CN114019544A - Navigation signal rapid capturing method based on low-earth-orbit satellite - Google Patents

Abstract

The invention discloses a navigation signal rapid acquisition method based on a low earth orbit satellite, and belongs to the field of satellite navigation signal processing. The method disclosed by the invention can be used for quickly judging the successful capture based on the FPGA to calculate the noise threshold, and meanwhile, the capture time is further shortened by additionally adding a fine capture module and adopting a rough capture and fine capture parallel processing algorithm. The method solves the problem that the conventional capturing algorithm has too long time for searching all the pseudo code phases, and further compresses capturing time by changing the conventional serial processing of coarse capturing and fine capturing into a parallel processing mode. The rapid capturing method only occupies a small amount of hardware logic resources additionally, and by optimizing a software algorithm, the capturing speed is far superior to that of a traditional algorithm, so that the rapid capturing method is favorable for popularization and application in a satellite-borne receiver.

Description

Navigation signal rapid capturing method based on low-earth-orbit satellite

Technical Field

The invention relates to a navigation signal rapid acquisition method based on a low earth orbit satellite, belonging to the technical field of signal processing in satellite navigation.

Background

With the development of commercial aerospace, the realization of communication, navigation and remote sensing on a near-earth orbit has become a research hotspot. Unlike the navigation and positioning means of medium and high orbit satellites, the low orbit satellite obtains position, speed and time information by carrying a GNSS receiver. Due to the low cost and excellent performance of the satellite-borne GNSS receiver, the satellite-borne GNSS receiver becomes a standard configuration of a low-orbit satellite platform, and besides providing information such as position and the like for the satellite platform, observation data of the satellite-borne GNSS receiver is also used for high-precision orbit determination and other applications.

The conventional satellite-borne receiver is divided into a coarse acquisition stage and a fine acquisition stage when acquiring signals, wherein the coarse acquisition stage searches all pseudo code phases from a first pseudo code phase, the maximum correlation value corresponding to each pseudo code phase is compared with an acquisition threshold, if the maximum correlation value exceeds the threshold, the acquisition is considered to be successful, and then the acquisition stage is switched to the fine acquisition stage. The fine capture stage realizes accurate estimation of carrier phase, and the fine capture stage and the coarse capture stage share the same FPGA capture module resource and are in serial computing relationship. The low-orbit satellite has high speed, the visible time length of a single satellite is short, the conventional acquisition process shows that the acquisition time of the navigation signal is long, and the visible satellite signal cannot be acquired in time, so that the cold start time of the receiver is prolonged, and the integrity rate of GNSS data is influenced.

Disclosure of Invention

The invention provides a navigation signal rapid capturing method based on a low-earth orbit satellite in order to overcome the problem of long GNSS signal capturing time of a traditional satellite-borne receiver.

In order to achieve the purpose, the invention adopts the technical scheme that:

a navigation signal rapid capturing method based on a low earth orbit satellite is realized based on an FPGA (field programmable gate array), wherein a digital down-conversion module, a coarse capturing module and a fine capturing module run in the FPGA, and the coarse capturing module and the fine capturing module work in parallel; the method comprises the following steps:

step 1, a satellite navigation receiver performs down-conversion and AD sampling on a radio frequency analog signal to obtain a digital intermediate frequency signal, and then the digital intermediate frequency signal is converted into a baseband signal by using a digital down-conversion module;

step 2, the receiver generates 100 groups of satellite pseudo code sequences with the length of 1ms to be captured, and each group of pseudo code sequences delays half of pseudo code phase in sequence;

step 3, starting coarse catching processing by the receiver, and performing coarse catching by a coarse catching module;

step 4, the receiver starts fine catching treatment, and fine catching is carried out through a fine catching module;

step 5, performing frequency estimation on the input baseband signal according to the fine capture result;

and 6, converting the frequency value estimation value into a frequency control word for generating a local carrier of the receiver, wherein the capturing stage is finished at the moment.

