Method for rapidly capturing enhanced MCSK signal in low orbit navigation

Abstract

The invention discloses a method for rapidly capturing a low-orbit navigation enhanced MCSK signal, the MCSK signal comprises a high-speed message signal and a low-speed message signal, the total number of code periods of the MCSK signal is N, and the number of the code periods of the low-speed message signal is nlow; nlow is less than N; the method specifically comprises the following steps: step 1, initializing a frequency search index, frequency search carrier NCO control information and a to-be-captured satellite number, and carrying out initialization processing on acquisition and storage of baseband signals based on an MBZP scheme; step 2, collecting, storing and reading baseband signals; the method comprises the following steps: acquiring a baseband signal, and performing data storage and reading on the baseband signal according to an MBZP scheme; and step 3, baseband signal frequency adjustment: starting from MCSK signal system characteristics, by setting a local time domain signal, it can be ensured that no peak shifting problem and coherent correlation loss exist in any combination of sampling time and Doppler, and the accuracy of a capture result is ensured.

Description

Technical Field

[0001] The invention belongs to the field of satellite navigation and relates to a method for quickly capturing a low-orbit navigation enhanced MCSK signal. Background Art

[0002] With the vigorous development of low-orbit satellite constellations, especially the proposal of low-orbit giant Internet constellation plans represented by Starlink, positioning, navigation and timing (PNT) services based on low-orbit satellite constellations have gradually become a research hotspot. Low-orbit satellite constellations and medium- and high-orbit navigation satellite constellations can complement each other's advantages and provide new ideas for breaking through the performance bottleneck of PNT services. PNT services based on low-orbit satellite constellations mainly utilize the fast changes in the geometric configuration of low-orbit satellites to broadcast dedicated low-orbit navigation enhancement signals to achieve an increase in PPP convergence speed. Different from traditional satellite navigation signals, Starlink's low-orbit navigation enhancement design and orbit use a new signal system - Multiplexed Code shift Keying (MCSK).

[0003] MCSK is composed of two components: one component mainly supports high-precision carrier measurement (measurement component), and the other component mainly supports high information rate broadcasting (data component). For the ranging component, data is not modulated or only low-speed telegrams are modulated, and the same direct spread spectrum modulation method as GNSS is adopted. For the data component, a CSK modulation signal is adopted. CSK is a multi-level information modulation signal that uses different code initial phases to modulate information, generally recorded as CSK (U, R), U represents the number of bits modulated by each CSK symbol, therefore, the number of different code initial phases required to be used is 2U, and R represents the number of code periods each CSK symbol lasts. MCSK signal is a signal generated by time-division multiplexing the above-mentioned measurement component and data component using code period time division multiplexing.

[0004] Capture is a rough estimation of the code phase and carrier frequency of the navigation signal. It is the first synchronization process between the receiver and the navigation signal and the prerequisite for the receiver to locate. Traditional navigation capture algorithms include serial code phase capture and parallel code phase capture. The former can achieve accurate phase estimation by correlating the input signal with the locally reproduced target signal, but it is calculated serially in the code phase dimension, which has a large time overhead. It has been gradually replaced by parallel code phase capture in traditional GNSS scenarios. When applied to low-orbit navigation scenarios, the number of target satellites has increased sharply, the satellite visibility time has shortened, and the availability of serial code phase capture has been further reduced; parallel code phase capture is based on the correspondence between the time domain correlation process and the spectrum multiplication, and uses FFT operation to achieve parallel output of code phase correlation results. Patents such as "GPS Signal Capture Method and System" (Application No.: CN20121012835) propose a method based on double block zero padding (Double Block Zero Padding DBZP) and its variants to expand the length and signal form of the local reproduced signal to eliminate the correlation loss caused by data bit flipping. However, the above method is only applicable to traditional GNSS signals. When applied to MCSK signals, not only does it have correlation losses, but in extreme cases it will introduce stable peak shifts, making the MCSK signal unable to synchronize correctly and the receiver lose its positioning capability. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for quickly capturing low-orbit navigation enhanced MCSK signals, so as to solve the technical problems in the prior art of slow capture speed and low accuracy of capture results when capturing signals in a low-orbit navigation enhanced scenario.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions to achieve the above problems:

[0007] A method for quickly capturing a low-orbit navigation enhanced MCSK signal, wherein the MCSK signal includes a high-speed message signal and a low-speed message signal, the total number of code periods of the MCSK signal is N, and the number of code periods of the low-speed message signal is nlow; nlow<N; specifically comprising the following steps:

