Low-orbit satellite signal capturing and tracking method and signal processing method

Abstract

The invention relates to the field of satellite navigation, and discloses a low earth orbit satellite signal capturing and tracking method and a signal processing method, comprising the following steps: periodically receiving a low earth orbit satellite signal of at least one time slice, and carrying out carrier separation on the low earth orbit satellite signal to obtain a stripped signal corresponding to the low earth orbit satellite signal; processing the stripped signal and a locally stored pseudo-random code sequence so as to find an amplitude maximum value of the stripped signal in a frequency domain, and calculating Doppler frequency after determining that the stripped signal is successfully captured according to the amplitude maximum value; and transferring the successfully captured stripped signal to a tracking channel, tracking the stripped signal regularly, and returning to the previous step. According to the invention, signals with relatively large Doppler frequency offset can be efficiently captured in real time for low-orbit signals, the number of navigation satellites which can be captured by the system at the same time can be greatly increased, and the system efficiency and the system coverage rate are improved.

Description

Technical Field

[0001] The present invention relates to the field of satellite navigation and can be used for capturing and tracking low-orbit navigation satellite signals in a low-orbit satellite receiver, and in particular to a low-orbit satellite signal capturing and tracking method and a signal processing method. Background Art

[0002] In current satellite communication systems and satellite navigation systems, pseudo-random spread spectrum codes inserted in navigation signals are generally used to capture signals. Once the signals are captured, they are switched to the tracking phase to perform signal time-frequency synchronization, demodulation, despreading, decoding, and other steps.

[0003] There are two common methods used in existing signal capture technology: parallel frequency search and parallel code phase search. Taking a pseudo-random spread spectrum code with 2046 chips as an example, when capturing a signal, the signal is usually decomposed into two dimensions: frequency and time. The time dimension is 2046 chips, and the frequency dimension is decomposed according to the required frequency accuracy. Here, it is assumed that the Doppler frequency of 10 kHz is decomposed into 21 frequency bands, each with 500 Hz. At this time, a total of 42966 (2046×21) search units need to be searched. The parallel frequency search method searches 21 frequency units at a time for each code phase. First, the digital intermediate frequency input signal is mixed with the carrier copied inside the receiver, and then correlated with the copied pseudo-random spread spectrum code to strip off the pseudo-code sequence modulated on the carrier. Then, the correlation results in the time domain are converted to the frequency domain through Fourier transform, which reflects the strength of the time domain correlation results in each frequency unit. Finally, the Doppler frequency value is found by comparing the results, such as Fig. 9 shown.

[0004] The parallel code phase search method searches for 2046 code phases on the frequency unit. First, the digital intermediate frequency input signal is mixed with the carrier copied inside the receiver, and then the mixed data is Fourier transformed to obtain the frequency domain value, and then conjugate multiplied with the Fourier change value of the copied pseudo code sequence, and finally the amplitude maximum is found through the inverse Fourier transform to the time domain to find the code phase value on the frequency search unit, such as Fig.10 shown.

[0005] Usually, when using the parallel frequency search method, in order to reduce the number of FFT points during calculation and increase the running speed, the PMF-FFT algorithm is used instead of the FFT transform at the cost of sacrificing a certain amount of energy loss. However, in the signal capture after low-orbit stripping, the existing technology has the following defects:

[0006] 1. The number of low-orbit satellites has greatly increased compared to GNSS satellites, which means that the number of navigation signals that the receiver can receive from different satellites has increased, greatly increasing the amount of data processing and requiring more efficient processing strategies.

[0007] 2. Low-orbit satellites generally operate at higher speeds, so the Doppler frequency range of their signals reaching the ground is wider, usually reaching ±40 kHz. Therefore, in order to obtain higher frequency resolution, the number of FFT points needs to be increased, which greatly increases the workload of the receiver.

[0008] 3. The Doppler frequency and code phase estimated by this method usually cannot meet the final demodulation requirements, and it is necessary to use the residual frequency offset estimation method in combination to obtain a more accurate frequency estimate. Summary of the invention

[0009] Under the existing receiver signal processing technology, the capture and tracking of low-orbit signals requires a large amount of hardware resources. Under the condition of limited receiver resources, the time overhead can only be increased, which will affect the real-time capture of low-orbit signals. The present invention provides a low-orbit satellite signal capture and tracking method and a signal processing method, which use a new signal capture and processing strategy to increase the real-time signal processing rate, obtain the Doppler frequency of the captured signal in real time, and then obtain a more accurate residual frequency deviation value through an adaptive residual frequency deviation estimation method to obtain demodulated data in real time.