Further, the specific manner of step 3 is as follows:

301, correlating each pseudo code sequence generated by the receiver with an input 1ms baseband signal to obtain 100-segment 10us short integration results; carrying out 512-point FFT processing on 100 sections of short integral values generated by each group of code phases to complete parallel frequency search, wherein the frequency resolution is 195.3 Hz;

step 302, performing the operation of step 3 on the next millisecond baseband data, and performing corresponding point incoherent accumulation on each group of 512-point FFT results to obtain 100 × 512 incoherent accumulation results;

step 303, searching the maximum value of 100 × 512 incoherent accumulation results, if the maximum value exceeds an acquisition threshold, then performing coarse acquisition successfully, at this time, not performing residual pseudo code phase search, recording a pseudo code sequence corresponding to the maximum value, starting fine acquisition processing by the receiver, and simultaneously starting coarse acquisition processing of the next satellite; if the maximum value does not exceed the capture threshold, the receiver delays 100 half-chip phases to regenerate 100 groups of satellite pseudo code sequences, and the operation of the steps 301 to 302 is carried out again; and if the incoherent accumulation maximum value does not exceed the capture threshold after the traversal search of all the pseudo code phases is completed, the coarse capture fails, and the coarse capture processing of the next satellite is performed at the moment.

Further, the specific manner of step 4 is as follows:

the receiver generates 10 groups of satellite pseudo code sequences with the length of 1ms based on the maximum pseudo code sequence obtained by coarse capture, and each group of pseudo codes delays half of the pseudo code phase in sequence; carrying out short integration, FFT and 10 times of incoherent accumulation processing on the 10 groups of pseudo code sequences and the input baseband signal, if the maximum incoherent accumulation result exceeds a capture threshold, then successfully capturing, recording the pseudo code sequence corresponding to the maximum value, and starting to carry out frequency estimation; otherwise, the fine capture fails, and the coarse capture processing of the next satellite is started.

Further, the specific manner of step 5 is as follows:

carrying out short integration, FFT and 10 times of incoherent accumulation processing on the pseudo code sequence corresponding to the maximum value obtained by fine capture and an input baseband signal, wherein the maximum correlation value of the FFT incoherent accumulation is y, and the corresponding FFT median is k; the left and right sides of the y value correspond to the incoherent accumulated values y1 and y2 respectively;

through quadratic fit, the frequency estimation value is obtained as follows:

compared with the prior art, the invention has the following beneficial effects:

1. the invention initializes the local pseudo code phase by combining the GNSS ephemeris and the receiver position, the coarse acquisition is considered to be successful as long as the correlation value of the input signal exceeds the acquisition threshold, and the acquisition time is saved without searching all pseudo code phases.

2. According to the invention, a new capture module is added in the FPGA to realize the parallel calculation of coarse capture and fine capture, so that the capture time is further saved.

3. From the aspect of engineering realization, the added capture module occupies small FPGA logic resources, does not influence the type selection of an FPGA device, does not increase the cost of a receiver, and is favorable for popularization and application in the satellite-borne receiver.

In a word, the method carries out local pseudo code phase prediction based on the GNSS ephemeris and the receiver position, compares the local pseudo code phase prediction with the noise threshold obtained by FPGA calculation, thereby rapidly carrying out successful acquisition judgment, and simultaneously, adopts a coarse acquisition/fine acquisition parallel processing mode to further shorten the acquisition time. The method is suitable for various satellite navigation receiving terminals.

Drawings

Various other advantages and benefits will become more apparent to those of ordinary skill in the art from the following detailed description of the embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. In the drawings:

fig. 1 is a flowchart illustrating a fast acquisition process of a low-earth-orbit-satellite-based navigation signal according to an embodiment of the invention.

Detailed Description

The invention will be further explained with reference to the drawings.

As shown in fig. 1, a method for fast acquiring a navigation signal based on a low earth orbit satellite includes the following steps:

s1, the satellite navigation receiver carries out down-conversion and AD sampling on the radio frequency analog signal to convert the radio frequency analog signal into a digital intermediate frequency signal, and the DDC digital down-conversion module is used for converting the digital intermediate frequency signal into a baseband signal;

s2, the receiver generates a pseudo code sequence of the satellite to be captured;

and S3, the receiver starts coarse acquisition processing and performs signal coarse acquisition by adopting a matched filtering method. The time domain searches the pseudo code phase in a sliding mode, and the frequency searches frequency in a parallel mode in an FFT (fast Fourier transform) mode;