[0008] Step 1: Initialize the frequency search index, frequency search carrier NCO control information and the number of the satellite to be captured, and initialize the acquisition and storage of the baseband signal based on the MBZP scheme;

[0009] Step 2: collecting, storing and reading baseband signals;

[0010] Collect baseband signals, store and read the baseband signals according to the MBZP scheme;

[0011] Step 3: Adjust the baseband signal frequency:

[0012] The frequency search carrier NCO control information provided by the capture control module is used to down-convert the baseband signal read in step 2 to obtain the baseband signal S after frequency adjustment. base_I_i (n) and S base_Q_i (n);

[0013] Step 4: Generate local frequency domain signals SI(n) and SQ(n) of the satellite to be captured according to the MBZP scheme;

[0014] Step 5: Use the frequency adjusted baseband signal S obtained in step 3 base_I_i (n) and S base_Q_i (n) is correlated with the local frequency domain signals SI(n) and SQ(n) based on MBZP obtained in step 4 to obtain the correlation result S corr_I_i (n) and

[0015] Step 6: The relevant result S obtained in step 5 corr_I_i (n) and Perform two-level correlation result processing to obtain the first-level correlation result Sncorr1(n) and the second-level correlation result S NCcorr2 (n);

[0016] Step 7: Use the decision threshold V TH2 The second level correlation result S obtained in step 6 NCcorr2 (n) Compare and judge, and obtain the capture result;

[0017] The capture result includes satellite number, satellite frequency Doppler, capture signal start time information, capture signal code phase M index2 and capture signal cycle start bit estimate value M index1 *f s *T+

[0018] M index2 .

[0019] Step 2 specifically includes the following steps:

[0020] Step 2.1, according to the storage instruction, starting from the local millisecond pulse, the baseband signal is sampled and stored, and the time information Tstamp corresponding to the sampling start time is recorded;

[0021] The baseband signal storage length is not less than [N×N n +1]f s T;

[0022] in:

[0023] N nIndicates the cumulative number of times;

[0024] f s Indicates the sampling rate;

[0025] T represents the code period;

[0026] Step 2.2, according to the read instruction, the continuous reading length is N C The baseband signal;

[0027] N C ≥[N+1]f s T

[0028] (i-1)Nf s T+1≥N n ;

[0029] in:

[0030] (i-1)Nf s T+1 indicates the starting position of the i-th reading of data.

[0031] Step three specifically includes the following steps:

[0032] Step 3.1, generating a sine sequence and a cosine sequence based on the frequency search carrier NCO control information of step 1;

[0033] The lengths of the sine sequence and the cosine sequence are not less than N C ;

[0034] Step 3.2: multiply the sine sequence and cosine sequence obtained in step 3.1 by the baseband signal read in step 2 to obtain the frequency-adjusted baseband signal S base_I_i (n) and S base_Q_i (n).

[0035] Step 4 specifically includes the following steps:

[0036] Step 4.1, generating a local time domain signal C'(n);

[0037] The length of the local time domain signal C'(n) is N C =2x, where

[0038] Step 4.2, with f s T is the segmentation length for the local time domain signal in step 4.1 [N+1]f s T is segmented to obtain N+1 local time domain signal segments;

[0039] Step 4.3, adding a pseudo code sequence C of the satellite to be captured with a code period to at least one local time domain signal segment obtained in step 4.2 m (nm ), and the remaining local time domain signal segments are set to zero to obtain the local time domain signal c(n), where c1(n1)~c N+1 (n N+1 ) cannot have two identical non-zero sequences and must have at least one non-zero sequence;

[0040] The pseudo code sequence C m (n m ) is generated by the satellite number and pseudo code NCO control information;

[0041]

[0042] in:

[0043] n m =n-(m-1)f s T;

[0044] Step 4.4, performing a fast Fourier transform on the local time domain signal c(n) obtained in step 4.3 to generate local frequency domain signals SI(n) and SQ(n) based on MBZP;

[0045] SI(n)+jSQ0(n)=FFT[c(n)]

[0046] SQ(n)=-SQ0(n).

