Low earth orbit satellite signal capturing method based on PMF-FFT

Abstract

The invention provides a low earth orbit satellite signal capturing method based on PMF-FFT. The low earth orbit satellite signal capturing method comprises the steps that firstly, low earth orbit satellite downlink signals are received and frequency mixing is carried out; carrying out adjustable parameter window adding processing on the frequency-mixed signal; inputting the windowed signal into a PMF module, and carrying out correlation calculation on the windowed signal and a local pseudo code to obtain PMF module output; carrying out adjustable parameter window adding processing on the output of each segmented matched filter in the PMF module; performing N-point FFT on the output result of the segmented matched filter; performing normalization processing on the FFT result of the # imgabs0 # point to obtain a normalized amplitude-frequency response; and performing threshold judgment according to the normalized amplitude-frequency response to realize capture. The method not only solves the problems of difficulty in broadband capture, long capture time and poor anti-Doppler effect of the traditional signal capture method, but also solves the problem that the traditional PMF-FFT method cannot break through a preset capture threshold and misses capture due to the fact that the normalized power spectral density at the target Doppler frequency shift is reduced due to energy loss under the condition of low signal-to-noise ratio. Therefore, the low-orbit satellite navigation signal can be quickly captured.

Description

Technical Field

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

[0002] In recent years, with the accelerated deployment of low-orbit satellite constellations, low-orbit satellite technology has ushered in explosive development in the fields of communications and navigation integration. Unlike traditional narrowband navigation signals, the new generation of low-orbit satellite network adopts a broadband signal system, which improves anti-interference ability and positioning accuracy through large-bandwidth transmission, and provides high-throughput services for the integrated space-ground network. This type of broadband signal has a more complex time-frequency structure, and due to the low satellite orbit altitude (500-1200 kilometers) and fast movement speed (7.5-7.8 km / s), its Doppler frequency shift range far exceeds that of traditional GNSS signals, and the frequency deviation change rate is high. 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, the signal capture technology of low-orbit satellites poses three challenges: high dynamics, wide frequency deviation, and low signal-to-noise ratio.

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

[0004] The signal capture algorithm based on PMF-FFT (segmented matched filter-fast Fourier transform) can improve the speed and accuracy of signal capture by combining the advantages of segmented matched filter and fast Fourier transform through a technical architecture that combines time domain segmented processing with frequency domain parallel search. However, this algorithm has inherent signal energy accumulation limitations in engineering implementation: due to the shortened integration time of each sub-segment due to signal segmentation processing, the coherent cumulative gain is correspondingly reduced. This decrease in processing gain is specifically manifested as the scalloping loss phenomenon in frequency domain detection and the main lobe energy attenuation effect. 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 it impossible for the detection statistic to break through the preset capture threshold, ultimately causing the problem of missed capture of StarNet low-orbit satellite signals. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention proposes a low-orbit satellite signal capture method based on PMF-FFT. On the basis of the traditional PMF-FFT method, the main lobe attenuation of the PMF part and the scallop loss of the FFT part are improved by adding adjustable parameter windows respectively. The technical difficulties of the traditional signal capture method, such as difficulty in capturing a wide band, long capture time and poor anti-Doppler effect, are solved, and the problem of the traditional PMF-FFT method being unable to break through the preset capture threshold and miss the capture under low signal-to-noise ratio conditions due to energy loss resulting in a decrease in the normalized power spectral density at the target Doppler frequency shift, is overcome, thereby achieving rapid capture of the new generation of star network low-orbit satellite navigation signals.

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

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

[0008] Step 1: The satellite receiver receives the downlink signal from the StarNet low-orbit satellite and mix it with the local carrier to get the mixed signal , Indicates Sample data;

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

[0010]

[0011] For adjustable parameter window:

[0012]

[0013] in are adjustable window function parameters, is the data point 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;

[0014] Step 3: Add the windowed signal Input the PMF module, perform correlation calculation with the local pseudo code in the PMF module, and obtain the output of the PMF module; the PMF module is composed of The first The output of the piecewise matched filter is ;

[0015] Step 4: Add an adjustable window to the output of each segmented matched filter in the PMF module to obtain the output result after windowing; The output of the piecewise matched filter The output result after adding adjustable parameter window processing is :

[0016]

[0017] Step 5: Perform N-point FFT on the output results of the P segmented matched filters after the adjustable window processing to obtain FFT result of point , The FFT result serial number, the value is ,and ;right After normalization, the normalized amplitude-frequency response is:

[0018]

[0019] Step 6: Normalize the amplitude-frequency response As the vertical axis of the coordinate system, the Doppler frequency shift As the horizontal axis of the coordinate system, the normalized amplitude-frequency response curve in the coordinate system is obtained, and it is determined whether the peak value of the normalized amplitude-frequency response curve exceeds the 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 low-orbit satellite downlink signal; wherein for:

[0020]

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

[0022] Furthermore, in step 1, the expression of the downlink intermediate frequency signal of the StarNet low-orbit satellite received by the satellite receiver is assumed to be:

[0023]

[0024] in is the intermediate frequency signal amplitude, for Pseudocode of time, Indicates Sample 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 length of half the pseudo code chip length, and is mixed with the local carrier after each sliding to remove the carrier frequency, and the mixed signal obtained is The expression is:

[0026]

[0027] in Is the imaginary number symbol.