[0010] The present invention is achieved through the following technical solutions:

[0011] A method for capturing and tracking low-orbit satellite signals, comprising:

[0012] Periodically receiving a low-orbit satellite signal of at least one time segment, and performing carrier separation on the low-orbit satellite signal, thereby obtaining a stripped signal corresponding to the low-orbit satellite signal;

[0013] The stripped signal is processed with the pseudo-random code sequence stored locally to find the maximum amplitude of the stripped signal in the frequency domain, and the Doppler frequency is calculated after the stripped signal is successfully captured based on the maximum amplitude. At the same time, the stripped signal after successful capture is transferred to the tracking channel and the stripped signal is tracked regularly, and at the same time, returns to the previous step.

[0014] As an optimization, each of the stripped signals includes a pseudo-random spread spectrum code sequence and a data code of the low-orbit satellite.

[0015] As an optimization, the low-orbit satellite signal is subjected to carrier separation, so that the specific process of obtaining the stripped signal corresponding to the low-orbit satellite signal is as follows:

[0016] A1. Preprocessing the low-orbit satellite signal to obtain a digital intermediate frequency signal, wherein the preprocessing includes filtering, down-conversion, analog-to-digital conversion, quantization, and AGC;

[0017] A2. Splitting the digital intermediate frequency signal into an I intermediate frequency signal and a Q intermediate frequency signal, and mixing the I intermediate frequency signal and the Q intermediate frequency signal with a local carrier respectively to complete carrier stripping and obtain the stripped signal.

[0018] As an optimization, in A2, the specific formula for mixing the I intermediate frequency signal and the Q intermediate frequency signal with the local carrier is:

[0019]

[0020] Where D(n) is the original data, i.e., zero-frequency signal or data code, C(n) is the pseudo-random spread spectrum code sequence, ω i (n) is the signal angular frequency, θ(n) is the signal phase, n is the noise sequence, the distribution follows a Gaussian distribution with a mean of 0, ω0(n) is the local carrier angular frequency, θ0 is the random initial phase, x i (n) represents the I branch signal after stripping, x q (n) represents the q signal of the stripped signal, a is the random gain, t(n) is the sampling time, θ e is the random phase error, ω e is the angular frequency error.

[0021] As an optimization, the stripped signal and the pseudo-random code sequence are processed to find the maximum amplitude of the stripped signal in the frequency domain, and the specific process of calculating the Doppler frequency after the stripped signal is successfully captured is determined by the maximum amplitude:

[0022] B1. Conjugate-multiplying the stripped signal with the pseudo-random code sequence of the low-orbit satellite signal stored locally, and then converting it into the frequency domain through Fourier transform to obtain a first frequency domain signal, wherein the pseudo-random code sequence stored locally corresponds to the same pseudo-random spread spectrum code sequence of the low-orbit satellite;

[0023] B2. Find the maximum amplitude point in the first frequency domain signal, which is the maximum amplitude value of the stripped signal in the frequency domain;

[0024] B3. Determine whether the maximum amplitude is greater than a preset amplitude threshold value. If so, determine that the stripped signal is captured successfully and calculate the Doppler frequency of the stripped signal. Otherwise, return to B1.

[0025] As an optimization, the specific process of regularly tracking the stripped signal in the tracking channel is:

[0026] The captured Doppler frequency is used at ten sampling time points before and after the interval time point of the low-orbit satellite signal broadcast to confirm whether there is a post-stripping signal that has entered the tracking channel. If the post-stripping signal is confirmed, the Doppler frequency of the post-stripping signal is obtained, and then the tracking process is directly carried out. If the post-stripping signal is not confirmed, it jumps to B1 again for re-capture.

[0027] The present invention also discloses a signal processing method for low-orbit satellite signals, comprising:

[0028] Capturing and tracking a preset low-orbit satellite signal using one of the aforementioned low-orbit satellite signal capture and tracking methods;

[0029] Calculating a final carrier-to-noise ratio of a stripped signal corresponding to the tracked low-orbit satellite signal, and performing time synchronization on the stripped signal;

[0030] For the stripped signal after time synchronization, different residual frequency offset estimation methods are selected according to the final carrier-to-noise ratio to calculate the residual frequency offset estimation value. Finally, the stripped signal is frequency offset compensated using the obtained residual frequency offset estimation value, and then phase compensation, demodulation, and channel decoding are performed to finally obtain the output signal.

[0031] As an optimization, before calculating the final carrier-to-noise ratio, the stripped signal is firstly subjected to coarse frequency compensation and then subjected to matched filtering.

