CN106526633B - A kind of catching method and device of GNSS baseband signal - Google Patents
A kind of catching method and device of GNSS baseband signal Download PDFInfo
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- G—PHYSICS
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- 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
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- 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
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Abstract
The invention discloses the catching methods and device of a kind of GNSS baseband signal, the catching method includes simultaneously scanning for same by multiple correlators first not capture satellite, after this, which does not capture satellite, is captured, other three are captured according to same method and does not capture satellite；Then the satellite number of other all Observable satellites is got according to the satellite navigation message of aforementioned four captured satellites；The corresponding frequency for not capturing satellite is obtained according to literary asterisk again, and satellite is not captured to other by N number of correlator and is successively scanned for, until all are not captured satellite capture, that is to say that GNSS baseband signal completes capture.By the present invention in that being scanned for multiple correlators to same satellite, capture time of the system to capture satellite-signal when is greatly reduced, the treatment effeciency of system is improved.
Description
Technical Field
The present invention relates to processing of GNSS baseband signals, and more particularly, to a method and apparatus for capturing GNSS baseband signals.
Background
With the rapid development of technology, a variety of navigation systems have appeared. A single navigation system is difficult to meet the navigation requirements in global, all-weather and various complex environments, and a combined navigation system formed by two or more systems becomes a hot spot of research in various countries.
However, the satellite transmits only a high frequency carrier wave modulated with information to the ground, and the satellite signal must be processed in order to obtain useful information such as a ranging code and a navigation message of the satellite. The structures and the modes of signals transmitted by Beidou, GPS and GLONASS satellites are different from each other, but the signal processing methods are similar.
The satellite signal is generally processed by first analyzing, capturing and tracking a signal structure to obtain a navigation message. With the disclosure of the domestic beidou navigation system, a processing system for GNSS baseband signals in the beidou navigation system is needed. In addition, the conventional satellite signal processing has a low processing speed, and cannot support multi-channel data acquisition particularly when acquiring a signal.
Disclosure of Invention
In order to overcome the defects of the prior art, an object of the present invention is to provide a method for capturing GNSS baseband signals, which can achieve multi-channel and fast capturing of GNSS baseband signals.
One of the purposes of the invention is realized by adopting the following technical scheme:
the invention provides a method for capturing a GNSS baseband signal, which comprises the following steps:
s1: dividing the frequency of the current uncaptured satellite into M first frequency bands, allocating a corresponding correlator to each first frequency band, searching the current uncaptured satellite in each first frequency band by a method of searching satellite signals through the correlators, and obtaining an output result of each correlator;
s2: judging whether the current uncaptured satellite is captured or not according to all output results, if so, marking the current uncaptured satellite as the captured satellite, switching to the next satellite and marking as the current uncaptured satellite, and executing S1; until four satellites are acquired, S3 is performed;
s3: acquiring the satellite numbers of all current observable satellites according to the satellite navigation messages of the four satellites;
s4: obtaining the frequency of all uncaptured satellites according to the satellite number, dividing the frequency of each uncaptured satellite into N second frequency bands, allocating a corresponding correlator to each second frequency band, sequentially searching each uncaptured satellite in each second frequency band by a method of searching satellite signals through the correlators, judging whether each uncaptured satellite is captured, and if so, marking the corresponding uncaptured satellite as the captured satellite;
s5: the GNSS baseband signals complete acquisition when all satellites are marked as acquired satellites.
Preferably, the method further comprises dividing the frequency of each acquired satellite into I third frequency bands, and performing secondary acquisition on each acquired satellite in turn by searching satellite signals through the correlator in each third frequency band until the secondary acquisition is completed on all the acquired satellites.
Preferably, I ═ 10.
