Visible satellite signal joint capturing method based on A-GPS

Abstract

The invention discloses a visible satellite signal joint capturing method based on A-GPS. The method comprises the following steps: carrying out square operation on visible satellite signals received by a receiver antenna; performing one-time fast Fourier transform on the squared signal to obtain Doppler spectrums of all visible satellite signals; doppler frequency capture of each visible satellite signal is completed; estimating code phase estimation values of all visible satellite signals; and completing visible satellite signal code phase capture and the like. The visible satellite signal joint capture method based on the A-GPS provided by the invention has the following advantages: Doppler frequencies and code phases of all visible satellite signals are respectively subjected to joint capture by using auxiliary information provided by the A-GPS; the related operation independently carried out on each search unit of each visible satellite in traditional capture is simplified into global related processing between a local joint reference signal and a received signal, so that the capture efficiency can be greatly improved, and the capture operand is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite navigation receivers, and in particular relates to a visible satellite signal joint capture method based on A-GPS. Background Art

[0002] The Global Navigation Satellite System (GNSS) can provide global users with all-weather, high-precision and real-time position, speed and time information, so it has been widely used in both military and civilian fields. Conventional GNSS receivers generally include three modules: capture, tracking and positioning. Among them, capture is the prerequisite for GNSS receivers to track and locate satellite signals. The purpose of capture is to obtain the spread spectrum code, Doppler frequency and code phase rough estimate of the currently visible satellite to provide initialization parameters for the tracking loop. However, the conventional capture method requires searching the Doppler frequency and code phase of each possible satellite signal spread spectrum code separately, that is, it is necessary to search in the three-dimensional space of unknown spread spectrum code, Doppler frequency and code phase, so the amount of computation required for this capture method is relatively large.

[0003] There have been a lot of research results on improving the receiver acquisition efficiency. Introducing auxiliary information to reduce the parameter search range is a commonly used acquisition method. Among them, the method of using local joint reference signals to estimate the code phase of multiple satellites simultaneously in each Doppler frequency band (hereinafter referred to as the multi-C / A code acquisition method) is a commonly used acquisition method. This method can estimate the code phase of multiple satellites at the same time, which improves the acquisition efficiency to a certain extent. However, this method still requires searching each frequency band, so it still has a large amount of calculation. Summary of the invention

[0004] In order to solve the above problems, the purpose of the present invention is to provide a visible satellite signal joint acquisition method based on A-GPS, which can reduce the acquisition speed of the satellite navigation receiver, reduce the acquisition calculation amount, and improve the acquisition efficiency.

[0005] In order to achieve the above object, the A-GPS-based visible satellite signal joint acquisition method provided by the present invention comprises the following steps performed in sequence:

[0006] 1) Performing a square operation on the visible satellite signal received by the receiver antenna to eliminate the influence of the spread spectrum code and obtain a squared signal;

[0007] 2) Performing a fast Fourier transform on the squared signal to obtain the Doppler spectra of all visible satellite signals, thereby estimating the Doppler frequency estimates of all visible satellite signals;

[0008] 3) The Doppler frequency value range of each visible satellite signal is estimated using the auxiliary information provided by A-GPS, and the Doppler frequency estimation value falling into the Doppler frequency value range of the corresponding visible satellite signal is used as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the Doppler frequency capture of each visible satellite signal;

[0009] 4) constructing a local joint reference signal using the Doppler frequency estimation values ​​and spread spectrum codes of the above-mentioned visible satellite signals;

[0010] 5) performing a parallel correlation operation on the local joint reference signal and the received signal to obtain a correlation function containing correlation peaks of all visible satellite signals, thereby estimating code phase estimation values ​​of all visible satellite signals;

[0011] 6) Use the auxiliary information provided by A-GPS to estimate the code phase value range of each visible satellite signal, and use the code phase estimate of the visible satellite signal that falls within the corresponding visible satellite signal code phase value range as the estimated value of the code phase of the visible satellite signal, thereby completing the visible satellite signal code phase capture.

