CN114563803A - High-dynamic high-sensitivity Beidou D1 signal capturing method - Google Patents

High-dynamic high-sensitivity Beidou D1 signal capturing method Download PDF

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CN114563803A
CN114563803A CN202210209207.4A CN202210209207A CN114563803A CN 114563803 A CN114563803 A CN 114563803A CN 202210209207 A CN202210209207 A CN 202210209207A CN 114563803 A CN114563803 A CN 114563803A
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吴思凡
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Chongqing Aerospace Launch Vehicle Electronic Technology Co ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D30/00—Reducing energy consumption in communication networks
    • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

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Abstract

The invention relates to a high-dynamic high-sensitivity Beidou D1 signal capturing method, and belongs to the technical field of satellites. After necessary preprocessing, the intermediate frequency signals are stored in two caches in parallel, data in the cache A are used for a strong satellite capturing mode that coherent integration time is 1ms, NH code jumping is avoided, but sensitivity is low, the cache B adopts fixed-point intermittent caching, the method is suitable for a weak satellite capturing mode that coherent integration time is 4ms, data time span is large, and sensitivity is high, and the strong and weak satellite modes work alternately until signal capturing succeeds. For successfully captured signals, the real-time Doppler and pseudo codes of the signals are captured (refined) for the second time at a software end by using a sliding window with a certain length so as to adapt to higher dynamic stress.

Description

High-dynamic high-sensitivity Beidou D1 signal capturing method
Technical Field
The invention belongs to the technical field of satellites and relates to a high-dynamic high-sensitivity Beidou D1 signal capturing method.
Background
The Beidou D1 signal capturing with high dynamic and high sensitivity is mainly used for realizing the rapid capturing of a D1 signal by a GNSS receiver under the conditions of high dynamic and weak received signal power under the influence of Neumann-Hofman code (NH code) for overcoming the D1 signal modulation, and is an important link in a navigation receiver with high dynamic and high sensitivity.
The Neumann-Hofman code period modulated by the Beidou D1 signal is 20ms, and the chip length is 1 ms.
The function is as follows:
1) reducing mutual interference of GNSS signals;
2) facilitating a bit synchronization process of the signal. However, since the receiver does not know the NH code phase of the D1 signal during the signal acquisition phase, if the coherent integration time is extended to increase the baseband gain, the risk of energy cancellation is very high, further reducing the sensitivity.
To achieve high sensitivity capture of the D1 signal, the strategy adopted at this time is generally:
(1) the coherent integration time of the D1 signal is fixed to 1ms, the problem of bit jump is avoided in a non-coherent integration mode, but the non-coherent loss cannot be ignored along with the increase of the integration time;
(2) the NH code phase is searched for in the fourth dimension of the signal search (the remaining three dimensions are carrier doppler, pseudo code phase and satellite PRN), but this greatly increases resource consumption.
Continuing with strategy (2) above: to achieve sufficient coherent gain, the NH code must be stripped, but since the receiver is not aware of the NH code phase of each satellite D1 signal at acquisition, a total of 20 code start points (corresponding to 20 different NH code phases) are searched, and the resource consumption for completing one acquisition cycle is expanded from 2M Ncode ndopsler in strategy (1) to 20M 2 ndopsler, where M is the total integration time, 2 Ncode is twice the code length of the D1 signal (half the number of chips in a code period), and ndopsler is the number of doppler frequency wells.
Disclosure of Invention
In view of this, the present invention provides a Beidou D1 signal acquisition method with high dynamic and high sensitivity.
