the computational complexity of Doppler frequency shift search while ensuring high sensitivity. However, due to the segmented processing of the signal, the integration time of each sub-segment is shortened, and the coherent accumulation gain is correspondingly reduced. It is prone to scalloping loss and main lobe attenuation. When facing the low signal-to-noise ratio condition of star network low-orbit satellite signals, the scalloping loss and main lobe attenuation will cause the normalized power spectral density at the target Doppler frequency shift to decrease, making the detection statistic unable to break through the preset acquisition threshold, and ultimately resulting in the problem of missed acquisition of star network low-orbit satellite signals.

[0048] To solve this problem, this embodiment proposes a low-orbit satellite signal acquisition method based on PMF-FFT. On the basis of the traditional PMF-FFT method, it improves the main lobe attenuation of the PMF part and the scalloping loss of the FFT part in turn by adding adjustable parameter windows. This method can effectively suppress noise interference, adapt to the wide-range dynamic frequency offset characteristics of low-orbit satellite signals, and complete the signal acquisition process within a short time in resource-constrained receiving terminals, greatly improving the acquisition speed and accuracy. In addition, this method also has good adaptability and robustness, can effectively cope with signal changes in different environments, ensures stable operation under various conditions, and provides key technical support for the actual deployment of low-orbit satellite navigation systems.

[0049] As Figure 1 shown, the low-orbit satellite signal acquisition method based on PMF-FFT in this embodiment mainly includes the following steps:

[0050] Step 1: The satellite receiver receives the star network low-orbit satellite downlink signal , and mixes it with the local carrier to obtain the mixed signal , denotes the th sampling data; the specific process is as follows:

[0051] Suppose the expression of the star network low-orbit satellite downlink intermediate frequency signal received by the satellite receiver is:

[0052]

[0053] where is the intermediate frequency signal amplitude, is the pseudo-code at moment, denotes the th sampling data, is the pseudo-code chip duration, is the pseudo-code phase offset, is the frequency of the intermediate-frequency signal, is the Doppler frequency shift of the intermediate-frequency signal, is the initial phase of the intermediate-frequency signal, is the noise term. The purpose of this embodiment is to obtain the Doppler frequency shift .

[0054] Sample the intermediate-frequency signal. The sampled data slides backward with a sliding step of half the pseudo-code chip length. After each slide, mix it with the local carrier to remove the carrier frequency. The resulting mixed signal The expression is:

[0055]

[0056] where is the imaginary symbol.

[0057] Step 2: Due to the inherent defects of the aforementioned typical PMF-FFT algorithm, which are mainly reflected in scalloping loss and main lobe attenuation, the method proposed in the present invention improves these two defects respectively. For the main lobe attenuation corresponding to the PMF part, this embodiment proposes an improved method of adding an adjustable parameter window: Adding a window can increase the main lobe bandwidth of the PMF, enabling the PMF to play a role at higher Doppler frequency shifts. The previous window functions were mainly Hanning window, Hamming window, and Blackman window, while the present invention proposes an adjustable parameter window to improve the main lobe bandwidth of the PMF. The expression of the adjustable parameter window is as follows:

[0058]

[0059] where is the adjustable window function parameter, is the data point sequence number in the window function, is the length of the window function, which is consistent with the length of a single segmented matched filter in the subsequent PMF module.

[0060] The main lobe bandwidths of the Hanning window, Hamming window, and Blackman window are fixed. However, the main lobe bandwidth of the window function of the adjustable parameter window proposed in the present invention can change with the parameter , greatly improving the flexibility in the main lobe bandwidth of the PMF and being able to effectively increase the main lobe bandwidth of the PMF according to actual needs.

[0061] By performing the adjustable parameter window processing on the mixed signal , the windowed signal is obtained:

[0062]

[0063] Step 3: The windowed signal is input into the PMF module, where correlation calculation is performed with the local pseudo-code to obtain the output of the PMF module;

[0064] Assume the chip length of the used pseudo-code is , and the PMF module includes segmented matched filters, each with a length of X, and the relationship among the three is .

[0065] The output of the segmented matched filter is :

[0066]

[0067] In the formula, is the pseudo-code at time

[0068] In the present invention, through Step 2, the signal is first multiplied by the window function before entering the PMF and then enters the PMF for processing; when X = 16, , and the frequency offset takes values from 0 to 30 kHz, the comparison diagram of the amplitude-frequency response of the PMF part before and after improvement is as shown in Figure 2 , and the result shows that the gain attenuation can be significantly reduced by adding an adjustable window.

[0069] Step 4: For the scallop loss of the FFT part, in traditional improvement algorithms, most use the method of padding zeros to the FFT for improvement. Although padding zeros can reduce the scallop loss, it will increase the number of FFT points, resulting in a significant increase in the computational complexity of the FFT; the present invention uses an adjustable window for windowing processing, which can reduce the scallop loss without increasing the number of FFT points and improve the gain of the FFT part.