[0047] Step 5 specifically includes the following steps:

[0048] Step 5.1: adjust the frequency of the baseband signal S obtained in step 3 base_l_i (n) and S base_Q_i (n) performing a fast Fourier transform to obtain frequency domain sequences Sib_i(n) and SQb_i(n) of the frequency-adjusted baseband signal;

[0049] Sib_i(n)+jSQb_i(n)=FFT[S base_I_i (n)+S base_Q_i (n)]

[0050] Step 5.2, perform correlation operation on the frequency domain sequences Sib_i(n) and SQb_i(n) of the frequency-adjusted baseband signal obtained in step 5.1 and the local frequency domain signals SI(n) and SQ(n) based on MBZP obtained in step 4 to obtain the frequency domain information SIR_i(n) and SQR_i(n) of the correlation results:

[0051] SIR_i(n)=SIb_i(n)×SI(n)-SQb_i(n)×SQ(n)

[0052] SQR_i(n)=SIb_i(n)×SQ(n)+SQb_i(n)×SI(n)

[0053] Step 5.3, perform inverse Fourier transform on the frequency domain information of the correlation result obtained in step 5.2 to obtain the correlation result S corr_I_i (n) and jS corr_Q_i (n);

[0054] S corr_l_i (n)+jS corr_Q_i (n)=IFFT[SIR_i(n)+j SQR_i(n)].

[0055] Step six specifically includes the following steps:

[0056] Step 6.1: Use the following formula to calculate the correlation result S obtained in step 5: corr_I_i (n) and jS corr_Q_i (n) Process the first-level correlation result as shown in the following formula to obtain the first-level correlation result Sncorr1(n);

[0057]

[0058] T n1 Indicates the cumulative number of times;

[0059] Step 6.2, determine whether the value of nlow of the MCSK signal to be captured is 1, if so, proceed to step 6, if not, proceed to step 6.3;

[0060] Step 6.3, use the following formula to process the first-level correlation processing result Sncorr1(n) obtained in step 6.1 as shown in the following formula to obtain the second-level correlation result S NCcorr2 (n);

[0061]

[0062] in:

[0063] T n2 Indicates the number of accumulations, T n2 =N+1;

[0064] Step seven specifically includes the following steps:

[0065] Step 7.1, record and store S NCcorr2 (n), determine whether the current frequency search index has completed all searches, if so, proceed to step 7.2, if not, update the current frequency search index, and update the frequency search carrier NCO control information based on the current frequency search index, and return to step 1;

[0066] Step 7.2, determine S NCcorr2 Is the maximum value of (n) greater than V TH2 If yes, the capture is successful, the capture result is obtained, and step 1 is entered; if no, step 1 is entered directly;

[0067] The capture result includes satellite number, satellite frequency Doppler, capture signal start time information, capture signal code phase M index2 and capture signal cycle start bit M index1 *f s *T+M index2 ;

[0068] In the capture result obtained in step 7.2, the capture signal cycle start position M index1 *f s *T+M index2 The acquisition method includes the following steps:

[0069] S1, with f s T is the distance to S NCcorr2 The maximum value of the frequency search index corresponds to the Sncorr1 segment, and the maximum value of each segment Sncorr1 is respectively TH1 Compare and get the symbol sequence S S (q):

[0070]

[0071] S2, for symbol sequence S S (q) and its first term S S(1) Process and obtain S S1 (q);

[0072]

[0073] S2, for S S1 (q) Calculate by the following formula to obtain the estimated value Mindex1 of the acquisition signal cycle start position, that is, obtain the acquisition signal cycle start position M index1 *f s *T+M index2 ;

[0074] M index1 =find(S S1 (m+1)=1and S S1 (m)=0)-1.

[0075] Compared with the prior art, the present invention has the following beneficial technical effects:

[0076] (I) Based on the characteristics of the MCSK signal system, the present invention can ensure that there is no peak shift problem and coherent correlation loss in any combination of sampling time and Doppler by setting a local time domain signal, thereby ensuring the accuracy of the capture result.

[0077] (II) The present invention introduces the correlation result of the local time domain signal, so that the capture can not only output the capture signal code phase and satellite frequency Doppler, but also output the estimated value of the capture signal period start bit, accelerate the capture and tracking conversion, and improve the capture speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 A fast acquisition scheme for low-orbit navigation enhanced MCSK signals disclosed in the present invention;

[0079] Figure 2 This is a schematic diagram of data storage and reading based on MBZP;

[0080] Figure 3 Schematic diagram of signal generation based on MBZP;

[0081] Figure 4 To capture the schematic diagram of the decision and control process;

[0082] Figure 5 As an example, a schematic diagram of the corresponding relationship between the local reference signal and each sampling time.