s4, carrying out multiple times of incoherent accumulation on the correlation value result;

and S5, in the pseudo code searching process, starting fine acquisition processing when the incoherent accumulated correlation value exceeds the acquisition threshold, and starting the coarse acquisition processing of the next satellite. If all pseudo code phases are traversed and searched, and the correlation value does not exceed the capture threshold, then the next satellite signal is roughly captured;

and S6, performing a fine capture processing stage, re-correlating the input satellite signals, and performing frequency estimation if the correlation value exceeds the capture threshold. Otherwise, after the fine capture is finished, starting the next satellite for coarse capture;

step 7, starting frequency accurate estimation, resetting local carrier frequency, ending the capturing phase and entering a signal traction phase;

wherein, step S5 specifically includes the following steps:

(501) the coarse capture processing uses a coarse capture module in the FPGA, the module supports M groups of pseudo code phases to be searched in parallel, the coarse capture FPGA processor adds correlation value results corresponding to the M groups of pseudo code phases and then calculates the average value of the correlation value results to obtain a noise substrate N, and the capture threshold is defined as 1.5 multiplied by N;

(502) when the maximum non-correlation accumulation value obtained during the parallel search of M groups of pseudo code phases exceeds the capture threshold, the coarse capture is successful, the search of the residual pseudo code phases is not carried out at the moment, and the coarse capture is finished;

(503) the fine capture processing uses a fine capture module in the FPGA, and the module supports parallel search of a small number of pseudo code phases. The rough capture and the fine capture correspond to different FPGA capture modules, and if the navigation signal is successfully rough captured, the traditional serial processing of the rough capture and the fine capture is changed into parallel processing.

According to the method, the pseudo code phase of a visible satellite is estimated and initialized according to a GNSS ephemeris and the position of a receiver, an FPGA calculates a GNSS signal capture threshold in real time in the pseudo code phase searching process, and if the correlation value of a local pseudo code and an input signal exceeds the capture threshold, coarse capture is considered to be successful. In addition, the invention enables the rough capture and the fine capture to be processed in parallel by adding the FPGA capture module, thereby further shortening the capture time.

The following is a more specific example:

a navigation signal fast acquisition method based on low earth orbit satellites comprises the following steps:

step 1, the satellite navigation receiver carries out down-conversion and AD sampling on the radio frequency analog signal to convert the radio frequency analog signal into a digital intermediate frequency signal, and the DDC module is utilized to convert the digital intermediate frequency signal into a baseband signal. The concrete mode is as follows:

1) performing navigation signal frequency conversion and digital sampling processing

The receiver is internally integrated with a radio frequency chip, and radio frequency signals are down-converted to intermediate frequency digital signals by configuring radio frequency chip parameters (filter bandwidth, local carrier frequency, AD sampling digit and the like) through a receiver control processor such as a DSP.

2) Secondary conversion of digital intermediate frequency signals to baseband signals

And a DDC module of the FPGA processor of the receiver multiplies the input digital intermediate frequency signal by a locally generated intermediate frequency signal to obtain a baseband signal.

Step 2, the receiver generates a satellite pseudo code sequence to be captured;

and 3, starting coarse capture processing by the receiver, and performing signal coarse capture by adopting a matched filtering method. The time domain searches the pseudo code phase in a sliding mode, and the frequency searches the frequency in a parallel mode in an FFT mode. The concrete mode is as follows:

1) obtaining short integral processing result

Because the low orbit satellite has fast motion speed of about 8000m/s, Doppler caused by dynamic is about-40 KHz, the receiver adopts 10ms short integral to process, and the corresponding frequency searching range is-50 KHz. For a 1ms full period pseudo code, there are 100 segments of short integration results.

The coarse capture module completes the search of 100 groups of parallel pseudo codes, so that the receiver generates 100 sections of short integration results corresponding to the 100 groups of pseudo codes at the moment.