In order to achieve the purpose, the invention provides the following technical scheme:
a Beidou D1 signal capturing method with high dynamic and high sensitivity comprises the following steps:
s1: determining a time window TDCAccumulating the digital intermediate frequency signals IF sampled by the analog-to-digital conversion ADC in a time window, calculating the average value at the end of the window to obtain the direct current component of the intermediate frequency signals, and subtracting the component from the subsequently transmitted intermediate frequency signals, wherein TDCIs taken as value of 2nFs and fs are 60MHz intermediate frequency signal sampling rates;
s2: multiplying the signal by a carrier signal output by a local oscillator through a frequency mixer to obtain a zero intermediate frequency I/Q signal;
s3: performing half-band filtering on the Beidou D1 signal to remove a noise component in the signal; for the B1I signal, 2 times down-sampling is carried out on the zero intermediate frequency signal after the zero intermediate frequency signal passes through first-stage half-band filtering, then 2 times down-sampling is carried out on the zero intermediate frequency signal after the zero intermediate frequency signal passes through second-stage half-band filtering, 3 times down-sampling is carried out on the intermediate frequency signal after the zero intermediate frequency signal passes through the first-stage half-band filtering, and finally the zero intermediate frequency signal with fs/12 being 5MHz is obtained;
s4: after several stages of half-band filters and samplers in step S3, the effective bit width of the signal is gradually accumulated, and a new time window T is set to reduce the complexity of subsequent operationsQCalculating the probability distribution of the sampling values in a window; due to normal distribution symmetry, only probability distribution of +1 sampling points and +3 sampling points is considered, if the number of +1 points is more than twice of the number of +3 points, the threshold is reduced, otherwise, the threshold is improved, and the sampling point values always present normal distribution;
s5: down-sampling the quantized signal to twice the pseudo code rate and transmitting the signal to a capturing circuit;
s6: the acquisition circuit finishes signal acquisition in a mode of alternating a strong satellite mode and a weak satellite mode; coherent integration is not carried out on the signals in the strong star mode, namely the coherent integration time is 1 ms; the coherent integration time of the signal in the weak satellite mode is selected to be 4 ms;
s7: capturing the D1 signal in the strong star mode, and storing the data entering the capturing circuit in the step S5 until the data capacity is VAIn cache A, VAThe value of (a) is equal to the number of sampling points 2 × N (T) required for completing one signal acquisition in the strong star modenoncoh+1), N being the number of chips in the pseudo code period, TnoncohIs a non-coherent integration time; at the same time, the data in step S5 is stored in parallel to a data capacity of VBIn cache B of (1), VBIs equal to the number of samples required to complete signal acquisition in the weak star mode of one time, 2 x N (T)coh*Tnoncoh+1), N being the number of chips in the pseudo code period, Tcoh4 is the coherent integration number in the weak star mode, TnoncohIs a non-coherent integration time;
s8: cache B is only paired with [ T ]0+K*TD1,T0+K*TD1+4]Buffering the signal points in the time interval, i.e. the above-mentioned fixed-point intermittent bufferingStore therein, TD120 navigation data period of D1 signal, T0For the initial time of the cache, K is a non-negative integer and the value range is more than or equal to 0 and less than K and less than TnoncohThe units are milliseconds;
s9: the time for finishing storing all the sampling points in the cache A is obviously shorter than that in the cache B, after the cache A finishes storing, the capture engine writes a PRN (pseudo random number) to be searched, and the capture circuit generates a pseudo code according to the PRN; after the pseudo code is generated, the circuit operates at the working frequency facqTaking out the data in the cache for playback;
s10: firstly, carrying out carrier stripping on signals according to a theoretical intermediate frequency value of a radio frequency chip to obtain I, Q two paths of zero intermediate frequency signals;
s11: transmitting the playback signal point into an N-bit shift register in a matched filtering module, wherein N is equal to a value which is twice the code length of the signal; when the count of the playback signal points is greater than or equal to N, the signal is subjected to short-time correlation matched filtering according to the pseudo code sequence generated in step S10 to obtain 2KA short-time correlation sum, where K is the radix of the FFT; because the coherent integration time is 1ms under the strong satellite mode, the algorithm is not influenced by NH code modulation, and 2 is usedKThe short-time correlation sum is directly transmitted to an FFT module;
s12: 2 in step S11KFFT conversion is carried out on the short-time correlation sum to obtain 2 of the frequency domainKThe signal segments are subjected to cache operation, and after the next correlation operation is finished, the new signal segments accumulate cache results; the accumulated times reach TnoncohThen, the average value E of the incoherent integral energy of N code phases is obtainedmeanTo find the maximum integral energy value EmaxThe ratio Emax/EmeanComparing with a preset threshold value to judge the energy;
s13: limited by hardware resources, the satellite PRN dimension must be serial search, and after the current satellite is captured, the playback V is repeatedAThe step S9-S12 is repeated, and the next star is captured;
s14: starting a tracking channel for the successfully captured satellite, and reading the dump timestamp T returned after the channel is integrated for the first timedumpBuffer out of the combined signalTime stamp T released when it is time to finishstoreCalculating the code starting point of the signal;
after BDS satellites on the searching list complete one round of acquisition in the strong star mode, V is usedBThe time for storing the sample points required by one round of capture is long, and V needs to be detectedBWhether or not to store to a specified capacity, e.g. VBIf not enough samples are stored, the strong star pattern is repeated to capture signals: will VAThe new sampling point in (1) is taken out, and the steps S9 to S14 are repeated; otherwise, starting a weak star mode to capture the D1 signal;