Preferably, the method for searching satellite signals by the correlator specifically comprises the following steps:
s11: converting a signal received through a GNSS antenna into an intermediate frequency digital signal s (t), which can be expressed by equation (1):
wherein P is the signal power; d (t) is a navigation message bit; c (t) is a C/A code; τ represents the time delay in the transmission of the satellite signal from the satellite to the receiver; f. ofdIs the doppler shift; t isObserving time; f. ofIF＝fNominal scale+fdRepresented as a carrier down-converted intermediate frequency signal; f. ofNominal scaleRepresenting a nominal frequency of the intermediate frequency digital signal; phi is an initial carrier phase; n (t) is white noise, the power spectral density of which is constant; f. ofL1A frequency indicating that the intermediate frequency digital signal is in the L1 band;
s12: multiplying the intermediate frequency digital signal s (t) by a local carrier signal to obtain an in-phase component I (t) and a normal-phase component Q (t);
the local carrier signal is represented asWherein Estimated Doppler shift at acquisition, flocRepresenting the actual frequency, Q, of the intermediate-frequency digital signalloc(t) signals representing the positive phase component of the local carrier signal, Iloc(t) a signal, phi, representing the in-phase component of the local carrier signallocRepresenting an initial phase of the intermediate frequency digital signal;
mixing (1) and (2) to obtain an in-phase component I (t) and a normal-phase component Q (t), which can be expressed by formula (3):
s13: processing the signal of the formula (3) through a low-pass filter to obtain a formula (4):
wherein,estimating a residual for the doppler shift;
s14: it is assumed that the local pseudo-code sequence can be represented asThe same phase component integral I can be obtained by carrying out correlation processing and integration on the same phase component I and the formula (4)PAnd positive phase component integral QPIt is specifically represented by formula (5):
where T is the pre-detection integration time,represents the propagation time of the satellite signal from the satellite to the receiver;
s15: the formula (5) is simplified to obtain a formula (6):
the final in-phase component integral IPAnd positive phase component integral QPI.e. the output of the correlator.
Preferably, T is 1ms, 2ms, 5ms or 10 ms.
Preferably, M ═ N + 4.
Preferably, M-32 and N-28.
In order to overcome the deficiencies of the prior art, it is a second object of the present invention to provide an apparatus for capturing GNSS baseband signals, which is capable of capturing GNSS baseband signals in multiple channels and at high speed.
The second purpose of the invention is realized by adopting the following technical scheme:
the invention also provides a device for capturing the GNSS baseband signal, which comprises:
the first satellite acquisition module is used for dividing the frequency of the current uncaptured satellite into M first frequency bands, allocating a corresponding correlator to each first frequency band, searching the current uncaptured satellite in each first frequency band by a method of searching satellite signals through the correlators, and obtaining the output result of each correlator;
the acquisition completion marking module is used for judging whether the current uncaptured satellite is acquired or not according to all the output results, if so, marking the current uncaptured satellite as the acquired satellite, switching to the next satellite and marking as the current uncaptured satellite, and executing the first satellite acquisition module; executing a satellite number acquisition module until four satellites are captured;
the satellite number acquisition module is used for acquiring the satellite numbers of all the current observable satellites according to the satellite navigation messages of the four satellites;
the other satellite capturing module is used for obtaining the frequencies of all the uncaptured satellites according to the satellite numbers, dividing the frequency of each uncaptured satellite into N second frequency bands, allocating a corresponding correlator to each second frequency band, sequentially searching each uncaptured satellite in each second frequency band by a method of searching satellite signals through the correlators, judging whether each uncaptured satellite is captured, and if so, marking the corresponding uncaptured satellite as the captured satellite;
and the acquisition completion module is used for completing acquisition of the GNSS baseband signal when all the satellites are marked as acquired satellites.
Compared with the prior art, the invention has the beneficial effects that: the invention searches the same satellite by using a plurality of correlators, thereby greatly reducing the capturing time of the system for capturing satellite signals and improving the processing efficiency of the system; in addition, the invention also carries out secondary acquisition, further positions the satellite signal to a more accurate frequency band, and provides more accurate data for the processing of the next step.