[0032] As an optimization, the specific process of calculating the carrier-to-noise ratio of the stripped signal corresponding to the tracked low-orbit satellite signal and performing time synchronization on the stripped signal is as follows:

[0033] Initial carrier-to-noise ratio calculation stage: calculating the initial carrier-to-noise ratio of the stripped signal;

[0034] Coarse synchronization stage: the stripped signal is integrated and cleared to obtain the coherent integration results of the I branch signal and the Q branch signal of the stripped signal, and the initial carrier-to-noise ratio of the stripped signal is improved by the coherent integration results to obtain the final carrier-to-noise ratio, and the leading, timely and lagging signals of the I branch signal and the leading, timely and lagging signals of the Q branch signal are compared to obtain the maximum amplitude point, which is the coarse optimal sampling point; in the coherent integration process, high-frequency noise components can be eliminated and the energy of the required signal frequency components can be increased.

[0035] Fine synchronization stage: the coarse optimal sampling point is corrected using an interpolation symbol time synchronization method to obtain a fine optimal sampling point.

[0036] As an optimization, the specific process of calculating the residual frequency offset estimation value by selecting different residual frequency offset estimation methods according to the final carrier-to-noise ratio for the stripped signal after time synchronization is as follows:

[0037] C1. Compare the final carrier-to-noise ratio with a preset carrier-to-noise ratio threshold value. If the final carrier-to-noise ratio is greater than the preset threshold value, jump to C2; otherwise, jump to C3;

[0038] C2. Calculate the residual frequency offset estimate according to the following formula:

[0039]

[0040] In the formula, To estimate the residual frequency offset, T is the sampling period, L is the data length used for calculation, r(k) represents the kth sampling point, r * (k) represents the conjugate of the kth sampling point,

[0041] C3. Take out the pilot segment of the original data, conjugate it with the local pilot and perform FFT transformation. After transformation, calculate the residual frequency offset estimation value according to the following formula:

[0042]

[0043] In the formula Represents the estimated residual frequency offset, T is the sampling time interval, k0 represents the index value corresponding to the maximum amplitude of the spectrum after FFT transformation, X(k0) represents the maximum amplitude, r represents the difference between the index value corresponding to the second maximum amplitude of the spectrum and k0, and X(r⊙0.5) represents the larger value at index r±0.5.

[0044] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0045] The present invention reduces the amount of system data processing through the signal processing flow and strategy from capture to confirmation, especially when more satellites need to be captured simultaneously.

[0046] The present invention effectively utilizes the signal-to-noise ratio of the signal when estimating the residual frequency offset, and adaptively uses different residual frequency offset estimation algorithms for weak signals and strong signals, thereby increasing the estimation range of the residual frequency offset and making the residual frequency offset more accurate.

[0047] The present invention can efficiently and real-time capture signals with large Doppler frequency deviation for low-orbit burst signals, and the number of navigation satellites that can be captured simultaneously by the system can be greatly increased, thereby improving system efficiency and system coverage. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0049] Figure 1 A flowchart of a method for capturing and tracking low-orbit satellite signals according to the present invention;

[0050] Figure 2 This is a flow chart of a signal processing method for a low-orbit satellite signal according to the present invention;

[0051] Figure 3 Flow chart of the acquisition phase of low-orbit satellite signals;

[0052] Figure 4 Flow chart for capturing signal to confirm;

[0053] Figure 5 is a flowchart of the interpolation symbol time synchronization algorithm;

[0054] Figure 6 Flow chart for residual frequency deviation calculation;

[0055] Figure 7 is a flow chart of the phase compensation algorithm;

[0056] Figure 8 A full flow chart of signal processing combining the two methods of the present invention; DETAILED DESCRIPTION

[0057] Fig. 9 A schematic diagram of the structure of a hardware device for performing a parallel frequency search method; Fig.10 A schematic diagram of the structure of the hardware device for performing the parallel code phase search method. In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0058] This embodiment 1 provides a method for capturing and tracking a low-orbit satellite signal, including:

[0059] S1. Periodically receive a low-orbit satellite signal of at least one time segment, and perform carrier separation on the low-orbit satellite signal to obtain a stripped signal corresponding to the low-orbit satellite signal; in some embodiments, each of the stripped signals includes a pseudo-random spread spectrum code sequence and a data code of the low-orbit satellite.

[0060] There may be only one or more low-orbit satellite signals. If there are more than one, then multiple low-orbit satellite signals will be received at the same time. Assume that there are 5 low-orbit satellite signals to be captured at the same time, and their pseudo-random spread spectrum code numbers are 1, 2, 3, 4, and 5 respectively. Correspondingly, the local pseudo-random spread spectrum code numbers are also 1, 2, 3, 4, and 5 respectively.

[0061] In some embodiments, the specific process of performing carrier separation on the low-orbit satellite signal to obtain a stripped signal corresponding to the low-orbit satellite signal is:

[0062] A1. Preprocessing the low-orbit satellite signal to obtain a digital intermediate frequency signal, wherein the preprocessing includes filtering, down-conversion, analog-to-digital conversion, quantization, and AGC;

[0063] The low-orbit satellite signal is received through the antenna, and then the RF link completes the steps of filtering, down-conversion, analog-to-digital conversion, quantization, AGC, etc., and converts the low-orbit satellite signal (low-orbit satellite navigation signal) into a digital intermediate frequency signal.

