CN110018500B - Beidou satellite signal capturing method based on circumferential shift - Google Patents
Beidou satellite signal capturing method based on circumferential shift Download PDFInfo
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- CN110018500B CN110018500B CN201910340444.2A CN201910340444A CN110018500B CN 110018500 B CN110018500 B CN 110018500B CN 201910340444 A CN201910340444 A CN 201910340444A CN 110018500 B CN110018500 B CN 110018500B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
- G01S19/29—Acquisition or tracking or demodulation of signals transmitted by the system carrier including Doppler, related
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
- G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
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Abstract
The application discloses a Beidou satellite signal capturing method based on circumferential shift, which comprises the following steps of: s1, multiplying the input digital intermediate frequency signal in each integration time with the in-phase component and the quadrature component of a local carrier generator respectively, carrying out carrier stripping to obtain a baseband complex signal sequence X (n), and carrying out fast Fourier transform FFT to obtain a spectrum sequence X (k); s2: the ranging code C (n) is obtained after the local NH code secondary modulation, and C is obtained after fast Fourier transform FFT and complex conjugation * (k) The method comprises the steps of carrying out a first treatment on the surface of the S3, each time the sequence X (k) is shifted circumferentially one time, the shift l bits are denoted as sequence X (k-l) and are compared with C * (k) Multiplying; s4, performing Inverse Fast Fourier Transform (IFFT) on the result in S3 to obtain Doppler frequency shift f corresponding to the l shift d Is related to the result r of the correlation l (m); s5, entering S1, reading the digital intermediate frequency signal in the next integration time, obtaining a correlation matrix of the next adjacent integration time according to the steps, carrying out conjugate multiplication on the correlation matrices of the two adjacent integration times, and accumulating to obtain a differential coherent integration result.
Description
Technical Field
The application relates to the technical field of satellite communication, in particular to a Beidou satellite signal capturing method based on circumferential shift.
Background
With the implementation of Beidou satellite navigation system in China, satellite communication technology application is more and more important. The acquisition of satellite signals is the first step in satellite digital signal processing. The method aims to obtain two parameters of the rough carrier Doppler frequency shift and the ranging code phase of all visible Beidou satellites, and provides more accurate conditions for follow-up tracking modules.
The capturing of carrier Doppler frequency and ranging code phase is a two-dimensional searching process, and the traditional satellite signal capturing algorithm mainly comprises a serial searching algorithm, a parallel frequency domain searching algorithm and a parallel code phase capturing algorithm. The serial search acquisition algorithm is the earliest conventional method proposed by satellite navigation systems to solve the acquisition problem. The method is to search the ranging code phase and Doppler shift in series. Since the step size of the code phase search is typically half a chip and one symbol period is long, the serial search is inefficient. The doppler frequency search step size is determined by the coherent integration time, usually the inverse of the coherent integration time, and most conventional hardware receivers employ a serial search algorithm. The parallel code phase search acquisition method is proposed to solve the problem of slow serial search speed. The method is based on fast Fourier transform (Fast Fourier Transform, FFT) and Inverse Fast Fourier Transform (IFFT) to convert the correlation operation of satellite signals in the time domain into multiplication operation of the frequency domain, so that the time for capturing is reduced, and a foundation is laid for realizing real-time processing.
The parallel code phase acquisition algorithm is a parallel processing method, and the schematic diagram of the algorithm is shown in fig. 1. The input digital intermediate frequency signals are multiplied by the in-phase component and the quadrature component of the local carrier generator respectively, carrier stripping is carried out, and I-path signals and Q-path signals are obtained respectively. The obtained complex signal is multiplied by the complex conjugate of the Fourier transform of the local ranging code after the fast Fourier transform, and then the obtained result is subjected to Inverse Fast Fourier Transform (IFFT) and modulo is carried out to obtain a correlation value result. And when the peak value exceeds a preset threshold, the acquisition is successful. From which two parameter values of the coarse ranging code phase and doppler shift of the input signal are obtained. If there is no obvious peak, the frequency needs to be adjusted to enable the local carrier generator to generate the sine and cosine signals of the next frequency point, and the above operation is repeated until all possible frequency units are searched. From simulations it can be seen that a signal with an integration time of 1ms can be acquired at-15 dB using the parallel code phase search algorithm. However, it is difficult to acquire the doppler frequency and ranging code phase of the satellite signal in a weak signal environment.