Drawings
FIG. 1 is a flow chart of a method according to an embodiment of the present invention;
FIG. 2 is a block diagram of an apparatus according to an embodiment of the present invention.
Detailed Description
The invention will be further described with reference to the accompanying drawings and the detailed description below:
as shown in fig. 1, for a receiver, the position is determined according to the satellites, that is, when determining the position of the receiver, it is first necessary to receive the satellite signals of all the satellites, that is, to search for and acquire the satellite signals. Generally, there are 32 satellites, the frequency of the satellite signal of each satellite is different, and when the satellite signals of the 32 satellites are captured, in order to improve the capture time, the invention provides a method for capturing GNSS baseband signals, which can rapidly capture the satellite signals of all satellites, the method comprising the following steps:
s1: dividing the frequency of the current uncaptured satellite into M first frequency bands, allocating a corresponding correlator to each first frequency band, searching the uncaptured satellite in each frequency band by a method of searching satellite signals through the correlators, and obtaining an output result of each correlator.
First, when the satellite signal is unknown, the frequency of an uncaptured satellite is generally set by an empirical value, that is, the frequency band of the satellite signal of a satellite is estimated. In the receiver, a plurality of correlators are provided, which are instruments that extract useful signals from interference and noise by using the correlation characteristics of the signals. The number of the correlators is 32, and when the same satellite is searched, the 32 correlators are used for simultaneously performing parallel processing, so that the satellite can be quickly searched. Since the frequency range of each uncaptured satellite may be large, each uncaptured satellite is first divided into a plurality of first frequency bands, and a corresponding correlator is allocated to each first frequency band, so that the uncaptured satellite is searched by a method of searching satellite signals through the correlators in each frequency band, so that the correlators operate simultaneously, and the searching time is greatly reduced. The method for searching satellite signals by the correlator specifically comprises the following steps:
s11: converting the signal received by the GNSS antenna into an intermediate frequency digital signal s (t), which can be expressed as:
wherein P is the signal power; d (t) is a navigation message bit; c (t) is a C/A code; τ represents the time delay in the transmission of the satellite signal from the satellite to the receiver; f. ofdIs the doppler shift; t is the observation time; f. ofIF＝fNominal scale+fdRepresented as a carrier down-converted intermediate frequency signal; f. ofNominal scaleRepresenting a nominal frequency of the intermediate frequency digital signal; phi is an initial carrier phase; n (t) is white noise, the power spectral density of which is constant; f. ofL1Indicating that the intermediate frequency digital signal is at a frequency in the L1 band.
S12: multiplying the intermediate frequency digital signal s (t) by a local carrier signal to obtain an in-phase component I (t) and a normal-phase component Q (t);
the local carrier signal is represented asWherein Estimated Doppler shift at acquisition, flocRepresenting the actual frequency, Q, of the intermediate-frequency digital signalloc(t) signals representing the positive phase component of the local carrier signal, Iloc(t) a signal, phi, representing the in-phase component of the local carrier signallocRepresenting the initial phase of the intermediate frequency digital signal.
Suppose thatTo estimate the residual for the doppler shift, when acquiring satellite signals, only if Δ f is as close to 0 as possible, the corresponding satellite signals can be acquired.
Preferably, in the present invention, in the acquisition of the signal, it is generally considered that the acquisition of the satellite is completed by setting the value of Δ f to be less than a certain threshold.
Mixing (1) and (2) to obtain an in-phase component I (t) and a positive-phase component Q (t), namely:
s13: the in-phase component i (t) and the normal-phase component q (t) are processed by a low-pass filter to obtain formula (4):
s14: it is assumed that the local pseudo-code sequence can be represented asThe obtained product is correlated and integrated with the formula (4) to obtain an in-phase component integral IPAnd positive phase component integral QPIt can be represented by formula (5):
wherein T is the pre-detection integration time, and T<20 ms; typically an integer multiple of 1ms of the chip period is taken for C/a codes,representing the propagation time of the satellite signal from the satellite to the receiver.