s15: when V isBThe amount of buffered data reaches 2 × N (T)coh*TnoncohAfter +1), starting a weak star capturing mode; the first step of the weak satellite mode is still carrier stripping and short-time correlation matched filtering, the steps S9, S10 and S11 are repeated, after the short-time correlation sum of the corresponding data of 1ms is generated, because the coherent integration time is more than 1ms, the influence of NH code modulation cannot be ignored, and the NH code stripping is carried out on the correlation sum;
s16: signals used by two adjacent rounds of coherent integration are discontinuous in time and are all located at fixed phases in a D1 data period, the NH code phase is not required to be searched by a method of sliding code starting points, and only the following 8 NH code combinations are used for carrying out correlation and NH code stripping: {1,1,1,1}, { -1,1,1,1}, {1, -1,1,1}, {1,1, -1,1}, {1,1,1, -1}, { -1, -1,1,1}, {1, -1,1, -1}, {1, -1, -1,1 };
s17: storing the stripped coherent sum of NH code into a buffer VC；VCThe accumulated data stored in the buffer and NH stripping data output by the next short-time correlation filter are subjected to coherent accumulation operation until the coherent times reach NcohUntil the end; passing the generated coherent sum to an FFT module;
s18: repeating the steps S11-S13 until all satellites on the satellite searching list complete signal acquisition in the weak satellite mode;
s19: due to the weak star mode, VBThe time span of the buffer signal is obviously lengthened, and under high dynamic condition, the difference value between the pseudo code phase and Doppler value searched when the signal is successfully captured and the actual code phase and Doppler value when the tracking channel is opened has a certain probabilityThe rate exceeds the loop tracking range; setting a window with the length of 1.5 chips to slide the pseudo code phase of the signal by utilizing the EPL three-way integral value returned by the baseband; the number of sliding is M, and the value of M is (dT a f)code/fcarrier) 1.5, a is the calculated Doppler change rate of the receiver PVT, assuming the receiver is in the positioning state, fcodeFor the pseudo-code frequency of the signal, fcarrierFor the signal carrier frequency, dT ═ Tdump-TstoreThe time span from acquisition to tracking described in step S14;
s20: after the M times of pseudo code sliding is finished, reading the integral energy values corresponding to 3 × M different pseudo code phases stored in the buffer, finding out the code phase corresponding to the maximum energy, and sliding the signal pseudo code to the position by the configuration baseband; sliding in the Doppler dimension of the signal, and finding out the real-time Doppler frequency of the signal according to the returned integral energy value; after the code phase and the Doppler are reconfigured, the signal is successfully switched to the tracking state.
Optionally, in the strong star mode, the signal capturing process includes:
continuous caching, carrier stripping, short-time correlation matched filtering, spectrum analysis, non-coherent integration and energy judgment;
in the weak satellite mode, the basic flow of signal acquisition is as follows:
fixed-point discontinuous caching, carrier stripping, short-time correlation matched filtering, NH code stripping, coherent integration, spectrum analysis, non-coherent integration and energy judgment, Doppler and pseudo code compensation.
Optionally, for the intermediate frequency signal transmitted by the radio frequency chip ADC, first, dc component removal is performed to prevent the distribution of signal samples from being affected to interfere with the decision threshold of the subsequent quantization circuit; after passing through a plurality of stages of filters and samplers, the effective bit width of the digital signal is gradually accumulated and needs to be re-quantized to 2 bits by Automatic Gain Control (AGC); and accumulating and counting MAG bits by AGC logic, calculating proportional distribution in corresponding time of AGC, and under the characteristic that intermediate frequency signals removed by direct current are in Gaussian distribution, if the proportional distribution exceeds 17%, indicating that the gain is too large, generating a feedback signal to reduce the PGA gain, and otherwise, if the proportional distribution is less than 17%, increasing the PGA gain.
Optionally, the transfer function of the filter is a conjugate of a signal spectrum, the filter must be perfectly matched with the spectrum of the input signal, and assuming that the sample sequence of the input signal is represented by s (n), the output of the matched filter is represented as:
Figure BDA0003532449560000041
in order to ensure that the accumulated sampling point sequence still contains frequency information, the sequence stripped by the pseudo code is only accumulated in a segmented manner, and thus, the accumulated sum of the parts is subjected to spectrum analysis by using FFT; consider the big dipper intermediate frequency data sampling sequence of 1ms length, note:
Figure BDA0003532449560000042
wherein T issFor the sampling interval, C is a pseudo-random code, omegaIFIs the intermediate frequency carrier frequency and is the initial phase; the value of M is 0 to N-1, which means that the number of samples in 1ms is N, and no navigation message bit jumps in a pseudo-random code period; each segment of short-time correlation integral has length of M sampling points, and T is usedpThe duration of the short-time correlation is represented, and the output of the ith short-time correlation accumulation is:
Figure BDA0003532449560000043
Figure BDA0003532449560000044
the result of the short-time correlation is a sampling interval of TpOf (a) a sequence of numbers In-jQnN ═ 1., L }, where L samples are present in the entire period, and the sequence is FFT transformed, the result is:
Figure BDA0003532449560000051
in the strong star mode, the data length corresponding to one FFT is 1ms, assuming that L is generated by short-time correlation matched filtering in total as 8 partial sums, and the number of signal points is increased to 16 by 0-complementing operation, i.e. the signal period is lengthened to 2ms, then the fundamental frequency of FFT transform
Figure BDA0003532449560000052
Namely, the capture resolution of the carrier Doppler dimension is 500Hz, and the step length is omega xL which is 4000 Hz; in the weak satellite mode, firstly, performing coherent integration on the partial sum generated by the short-time correlation matched filtering, obtaining L partial sums after further accumulation every 4ms, and then sending the partial sums into an FFT module; coherent integration only folds the sampling points with the same phase in the code period, and the frequency resolution and the step length at the moment are consistent with those in the strong star mode.