[0012] In step 1), the method of performing a square operation on the visible satellite signal received by the receiver antenna to eliminate the influence of the spread spectrum code and obtaining the squared signal is:

[0013] First, the visible satellite signal received by the receiver antenna is expressed as:

[0014]

[0015] Where, the superscript q represents the parameter of the qth visible satellite, Q represents the number of visible satellites; P q Indicates the power of visible satellite signals; c q (t) represents the spreading code of the visible satellite signal; τ q Indicates the code delay of the visible satellite signal; represents the Doppler frequency of the visible satellite signal; η(t) represents the observation noise, which is assumed to be Gaussian white noise;

[0016] Then, the visible satellite signal is squared to obtain the squared signal:

[0017]

[0018] In step 2), the method of obtaining the Doppler spectra of all visible satellite signals by performing a fast Fourier transform (FFT) on the squared signal to estimate the Doppler frequency estimation value of all visible satellite signals is:

[0019] First, the Doppler spectrum obtained by performing a fast Fourier transform operation on the squared signal is expressed as:

[0020]

[0021] Where, δ(·) represents the exponential function after FFT operation; f represents the independent variable of Doppler spectrum;

[0022] Then, the Doppler frequencies of all visible satellites are estimated from the above Doppler spectrum. The Doppler spectrum obtained by formula (3) will form a peak at twice the Doppler frequency of all visible satellite signals. Therefore, the Doppler frequencies of all visible satellites are estimated directly based on the peak value of the Doppler spectrum. The peak value is obtained by performing constant false alarm detection on the Doppler spectrum of the visible satellite signal. The constant false alarm detection threshold T h It is expressed as:

[0023]

[0024] Where σ represents the standard deviation of Gaussian white noise; P f Indicates the false alarm probability; exceeds the constant false alarm detection threshold T h The Doppler frequency corresponding to the peak value is twice the estimated value of the Doppler frequency of all visible satellite signals. The obtained set of Doppler frequency estimates is expressed as

[0025] In step 3), the method of using the auxiliary information provided by A-GPS to estimate the Doppler frequency value range of each visible satellite signal, and using the Doppler frequency estimation value falling into the Doppler frequency value range of the corresponding visible satellite signal as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the Doppler frequency capture of each visible satellite signal is:

[0026] First, the Doppler frequency provided by the A-GPS base station is used as a reference, and the Doppler frequency range of each visible satellite signal received by the receiver antenna is estimated in combination with parameters including the base station coverage range. The formula is:

[0027]

[0028] in, Indicates the Doppler frequency value range obtained by the A-GPS base station; It represents the Doppler frequency of the qth satellite signal provided by the A-GPS base station, in Hz; l represents the horizontal coverage of the base station, h represents the height of the base station, both in km; v represents the movement speed of the receiver, in km / h;

[0029] Then, the Doppler frequency estimation value that falls within the Doppler frequency value range of the corresponding visible satellite signal is used as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the capture of the Doppler frequency of each visible satellite signal.

[0030] In step 4), the method of constructing a local joint reference signal using the Doppler frequency estimation value and the spread spectrum code of each visible satellite signal is:

[0031] Using the estimated Doppler frequency of each visible satellite signal and the known spreading code c of each satellite signal q (t) to generate the local joint reference signal, the formula is:

[0032]

[0033] In step 5), the method of performing a parallel correlation operation on the local joint reference signal and the received signal to obtain a correlation function containing correlation peaks of all visible satellite signals, thereby estimating the code phase estimation values ​​of all visible satellite signals is:

[0034] First, a parallel correlation operation is performed on the local joint reference signal x(t) and the received signal s(t). The formula of the correlation operation process is:

[0035]

[0036] Among them, R q (·) represents the autocorrelation function of the visible satellite signal; R ∑ (·) represents the superposition of autocorrelation functions, which contains the correlation peaks of all visible satellite signals; x * (t) represents the complex conjugate of the local joint reference signal x(t);

[0037] Then, the code phases of all visible satellites are estimated from the correlation function; the satellites exceeding the detection threshold T h The code phase corresponding to the peak value of the correlation function is taken as the code phase estimation value, and the obtained set of code phase estimation values ​​is recorded as

[0038] In step 6), the method of using the auxiliary information provided by A-GPS to estimate the code phase value range of each visible satellite signal, and using the estimated code phase value of the visible satellite signal falling within the corresponding visible satellite signal code phase value range as the estimated value of the code phase of the visible satellite signal, thereby completing the visible satellite signal code phase capture is:

[0039] First, the code phase value provided by the A-GPS base station is used as a benchmark, and the code phase value range of each visible satellite signal received by the receiver is estimated by combining parameters including mobile network delay and A-GPS base station coverage. The formula is:

[0040]

[0041] in, Indicates the code phase value range of the visible satellite signal obtained by the A-GPS base station; represents the code phase of the qth visible satellite signal provided by the A-GPS base station; t0 represents the network delay;

[0042] Then, the code phase estimation value of the visible satellite signal that falls within the corresponding visible satellite signal code phase value range is used as the estimated value of the code phase of the visible satellite signal, thereby completing the capture of the visible satellite signal code phase.