S15: since d (T) is a navigation message bit with a bit rate of 50bps, d (T) is considered to be unchanged during the integration time T, and therefore, the formula (5) can be simplified to the formula (6) except for the integration number:
that is, each correlator outputs a set of in-phase component integrals I after searching the satellite signalPAnd positive phase component integral QPI.e. the output of the correlator.
S2: judging whether the current uncaptured satellite is captured or not according to all output results, if so, recording the current uncaptured satellite as the captured satellite by the uncaptured satellite, switching to the next satellite and recording as the current uncaptured satellite, and executing S1; until four satellites are acquired, S3 is performed.
When each uncaptured satellite is searched, the invention adopts a plurality of correlators to search the same uncaptured satellite at the same time in different frequency bands, so that the correlator in each frequency band has an output result. I.e. the integral I of the in-phase component obtained in each frequency bandPIntegral Q of the sum positive phase componentPAnd comparing the values obtained by the square sum calculation, and selecting the frequency band corresponding to the maximum value as the frequency band of the satellite signal of the uncaptured satellite, namely that the uncaptured satellite is captured currently. When the acquisition of one satellite is completed, the same is continuedThe method of (3) acquires additional non-acquired satellites until acquisition of four satellites is completed.
S3: and acquiring the satellite numbers of all the current observable satellites according to the satellite navigation messages of the four satellites.
Generally speaking, as long as there are four satellites, the feature that the positioning result can be generated is that the satellite numbers of all observable satellites can be obtained according to the navigation messages in the satellite signals of the four satellites. And each satellite has a corresponding satellite number, so that the information such as the position, the frequency and the like of the corresponding satellite can be obtained according to the satellite number. Therefore, when searching for other non-acquired satellites, the searching frequency range is determined, and the searching time for other non-acquired satellites can be greatly reduced.
S4: obtaining the frequency of all the uncaptured satellites according to the satellite number, dividing the frequency of each uncaptured satellite into N second frequency bands, allocating a corresponding correlator to each second frequency band, searching each uncaptured satellite in sequence in each second frequency band by a method of searching satellite signals through the correlators, judging whether each uncaptured satellite is captured, and if so, marking the corresponding uncaptured satellite as the captured satellite.
By acquiring the satellite number of each of the uncaptured satellites, the search frequency of the uncaptured satellite can be determined. The term "non-captured satellite" refers to a satellite other than 4 satellites that have already been captured. Since there are 32 correlators used in the present invention, and 4 correlators have already completed the acquisition of the above four satellites, there are 28 correlators left, so that in order to ensure the search time, 28 correlators can also search for one satellite which is not acquired at the same time. That is, the same correlator is used to search for satellite signals, and each satellite that is not acquired is searched. When one non-captured satellite is captured, marking the non-captured satellite as a captured satellite, and continuing to acquire other non-captured satellites; until all the non-acquired satellites are fully acquired, i.e., all the satellites are marked as acquired satellites.
S5: the GNSS baseband signals complete acquisition when all satellites are marked as acquired satellites.
When all the satellites are acquired, that is, the satellite signals of all the satellites are determined to be in the corresponding frequency bands, so that the satellite signals of each satellite can be determined to be in the known frequency bands when the satellite signals are tracked in the next step, and the position information of the receiver can be quickly obtained.