The invention has the beneficial effects that: the invention converts the search of the NH code starting point into the search of a plurality of truncated 4-bit NH code sequences, not only maintains a coherent integration strategy, but also reduces the resource consumption for completing one-time capture, and improves the capture sensitivity and the capture time. The method adopts a base band working mode of cache-playback, so in order to make up for the inconsistency of signal acquisition parameters and real-time signal parameters caused by high dynamics, the method utilizes an EPL correlator of a tracking channel to slide the pseudo code phase of a signal so as to achieve the aim of successfully pulling the signal under the high dynamics.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.
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For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a flow chart of the present invention.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.
Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.
Please refer to fig. 1, which shows a high dynamic and high sensitivity Beidou D1 signal capturing method.
(1) Determining a time window TDCAccumulating the digital Intermediate Frequency (IF) signals sampled by an analog-to-digital converter (ADC) within a time window, averaging at the end of the window to obtain a DC component of the IF signal, and subtracting the DC component from a subsequent incoming IF signal, wherein TDCIs taken as value of 2nFs, fs is the 60MHz intermediate frequency signal sampling rate.
(2) The signal is multiplied by a carrier signal output by a local oscillator through a frequency mixer to obtain a zero intermediate frequency I/Q signal.
(3) The Beidou D1 signal is half-band (Halfband) filtered to remove noise components in the signal. Taking the B1I signal as an example, the zero intermediate frequency signal is down-sampled by 2 times after being filtered by a first half band (Halfband), then down-sampled by 2 times after being filtered by a second half band, and then down-sampled by 3 times after being filtered by the first half band, so as to finally obtain the zero intermediate frequency signal with fs/12 being 5 MHz.
(4) After passing through the plurality of stages of half-band filters and samplers in the step (3), the effective bit width of the signal is gradually accumulated, and a new time window T is set for reducing the complexity of subsequent operationQAnd carrying out probability distribution calculation of the sampling values in the window. Due to the symmetry of normal distribution, only the probability distribution of the +1 sampling point and the +3 sampling point needs to be considered, if the +1 point is more than twice of the +3 point, the threshold is reduced, otherwise, the threshold is improved, and the sampling point value always presents normal distribution.
(5) The quantized signal is down-sampled to twice the pseudo code rate and passed to the acquisition circuit.
(6) The acquisition circuit finishes signal acquisition in a mode of alternating a strong star mode and a weak star mode. Coherent integration is not performed on the signal in the strong star mode (i.e., coherent integration time is 1ms), and the effect of NH code modulation is negligible at this time. The coherent integration time of the signal in weak star mode is chosen to be 4 ms. Specifically, the basic flow of signal acquisition in the strong star mode is
Continuous buffering- > carrier stripping- > short-time correlation matched filtering- > spectral analysis (FFT) - > non-coherent integration and energy decision,
the basic flow of signal acquisition in the weak satellite mode is as follows:
fixed-point discontinuous caching- > carrier stripping- > short-time correlation matched filtering- > NH code stripping- > coherent integration- > spectrum analysis (FFT) - > non-coherent integration and energy judgment- > Doppler and pseudo-code compensation.
(7) Capturing the D1 signal in a strong star mode, specifically, storing the data entering the capturing circuit in the step (5) to a data capacity VAIn cache A, VAShould equal the number of samples required to complete a signal acquisition in the strong star mode, 2 x N (T)noncoh+1), N is the number of chips in the pseudo-code period, TnoncohIs the non-coherent integration time. At the same time, storing the data in the step (5) to the data capacity V in parallelBIn cache B of (1), VBShould equal the number of samples required to complete signal acquisition in the weak star mode 2 x N (T)coh*Tnoncoh+1), N is the number of chips in the pseudo-code period, Tcoh4 is the number of coherent integrations in the weak star mode, TnoncohIs the non-coherent integration time.
(8) In particular, cache B is only paired with [ T ]0+K*TD1,T0+K*TD1+4]Buffering signal points in time intervals, i.e. fixed-point discontinuous buffering as described above, where TD120 navigation data period of D1 signal, T0For the starting time of the cache, K is a non-negative integer and has a value range of 0-K < TnoncohThe units are milliseconds.
(9) Because the time for the cache A to finish storing all the sampling points is obviously shorter than that of the cache B, after the cache A finishes storing, the capturing engine writes in a PRN (pseudo random number) to be searched, and the capturing circuit generates a pseudo code according to the PRN. After the pseudo code is generated, the circuit works at the working frequency facqAnd taking out the data in the buffer for playback.
(10) Firstly, carrying out carrier stripping on signals according to a theoretical intermediate frequency value of a radio frequency chip to obtain I, Q two paths of zero intermediate frequency signals.