[0043] The A-GPS-based visible satellite signal joint acquisition method provided by the present invention has the following advantages:

[0044] Using the auxiliary information provided by A-GPS, the Doppler frequencies and code phases of all visible satellite signals are jointly captured respectively, and the correlation operation performed independently on each search unit of each visible satellite in traditional capture is simplified to a global correlation processing between the local joint reference signal and the received signal. Therefore, the capture efficiency can be greatly improved and the capture calculation amount can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a flow chart of the A-GPS based visible satellite signal joint acquisition method provided by the present invention.

[0046] Figure 2 The figure compares the changes in the number of visible satellites among the traditional parallel code phase capture method, the multi-C / A code capture method and the A-GPS based visible satellite signal joint capture method provided by the present invention.

[0047] Figure 3 The amount of computation when processing each satellite signal separately for the traditional parallel code phase capture method, the multi-C / A code capture method and the A-GPS-based visible satellite signal joint capture method provided by the present invention varies with the number of visible satellites.

[0048] Figure 4 The invention discloses a method for obtaining a conventional parallel code phase capture method, a method for obtaining a multi-C / A code, and a joint capture method for obtaining a visible satellite signal based on A-GPS provided by the present invention.

[0049] Figure 5The relationship between the amount of computation and the number of frequency bands of the traditional parallel code phase capture method, the multi-C / A code capture method and the A-GPS based visible satellite signal joint capture method provided by the present invention.

[0050] Figure 6 (a) is the detection probability of the method of the present invention and the multi-C / A code acquisition method when the signal-to-noise ratio of the satellite signal is -24dB to -17dB, the number of visible satellites captured is 2, and the Doppler frequency shift Δf is 0Hz to 200Hz.

[0051] Figure 6 (b) is the detection probability of the method of the present invention and the multi-C / A code acquisition method when the signal-to-noise ratio of the satellite signal is -24dB to -17dB, the number of captured visible satellites is 3, and the Doppler frequency shift Δf is 0Hz to 200Hz.

[0052] Figure 6 (c) is the detection probability of the method of the present invention and the multi-C / A code acquisition method when the signal-to-noise ratio of the satellite signal is -24dB to -17dB, the number of visible satellites captured is 4, and the Doppler frequency shift Δf is 0Hz to 200Hz.

[0053] Figure 6 (d) is the detection probability of the method of the present invention and the multi-C / A code acquisition method when the signal-to-noise ratio of the satellite signal is -24dB to -17dB, the number of captured visible satellites is 5, and the Doppler frequency shift Δf is 0Hz to 200Hz. DETAILED DESCRIPTION

[0054] The A-GPS-based visible satellite signal joint acquisition method provided by the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0055] like Figure 1 As shown, the A-GPS-based visible satellite signal joint acquisition method provided by the present invention comprises the following steps performed in sequence:

[0056] 1) Performing a square operation on the visible satellite signal received by the receiver antenna to eliminate the influence of the spread spectrum code and obtain a squared signal;

[0057] First, the visible satellite signal received by the receiver antenna is expressed as:

[0058]

[0059] Where, the superscript q represents the parameter of the qth visible satellite, Q represents the number of visible satellites; P q Indicates the power of visible satellite signals; c q (t) represents the spreading code of the visible satellite signal; τ qIndicates the code delay of the visible satellite signal; represents the Doppler frequency of the visible satellite signal; η(t) represents the observation noise, which is generally assumed to be Gaussian white noise;

[0060] Then, the visible satellite signal is squared to obtain the squared signal:

[0061]

[0062] The first term after the equal sign in formula (2) is a square term. Since the spreading code c of the visible satellite signal q (t) takes the value of ±1, so the influence of the spread spectrum code can be eliminated by squaring equation (1).