[0064] A2. Splitting the digital intermediate frequency signal into an I intermediate frequency signal and a Q intermediate frequency signal, and mixing the I intermediate frequency signal and the Q intermediate frequency signal with a local carrier respectively to complete carrier stripping and obtain the stripped signal.

[0065] In some embodiments, the specific formula for mixing the I intermediate frequency signal and the Q intermediate frequency signal with the local carrier is:

[0066]

[0067] Where D(n) is the original data, i.e., the zero-frequency signal, C(n) is the pseudo-random spread spectrum code sequence, ω i (n) is the signal angular frequency, θ(n) is the signal phase, n is the noise sequence, the distribution follows a Gaussian distribution with a mean of 0, ω0(n) is the local carrier angular frequency, θ0 is the random initial phase, x i (n) represents the I branch signal after stripping, x q (n) represents the q signal of the stripped signal, a is the random gain, t(n) is the sampling time, θ e is the random phase error, ω e is the angular frequency error.

[0068] S2. Process the stripped signal with the pseudo-random code sequence stored locally to find the maximum amplitude of the stripped signal in the frequency domain, and calculate the Doppler frequency after the stripped signal is successfully captured by determining the maximum amplitude. At the same time, transfer the stripped signal after successful capture to the tracking channel and track the stripped signal regularly, and at the same time, return to the previous step.

[0069] In some embodiments, the stripped signal and the pseudo-random code sequence are processed to find the maximum amplitude of the stripped signal in the frequency domain, and the specific process of calculating the Doppler frequency after the stripped signal is successfully captured is determined by the maximum amplitude:

[0070] B1. Conjugate-multiplying the stripped signal with the pseudo-random code sequence of the low-orbit satellite signal stored locally, and then converting it into the frequency domain through Fourier transform to obtain a first frequency domain signal, wherein the pseudo-random code sequence stored locally corresponds to the same pseudo-random spread spectrum code sequence of the low-orbit satellite;

[0071] B2. Find the maximum amplitude point in the first frequency domain signal, which is the maximum amplitude value of the stripped signal in the frequency domain;

[0072] B3. Determine whether the maximum amplitude is greater than a preset amplitude threshold value. If so, determine that the stripped signal is captured successfully and calculate the Doppler frequency of the stripped signal. Otherwise, return to B1.

[0073] In some embodiments, the specific process of periodically tracking the post-stripping signal in the tracking channel is:

[0074] Near the interval time point of the low-orbit satellite signal broadcast (the ten sampling time points before and after the interval time point of the low-orbit satellite signal broadcast), the captured Doppler frequency is used to confirm whether there is a stripped signal that has entered the tracking channel. If the stripped signal is confirmed, the Doppler frequency of the stripped signal is obtained, and then the tracking process is directly carried out. If the stripped signal is not confirmed, jump to B1 again for re-capture.

[0075] Take Satellite 1 as an example.

[0076] 1) The signal after carrier stripping (stripped signal) is conjugate-multiplied with the local pseudo-random code sequence of satellite 1, and then converted to the frequency domain through Fourier transform. The frequency domain amplitude is taken to obtain the position where the maximum amplitude occurs. If it is higher than the preset threshold value, the signal is captured successfully, and the Doppler frequency at this time is calculated.

[0077] y i (n) = aD(n)C(n)cos(ω e (n)t(n)+θ e )*C(n)

[0078] =aD(n)cos(ω e (n)t(n)+θ e )

[0079] y q(n) = aD(n)C(n)sin(ω e (n)t(n)+θ e )*C(n)

[0080] =aD(n)sin(ω e (n)t(n)+θ e )

[0081] y i (n) and y q (n) corresponds to the previous x i (n) and x q (n).

[0082] 2) Transfer satellite signal 1 to the tracking channel. After that, the satellite signal will no longer be captured separately. Only the captured Doppler frequency will be used for confirmation near the interval time point of the signal broadcast. If the signal is confirmed, its coarse Doppler frequency deviation estimate can be obtained, and then the tracking process will be directly carried out. If the signal is not confirmed, it will be switched back to step 1).

[0083] According to different satellites, the local pseudo-random code sequences 2, 3, 4, and 5 are used to repeat steps 1) and 2) until all preset low-orbit satellite signals are captured.