The acquisition of satellite signals is complicated and difficult due to the doppler shift of the satellite signal carrier frequency caused by the high speed motion of the space satellites. Meanwhile, the influence of NH (Neumann-Hoffman) code modulation exists in the Beidou satellite B1 frequency point signal, and the correlation integration time cannot exceed 1ms under the condition that the influence of NH code phase jump is not removed. Therefore, the gain of the signal cannot be improved by prolonging the integration time under the condition of low signal-to-noise ratio, and the capturing sensitivity is low. Meanwhile, the related operation amount is overlarge due to the overlarge FFT operation times in the parallel code phase algorithm, so that the speed and the sensitivity still need to be improved.
Disclosure of Invention
According to the problems existing in the prior art, the application discloses a Beidou satellite signal capturing method based on circumferential shift, which specifically comprises the following steps:
s1, multiplying the input digital intermediate frequency signal in each integration time with the in-phase component and the quadrature component of a local carrier generator respectively, carrying out carrier stripping to obtain a baseband complex signal sequence X (n), and carrying out fast Fourier transform FFT to obtain a spectrum sequence X (k);
s2: the ranging code C (n) is obtained after the local NH code secondary modulation, and C is obtained after fast Fourier transform FFT and complex conjugation * (k)；
S3, each time the sequence X (k) is shifted circumferentially one time, the shift l bits are denoted as sequence X (k-l) and are compared with C * (k) Multiplying;
s4, performing Inverse Fast Fourier Transform (IFFT) on the result in S3 to obtain Doppler frequency shift f corresponding to the l shift d Is related to the result r of the correlation l (m) repeating S3 until l=n-1, combining all r l (m) obtaining a correlation matrix Y (m, l), wherein m is the corresponding ranging code phase, and l is the number of circumferential shift bits. The correlation matrix can also be expressed as Y (τ, f d ) Where τ is the code phase delay, represented by τ=m/f s Determining f d For Doppler shift, from f d ＝l·f s N is determined, wherein f s Is the sampling frequency;
s5, entering S1, reading the digital intermediate frequency signal in the next integration time, obtaining the correlation matrix of the next adjacent integration time according to the steps, carrying out conjugate multiplication and accumulation on the correlation matrices of the two adjacent integration times, and obtaining a differential coherent integration resultJudging the obtained matrix Z (tau, f d ) Whether the maximum correlation value of (c) is greater than the acquisition threshold, if the acquisition threshold is exceeded then the current satellite is visible,recording and storing the corresponding Doppler frequency value and code phase delay value; if matrix Z (τ, f d ) If the maximum correlation value of (2) is smaller than the acquisition threshold, the acquisition failure is judged.
In the above step, the cyclic circumference shift operation of the input frequency domain signal in S2 is equivalent to the fast fourier transform after the frequency shift of the time domain signal, where the circumference shift is shown in the formula:
where X (k) is the FFT of X (N), N is the data length of X (N), n=0, 1,..n-1, l is the number of circumferentially shifted bits, where the correlation r of the input digital signal X (N) with the local ranging code c (N) l (m) is as follows:
where N is the number of x (N) bits, m=0, 1,..n-1, l is the number of cyclic circumferential shifts, for the correlation sequence result r l (m) taking the modulus to complete the range code correlation process in a Doppler unit.
By adopting the technical scheme, the Beidou satellite signal capturing method based on circumferential shift provided by the application has the advantages that the frequency spectrum sequence after carrier stripping of the input intermediate frequency signal is circularly and circumferentially shifted to replace the repeated carrier stripping and Fast Fourier Transform (FFT) operation of the input signal in a parallel code phase capturing algorithm, so that the operand is reduced; meanwhile, the idea of differential integration is adopted to carry out conjugate multiplication and accumulation on the coherent result in each integration time, so that the signal-to-noise ratio is enhanced, and the capturing sensitivity is improved.
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In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings may be obtained according to the drawings without inventive effort to those skilled in the art.
FIG. 1 is a flow chart of the method of the present application.
Fig. 2 is a graph showing the effect of differential coherent integration using the present application under different conditions.
Fig. 3 is an effect diagram of using a non-coherent integration method.