(11) And transmitting the playback signal point into an N-bit shift register in the matched filtering module, wherein N is equal to the value of twice the code length of the signal. When the number of the playback signal points is more than or equal to N, carrying out short-time correlation matched filtering on the signals according to the pseudo code sequence generated in the step (10) to obtain 2KA short timeCorrelation sum, where K is the radix of the FFT. The coherent integration time is 1ms under the strong satellite mode, and the algorithm is not influenced by NH code modulation, so that the interference ratio of 2 is increasedKThe short-time correlation sum is passed directly to the FFT block.
(12) 2 in the step (11)KFFT conversion is carried out on the short-time correlation sum to obtain 2 of the frequency domainKAnd each signal segment is subjected to cache operation, and after the next correlation operation is finished, the new signal segment accumulates cache results. The accumulated times reach TnoncohThen, the average value E of the incoherent integral energy of N code phases is obtainedmeanTo find the maximum integral energy value EmaxThe ratio Emax/EmeanAnd comparing the energy with a preset threshold value to judge the energy.
(13) Because of the limitation of hardware resources, the satellite PRN dimension is necessarily serial search, and therefore, after the current satellite is captured, the playback V is replayed againAAnd (5) the stored sampling points are stored, and the steps (9) to (12) are repeated, and the process of capturing the next star is entered.
(14) Starting a tracking channel for the successfully captured satellite, and reading the dump timestamp T returned after the channel is integrated for the first timedumpCombined with time stamp T released when signal buffering is finishedstoreAnd calculating the code starting point of the signal.
After BDS satellites on the searching list complete one round of acquisition in the strong star mode, V is usedBThe time for storing the sample points required by one round of capture is long, and V needs to be detectedBWhether or not to store to a specified capacity, e.g. VBIf not enough samples are stored, the strong star pattern is repeated to capture signals: will VAAnd (4) taking out the new sample point and repeating the steps (9) to (14). Otherwise, the weak star mode is turned on to capture the D1 signal.
(15) When V isBThe amount of buffered data reaches 2 × N (T)coh*Tnoncoh+1), the weak star capture mode is turned on. The first step of the weak star mode is still carrier stripping and short-time correlation matched filtering, so the steps (9), (10) and (11) are repeated, after the short-time correlation sum of the corresponding data of 1ms is generated, because the coherent integration time is more than 1ms, the influence of NH code modulation is not negligible, and therefore the correlation sum needs to be stripped by NH codesAnd (5) separating.
(16) Specifically, since the signals used by two adjacent rounds of coherent integration are discontinuous in time and are all located at fixed phases in the D1 data period, there is no need to search for the NH code phase by using the method of sliding code starting point, and only the following 8 NH code combinations are used to perform NH code stripping on the correlation sum: {1,1,1,1}, { -1,1,1,1}, {1, -1,1,1}, {1,1, -1,1}, {1,1,1, -1}, { -1, -1,1,1}, {1, -1,1, -1}, {1, -1, -1,1}, and {1 }.
(17) Storing the stripped coherent sum of NH code into a buffer VC。VCThe accumulated data stored in the buffer and NH stripping data output by the next short-time correlation filter are subjected to coherent accumulation operation until the coherent times reach NcohUntil now. The resulting coherent sum is passed to the FFT module.
(18) And (5) repeating the steps (11) to (13) until all the satellites on the searching list complete signal acquisition in a weak satellite mode.
(19) Due to the weak star mode, VBThe time span of the buffer signal is obviously lengthened, and under high dynamic condition, the difference value between the pseudo code phase and Doppler value searched when the signal is successfully captured and the actual code phase and Doppler value when the tracking channel is opened exceeds the loop tracking range with a certain probability. Therefore, a window with the length of 1.5 chips needs to be set to slide the pseudo code phase of the signal by further utilizing the three EPL integral values returned by the baseband. The number of sliding is M, and the value of M is (dT a f)code/fcarrier) 1.5, a is the calculated Doppler Change Rate for the receiver PVT (assuming the receiver is in a position fix state at this time), fcodeFor the pseudo-code frequency of the signal, fcarrierFor the signal carrier frequency, dT ═ Tdump-TstoreThe time span from acquisition to tracking described in step (14).
(20) And after the M times of pseudo code sliding is finished, reading the integral energy values corresponding to the 3 × M different pseudo code phases stored in the buffer, finding the code phase corresponding to the maximum energy, and sliding the signal pseudo code to the position by the configuration baseband. And in the same way, sliding is carried out in the Doppler dimension of the signal, and the real-time Doppler frequency of the signal is found according to the returned integral energy value. After the code phase and the Doppler are reconfigured, the signal is successfully switched to the tracking state.