[0084] Embodiment 2 further discloses a signal processing method for a low-orbit satellite signal, including:

[0085] Capturing and tracking the preset low-orbit satellite signal by using a low-orbit satellite signal capturing and tracking method of Example 1;

[0086] Calculating a final carrier-to-noise ratio of a stripped signal corresponding to the tracked low-orbit satellite signal, and performing time synchronization on the stripped signal;

[0087] In some embodiments, before calculating the final carrier-to-noise ratio, the stripped signal is firstly subjected to coarse frequency compensation (the coarse Doppler frequency is generally large, and matched filtering cannot be performed without coarse compensation), and then matched filtering is performed.

[0088] In some embodiments, the specific process of calculating the carrier-to-noise ratio of the stripped signal corresponding to the tracked low-orbit satellite signal and performing time synchronization on the stripped signal is:

[0089] Initial carrier-to-noise ratio calculation stage: calculating the initial carrier-to-noise ratio of the stripped signal;

[0090] Coarse synchronization stage: perform integration and zeroing processing on the stripped signal to obtain the coherent integration results of the I branch signal and the Q branch signal of the stripped signal, improve the initial carrier-to-noise ratio of the stripped signal by the coherent integration result to obtain the final carrier-to-noise ratio, and compare the leading, timely and lagging three-way signals of the I branch signal and the leading, timely and lagging three-way signals of the Q branch signal, and the maximum amplitude point obtained is the coarse optimal sampling point;

[0091] Fine synchronization stage: the coarse optimal sampling point is corrected using an interpolation symbol time synchronization method to obtain a fine optimal sampling point.

[0092] That is, the tracking channel first calculates the initial carrier-to-noise ratio of the signal, then performs integration and zeroing processing on the signal stripped of the carrier and pseudo-random spread spectrum code to obtain the coherent integration results on the I and Q branches, and then improves the accuracy of the initial carrier-to-noise ratio through the coherent integration results to obtain the final carrier-to-noise ratio, where the I and Q branches are divided into three paths: leading, timely, and lagging, namely I E ,I P ,I L , Q E , Q P , Q L , the code phase is delayed by 1 / 2 code chip, the maximum amplitude point is the coarse optimal sampling point, and then the interpolation symbol time synchronization method is used for time synchronization.

[0093] For the stripped signal after time synchronization, different residual frequency offset estimation methods are selected according to the final carrier-to-noise ratio to calculate the residual frequency offset estimation value. Finally, the stripped signal is frequency offset compensated using the obtained residual frequency offset estimation value, and then phase compensation, demodulation, and channel decoding are performed to finally obtain the output signal.

[0094] In some embodiments, the specific process of calculating the residual frequency offset estimation value by selecting different residual frequency offset estimation methods according to the final carrier-to-noise ratio for the stripped signal after time synchronization is as follows:

[0095] C1. Compare the final carrier-to-noise ratio with a preset carrier-to-noise ratio threshold value. If the final carrier-to-noise ratio is greater than the preset threshold value, jump to C2; otherwise, jump to C3;

[0096] C2. Calculate the residual frequency offset estimate according to the following formula:

[0097]

[0098] In the formula, To estimate the residual frequency offset, T is the sampling period, L is the data length used for calculation, r(k) represents the kth sampling point, r * (k) represents the conjugate of the kth sampling point,

[0099] C3. Take out the pilot segment of this segment of data, conjugate it with the local pilot and perform FFT transformation. After transformation, calculate the residual frequency offset estimation value according to the following formula:

[0100]

[0101] In the formula Represents the estimated residual frequency offset, T is the sampling time interval, k0 represents the index value corresponding to the maximum amplitude of the spectrum after FFT transformation, X(k0) represents the maximum amplitude, r represents the difference between the index value corresponding to the second maximum amplitude of the spectrum and k0, and X(r⊙0.5) represents the larger value at index r±0.5.

[0102] Next, the present invention will be described by using more detailed examples.

[0103] 1. When the terminal is cold started, the intermediate frequency signal is down-converted and then an integral operation is performed for a time of one chip. Then, according to the list of satellite signals to be captured, the corresponding pseudo-random codes are selected in turn. Figure 3 The satellite signal capture process shown in the figure, after successfully capturing a satellite signal, updates the satellite capture status of the status register until all satellites are successfully captured and the data is transferred to the signal tracking buffer.

[0104] 2. It is not necessary to capture the signal every sampling point. Instead, the satellite signal capture status is confirmed every T signal period (T signal period refers to the interval time point of satellite signal broadcasting, which is the "regular" in S1) to determine whether to perform the signal capture process or the signal confirmation process. The signal confirmation process is as follows: Figure 3 If the signal is not confirmed, the satellite capture status is changed from confirmed to captured, and the signal is captured again.

[0105] 3. Before performing residual frequency estimation, extract the data about the carrier-to-noise ratio in the status register, compare the carrier-to-noise ratio with the preset threshold value, and use different frequency estimation algorithms according to the comparison results to obtain the optimal estimation value.