Detailed Description
In order to make the technical scheme and advantages of the present application more clear, the technical scheme in the embodiment of the present application is clearly and completely described below with reference to the accompanying drawings in the embodiment of the present application:
in the Beidou satellite signal capturing method based on circumferential shift shown in fig. 1, actually received satellite signals are converted into digital intermediate frequency signals through A/D after passing through a radio frequency front end. The capturing algorithm process of the Beidou satellite signals specifically comprises the following steps:
s1, multiplying the input digital intermediate frequency signal in each integration time with the in-phase component and the quadrature component of a local carrier generator respectively, carrying out carrier stripping to obtain a baseband complex signal sequence X (n), and carrying out fast Fourier transform FFT to obtain a spectrum sequence X (k);
s2: the ranging code C (n) is obtained after the local NH code secondary modulation, and C is obtained after fast Fourier transform FFT and complex conjugation * (k)；
S3, each time the sequence X (k) is shifted circumferentially one time, the shift l bits are denoted as sequence X (k-l) and are compared with C * (k) Multiplying;
s4, performing Inverse Fast Fourier Transform (IFFT) on the result in S3 to obtain Doppler frequency shift f corresponding to the l shift d Is related to the result r of the correlation l (m) repeating S3 until l=n-1, combining all r l (m) obtaining a correlation matrix Y (m, l), wherein m is the corresponding ranging code phase, and l is the number of circumferential shift bits. The correlation matrix can also be expressed as Y (τ, f d ) Where τ is the code phase delay, represented by τ=m/f s Determining f d For Doppler shift, from f d ＝l·f s N is determined, wherein f s For samplingA frequency;
s5, entering S1, reading the digital intermediate frequency signal in the next integration time, obtaining the correlation matrix of the next adjacent integration time according to the steps, carrying out conjugate multiplication and accumulation on the correlation matrices of the two adjacent integration times, and obtaining a differential coherent integration resultJudging the obtained matrix Z (tau, f d ) If the maximum correlation value of the code phase delay value is larger than the acquisition threshold, if the maximum correlation value exceeds the acquisition threshold, the current satellite is visible, and the corresponding Doppler frequency value and code phase delay value are recorded and stored; if matrix Z (τ, f d ) If the maximum correlation value of (2) is smaller than the acquisition threshold, the acquisition failure is judged.
In the above steps, the cyclic circumference shift operation of the input frequency domain signal is equivalent to the fast fourier transform after the frequency shift of the time domain signal, wherein the circumference shift is as shown in the formula:
where X (k) is the FFT of X (N), N is the data length of X (N), n=0, 1,..n-1, l is the number of circumferentially shifted bits, where the correlation r of the input digital signal X (N) with the local ranging code c (N) l (m) is as follows:
where N is the number of x (N) bits, m=0, 1,..n-1, l is the number of cyclic circumferential shifts, for the correlation sequence result r l (m) taking the modulus to complete the range code correlation process in a Doppler unit.
Therefore, when searching for satellite signals, all possible Doppler frequency units can be obtained by performing FFT operation on the input signals only once and performing cyclic circumference shift operation. Compared with the traditional parallel code phase acquisition algorithm, the FFT frequency is reduced, and therefore the searching speed is improved.
In the conventional GPS satellite signal acquisition algorithm, a method of coherent integration and incoherent integration is generally adopted to improve the signal gain. Coherent integration can improve signal-to-noise ratio by increasing data integration time, but can be affected by navigation data hopping and increased computation due to longer integration time. Incoherent integration may not be limited by navigation data hopping, but introduces squaring losses, which become larger as integration time increases. The article therefore uses a differential coherent integration algorithm whose basic idea is to conjugate multiply the correlation integration matrices over multiple integration times and then accumulate. The mathematical model is as follows:
wherein Y is i ,Y i+1 Two adjacent coherent integration matrices can be represented as useful signal V i And noise signal N i And the result of the useful signal matrix at adjacent time is correlated, while the random noise is uncorrelated, and meanwhile, the method has the characteristic of Gaussian noise, and can reduce the influence of the noise by superposition. Therefore, the signal to noise ratio can be enhanced and the square loss caused by incoherent integration can be restrained by conjugate multiplication of the coherent integration matrix.