[0063] 2) Perform a fast Fourier transform (FFT) on the squared signal to obtain the Doppler spectra of all visible satellite signals, thereby estimating the Doppler frequency estimates of all visible satellite signals;

[0064] First, the squared signal shown in equation (2) is subjected to a fast Fourier transform operation. Since the second term after the equal sign in equation (2) is the product term of visible satellite signals with different code delays and different spread spectrum codes, this term will contain multiple spectral components after FFT operation and cannot form a peak value of coherent accumulation. Therefore, the Doppler spectrum obtained after FFT operation of equation (2) can be expressed as:

[0065]

[0066] Where, δ(·) represents the exponential function after FFT operation; f represents the independent variable of Doppler spectrum;

[0067] Then, the Doppler frequencies of all visible satellites are estimated from the above Doppler spectrum. The Doppler spectrum obtained by formula (3) will form a peak at twice the Doppler frequency of all visible satellite signals, so the Doppler frequencies of all visible satellites can be directly estimated based on the peak value of the Doppler spectrum. The peak value is obtained by performing constant false alarm detection on the Doppler spectrum of the visible satellite signal. The constant false alarm detection threshold T h It is expressed as:

[0068]

[0069] Where σ represents the standard deviation of Gaussian white noise; P f Indicates the false alarm probability; exceeds the constant false alarm detection threshold T h The Doppler frequency corresponding to the peak value is twice the estimated value of the Doppler frequency of all visible satellite signals. The obtained set of Doppler frequency estimates is expressed as Since the Doppler spectrum does not include the code phase information of each visible satellite signal at this time, it is impossible to confirm the specific visible satellite signal corresponding to each estimated Doppler frequency.

[0070] 3) The Doppler frequency value range of each visible satellite signal is estimated using the auxiliary information provided by A-GPS, and the Doppler frequency estimation value falling into the Doppler frequency value range of the corresponding visible satellite signal is used as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the Doppler frequency capture of each visible satellite signal;

[0071] First, the Doppler frequency provided by the A-GPS base station is used as a reference, and the Doppler frequency range of each visible satellite signal received by the receiver antenna is estimated in combination with parameters including the base station coverage range. The formula is:

[0072]

[0073] in, Indicates the Doppler frequency value range obtained by the A-GPS base station; It represents the Doppler frequency of the qth satellite signal provided by the A-GPS base station, in Hz; l represents the horizontal coverage of the base station, h represents the height of the base station, both in km; v represents the movement speed of the receiver, in km / h;

[0074] Then, the Doppler frequency estimation value that falls within the Doppler frequency value range of the corresponding visible satellite signal is used as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the capture of the Doppler frequency of each visible satellite signal.

[0075] 4) constructing a local joint reference signal using the Doppler frequency estimation values ​​and spread spectrum codes of the above-mentioned visible satellite signals;

[0076] Using the estimated Doppler frequency of each visible satellite signal and the known spreading code c of each satellite signal q (t) to generate the local joint reference signal, the formula is:

[0077]

[0078] 5) performing a parallel correlation operation on the local joint reference signal and the received signal to obtain a correlation function containing correlation peaks of all visible satellite signals, thereby estimating code phase estimation values ​​of all visible satellite signals;

[0079] First, a parallel correlation operation is performed on the local joint reference signal x(t) and the received signal s(t). The formula of the correlation operation process is:

[0080]

[0081] Among them, R q (·) represents the autocorrelation function of the visible satellite signal; R ∑ (·) represents the superposition of autocorrelation functions, which contains the correlation peaks of all visible satellite signals; x * (t) represents the complex conjugate of the local joint reference signal x(t);

[0082] Then, the code phases of all visible satellites are estimated from the correlation function; similar to the estimation of Doppler frequency, the detection threshold T is also exceeded. h The code phase corresponding to the peak value of the correlation function is taken as the code phase estimation value, and the obtained set of code phase estimation values ​​is recorded as The code phase estimate cannot be confirmed at this time The visible satellite signal needs to be further linked to the corresponding specific visible satellite signal through the auxiliary information provided by A-GPS.