[0106] 4. The time and range of signal confirmation can be adjusted appropriately, such as confirming once every T±t0. The sampling data required for signal confirmation can be appropriately extended to prevent signal leakage.

[0107] 5. The entire signal processing process is aimed at low-orbit satellite navigation burst signals. The signal processing process and signal capture and tracking strategy have the advantages of low resource consumption and good system performance for low-orbit navigation burst signals.

[0108] Figure 3The signal capture phase process is shown below as a specific implementation example: At t = t0, the rate at which the intermediate frequency sampling data enters the capture buffer after down-conversion is f s =320khz.

[0109] 1. Take out the data cached in the capture buffer and perform a whole chip integration of the length of a chip time, that is, accumulate every 4 sampling points to obtain 80kHz baseband spread spectrum data x(n).

[0110] 2. Select the pilot segment p(n) of the spread spectrum data and compare it with the local pseudo-random spread spectrum code p local (n) Conjugate multiplication to obtain the original data without pseudo code (D(n) above), perform a 512-point Fast Fourier Transform on z(n) to obtain Z(n) = FFT{z(n)}, and find the maximum amplitude.

[0111] 3. Set the threshold value to the ratio of the maximum amplitude to the 8th amplitude (i.e. remove the 3 amplitude points on the left and right of the maximum amplitude point to ensure the signal energy and noise energy of the selected point). If the value is less than the preset threshold, set Repeat the previous two steps.

[0112] 3. If the maximum amplitude is greater than the threshold, the captured data is converted to the tracking channel Buffer and the Doppler frequency captured at this time is recorded.

[0113] Figure 4 As the signal confirmation phase process, a specific implementation case is given below: Assuming that the navigation burst signal broadcast period is T, the rate at which the intermediate frequency sampling data enters the capture buffer Buffer at time t = t0 is f s =320khz.

[0114] 1. Confirm the capture status of the preset capture satellite list every time T. If the satellite status is confirmed, cache the data in the tracking cache Buffer channel.

[0115] 2. Take out the tracking channel buffer, t = t0-t e The data at the moment is integrated over the entire code chip to obtain the 80 kHz baseband spread spectrum data x(n).

[0116] 3. Select the pilot band of the spread spectrum data, conjugate and multiply it with the local pseudo-random spread spectrum code to obtain the original data without the pseudo-code, perform a 512-point fast Fourier transform, and find the maximum amplitude point. If the maximum amplitude is greater than the preset signal confirmation threshold (the threshold can be slightly lower than the capture threshold), the signal tracking process can be carried out.

[0117] 4. If the maximum amplitude is less than the preset signal threshold, set t = t0 + T and repeat the above steps 1-3. If the signal cannot be captured for three consecutive times, it is determined that the signal is lost, and the satellite status is changed to capture, and the signal capture process of the satellite is repeated at the next moment.

[0118] Figure 8 The whole low-orbit navigation signal processing process is as follows. The specific implementation case is set as f s =320khz, the navigation signal is a periodic burst signal, the signal period is T, the Doppler change rate is f d ′=300Hz / s:

[0119] 1. After the antenna receives the RF signal, it undergoes RF link preprocessing to complete the transformation from RF signal to IF signal, including RF front-end filtering, down-conversion, analog-to-digital conversion, 12-bit quantization, automatic gain control AGC, etc.

[0120] 2. The local carrier register generates a local orthogonal carrier and performs complex mixing with the intermediate frequency data to obtain a zero intermediate frequency signal, thus completing carrier stripping.

[0121] 3. The signal after carrier stripping is complex-correlated with the corresponding pseudo-random code to complete pseudo-code stripping, and then the signal-to-noise ratio is calculated according to Figure 3 The capture process performs signal capture to obtain the estimated Doppler and the approximate starting point of the signal.

[0122] 4. After completing the signal capture, the data is converted to the tracking channel. In the next signal cycle, the tracking channel first uses the captured Doppler frequency for frequency compensation, and then obtains the Doppler coarse frequency estimate of the current signal through the tracking confirmation process (according to the assumption, the Doppler change of the signal in the current cycle is △f≈300*T compared with the signal in the previous cycle). After frequency compensation, the signal only has residual Doppler frequency deviation and system error caused by system time jitter. The frequency deviation range is approximately 50 to 200 Hz.

[0123] 5. Use a 129-order raised cosine filter for matched filtering to maximize the signal-to-noise ratio. The raised cosine filter uses a roll-off factor of 0.35.