The invention provides a high-dynamic high-sensitivity Beidou D1 signal capturing method, which comprises the steps of firstly removing direct current components of intermediate frequency signals transmitted by a radio frequency chip ADC (analog to digital converter) as shown in figure 1, and preventing signal sample distribution from being influenced to interfere with a decision threshold of a subsequent quantization circuit. After several stages of filters and samplers, the effective bit width of the digital signal will gradually accumulate and need to be re-quantized to 2 bits through Automatic Gain Control (AGC). And accumulating and counting MAG bits by AGC logic, calculating proportional distribution in corresponding time of AGC, and generating a feedback signal to reduce PGA gain if the proportional distribution exceeds 17% and the gain is too large under the characteristic that intermediate frequency signals removed by direct current are in Gaussian distribution, otherwise, increasing the PGA gain if the proportional distribution is less than 17%.
TABLE 12 sample distribution of bits
Figure BDA0003532449560000081
A half-band filter is a special FIR filter with an even number of orders and an odd length. Except for the intermediate value of 0.5, the coefficients of the filter are all 0 in the other even numbered coefficients (so that multiplication and addition operations during filtering are greatly saved).
Taking the beidou B1I signal as an example, the filter parameters are designed according to the effective signal bandwidth as shown in table 2.
TABLE 2 Filter parameters
Sampling rate Bandwidth of Transition zone Stop band attenuation
60MHz 4MHz 28MHz 65dB
The related matched filtering adopted by the method is a linear filter with the maximum ratio of the instantaneous power of the signal to the average power of the noise under the white noise condition, and the related matched filtering is an optimal filter. The transfer function of the matched filter is the conjugate of the signal spectrum, so the matched filter must be perfectly matched to the spectrum of the input signal, and assuming that the sample sequence of the input signal is represented by s (n), the output of the matched filter can be represented as:
Figure BDA0003532449560000091
in order to ensure that the accumulated sample point sequence still contains frequency information, the sequence stripped by the pseudo code is only accumulated in a segmented mode, and therefore the partial accumulated sum can be subjected to spectrum analysis through FFT. Consider the big dipper intermediate frequency data sampling sequence of 1ms length, can record as:
Figure BDA0003532449560000092
wherein T issFor the sampling interval, C is a pseudo-random code, omegaIFIs the intermediate frequency carrier frequency and is the initial phase. The value of M is 0 to N-1, which means that the number of samples in 1ms is N, and the navigation message bit item is ignored because no navigation message bit jumps in one pseudo random code period. Each short correlation integral has a length of M samples, here TpThe duration of the short-time correlation is represented, and the output of the ith short-time correlation accumulation is:
Figure BDA0003532449560000093
Figure BDA0003532449560000094
the result of the short-time correlation is a sampling interval of TpOf (a) a sequence of numbers In-jQnN ═ 1., L }, where L samples are present in the entire period, and the sequence is FFT transformed, the result is:
Figure BDA0003532449560000095
in the strong star mode, the data length corresponding to one FFT is 1ms, assuming that L is generated by short-time correlation matched filtering and is equal to 8 partial sums, and the number of signal points is increased to 16 by 0-complementing operation (i.e. the signal period is lengthened to 2ms), the fundamental frequency of FFT is changed
Figure BDA0003532449560000096
I.e. the acquisition resolution in the carrier doppler dimension is 500Hz and the step size is ω × L4000 Hz. In the weak star mode, coherent integration is performed on the partial sum generated by the short-time correlation matched filtering, L partial sums after further accumulation are obtained every 4ms, and then the partial sums are sent to the FFT module. Because coherent integration only folds the same phase of sampling points in the code period, the frequency resolution and step length at this time are both consistent with those in the strong star mode.
Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.