[0083] The specific contents of the present invention are further explained in detail below in conjunction with embodiments. DETAILED DESCRIPTION

[0084] It should be noted that, unless otherwise specified, all components in the present invention are components known in the art.

[0085] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent changes made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0086] The present invention provides a method for quickly capturing a low-orbit navigation enhanced MCSK signal, wherein the MCSK signal includes a high-speed message signal and a low-speed message signal, the total number of code periods of the MCSK signal is N, and the number of code periods of the low-speed message signal is nlow; nlow<N; the method specifically includes the following steps: step 1, initializing a frequency search index, a frequency search carrier NCO control information and a satellite number to be captured, and initializing the acquisition and storage of a baseband signal based on an MBZP scheme;

[0087] Step 2: collecting, storing and reading baseband signals;

[0088] Collect baseband signals, store and read the baseband signals according to the MBZP scheme;

[0089] Step 3: Adjust the baseband signal frequency:

[0090] The frequency search carrier NCO control information provided by the capture control module is used to down-convert the baseband signal read in step 2 to obtain the baseband signal S after frequency adjustment. base_I_i (n) and S base_Q_i (n);

[0091] Step 4: Generate local frequency domain signals SI(n) and SQ(n) of the satellite to be captured according to the MBZP scheme;

[0092] Step 5: Use the frequency adjusted baseband signal S obtained in step 3 base_I_i (n) and S base_Q_i (n) is correlated with the local frequency domain signals SI(n) and SQ(n) based on MBZP obtained in step 4 to obtain the correlation result S corr_I_i (n) and

[0093] Step 6: The relevant result S obtained in step 5 corr_I_i (n) and Perform two-level correlation result processing to obtain the first-level correlation result Sncorr1(n) and the second-level correlation result S NCcorr2 (n);

[0094] Step 7: Use the decision threshold V TH2 The second level correlation result S obtained in step 6 NCcorr2 (n) Compare and judge, and obtain the capture result;

[0095] The capture results include satellite number, satellite frequency Doppler, capture signal start time information, capture signal code phase M index2 and capture signal cycle start bit estimate value M index1 *f s *T+M index2 .

[0096] In the above technical solution, starting from the characteristics of the MCSK signal system, by setting the local time domain signal, it can be ensured that there is no peak offset problem and coherent correlation loss in any combination of sampling time and Doppler, thereby ensuring the accuracy of the capture result.

[0097] The correlation results of introducing local time domain signals enable the capture to not only output the capture signal code phase and satellite frequency Doppler, but also output the estimated value of the capture signal period start bit, which speeds up the capture and tracking conversion and improves the capture speed.

[0098] Step 2 specifically includes the following steps:

[0099] Step 2.1, according to the storage instruction, starting from the local millisecond pulse, the baseband signal is sampled and stored, and the time information Tstamp corresponding to the sampling start time is recorded;

[0100] The baseband signal storage length is not less than [N×N n +1]f s T;

[0101] in:

[0102] N n Indicates the cumulative number of times;

[0103] f s Indicates the sampling rate;

[0104] T represents the code period;

[0105] Step 2.2, according to the read instruction, the continuous reading length is N C The baseband signal;

[0106] N C ≥[N+1]f s T

[0107] (i-1)Nf s T+1≥N n ;

[0108] in:

[0109] (i-1)Nf s T+1 indicates the starting position of the i-th reading of data.

[0110] Step three specifically includes the following steps:

[0111] Step 3.1, generating a sine sequence and a cosine sequence based on the frequency search carrier NCO control information of step 1;

[0112] The length of both the sine sequence and the cosine sequence is not less than N C ;

[0113] Step 3.2: multiply the sine sequence and cosine sequence obtained in step 3.1 by the baseband signal read in step 2 to obtain the frequency-adjusted baseband signal S base_I_i (n) and S base_Q_i (n).