Preferably, when the correlator searches for the satellite signal, since the satellite signal passes through an ionosphere, a troposphere and the like before reaching the ground, there is a large energy loss, the degree of the loss varies with the distance between the receiver and the satellite, and the received signal energy is strong or weak. For strong signals, the correlator can capture the corresponding signal only in a short time; for weak signals, it is difficult for the correlator to acquire the corresponding signal in a short time. Therefore, the setting of the pre-detection integration time is in a rotating manner when the satellite signal is searched by the correlator. The pre-detection integration time selected in the present invention is 1ms, 2ms, 5ms and 10 ms. That is, for example, for a strong signal, the pre-detection integration time that can be adopted is 1ms, 2ms, 5 ms; whereas for weak signals a pre-detection integration time of 10ms may be taken. PRN codes and satellite navigation messages are typically included for satellite signals; when a signal is captured, a local carrier signal and a satellite signal are mixed, and the local carrier signal is continuously adjusted, so that when the local carrier signal is close to a PRN code in the satellite signal, the satellite signal is searched, and the PRN code of the satellite signal is changed every a period of time, that is, when the signal is captured, a search must be completed within a period of time when the PRN code of a satellite line is changed once, and if the search cannot be completed, the local carrier needs to be readjusted to search the satellite signal again, which causes a great amount of waste in time. For strong signals, the satellite signals can be searched only in a short time, so that the search can be completed by setting the integration time of the search to be shorter, thereby saving more time, and for weak signals, the corresponding satellite signals are difficult to search in a short time, so that the integration time can be gradually increased until the satellite signals are searched. Of course, the pre-detection integration time does not exceed the PRN code variation time of the satellite signal. In the invention, when the satellite is captured, a self-adaptive capturing method is adopted, and the satellite is captured in a rotating mode. Firstly, setting the pre-detection integration time to be 1ms, and automatically setting the pre-detection integration time to be 2ms if any satellite cannot be captured under the integration time; for the same reason, the pre-detection integration time is set to 5ms and 10ms until the satellite can be captured.
In addition, the strength of the judgment signal is related to the altitude angle of the satellite, and the altitude angle of the satellite can be calculated according to the position of the satellite. That is, the greater the altitude angle of the satellite, the stronger the intensity of the corresponding signal; and the smaller the altitude angle of the satellite, the weaker the strength of its corresponding signal. When the position of the satellite is obtained, whether the satellite signal corresponding to the satellite is a strong signal or a weak signal can be judged, and then the satellite can be further and quickly acquired by setting the integration time.
Further, secondary acquisition is included, that is, for all the satellites already acquired in S1 to S4, the satellite signal of each satellite is determined to be within a fixed frequency band, for example, the satellite signals of the first searched 4 satellites are determined to be within a corresponding first frequency band, and the satellite signals of the next 28 satellites are determined to be within a corresponding second frequency band. In order to further make the frequency band of the satellite row of each satellite more accurate, the determined frequency band is divided again, and then all satellites are acquired by the acquisition method, so that the frequency of the satellite signal of each satellite can be obtained more accurately. For example, after the acquisition of all satellites is completed by using the above-mentioned S1 to S4, the frequency range of the satellite signal is determined to be 500Hz, and then the frequency of the above-mentioned 500Hz is divided into a plurality of frequency bands, for example, the frequency band is divided into 10 frequency bands, that is, the frequency of each frequency band is 50 Hz; and then capturing all the satellites according to the capturing method, so that the frequency precision of the satellite signals of all the final satellites is 50Hz, namely the precision of the GNSS baseband signals is greatly increased.
As shown in fig. 2, the present invention further provides a GNSS baseband signal capturing apparatus, including:
the first satellite capturing module is used for dividing the frequency of the current uncaptured satellite into M first frequency bands, allocating a corresponding correlator to each first frequency band, searching the current uncaptured satellite in each first frequency band by a method of searching satellite signals through the correlators, and obtaining the output result of each correlator;
the acquisition completion marking module is used for judging whether the current uncaptured satellite is acquired or not according to all the output results, if so, marking the current uncaptured satellite as the acquired satellite, switching to the next satellite and marking as the current uncaptured satellite, and executing the first satellite acquisition module; executing a satellite number acquisition module until four satellites are captured;
the satellite number acquisition module is used for acquiring the satellite numbers of all the current observable satellites according to the satellite navigation messages of the four satellites;
the other satellite capturing module is used for obtaining the frequencies of all the uncaptured satellites according to the satellite numbers, dividing the frequency of each uncaptured satellite into N second frequency bands, allocating a corresponding correlator to each second frequency band, sequentially searching each uncaptured satellite in each second frequency band by a method of searching satellite signals through the correlators, judging whether each uncaptured satellite is captured, and if so, marking the corresponding uncaptured satellite as the captured satellite;
and the acquisition completion module is used for completing acquisition of the GNSS baseband signal when all the satellites are marked as acquired satellites.