[0070] Specifically, the output of each segmented matched filter in the PMF module is processed with an adjustable window to obtain the windowed output result; among them, the output of the segmented matched filter after being processed with an adjustable window is the output result :

[0071]

[0072] Step 5: Perform an FFT of points on the output results of the P segmented matched filters after being processed with an adjustable window to obtain the FFT result of points , is the FFT result serial number, and the value range is and ; Normalize to obtain the normalized amplitude-frequency response as follows:

[0073]

[0074] Step 6: Use the normalized amplitude-frequency response as the vertical axis of the coordinate system and the Doppler shift as the horizontal axis of the coordinate system to obtain the normalized amplitude-frequency response curve in the coordinate system. Determine whether the peak of the normalized amplitude-frequency response curve exceeds the set threshold. If it exceeds, it is determined that the capture is successful, and the current pseudo-code phase is obtained. The horizontal axis coordinate corresponding to the peak is the Doppler shift of the downlink signal of the low-earth orbit satellite; otherwise, continue to slide backward with a sliding step of half the pseudo-code chip length for the next detection; where is:

[0075]

[0076] where is the rate of the pseudo-code, is the sampling rate of the pseudo-code, that is, the sliding step of the chip. In practice, generally, half a chip is used as the data sliding step.

[0077] The comparison diagram of the amplitude-frequency response results of the FFT part before and after adding an adjustable parameter window is as shown in Figure 3 .

[0078] The complete gain of the normalized amplitude-frequency response obtained by the PMF-FFT method without adding an adjustable parameter window improvement is as shown in Figure 4 . It can be seen from Figure 4 that PMF-FFT combines PMF and FFT. The outer envelope is affected by PMF, and the internal attenuation is affected by FFT. The complete gain of the normalized amplitude-frequency response obtained by the PMF-FFT method with an adjustable parameter window improvement is as shown in Figure 5 . It can be seen from the figure that the overall gain of the improved PMF-FFT has been greatly improved, the main lobe bandwidth has increased significantly, and the scallop attenuation has been greatly reduced, indicating that the improvement is very effective.

[0079] The result of capturing the star network low-earth orbit satellite signal using the improved PMF-FFT signal capture algorithm is as shown in Figure 6 . It can be seen that the peak is very obvious, and the Doppler shift of the signal can be captured. The improved PMF-FFT signal capture algorithm obtains the carrier frequency in parallel through one-dimensional search along the pseudo-code phase, has the advantage of fast capture speed, and occupies moderate hardware resources, and is especially suitable for the capture of direct-sequence spread-spectrum pseudo-codes in low-earth orbit satellite environments.

[0080] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A method for capturing low-earth orbit satellite signals based on PMF-FFT, characterized in that: including the following steps: Step 1: The satellite receiver receives the downlink signal of the satellite network's low-earth orbit satellite and mixes it with the local carrier to obtain the mixed signal , denotes the th sampled data; Step 2: Process the mixed-frequency signal by applying an adjustable window to obtain the windowed signal : It is an adjustable window: wherein is an adjustable window function parameter, is the data point serial number in the window function, is the length of the window function, which is consistent with the length of the segmented matched filter in the PMF module; Step 3: The windowed signal is input into the PMF module, where correlation calculation is performed with the local pseudo-code to obtain the output of the PMF module; the PMF module consists of segmented matched filters, and the output of the th segmented matched filter is ; Step 4: Perform an adjustable window processing on the outputs of each segmented matching filter in the PMF module to obtain the windowed output results; among which, the output of the th segmented matching filter after the adjustable window processing is : Step 5: Perform an N-point FFT on the output results of the P segmented matched filters after the adjustable parameter window processing to obtain the FFT results of , where is the FFT result sequence number, with values ranging from ; perform normalization on to obtain the normalized amplitude-frequency response as: Step 6: Using the normalized amplitude-frequency response as the vertical axis of the coordinate system, and using the Doppler frequency shift as the horizontal axis of the coordinate system, a normalized amplitude-frequency response curve in the coordinate system is obtained. Determine whether the peak value of the normalized amplitude-frequency response curve exceeds a set threshold value. If it exceeds, it is determined that the capture is successful, the current pseudo-code phase is obtained, and the horizontal axis coordinate corresponding to the peak value is the Doppler frequency shift of the downlink signal of the low-earth orbit satellite; where is: where is the rate of the pseudo-code, is the sampling rate of the pseudo-code.

2. The method for capturing low-orbit satellite signals based on PMF-FFT according to claim 1, wherein: In step 1, assume that the expression of the downlink intermediate frequency signal of the satellite network's low-earth orbit satellite received by the satellite receiver is: wherein is the amplitude of the intermediate frequency signal, is the pseudo-code at time indicating the th sampling data, is the pseudo-code chip duration, is the pseudo-code phase offset, is the frequency of the intermediate frequency signal, is the Doppler frequency shift of the intermediate frequency signal, is the initial phase of the intermediate frequency signal, is the noise term.

3. The method for capturing signals of low-earth orbit satellites based on PMF-FFT according to claim 2, characterized in that: In step 1, the intermediate frequency signal is sampled. The sampled data slides backward with a sliding step of half the length of a pseudo-code chip. After each slide, it is mixed with the local carrier to remove the carrier frequency, and the resulting mixed signal The expression is: wherein is the imaginary symbol.

4. The method for capturing signals of low-earth orbit satellites based on PMF-FFT according to claim 3, characterized in that: In step 3, assume that the length of the used pseudo-code chip is , and the PMF module includes segmented matched filters, and the length of each segmented matched filter is X. The relationship among the three is .

5. The method for capturing low-earth orbit satellite signals based on PMF-FFT according to claim 4, wherein: In step 3, the output of the th segment matching filter is : In the formula, is the pseudocode at the moment.

6. An electronic device, comprising a processor and a memory, the memory being configured to store one or more programs; characterized in that: When the one or more programs are executed by the processor, the method according to any one of claims 1 to 5 is implemented.

7. A readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, the method according to any one of claims 1 to 5 is implemented.

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