[0114] Step 4 specifically includes the following steps:

[0115] Step 4.1, generating a local time domain signal C'(n);

[0116] The length of the local time domain signal C'(n) is N C =2x, where

[0117] Step 4.2, with f s T is the segmentation length for the local time domain signal in step 4.1 [N+1]f s T is segmented to obtain N+1 local time domain signal segments;

[0118] Step 4.3, adding a pseudo code sequence C of the satellite to be captured with a code period to at least one local time domain signal segment obtained in step 4.2 m (n m ), and the remaining local time domain signal segments are set to zero to obtain the local time domain signal c(n), where c1(n1)~c N+1 (n N+1 ) cannot have two identical non-zero sequences and must have at least one non-zero sequence;

[0119] Pseudocode sequence C m (n m ) is generated by the satellite number and pseudo code NCO control information;

[0120]

[0121] in:

[0122] n m =n-(m-1)f s T;

[0123] Step 4.4, performing a fast Fourier transform on the local time domain signal c(n) obtained in step 4.3 to generate local frequency domain signals SI(n) and SQ(n) based on MBZP;

[0124] SI(n)+jSQ0(n)=FFT[c(n)]

[0125] SQ(n)=-SQ0(n).

[0126] Step 5 specifically includes the following steps:

[0127] Step 5.1: adjust the frequency of the baseband signal S obtained in step 3 base_l_i (n) and S base_Q_i (n) performing a fast Fourier transform to obtain frequency domain sequences Sib_i(n) and SQb_i(n) of the frequency-adjusted baseband signal;

[0128] Sib_i(n)+jSQb_i(n)=FFT[S base_I_i (n)+S base_Q_i (n)]

[0129] Step 5.2, perform correlation operation on the frequency domain sequences Sib_i(n) and SQb_i(n) of the frequency-adjusted baseband signal obtained in step 5.1 and the local frequency domain signals SI(n) and SQ(n) based on MBZP obtained in step 4 to obtain the frequency domain information SIR_i(n) and SQR_i(n) of the correlation results:

[0130] SIR_i(n)=SIb_i(n)×SI(n)-SQb_i(n)×SQ(n)

[0131] SQR_i(n)=SIb_i(n)×SQ(n)+SQb_i(n)×SI(n)

[0132] Step 5.3, perform inverse Fourier transform on the frequency domain information of the correlation result obtained in step 5.2 to obtain the correlation result S corr_I_i (n) and jS corr_Q_i (n);

[0133] S corr_I_i (n)+jS corr_Q_i (n)=IFFT[SIR_i(n)+j SQR_i(n)].

[0134] Step six specifically includes the following steps:

[0135] Step 6.1: Use the following formula to calculate the correlation result S obtained in step 5: corr_I_i (n) and jS corr_Q_i (n) Process the first-level correlation result as shown in the following formula to obtain the first-level correlation result Sncorr1(n);

[0136]

[0137] T n1 Indicates the cumulative number of times;

[0138] Step 6.2, determine whether the value of nlow of the MCSK signal to be captured is 1, if so, proceed to step 6, if not, proceed to step 6.3;

[0139] Step 6.3, use the following formula to process the first-level correlation processing result Sncorr1(n) obtained in step 6.1 as shown in the following formula to obtain the second-level correlation result S NCcorr2 (n);

[0140]

[0141] in:

[0142] T n2 Indicates the number of accumulations, Tn2 =N+1.

[0143] Step seven specifically includes the following steps:

[0144] Step 7.1, record and store S NCcorr2 (n), determine whether the current frequency search index has completed all searches, if so, proceed to step 7.2, if not, update the current frequency search index, and update the frequency search carrier NCO control information based on the current frequency search index, and return to step 1;

[0145] Step 7.2, determine S NCcorr2 Is the maximum value of (n) greater than V TH2 If yes, the capture is successful, the capture result is obtained, and step 1 is entered; if no, step 1 is entered directly;

[0146] The capture results include satellite number, satellite frequency Doppler, capture signal start time information, capture signal code phase M index2 and capture signal cycle start bit M index1 *f s *T+M index2 ;

[0147] In the capture result obtained in step 7.2, the capture signal cycle start position M index1 *f s *T+M index2 The acquisition method includes the following steps:

[0148] S1, with f s T is the distance to S NCcorr2 The maximum value of the frequency search index corresponds to the Sncorr1 segment, and the maximum value of each segment Sncorr1 is respectively TH1 Compare and get the symbol sequence S S (q):

[0149]

[0150] S2, for symbol sequence S S (q) and its first term S S(1) Process and obtain S S1 (q);

[0151]

[0152] S2, for S S1 (q) Calculate by the following formula to obtain the estimated value Mindex1 of the acquisition signal cycle start position, that is, obtain the acquisition signal cycle start position M index1 *f s *T+M index2 ;

[0153] M index1 =find(S S1 (m+1)=1and S S1 (m)=0)-1.