[0083] 6) Use the auxiliary information provided by A-GPS to estimate the code phase value range of each visible satellite signal, and use the code phase estimate of the visible satellite signal that falls within the corresponding visible satellite signal code phase value range as the estimated value of the code phase of the visible satellite signal, thereby completing the visible satellite signal code phase capture.

[0084] First, the code phase value provided by the A-GPS base station is used as a benchmark, and the code phase value range of each visible satellite signal received by the receiver is estimated by combining parameters including mobile network delay and A-GPS base station coverage. The formula is:

[0085]

[0086] in, Indicates the code phase value range of the visible satellite signal obtained by the A-GPS base station; represents the code phase of the qth visible satellite signal provided by the A-GPS base station; t0 represents the network delay;

[0087] Then, the code phase estimation value of the visible satellite signal that falls within the corresponding visible satellite signal code phase value range is used as the estimated value of the code phase of the visible satellite signal, thereby completing the capture of the visible satellite signal code phase.

[0088] The effect of the present invention can be further illustrated by the following simulation results.

[0089] Experimental description: First, the ephemeris information downloaded from the CDDIS website, the given base station location and the receiver location are used to simulate the auxiliary data provided by the A-GPS base station. Then, the satellite signal entering the user receiver is further simulated based on the ephemeris information and the set user receiver location. The environmental parameter settings of this experiment are shown in Table 1.

[0090] Table 1 Environmental parameter settings

[0091]

[0092] Figure 2 The figure compares the changes in the number of visible satellites among the traditional parallel code phase capture method, the multi-C / A code capture method and the A-GPS based visible satellite signal joint capture method provided by the present invention.

[0093] It can be seen from the figure that as the number of visible satellites increases, the amount of computation required by the method of the present invention is gradually reduced compared with the amount of computation required by the parallel code phase capture method. When 10 visible satellites are captured, the amount of computation generated by the method of the present invention is only 0.24% of the amount of computation generated by the parallel code phase capture method. At the same time, since the method of the present invention realizes parallel search of Doppler frequency, more C / A code capture methods have obvious advantages.

[0094] Figure 3 The amount of computation when processing each visible satellite signal separately for the traditional parallel code phase capture method, the multi-C / A code capture method and the A-GPS-based visible satellite signal joint capture method provided by the present invention varies with the number of visible satellites.

[0095] It can be seen from the figure that the normalized computational complexity of the method of the present invention is always 2.44% of that of the parallel code phase capture. When the number of visible satellites is 2, the computational complexity of the method of the present invention is 4.88% of that of the multi-C / A code capture method. When the number of visible satellites is 10, the computational complexity of the method of the present invention is 2.44% of that of the parallel code phase capture method, and the computational complexity of the multi-C / A code capture method is 10% of that of the parallel code phase capture method.

[0096] Figure 4 The invention discloses a method for obtaining a conventional parallel code phase capture method, a method for obtaining a multi-C / A code, and a joint capture method for obtaining a visible satellite signal based on A-GPS provided by the present invention.

[0097] It can be seen from the figure that as the integration time increases, the computational complexity of the three acquisition methods increases linearly. The relative computational complexity after normalization with the traditional parallel code phase acquisition method corresponding to the coherent integration time does not change with the integration time. The normalized computational complexity of the multi-C / A code acquisition method is 12.5%, while the normalized computational complexity of the method of the present invention is only 0.3% of that of the parallel code phase acquisition method.

[0098] Figure 5 The relationship between the amount of computation and the number of frequency bands of the traditional parallel code phase capture method, the multi-C / A code capture method and the A-GPS based visible satellite signal joint capture method provided by the present invention.

[0099] It can be seen from the figure that the computational complexity of the conventional parallel code phase capture and multi-C / A code capture methods increases with the increase of the number of frequency bands, while the computational complexity of the method of the present invention does not change with the number of frequency bands. When the number of frequency bands is 21, the normalized computational complexity of the method of the present invention and the multi-C / A code capture method is 0.59% and 12.5% ​​respectively. When the number of frequency bands is 201, the normalized computational complexity of the multi-C / A code capture method is still 12.5%. However, the normalized computational complexity of the method of the present invention is only 0.062%.

[0100] Figure 6 The detection probability of the method of the present invention and the multi-C / A code acquisition method is obtained when the signal-to-noise ratio of the visible satellite signal is -24dB to -17dB, the number of captured visible satellites is 2 to 5, and the Doppler frequency shift Δf is 0Hz to 200Hz.