[0028] Further, in step 3, the pseudo code chip length used is assumed to be ,PMF module includes Segmented matched filters, each segmented matched filter has a length of X, and the relationship between the three is: .

[0029] Furthermore, in step 3, The output of the piecewise matched filter is :

[0030]

[0031] In the formula, for Pseudocode of time.

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

[0033] An electronic device comprises a processor and a memory, wherein 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, combines the advantages of segmented matched filter and fast Fourier transform, significantly improves the speed and accuracy of signal capture, and sequentially improves the main lobe attenuation of the PMF part and the scallop loss of the FFT part by adding adjustable parameter windows, respectively, which not only solves the technical problems of wide-band difficult to capture, long capture time, and poor anti-Doppler effect in traditional signal capture methods, but also overcomes the problem that the normalized power spectral density at the target Doppler frequency shift decreases due to energy loss under low signal-to-noise ratio conditions, and the traditional PMF-FFT method cannot break through the preset capture threshold and misses the capture, thus realizing the rapid capture of the new generation of star network low-orbit satellite navigation signals. 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 continued 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 and in part will be obvious from the following description, or will be learned through 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 easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0040] Figure 1 It is the PMF-FFT capture principle diagram in the embodiment;

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

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

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

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

[0045] Figure 6 This is a diagram of the signal capture results in the embodiment. DETAILED DESCRIPTION

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

[0047] At present, in response to the demand for high-speed dynamic capture of StarNet low-orbit satellite signals, traditional signal capture methods have technical difficulties such as difficulty in capturing wide bandwidths, long capture time, and poor anti-Doppler effect. The typical PMF-FFT (segmented matched filter-fast Fourier transform) method, although it combines the segmented coherent accumulation of the segmented matched filter with the FFT frequency domain parallel processing, can reduce the computational complexity of Doppler frequency shift search while ensuring high sensitivity. However, due to the shortened integration time of each sub-segment and the corresponding reduction of coherent accumulation gain due to signal segmentation processing, scallop loss and mainlobe attenuation are prone to occur. When faced with the low signal-to-noise ratio conditions of StarNet low-orbit satellite signals, scallop loss and mainlobe attenuation will cause the normalized power spectral density at the target Doppler frequency shift to decrease, making it impossible for the detection statistic to break through the preset capture threshold, ultimately causing the problem of missed capture of StarNet low-orbit satellite signals.

[0048] To address this problem, this embodiment proposes a method for capturing low-orbit satellite signals based on PMF-FFT. Based on the traditional PMF-FFT method, the main lobe attenuation of the PMF part and the scallop loss of the FFT part are improved by adding adjustable windows respectively; this method can effectively suppress noise interference, adapt to the wide range of dynamic frequency deviation characteristics of low-orbit satellite signals, and complete the signal capture process in a short time in a resource-constrained receiving terminal, greatly improving the capture speed and accuracy. In addition, this method also has good adaptability and robustness, can effectively cope with signal changes in different environments, ensure stable operation under various conditions, and provide key technical support for the actual deployment of low-orbit satellite navigation systems.

[0049] like Figure 1 As 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 downlink signal from the StarNet low-orbit satellite and mix it with the local carrier to get the mixed signal , Indicates sampling data; the specific process is:

[0051] Assume that the expression of the downlink intermediate frequency signal of the low-orbit satellite of the StarNet received by the satellite receiver is:

[0052]

[0053] in is the intermediate frequency signal amplitude, for Pseudocode of time, Indicates Sample 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. This embodiment is to obtain the Doppler frequency shift .

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

[0055]

[0056] in Is the imaginary number symbol.

[0057] Step 2: Due to the inherent defects of the aforementioned typical PMF-FFT algorithm, which are mainly reflected in scallop loss and main lobe attenuation, the method proposed in the present invention improves these two defects respectively. In view of 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, so that the PMF can play a role in higher Doppler frequency shifts. The previous window functions are mainly Hanning window, Hamming window, and Blackman window, and the present invention proposes an adjustable parameter window for improving the main lobe bandwidth of the PMF. The expression of the adjustable parameter window is as follows:

[0058]

[0059] in are adjustable window function parameters, is the data point number in the window function, is the length of the window function, which is consistent with the length of a single segment matched filter in the subsequent PMF module.