[0154] Example:

[0155] This embodiment uses the above-mentioned fast acquisition method of low-orbit navigation enhanced MCSK signal to capture the MCSK (U, 2, 1) signal. Figure 5 As shown, Figure 5 The corresponding relationship between the local reference signal and each sampling moment is depicted, taking MCSK (U, 2, 1) as an example. It can be seen that the signal locally reproduces 1 code period of low-speed measurement component to be correlated with the target low-speed measurement component in the input signal to form a correlation peak, and reproduces nlow code periods 0 to eliminate the correlation loss introduced by the low-speed telegram flipping of the measurement component. Further, N-nlow code periods 0 are used for the correlation between the high-speed telegram component and the local low-speed measurement component to eliminate the correlation loss. It is based on the above process that the accuracy of the correlation peak is guaranteed regardless of the sampling moment, and no coherent correlation loss is introduced. The present invention realizes fast capture based on the fft framework, and further, by selecting N+1 signals to be captured and adding them to the N+1 local signal segments, it can realize parallel search of different satellites, further improving the capture speed.

Claims

1. A method for quickly capturing a low-orbit navigation enhanced MCSK signal, wherein the MCSK signal includes a high-speed message signal and a low-speed message signal, the total number of code periods of the MCSK signal is N, and the number of code periods of the low-speed message signal is nlow; nlow<N; characterized in that, The specific steps include: Step 1: Initialize the frequency search index, frequency search carrier NCO control information and the number of the satellite to be captured, and initialize the acquisition and storage of the baseband signal based on the MBZP scheme; Step 2: collecting, storing and reading baseband signals; Collect baseband signals, store and read the baseband signals according to the MBZP scheme; Step 3: Adjust the baseband signal frequency: The frequency search carrier NCO control information obtained in step 1 is used to down-convert the baseband signal read in step 2 to obtain a baseband signal S after frequency adjustment. base_I_i (n) and S base_Q_i (n); Step 4: Generate local frequency domain signals SI(n) and SQ(n) of the satellite to be captured according to the MBZP scheme; Step 5: Use the frequency adjusted baseband signal S obtained in step 3 base_I_i (n) and S base_Q_i (n) is correlated with the local frequency domain signals SI(n) and SQ(n) based on MBZP obtained in step 4 to obtain the correlation result S corr_I_i (n) and Step 6: The relevant result S obtained in step 5 corr_I_i (n) and Perform two-level correlation result processing to obtain the first-level correlation result Sncorr1(n) and the second-level correlation result S NCcorr2 (n); Step 7: Use the decision threshold V TH2 The second level correlation result S obtained in step 6 NCcorr2 (n) Compare and judge, and obtain the capture result; The capture result includes satellite number, satellite frequency Doppler, capture signal start time information, capture signal code phase M index2 and capture signal cycle start bit estimation value M index1 *f s MT+M index2 .

2. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 1, characterized in that: Step 2 specifically includes the following steps: Step 2.1, according to the storage instruction, starting from the local millisecond pulse, the baseband signal is sampled and stored, and the time information Tstamp corresponding to the sampling start time is recorded; The baseband signal storage length is not less than [N×N n +1]f s T; in: N n Indicates the cumulative number of times; f s Indicates the sampling rate; T represents the code period; Step 2.2, according to the read instruction, the continuous reading length is N C The baseband signal; N C ≥[N+1]f s T (i-1)Nf s T+1≥N n ; in: (i-1)Nf s T+1 indicates the starting position of the i-th reading of data.

3. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 1, characterized in that: Step three specifically includes the following steps: Step 3.1, generating a sine sequence and a cosine sequence based on the frequency search carrier NCO control information of step 1; The lengths of the sine sequence and the cosine sequence are not less than N C ; Step 3.2: multiply the sine sequence and cosine sequence obtained in step 3.1 by the baseband signal read in step 2 to obtain the frequency-adjusted baseband signal S base_l_i (n) and S base_Q_i (n).

4. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 1, characterized in that: Step 4 specifically includes the following steps: Step 4.1, generating a local time domain signal C'(n); The length of the local time domain signal C'(n) is N C =2x, where Step 4.2, with f s T is the segmentation length for the local time domain signal in step 4.1 [N+1]f s T is segmented to obtain N+1 local time domain signal segments; Step 4.3, adding a pseudo code sequence C of the satellite to be captured with a code period to at least one local time domain signal segment obtained in step 4.2 m (n m ), and the remaining local time domain signal segments are set to zero to obtain the local time domain signal c(n), where c1(n1)~c N+1 (n N+1 ) cannot have two identical non-zero sequences and must have at least one non-zero sequence; The pseudo code sequence C m (n m ) is generated by the satellite number and pseudo code NCO control information; in: n m =n-(m-1)f s T; Step 4.4, performing a fast Fourier transform on the local time domain signal c(n) obtained in step 4.3 to generate local frequency domain signals SI(n) and SQ(n) based on MBZP; SI(n)+jSQ0(n)=FFT[c(n)] SQ(n)=-SQ0(n).

5. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 1, characterized in that: Step 5 specifically includes the following steps: Step 5.1: adjust the frequency of the baseband signal S obtained in step 3 base_I_i (n) and S base_Q_i (n) performing a fast Fourier transform to obtain frequency domain sequences Sib_i(n) and SQb_i(n) of the frequency-adjusted baseband signal; Sib_i(n)+jSQb_i(n)=FFT[S base_I_i (n)+S base_Q_i (n)] Step 5.2, perform correlation operation on the frequency domain sequences Sib_i(n) and SQb_i(n) of the frequency-adjusted baseband signal obtained in step 5.1 and the local frequency domain signals SI(n) and SQ(n) based on MBZP obtained in step 4 to obtain the frequency domain information SIR_i(n) and SQR_i(n) of the correlation results: SIR_i(n)=SIb_i(n)×SI(n)-SQb_i(n)×SQ(n) SQR_i(n)=SIb_i(n)×SQ(n)+SQb_i(n)×SI(n) Step 5.3, perform inverse Fourier transform on the frequency domain information of the correlation result obtained in step 5.2 to obtain the correlation result S corr_I_i (n) and jS corr_Q_i (n); S corr_I_i (n)+jS corr_Q_i (n)=IFFT[SIR_i(n)+j SQR_i(n)]。 6. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 1, characterized in that: Step six specifically includes the following steps: Step 6.1: Use the following formula to calculate the correlation result S obtained in step 5: corr_I_i (n) and jS corr_Q_i (n) Process the first-level correlation result as shown in the following formula to obtain the first-level correlation result Sncorr1(n); T n1 Indicates the cumulative number of times; Step 6.2, determine whether the value of nlow of the MCSK signal to be captured is 1, if so, proceed to step 6, if not, proceed to step 6.3; Step 6.3, use the following formula to process the first-level correlation processing result Sncorr1(n) obtained in step 6.1 as shown in the following formula to obtain the second-level correlation result S NCcorr2 (n); in: T n2 Indicates the number of accumulations, T n2 =N+1.

7. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 1, characterized in that: Step seven specifically includes the following steps: Step 7.1, record and store S NCcorr2 (n), determine whether the current frequency search index has completed all searches, if so, proceed to step 7.2, if not, update the current frequency search index, and update the frequency search carrier NCO control information based on the current frequency search index, and return to step 1; Step 7.2, determine S NCcorr2 Is the maximum value of (n) greater than V TH2 If yes, the capture is successful, the capture result is obtained, and step 1 is entered; if no, step 1 is entered directly; The capture result includes satellite number, satellite frequency Doppler, capture signal start time information, capture signal code phase M index2 and capture signal cycle start bit M index1 *f s *T+M index2 .

8. The method for rapidly capturing low-orbit navigation enhanced MCSK signals as claimed in claim 7, characterized in that: In the capture result obtained in step 7.2, the capture signal cycle start position M index1 *f s *T+M index2 The acquisition method includes the following steps: S1, with f s T is the distance to S NCcorr2 The maximum value of the frequency search index corresponds to the Sncorr1 segment, and the maximum value of each segment Sncorr1 is respectively TH1 Compare and get the symbol sequence S S (q): S2, for symbol sequence S S (q) and its first term S S(1) Process and obtain S S1 (q); S2, for S S1 (q) Calculate by the following formula to obtain the estimated value Mindex1 of the acquisition signal cycle start position, that is, obtain the acquisition signal cycle start position M index1 *f s *T+M index2 ; M index1 =find(S S1 (m+1)=1and S S1 (m)=0)-1。

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