[0101] in Figure 6 (a) Figure 6 (b) Figure 6 (c) and Figure 6 (d) respectively represents the detection probability of the method of the present invention and the multi-C / A code acquisition method when the number of captured visible satellites is 2, the number of captured visible satellites is 3, the number of captured visible satellites is 4, and the number of captured visible satellites is 5, when the signal-to-noise ratio is -24dB to -17dB and the Doppler frequency shift Δf is 0Hz to 200Hz. The detection probability presented in the figure is the average detection probability of all visible satellites in the group. It can be seen from the figure that when the number of captured visible satellites is 2 and the signal-to-noise ratio is greater than -22dB, the detection probability of the method of the present invention and the multi-C / A code acquisition method is 100%, and when the signal-to-noise ratio is further reduced, the detection probabilities of the two acquisition methods begin to show differences. When the signal-to-noise ratio is -24dB, the detection probability of the method of the present invention is 98.9%, while the detection probability of the multi-C / A code acquisition method is distributed between 95.5% and 99.5%. The specific detection probability changes with the Doppler frequency shift Δf set in the experiment. When the number of captured visible satellites increases, the detection probabilities of the method of the present invention and the multi-C / A code acquisition method both decrease. When the number of visible satellites captured is 3 and the signal-to-noise ratio is higher than -20dB, the detection probability of the two acquisition methods will reach 100%, and when the signal-to-noise ratio is -24dB, the detection probability of the method of the present invention is 77.8%, and the detection probability of the multi-C / A code acquisition method is distributed in the range of 64.2% to 83.9%. When four or five visible satellites are captured at the same time, the detection probability of the method of the present invention and the multi-C / A code acquisition method will further decrease.

Claims

1. A method for joint acquisition of visible satellite signals based on A-GPS, characterized by: The method comprises the following steps performed in sequence: 1) Performing a square operation on the visible satellite signal received by the receiver antenna to eliminate the influence of the spread spectrum code and obtain a squared signal; 2) Performing a fast Fourier transform on the squared signal to obtain the Doppler spectra of all visible satellite signals, thereby estimating the Doppler frequency estimates of all visible satellite signals; 3) The Doppler frequency value range of each visible satellite signal is estimated using the auxiliary information provided by A-GPS, and the Doppler frequency estimation value falling into the Doppler frequency value range of the corresponding visible satellite signal is used as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the Doppler frequency capture of each visible satellite signal; 4) constructing a local joint reference signal using the Doppler frequency estimation values ​​and spread spectrum codes of the above-mentioned visible satellite signals; 5) performing a parallel correlation operation on the local joint reference signal and the received signal to obtain a correlation function containing correlation peaks of all visible satellite signals, thereby estimating code phase estimation values ​​of all visible satellite signals; 6) Use the auxiliary information provided by A-GPS to estimate the code phase value range of each visible satellite signal, and use the code phase estimate of the visible satellite signal that falls within the corresponding visible satellite signal code phase value range as the estimated value of the code phase of the visible satellite signal, thereby completing the visible satellite signal code phase capture.

2. The method for joint acquisition of visible satellite signals based on A-GPS according to claim 1, characterized in that: In step 1), the method of performing a square operation on the visible satellite signal received by the receiver antenna to eliminate the influence of the spread spectrum code and obtaining the squared signal is: First, the visible satellite signal received by the receiver antenna is expressed as: Where, the superscript q represents the parameter of the qth visible satellite, Q represents the number of visible satellites; P q Indicates the power of visible satellite signals; c q (t) represents the spreading code of the visible satellite signal; τ q Indicates the code delay of the visible satellite signal; represents the Doppler frequency of the visible satellite signal; η(t) represents the observation noise, which is assumed to be Gaussian white noise; Then, the visible satellite signal is squared to obtain the squared signal:

3. The method for joint acquisition of visible satellite signals based on A-GPS according to claim 1, characterized in that: In step 2), the method of obtaining the Doppler spectra of all visible satellite signals by performing a fast Fourier transform (FFT) on the squared signal to estimate the Doppler frequency estimation value of all visible satellite signals is: First, the Doppler spectrum obtained by performing a fast Fourier transform operation on the squared signal is expressed as: Where, δ(·) represents the exponential function after FFT operation; f represents the independent variable of Doppler spectrum; Then, the Doppler frequencies of all visible satellites are estimated from the above Doppler spectrum. The Doppler spectrum obtained by formula (3) will form a peak at twice the Doppler frequency of all visible satellite signals. Therefore, the Doppler frequencies of all visible satellites are estimated directly based on the peak value of the Doppler spectrum. The peak value is obtained by performing constant false alarm detection on the Doppler spectrum of the visible satellite signal. The constant false alarm detection threshold T h It is expressed as: Where σ represents the standard deviation of Gaussian white noise; P f Indicates the false alarm probability; exceeds the constant false alarm detection threshold T h The Doppler frequency corresponding to the peak value is twice the estimated value of the Doppler frequency of all visible satellite signals. The obtained set of Doppler frequency estimates is expressed as 4. The method for joint acquisition of visible satellite signals based on A-GPS according to claim 1, characterized in that: In step 3), the method of using the auxiliary information provided by A-GPS to estimate the Doppler frequency value range of each visible satellite signal, and using the Doppler frequency estimation value falling into the Doppler frequency value range of the corresponding visible satellite signal as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the Doppler frequency capture of each visible satellite signal is: First, the Doppler frequency provided by the A-GPS base station is used as a reference, and the Doppler frequency range of each visible satellite signal received by the receiver antenna is estimated in combination with parameters including the base station coverage range. The formula is: in, Indicates the Doppler frequency value range obtained by the A-GPS base station; It represents the Doppler frequency of the qth satellite signal provided by the A-GPS base station, in Hz; l represents the horizontal coverage of the base station, h represents the height of the base station, both in km; v represents the movement speed of the receiver, in km / h; Then, the Doppler frequency estimation value that falls within the Doppler frequency value range of the corresponding visible satellite signal is used as the estimation value of the Doppler frequency of the visible satellite signal, thereby completing the capture of the Doppler frequency of each visible satellite signal.

5. The method for joint acquisition of visible satellite signals based on A-GPS according to claim 1, characterized in that: In step 4), the method of constructing a local joint reference signal using the Doppler frequency estimation value and the spread spectrum code of each visible satellite signal is: Using the estimated Doppler frequency of each visible satellite signal and the known spreading code c of each satellite signal q (t) to generate the local joint reference signal, the formula is:

6. The method for joint acquisition of visible satellite signals based on A-GPS according to claim 1, characterized in that: In step 5), the method of performing a parallel correlation operation on the local joint reference signal and the received signal to obtain a correlation function containing correlation peaks of all visible satellite signals, thereby estimating the code phase estimation values ​​of all visible satellite signals is: First, a parallel correlation operation is performed on the local joint reference signal x(t) and the received signal s(t). The formula of the correlation operation process is: Among them, R q (·) represents the autocorrelation function of the visible satellite signal; R ∑ (·) represents the superposition of autocorrelation functions, which contains the correlation peaks of all visible satellite signals; x * (t) represents the complex conjugate of the local joint reference signal x(t); Then, the code phases of all visible satellites are estimated from the correlation function; the satellites exceeding the detection threshold T h The code phase corresponding to the peak value of the correlation function is taken as the code phase estimation value, and the obtained set of code phase estimation values ​​is recorded as 7. The method for joint acquisition of visible satellite signals based on A-GPS according to claim 1, characterized in that: In step 6), the method of using the auxiliary information provided by A-GPS to estimate the code phase value range of each visible satellite signal, and using the estimated code phase value of the visible satellite signal falling within the corresponding visible satellite signal code phase value range as the estimated value of the code phase of the visible satellite signal, thereby completing the visible satellite signal code phase capture is: First, the code phase value provided by the A-GPS base station is used as a benchmark, and the code phase value range of each visible satellite signal received by the receiver is estimated by combining parameters including mobile network delay and A-GPS base station coverage. The formula is: in, Indicates the code phase value range of the visible satellite signal obtained by the A-GPS base station; represents the code phase of the qth visible satellite signal provided by the A-GPS base station; t0 represents the network delay; Then, the code phase estimation value of the visible satellite signal that falls within the corresponding visible satellite signal code phase value range is used as the estimated value of the code phase of the visible satellite signal, thereby completing the capture of the visible satellite signal code phase.

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