2) FFT calculation of short integration results

Carrying out 512-point pipeline FFT processing on 100 sections of short integral values generated by each code phase, wherein the frequency resolution is 195.3 Hz;

step 4, carrying out multiple times of incoherent accumulation on the correlation value result;

performing corresponding point power accumulation on 512 correlation value results obtained by processing input data of each code phase every 1ms, and selecting 10 times of incoherent accumulation times in consideration of meeting a certain receiving sensitivity requirement;

and 5, in the pseudo code searching process, starting fine acquisition processing when the incoherent accumulated correlation value exceeds an acquisition threshold, and starting the coarse acquisition processing of the next satellite. And if the traversal search of all the pseudo code phases is completed and the correlation value does not exceed the acquisition threshold, performing the coarse acquisition processing of the next satellite signal. The concrete mode is as follows:

1) pseudo code correlation capture result correlation

A coarse capture module in the FPGA is used, the module supports 100 groups of pseudo code phases to be searched in parallel, correlation value results corresponding to 100 groups of pseudo code phases are added, then the average value of the correlation value results is obtained to obtain a noise substrate N, and a capture threshold is defined as 1.5 multiplied by N;

when the maximum non-correlation accumulation value obtained by 100 groups of pseudo code phase parallel search exceeds the capture threshold, the coarse capture is successful, the search of the residual pseudo code phases is not carried out at the moment, and the coarse capture is finished;

when the maximum uncorrelated accumulation value does not exceed the capture threshold in the parallel search, after 100 pseudo code phases are locally arranged in the sliding receiver, the next 100 groups of pseudo code phases are searched in parallel, and 20 sliding times are needed for completing all the search on all the pseudo codes of the GPS L1 frequency point signals;

2) if the rough catching is successful, the fine catching treatment is started

A fine capture module in the FPGA is used, and the module supports parallel search of a small number of pseudo code phases. The rough capture and the fine capture correspond to different FPGA capture modules, and if the navigation signal is successfully rough captured, the traditional serial processing of the rough capture and the fine capture is changed into parallel processing. Finely capturing the satellite signals which are successfully coarsely captured, and coarsely capturing the next satellite to be captured;

3) if the coarse capture fails, starting the coarse capture processing of the next satellite;

and 6, in the fine capture processing stage, re-correlating the input satellite signals, and if the correlation value exceeds a capture threshold, performing frequency estimation. Otherwise, the fine capture is finished, and the next satellite coarse capture is started. The concrete mode is as follows:

1) re-confirmation of captured results

The fine capture module completes search of 10 groups of parallel pseudo code phases, maximum correlation values corresponding to the 10 groups of pseudo code phases are obtained by a matched filtering method, correct capture is carried out if the maximum correlation values exceed a capture threshold, and false capture is carried out if the maximum correlation values exceed the capture threshold;

2) if correct acquisition is achieved, frequency estimation is started;

if the local pseudo code is correctly captured, delaying the local pseudo code, wherein the delayed pseudo code phase number is the pseudo code phase number corresponding to the maximum correlation value, and then starting to carry out frequency estimation;

3) if the satellite is mistakenly captured, closing the fine capture process and starting the next satellite for coarse capture;

and 7, starting frequency accurate estimation, resetting local carrier frequency, ending the acquisition stage and entering a signal traction stage.

The concrete mode is as follows:

1) performing accurate frequency estimation

Performing matched filtering on the input signal again, wherein the maximum correlation value after FFT is y (corresponding to the position k in the FFT), the correlation values corresponding to the left side and the right side of the maximum correlation value are y1 and y2 respectively, and the estimated frequency is y after quadratic fitting

2) Resetting local carrier frequency

Converting the frequency value obtained by accurate estimation into a frequency control word for generating a local carrier;

3) start signal pull processing

And ending the signal acquisition phase and entering a traction phase.

In a word, the method for generating the local pseudo code by combining the satellite position is adopted, the successful capturing judgment is rapidly carried out on the basis of the FPGA calculation noise threshold, and meanwhile, the capturing time is further shortened by additionally adding a fine capturing module and adopting a rough capturing and fine capturing parallel processing algorithm. The method solves the problem that the conventional capturing algorithm has too long time for searching all the pseudo code phases, and further compresses capturing time by changing the conventional serial processing of coarse capturing and fine capturing into a parallel processing mode. The rapid capturing method only occupies a small amount of hardware logic resources additionally, and by optimizing a software algorithm, the capturing speed is far superior to that of a traditional algorithm, so that the rapid capturing method is favorable for popularization and application in a satellite-borne receiver.