Various other modifications and changes may be made by those skilled in the art based on the above-described technical solutions and concepts, and all such modifications and changes should fall within the scope of the claims of the present invention.
Claims (7)
1. A method for capturing a GNSS baseband signal comprises the following steps:
s1: dividing the frequency of the current uncaptured satellite into M first frequency bands, allocating a corresponding correlator to each first frequency band, searching the current uncaptured satellite in each first frequency band by a method of searching satellite signals through the correlators, and obtaining an output result of each correlator;
s2: judging whether the current uncaptured satellite is captured or not according to all output results, if so, marking the current uncaptured satellite as the captured satellite, switching to the next satellite and marking as the current uncaptured satellite, and executing S1; until four satellites are acquired, S3 is performed;
s3: acquiring the satellite numbers of all current observable satellites according to the satellite navigation messages of the four satellites;
s4: obtaining the frequency of all uncaptured satellites according to the satellite number, dividing the frequency of each uncaptured satellite into N second frequency bands, allocating a corresponding correlator to each second frequency band, sequentially searching each uncaptured satellite in each second frequency band by a method of searching satellite signals through the correlators, judging whether each uncaptured satellite is captured, and if so, marking the corresponding uncaptured satellite as the captured satellite;
s5: when all the satellites are marked as the captured satellites, the GNSS baseband signals are captured;
the method for searching satellite signals by the correlator specifically comprises the following steps:
s11: converting a signal received through a GNSS antenna into an intermediate frequency digital signal s (t), which can be expressed by equation (1):
wherein P is the signal power; d (t) is a navigation message bit; c (t) is a C/A code; τ represents the time delay in the transmission of the satellite signal from the satellite to the receiver; f. ofdIs the doppler shift; t is the observation time; f. ofIF＝fNominal scale+fdRepresented as a carrier down-converted intermediate frequency signal; f. ofNominal scaleRepresenting a nominal frequency of the intermediate frequency digital signal; phi is an initial carrier phase; n (t) is white noise, the power spectral density of which is constant; f. ofL1A frequency indicating that the intermediate frequency digital signal is in the L1 band;
s12: multiplying the intermediate frequency digital signal s (t) by a local carrier signal to obtain an in-phase component I (t) and a normal-phase component Q (t);
the local carrier signal is represented asWherein Estimated Doppler shift at acquisition, flocRepresenting the actual frequency, Q, of the intermediate-frequency digital signalloc(t) signals representing the positive phase component of the local carrier signal, Iloc(t) a signal, phi, representing the in-phase component of the local carrier signallocRepresenting an initial phase of the intermediate frequency digital signal;
mixing (1) and (2) to obtain an in-phase component I (t) and a normal-phase component Q (t), which can be expressed by formula (3):
s13: processing the signal of the formula (3) through a low-pass filter to obtain a formula (4):
wherein,estimating a residual for the doppler shift;
s14: it is assumed that the local pseudo-code sequence can be represented asThe same phase component integral I can be obtained by carrying out correlation processing and integration on the same phase component I and the formula (4)PAnd positive phase component integral QPIt is specifically represented by formula (5):
where T is the pre-detection integration time,represents the propagation time of the satellite signal from the satellite to the receiver;
s15: the formula (5) is simplified to obtain a formula (6):
the final in-phase component integral IPAnd positive phase component integral QPI.e. the output of the correlator.