Claims (4)

1. A Beidou D1 signal capturing method with high dynamic and high sensitivity is characterized by comprising the following steps: the method comprises the following steps:
s1: determining a time window TDCAccumulating the digital intermediate frequency signals IF sampled by the analog-to-digital conversion ADC in a time window, calculating the average value at the end of the window to obtain the direct current component of the intermediate frequency signals, and subtracting the component from the subsequently transmitted intermediate frequency signals, wherein TDCIs taken as value of 2nFs and fs are 60MHz intermediate frequency signal sampling rates;
s2: multiplying the signal by a carrier signal output by a local oscillator through a frequency mixer to obtain a zero intermediate frequency I/Q signal;
s3: performing half-band filtering on the Beidou D1 signal to remove a noise component in the signal; for the B1I signal, 2 times down-sampling is carried out on the zero intermediate frequency signal after the zero intermediate frequency signal passes through first-stage half-band filtering, then 2 times down-sampling is carried out on the zero intermediate frequency signal after the zero intermediate frequency signal passes through second-stage half-band filtering, 3 times down-sampling is carried out on the intermediate frequency signal after the zero intermediate frequency signal passes through the first-stage half-band filtering, and finally the zero intermediate frequency signal with fs/12 being 5MHz is obtained;
s4: after several stages of half-band filters and samplers in step S3, the effective bit width of the signal is gradually accumulated, and a new time window T is set to reduce the complexity of subsequent operationsQCalculating the probability distribution of the sampling values in a window; due to normal distribution symmetry, only probability distribution of +1 sampling points and +3 sampling points is considered, if the number of +1 points is more than twice of the number of +3 points, the threshold is reduced, otherwise, the threshold is improved, and the sampling point values always present normal distribution;
s5: down-sampling the quantized signal to twice the pseudo code rate and transmitting the signal to a capturing circuit;
s6: the acquisition circuit finishes signal acquisition in a mode of alternating a strong satellite mode and a weak satellite mode; coherent integration is not carried out on the signals in the strong star mode, namely the coherent integration time is 1 ms; the coherent integration time of the signal in the weak satellite mode is selected to be 4 ms;
s7: capturing the D1 signal in the strong star mode, and storing the data entering the capturing circuit in the step S5 until the data capacity is VAIn cache A, VAThe value of (a) is equal to the number of sampling points 2 × N (T) required for completing one signal acquisition in the strong star modenoncoh+1), N being the number of chips in the pseudo code period, TnoncohIs a non-coherent integration time; at the same time, the data in step S5 is stored in parallel to a data capacity of VBIn cache B of (1), VBIs equal to the number of samples required to complete signal acquisition in the weak star mode of one time, 2 x N (T)coh*Tnoncoh+1), N being the number of chips in the pseudo code period, Tcoh4 is the coherent integration number in the weak star mode, TnoncohIs a non-coherent integration time;
s8: cache B is only paired with [ T ]0+K*TD1,T0+K*TD1+4]Buffering signal points in time intervals, i.e. fixed-point discontinuous buffering as described above, where TD120 navigation data period of D1 signal, T0For the starting time of the cache, K is a non-negative integer and has a value range of 0-K < TnoncohThe units are milliseconds;
s9: the time for finishing storing all the sampling points in the cache A is obviously shorter than that in the cache B, after the cache A finishes storing, the capture engine writes a PRN (pseudo random number) to be searched, and the capture circuit generates a pseudo code according to the PRN; after the pseudo code is generated, the circuit works at the working frequency facqTaking out the data in the cache for playback;
s10: firstly, carrying out carrier stripping on signals according to a theoretical intermediate frequency value of a radio frequency chip to obtain I, Q two paths of zero intermediate frequency signals;
s11: transmitting the playback signal point into an N-bit shift register in a matched filtering module, wherein N is equal to a value which is twice the code length of the signal; when the count of the playback signal points is greater than or equal to N, the signal is subjected to short-time correlation matched filtering according to the pseudo code sequence generated in step S10 to obtain 2KA short-time correlation sum, where K is the radix of the FFT; because the coherent integration time is 1ms under the strong satellite mode, the algorithm is not influenced by NH code modulation, and 2 is usedKThe short-time correlation sum is directly transmitted to an FFT module;
s12: 2 in step S11KFFT conversion is carried out on the short-time correlation sum to obtain 2 of the frequency domainKThe signal segments are subjected to cache operation, and after the next correlation operation is finished, the new signal segments accumulate cache results; the accumulated times reach TnoncohThen, the non-coherent integration energy of N code phases is obtainedAverage value of quantity EmeanTo find the maximum integral energy value EmaxThe ratio Emax/EmeanComparing with a preset threshold value to judge the energy;
s13: limited by hardware resources, the satellite PRN dimension must be serial search, and after the current satellite is captured, the playback V is repeatedAThe step S9-S12 is repeated, and the next star is captured;
s14: starting a tracking channel for the successfully captured satellite, and reading the dump timestamp T returned after the channel is integrated for the first timedumpCombined with time stamp T released when signal buffering is finishedstoreCalculating the code starting point of the signal;
after BDS satellites on the searching list complete one round of acquisition in the strong star mode, V is usedBThe time for storing the sample points required by one round of capture is long, and V needs to be detectedBWhether or not to store to a specified capacity, e.g. VBIf not storing enough sampling points, repeating the strong star mode to capture signals: will VAThe new sampling point in (1) is taken out, and the steps S9 to S14 are repeated; otherwise, starting a weak star mode to capture the D1 signal;