It should be noted that although the embodiments of the present invention have been described with reference to the accompanying drawings, the scope of the present invention is not limited thereto, and it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the principle of the present invention, and these should also be construed as falling within the scope of the present invention. Those skilled in the art will appreciate that the invention may be practiced without these specific details.

Claims (4)

1. A navigation signal rapid capturing method based on a low-earth orbit satellite is characterized by being realized based on an FPGA (field programmable gate array), wherein a digital down-conversion module, a coarse capturing module and a fine capturing module run in the FPGA, and the coarse capturing module and the fine capturing module work in parallel; the method comprises the following steps:

step 1, a satellite navigation receiver performs down-conversion and AD sampling on a radio frequency analog signal to obtain a digital intermediate frequency signal, and then the digital intermediate frequency signal is converted into a baseband signal by using a digital down-conversion module;

step 2, the receiver generates 100 groups of satellite pseudo code sequences with the length of 1ms to be captured, and each group of pseudo code sequences delays half of pseudo code phase in sequence;

step 3, starting coarse catching processing by the receiver, and performing coarse catching by a coarse catching module;

step 4, the receiver starts fine catching treatment, and fine catching is carried out through a fine catching module;

step 5, performing frequency estimation on the input baseband signal according to the fine capture result;

and 6, converting the frequency value estimation value into a frequency control word for generating a local carrier of the receiver to finish capturing.

2. The method for rapidly acquiring a navigation signal based on a low earth orbit satellite according to claim 1, wherein the specific manner of step 3 is as follows:

301, correlating each pseudo code sequence generated by the receiver with an input 1ms baseband signal to obtain 100-segment 10us short integration results; carrying out 512-point FFT processing on 100 sections of short integral values generated by each group of code phases to complete parallel frequency search, wherein the frequency resolution is 195.3 Hz;

step 302, performing the operation of step 3 on the next millisecond baseband data, and performing corresponding point incoherent accumulation on each group of 512-point FFT results to obtain 100 × 512 incoherent accumulation results;

step 303, searching the maximum value of 100 × 512 incoherent accumulation results, if the maximum value exceeds an acquisition threshold, then performing coarse acquisition successfully, at this time, not performing residual pseudo code phase search, recording a pseudo code sequence corresponding to the maximum value, starting fine acquisition processing by the receiver, and simultaneously starting coarse acquisition processing of the next satellite; if the maximum value does not exceed the capture threshold, the receiver delays 100 half-chip phases to regenerate 100 groups of satellite pseudo code sequences, and the operation of the steps 301 to 302 is carried out again; and if the incoherent accumulation maximum value does not exceed the capture threshold after the traversal search of all the pseudo code phases is completed, the coarse capture fails, and the coarse capture processing of the next satellite is performed at the moment.

3. The method for rapidly acquiring a navigation signal based on a low earth orbit satellite according to claim 2, wherein the specific manner of the step 4 is as follows:

the receiver generates 10 groups of satellite pseudo code sequences with the length of 1ms based on the maximum pseudo code sequence obtained by coarse capture, and each group of pseudo codes delays half of the pseudo code phase in sequence; carrying out short integration, FFT and 10 times of incoherent accumulation processing on the 10 groups of pseudo code sequences and the input baseband signal, if the maximum incoherent accumulation result exceeds a capture threshold, then successfully capturing, recording the pseudo code sequence corresponding to the maximum value, and starting to carry out frequency estimation; otherwise, the fine capture fails, and the coarse capture processing of the next satellite is started.

4. The method for rapidly acquiring a navigation signal based on a low earth orbit satellite according to claim 3, wherein the specific manner of the step 5 is as follows:

carrying out short integration, FFT and 10 times of incoherent accumulation processing on the pseudo code sequence corresponding to the maximum value obtained by fine capture and an input baseband signal, wherein the maximum correlation value of the FFT incoherent accumulation is y, and the corresponding FFT median is k; the left and right sides of the y value correspond to the incoherent accumulated values y1 and y2 respectively;

through quadratic fit, the frequency estimation value is obtained as follows:

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