2. The method of claim 1, further comprising dividing the frequency of each acquired satellite into I third frequency bands, and performing a second acquisition on each acquired satellite in turn by searching satellite signals through a correlator in each third frequency band until the second acquisition is completed for all the acquired satellites.
3. The GNSS baseband signal acquisition method according to claim 2, wherein I is 10.
4. The method for acquiring GNSS baseband signals according to claim 1, wherein T is 1ms, 2ms, 5ms, or 10 ms.
5. The method for acquiring GNSS baseband signals according to claim 1, wherein M is N + 4.
6. The method for acquiring GNSS baseband signals according to claim 5, wherein M is 32 and N is 28.
7. An apparatus for capturing GNSS baseband signals, comprising:
the first satellite acquisition module is used for dividing the frequency of the current uncaptured satellite into M first frequency bands, allocating a corresponding correlator to each first frequency band, searching the current uncaptured satellite in each first frequency band by a method of searching satellite signals through the correlators, and obtaining the output result of each correlator;
the acquisition completion marking module is used for judging whether the current uncaptured satellite is acquired or not according to all the output results, if so, marking the current uncaptured satellite as the acquired satellite, switching to the next satellite and marking as the current uncaptured satellite, and executing the first satellite acquisition module; executing a satellite number acquisition module until four satellites are captured;
the satellite number acquisition module is used for acquiring the satellite numbers of all the current observable satellites according to the satellite navigation messages of the four satellites;
the other satellite capturing module is used for obtaining the frequencies of all the uncaptured satellites according to the satellite numbers, dividing the frequency of each uncaptured satellite into N second frequency bands, allocating a corresponding correlator to each second frequency band, sequentially searching each uncaptured satellite in each second frequency band by a method of searching satellite signals through the correlators, judging whether each uncaptured satellite is captured, and if so, marking the corresponding uncaptured satellite as the captured satellite;
the acquisition completion module is used for completing acquisition of the GNSS baseband signals when all the satellites are marked as acquired satellites;
the correlator searches for satellite signals by:
s11: converting a signal received through a GNSS antenna into an intermediate frequency digital signal s (t), which can be expressed by equation (1):
wherein P is the signal power; d (t) is a navigation message bit; c (t) is a C/A code; tau represents the transmission of the satellite signal from the satellite to the receiverA time delay in the process; f. ofdIs the doppler shift; t is the observation time; f. ofIF＝fNominal scale+fdRepresented as a carrier down-converted intermediate frequency signal; f. ofNominal scaleRepresenting a nominal frequency of the intermediate frequency digital signal; phi is an initial carrier phase; n (t) is white noise, the power spectral density of which is constant; f. ofL1A frequency indicating that the intermediate frequency digital signal is in the L1 band;
s12: multiplying the intermediate frequency digital signal s (t) by a local carrier signal to obtain an in-phase component I (t) and a normal-phase component Q (t);
the local carrier signal is represented asWherein Estimated Doppler shift at acquisition, flocRepresenting the actual frequency, Q, of the intermediate-frequency digital signalloc(t) signals representing the positive phase component of the local carrier signal, Iloc(t) a signal, phi, representing the in-phase component of the local carrier signallocRepresenting an initial phase of the intermediate frequency digital signal;
mixing (1) and (2) to obtain an in-phase component I (t) and a normal-phase component Q (t), which can be expressed by formula (3):
s13: processing the signal of the formula (3) through a low-pass filter to obtain a formula (4):
wherein,estimating a residual for the doppler shift;
s14: it is assumed that the local pseudo-code sequence can be represented asThe same phase component integral I can be obtained by carrying out correlation processing and integration on the same phase component I and the formula (4)PAnd positive phase component integral QPIt is specifically represented by formula (5):
where T is the pre-detection integration time,represents the propagation time of the satellite signal from the satellite to the receiver;
s15: the formula (5) is simplified to obtain a formula (6):
the final in-phase component integral IPAnd positive phase component integral QPI.e. the output of the correlator.
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