And performing simulation experiments on the capturing algorithm by using an MATLAB platform. And simulating and generating a 20ms simulation intermediate frequency signal according to the ranging code structural characteristic of the actual Beidou satellite signal. And carrying out operation on the data in each integration time according to an algorithm block diagram. The intermediate frequency signal frequency is set to be 4.092MHz, the sampling rate is set to be 20.46MHz, and the ranging code rate is set to be 2.046MHz. The navigation message speed is 50b/s, the Doppler frequency range is-10 KHz, and the integration time is 1ms. The results of the two acquisition algorithms at a signal to noise ratio of-32 dB are shown in fig. 2 and 3. By comparing fig. 2 and 3, it can be seen that by processing the 20ms simulation data, when the signal-to-noise ratio is-32 dB, a difference algorithm can be used to obtain an obvious peak value, and the peak value is the maximum value of the integration obtained by the input signal during the two-dimensional search of the pseudo-random code phase and the doppler shift, and the peak value is generated by simulationThe signal parameters are the same. The incoherent algorithm has no obvious peak value, and the Doppler frequency shift and the ranging code phase value in the analog signal cannot be determined. For 20ms of simulation data, an algorithmic operation is performed at each integration time. When parallel code phase search is adopted for satellite signal data, if the Doppler frequency range is-10 KHz, the frequency search interval is 250Hz, and the integration time is 1ms, 20 x 82 FFT operations and 20 x 81 IFFT operations are required to be carried out on the digital signal. The improved circumference shift algorithm then requires 20 x 2 FFT operations and 20 x 81 IFFT operations. Wherein the FFT operation of N points at a time requires (Nlg 2 N)/2 complex multiplication operations and Nlg 2 N complex addition operations. The comparison of the algorithm calculation amounts is shown in table 1. It follows that the total computational effort of the acquisition algorithm is significantly reduced.
Table 1 algorithm operand comparison
As described above, the present application is not limited to the preferred embodiments, but any person skilled in the art, having the benefit of this disclosure, can apply to the present application to any equivalent or modified embodiment, and all such modifications, alterations, and equivalents thereof are intended to be included in the scope of the present application.
Claims (2)
1. The Beidou satellite signal capturing method based on circumferential shift is characterized by comprising the following steps of:
s1, multiplying the input digital intermediate frequency signal in each integration time with the in-phase component and the quadrature component of a local carrier generator respectively, carrying out carrier stripping to obtain a baseband complex signal sequence X (n), and carrying out fast Fourier transform FFT to obtain a spectrum sequence X (k);
s2: the ranging code is obtained after the local NH code is modulated againC (n), performing fast Fourier transform FFT and complex conjugation to obtain C * (k)；
S3, each time the sequence X (k) is shifted circumferentially one time, the shift l bits are denoted as sequence X (k-l) and are compared with C * (k) Multiplying;
s4, performing Inverse Fast Fourier Transform (IFFT) on the result in S3 to obtain Doppler frequency shift f corresponding to the l shift d Is related to the result r of the correlation l (m) repeating S3 until l=n-1, combining all r l (m) obtaining a correlation matrix Y (m, l), wherein m is the corresponding ranging code phase, l is the number of circumferential shift bits, and the correlation matrix can be converted into Y (τ, f) d ) Where τ is the code phase delay, represented by τ=m/f s Determining f d For Doppler shift, from f d ＝l·f s N is determined, wherein f s Is the sampling frequency;
s5, entering S1, reading the digital intermediate frequency signal in the next integration time, obtaining the correlation matrix of the next adjacent integration time according to the steps, carrying out conjugate multiplication and accumulation on the correlation matrices of the two adjacent integration times, and obtaining a differential coherent integration resultJudging the obtained matrix Z (tau, f d ) If the maximum correlation value of the code phase delay value is larger than the acquisition threshold, if the maximum correlation value exceeds the acquisition threshold, the current satellite is visible, and the corresponding Doppler frequency value and code phase delay value are recorded and stored; if matrix Z (τ, f d ) If the maximum correlation value of (2) is smaller than the acquisition threshold, the acquisition failure is judged.
2. The circumference shift-based Beidou satellite signal capturing method of claim 1, further characterized by: wherein the cyclic circumference shift operation of the input frequency domain signal is equivalent to the fast fourier transform of the time domain signal after frequency shifting, wherein the circumference shift is as shown in the formula:
where X (k) is the FFT of X (N), N is the data length of X (N), n=0, 1,..n-1, l is the number of circumferentially shifted bits, where the correlation r of the input digital signal X (N) with the local ranging code c (N) l (m) is as follows:
where N is the number of x (N) bits, m=0, 1,..n-1, l is the number of cyclic circumferential shifts, for the correlation sequence result r l (m) taking the modulus to complete the range code correlation process in a Doppler unit.
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