[0060] The main lobe bandwidth of the Hanning window, Hamming window and Blackman window is fixed, but the main lobe bandwidth of the window function of the adjustable window proposed in the present invention can be adjusted with the parameter The change greatly improves the flexibility of the main lobe bandwidth of PMF, and can effectively increase the main lobe bandwidth of PMF according to actual needs.

[0061] By mixing the signal Perform adjustable window processing to obtain the windowed signal :

[0062]

[0063] Step 3: Add the windowed signal Input the PMF module, perform correlation calculation with the local pseudo code in the PMF module, and obtain the output of the PMF module;

[0064] Assume the pseudo code chip length used is ,PMF module includes Segmented matched filters, each segmented matched filter has a length of X, and the relationship between the three is: .

[0065] No. The output of the piecewise matched filter is :

[0066]

[0067] In the formula, for Pseudocode of time.

[0068] In the present invention, the signal is multiplied by the window function before entering the PMF through step 2, and then enters the PMF for processing; when X=16, , frequency deviation When the value is 0 ~ 30kHz, the comparison of the amplitude-frequency response of the PMF part before and after improvement is shown in the figure below: Figure 2 As shown in Figure 3, the results show that the gain attenuation can be greatly reduced by adding an adjustable parameter window.

[0069] Step 4: For the scallop loss of the FFT part, most of the traditional improved algorithms use the method of padding the FFT with zeros. Although zero padding can reduce the scallop loss, it will lead to an increase in the number of FFT points, which greatly increases the amount of FFT calculation. The present invention uses an adjustable parameter 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 parameter window to obtain a windowed output result; The output of the piecewise matched filter The output result after adding adjustable parameter window processing is :

[0071]

[0072] Step 5: Perform the following operations on the output results of the P segmented matched filters after the adjustable window processing: The FFT of the point is FFT result of point , The FFT result serial number, the value is ,and ;right After normalization, the normalized amplitude-frequency response is:

[0073]

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

[0075]

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

[0077] The comparison of the amplitude-frequency response results of the FFT part before and after the improvement with the adjustable parameter window is shown in the figure below. Figure 3 shown.

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

[0079] The results of using the improved PMF-FFT signal acquisition algorithm to capture the low-orbit satellite signal of StarNet are as follows: Figure 6 As shown, it can be seen that the peak is very obvious and the Doppler frequency shift of the signal can be captured. The improved PMF-FFT signal capture algorithm acquires the carrier frequency in parallel through one-dimensional search along the pseudo code phase. It has the advantages of fast capture speed and moderate hardware resources occupied. It is particularly suitable for the capture of direct spread pseudo code in low-orbit satellite environment.

[0080] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention.

Claims

1. A method for capturing low-orbit satellite signals based on PMF-FFT, characterized in that: The following steps are involved: Step 1: The satellite receiver receives the downlink signal from the StarNet low-orbit satellite and mix it with the local carrier to get the mixed signal , Indicates Sample data; Step 2: Mix the signal Perform adjustable window processing to obtain the windowed signal : For adjustable parameter window: in are adjustable window function parameters, is the data point 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: Add the windowed signal Input the PMF module, perform correlation calculation with the local pseudo code in the PMF module, and obtain the output of the PMF module; the PMF module is composed of The first The output of the piecewise matched filter is ; Step 4: Add an adjustable window to the output of each segmented matched filter in the PMF module to obtain the output result after windowing; The output of the piecewise matched filter The output result after adding adjustable parameter window processing is : Step 5: Perform N-point FFT on the output results of the P segmented matched filters after the adjustable window processing to obtain FFT result of point , The FFT result serial number, the value is ,and ;right After normalization, the normalized amplitude-frequency response is: Step 6: Normalize the amplitude-frequency response As the vertical axis of the coordinate system, the Doppler frequency shift As the horizontal axis of the coordinate system, the normalized amplitude-frequency response curve in the coordinate system is obtained, and it is determined whether the peak value of the normalized amplitude-frequency response curve exceeds the 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 low-orbit satellite downlink signal; wherein for: in is the pseudo code rate, 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, characterized in that: In step 1, the expression of the downlink intermediate frequency signal of the StarNet low-orbit satellite received by the satellite receiver is: in is the intermediate frequency signal amplitude, for Pseudocode of time, Indicates Sample 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.

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

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

5. The method for capturing low-orbit satellite signals based on PMF-FFT according to claim 4, characterized in that: In step 3, The output of the piecewise matched filter is : In the formula, for Pseudocode of time.

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

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