[0124] 6. The signal after matched filtering has a higher signal-to-noise ratio. Figure 5The interpolation symbol timing synchronization algorithm shown in the figure is used for time synchronization. First, the data passes through a third-order Farrow filter with variable fractional delay to obtain sampled data synchronized with the transmitter. The fractional delay is determined according to the phase of the current NCO, and the initial overflow frequency of the NCO is 2 times the symbol rate. At the overflow moment of the NCO, the output y(k) of the Farrow filter is sampled. At this time, the rate of y(k) is 4 times the symbol rate, and y(m) with a symbol rate of 2 times is obtained. y(m) is sent to the secondary shift register to obtain y(m-1) and y(m-2). Then, the timing error is calculated using the Gardner timing error detection algorithm. The error is filtered by a second-order loop filter, and then fed back to the NCO after being added to the initial step control word of the NCO to complete the closed-loop control. After time synchronization, the data sampling rate is reduced to 1 / 4 times the symbol rate, and the sampled data with precise time synchronization is obtained.

[0125] 7. After symbol synchronization, the sampled data is selected to use the residual frequency offset estimation algorithm according to the signal-to-noise ratio. When the signal-to-noise ratio is higher than the threshold, the sampling point is first calculated and input into the first-level shift register to obtain y(k) and y(k-1), and the conjugate value of the first-level shift register is multiplied by the current input value, and the phase of the result is taken, that is, r(k) = arg(y(k)y * (k-1)), the specific implementation formula for taking the phase is arg(r(k))=imag(r(k)) / real(r(k)), and then the coefficient weighted value is generated locally Finally, multiply the coefficient weighted value by the phase value and calculate the average to obtain the optimal estimated frequency. When the signal-to-noise ratio is lower than the threshold, take out the pilot signal, conjugate it with the local pilot sequence and perform FFT. Then calculate the amplitude of the maximum amplitude point to the left and right of the amplitude point and the amplitude of the amplitude point to the left and right of 0.5. According to the formula Calculate the estimated frequency deviation. Since the residual frequency deviation after tracking confirmation and coarse frequency compensation is about 50-200 Hz, which is within the frequency pulling range of the two algorithms, both algorithms are applicable.

[0126] 8. Use the estimated frequency offset to perform frequency compensation. At this time, the signal has basically no frequency offset, but the phase may deflect due to the unknown initial phase, so phase compensation is required. Figure 7 As shown, take the pilot sequence and calculate the compensation factor according to the following formula: The compensation factor is actually a complex number. Normalize its amplitude and conjugate it with the sampled data to obtain the phase compensation data.

[0127] 9. Finally, the data is constellation demapped, and the demapped bit data is sent to the channel decoder to complete the channel decoding.

[0128] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection 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 scope of protection of the present invention.

Claims

1. A method for capturing and tracking low-orbit satellite signals, characterized in that: include: Periodically receiving a low-orbit satellite signal of at least one time segment, and performing carrier separation on the low-orbit satellite signal, thereby obtaining a stripped signal corresponding to the low-orbit satellite signal; The stripped signal is processed with the pseudo-random code sequence stored locally to find the maximum amplitude of the stripped signal in the frequency domain, and the Doppler frequency is calculated after the stripped signal is successfully captured based on the maximum amplitude. At the same time, the stripped signal after successful capture is transferred to the tracking channel and the stripped signal is tracked regularly, and at the same time, returns to the previous step.

2. The method for capturing and tracking low-orbit satellite signals according to claim 1, characterized in that: Each of the stripped signals includes a pseudo-random spread spectrum code sequence and a data code of the low-orbit satellite.

3. The method for capturing and tracking low-orbit satellite signals according to claim 1, characterized in that: The specific process of performing carrier separation on the low-orbit satellite signal to obtain a stripped signal corresponding to the low-orbit satellite signal is as follows: A1. Preprocessing the low-orbit satellite signal to obtain a digital intermediate frequency signal, wherein the preprocessing includes filtering, down-conversion, analog-to-digital conversion, quantization, and AGC; A2. Splitting the digital intermediate frequency signal into an I intermediate frequency signal and a Q intermediate frequency signal, and mixing the I intermediate frequency signal and the Q intermediate frequency signal with a local carrier respectively to complete carrier stripping and obtain the stripped signal.

4. The method for capturing and tracking low-orbit satellite signals according to claim 3, characterized in that: In A2, the specific formula for mixing the I intermediate frequency signal and the Q intermediate frequency signal with the local carrier is: Where D(n) is the original data, i.e., zero-frequency signal, C(n) is the pseudo-random spread spectrum code sequence, ω i (n) is the signal angular frequency, θ(n) is the signal phase, n is the noise sequence, the distribution follows a Gaussian distribution with a mean of 0, ω0(n) is the local carrier angular frequency, θ0 is the random initial phase, x i (n) represents the I branch signal after stripping, x q (n) represents the q signal of the stripped signal, a is the random gain, t(n) is the sampling time, θ e is the random phase error, ω e is the angular frequency error.

5. The method for capturing and tracking low-orbit satellite signals according to claim 4, characterized in that: The specific process of processing the stripped signal and the pseudo-random code sequence to find the maximum amplitude of the stripped signal in the frequency domain, and determining the Doppler frequency after the stripped signal is successfully captured by the maximum amplitude is as follows: B1. Conjugate-multiplying the stripped signal with the pseudo-random code sequence of the low-orbit satellite signal stored locally, and then converting it into the frequency domain through Fourier transform to obtain a first frequency domain signal, wherein the pseudo-random code sequence stored locally corresponds to the same pseudo-random spread spectrum code sequence of the low-orbit satellite; B2. Find the maximum amplitude point in the first frequency domain signal, which is the maximum amplitude value of the stripped signal in the frequency domain; B3. Determine whether the maximum amplitude is greater than a preset amplitude threshold value. If so, determine that the stripped signal is captured successfully and calculate the Doppler frequency of the stripped signal. Otherwise, return to B1.

6. The method for capturing and tracking low-orbit satellite signals according to claim 5, characterized in that: The specific process of regularly tracking the stripped signal in the tracking channel is as follows: The captured Doppler frequency is used at ten sampling time points before and after the interval time point of the low-orbit satellite signal broadcast to confirm whether there is a post-stripping signal that has entered the tracking channel. If the post-stripping signal is confirmed, the Doppler frequency of the post-stripping signal is obtained, and then the tracking process is directly carried out. If the post-stripping signal is not confirmed, it jumps to B1 again for re-capture.

7. A signal processing method for low-orbit satellite signals, characterized in that: include: Capturing and tracking a preset low-orbit satellite signal by using a low-orbit satellite signal capturing and tracking method according to any one of claims 1 to 6; Calculating a final carrier-to-noise ratio of a stripped signal corresponding to the tracked low-orbit satellite signal, and performing time synchronization on the stripped signal; For the stripped signal after time synchronization, different residual frequency offset estimation methods are selected according to the final carrier-to-noise ratio to calculate the residual frequency offset estimation value. Finally, the stripped signal is frequency offset compensated using the obtained residual frequency offset estimation value, and then phase compensation, demodulation, and channel decoding are performed to finally obtain the output signal.

8. The signal processing method of a low-orbit satellite signal according to claim 7, characterized in that: Before calculating the final carrier-to-noise ratio, the stripped signal is firstly subjected to coarse frequency compensation, and then matched filtering is performed.

9. The signal processing method of a low-orbit satellite signal according to claim 7, characterized in that: The specific process of calculating the carrier-to-noise ratio of the stripped signal corresponding to the tracked low-orbit satellite signal and performing time synchronization on the stripped signal is as follows: Initial carrier-to-noise ratio calculation stage: calculating the initial carrier-to-noise ratio of the stripped signal; Coarse synchronization stage: perform integration and zeroing processing on the stripped signal to obtain the coherent integration results of the I branch signal and the Q branch signal of the stripped signal, improve the initial carrier-to-noise ratio of the stripped signal by the coherent integration result to obtain the final carrier-to-noise ratio, and compare the leading, timely and lagging three-way signals of the I branch signal and the leading, timely and lagging three-way signals of the Q branch signal, and the maximum amplitude point obtained is the coarse optimal sampling point; Fine synchronization stage: the coarse optimal sampling point is corrected using an interpolation symbol time synchronization method to obtain a fine optimal sampling point.

10. The signal processing method of a low-orbit satellite signal according to claim 7, characterized in that: The specific process of calculating the residual frequency offset estimation value by selecting different residual frequency offset estimation methods according to the final carrier-to-noise ratio for the stripped signal after time synchronization is as follows: C1. Compare the final carrier-to-noise ratio with a preset carrier-to-noise ratio threshold value. If the final carrier-to-noise ratio is greater than the preset threshold value, jump to C2; otherwise, jump to C3; C2. Calculate the residual frequency offset estimate according to the following formula: In the formula, To estimate the residual frequency offset, T is the sampling period, L is the data length used for calculation, r(k) represents the kth sampling point, r * (k) represents the conjugate of the kth sampling point, C3. Take out the pilot segment of the original data, conjugate it with the local pilot and perform FFT transformation. After transformation, calculate the residual frequency offset estimation value according to the following formula: In the formula Represents the estimated residual frequency offset, T is the sampling time interval, k0 represents the index value corresponding to the maximum amplitude of the spectrum after FFT transformation, X(k0) represents the maximum amplitude, r represents the difference between the index value corresponding to the second maximum amplitude of the spectrum and k0, and X(r⊙0.5) represents the larger value at index r±0.5.

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