s15: when V isBThe amount of buffered data reaches 2 × N (T)coh*Tnoncoh+1), starting a weak satellite capturing mode; the first step of the weak satellite mode is still carrier stripping and short-time correlation matched filtering, the steps S9, S10 and S11 are repeated, after the short-time correlation sum of the corresponding data of 1ms is generated, because the coherent integration time is more than 1ms, the influence of NH code modulation cannot be ignored, and the NH code stripping is carried out on the correlation sum;
s16: signals used by two adjacent rounds of coherent integration are discontinuous in time and are all located at fixed phases in a D1 data period, the NH code phase is not required to be searched by a method of sliding code starting points, and only the following 8 NH code combinations are used for carrying out correlation and NH code stripping: {1,1,1,1}, { -1,1,1,1}, {1, -1,1,1}, {1,1, -1,1}, {1,1,1, -1}, { -1, -1,1,1}, {1, -1,1, -1}, {1, -1, -1,1 };
s17: storing the stripped coherent sum of NH code into a buffer VC；VCThe accumulated data stored in the buffer will be filtered with the next short-time correlationPerforming coherent accumulation operation on the NH stripping data output by the wave filter until the coherent times reach NcohUntil the end; passing the generated coherent sum to an FFT module;
s18: repeating the steps S11-S13 until all satellites on the satellite searching list complete signal acquisition in the weak satellite mode;
s19: due to the weak star mode, VBThe time span of the buffer signal is obviously lengthened, and under high dynamic condition, the difference value between the pseudo code phase and Doppler value searched when the signal is successfully captured and the actual code phase and Doppler value when the tracking channel is opened exceeds the loop tracking range with a certain probability; setting a window with the length of 1.5 chips to slide the pseudo code phase of the signal by utilizing the EPL three-way integral value returned by the baseband; the number of sliding is M, and the value of M is (dT a f)code/fcarrier) 1.5, a is the calculated Doppler change rate of the receiver PVT, assuming the receiver is in the positioning state, fcodeFor the pseudo-code frequency of the signal, fcarrierFor the signal carrier frequency, dT ═ Tdump-TstoreThe time span from acquisition to tracking described in step S14;
s20: after the M times of pseudo code sliding is finished, reading the integrated energy values corresponding to 3 x M different buffered pseudo code phases, finding the code phase corresponding to the maximum energy, and sliding the signal pseudo code to the position by the configuration baseband; sliding in the Doppler dimension of the signal, and finding out the real-time Doppler frequency of the signal according to the returned integral energy value; after the code phase and the Doppler are reconfigured, the signal is successfully switched to the tracking state.
2. The high-dynamic high-sensitivity Beidou D1 signal capturing method according to claim 1, characterized in that: in the strong star mode, the signal capturing process is as follows:
continuous caching, carrier stripping, short-time correlation matched filtering, spectrum analysis, non-coherent integration and energy judgment;
in the weak satellite mode, the basic flow of signal acquisition is as follows:
fixed-point discontinuous caching, carrier stripping, short-time correlation matched filtering, NH code stripping, coherent integration, spectrum analysis, non-coherent integration and energy judgment, Doppler and pseudo code compensation.
3. The high-dynamic high-sensitivity Beidou D1 signal capturing method according to claim 1, characterized in that: for the intermediate frequency signal transmitted by the radio frequency chip ADC, firstly, removing a direct current component to prevent the distribution of signal samples from being influenced to interfere with a decision threshold of a subsequent quantization circuit; after passing through a plurality of stages of filters and samplers, the effective bit width of the digital signal is gradually accumulated and needs to be re-quantized to 2 bits by Automatic Gain Control (AGC); and accumulating and counting the MAG bits of the amplitude value by the AGC logic, calculating the proportional distribution within the corresponding time of the AGC, and under the characteristic that the intermediate frequency signals removed by the direct current are in Gaussian distribution, if the proportional distribution exceeds 17 percent, indicating that the gain is too large, generating a feedback signal to reduce the PGA gain of the programmable gain amplifier, and otherwise, if the proportional distribution is less than 17 percent, increasing the PGA gain.
4. The high-dynamic high-sensitivity Beidou D1 signal capturing method according to claim 1, characterized in that: the transfer function of the filter is the conjugate of the signal spectrum, the filter must be perfectly matched with the spectrum of the input signal, and assuming that the sample sequence of the input signal is represented by s (n), the output of the matched filter is represented as:
Figure FDA0003532449550000031
in order to ensure that the accumulated sample point sequence still contains frequency information, the sequence after the pseudo code stripping is only accumulated in a segmented way, so that the FFT is used for carrying out frequency spectrum analysis on the accumulated sum of the parts; consider the big dipper intermediate frequency data sampling sequence of 1ms length, note:
Figure FDA0003532449550000032
wherein T issFor the sampling interval, C is a pseudo-random code, omegaIFIs the intermediate frequency carrier frequency and is the initial phase; the value of M is 0 to N-1, which means that the number of samples in 1ms is N, and no navigation message bit jumps in a pseudo-random code period; each segment of short-time correlation integral has length of M sampling points, and T is usedpThe duration of the short-time correlation is represented, and the output of the ith short-time correlation accumulation is:
Figure FDA0003532449550000041
Figure FDA0003532449550000042
the result of the short-time correlation is a sampling interval of TpOf (a) a sequence of numbers In-jQnN is 1,.. L }, where L samples are in all periods, and the sequence is FFT-transformed, the result is:
Figure FDA0003532449550000043
in the strong star mode, the data length corresponding to one FFT is 1ms, assuming that L is generated by short-time correlation matched filtering in total as 8 partial sums, and the number of signal points is increased to 16 by 0-complementing operation, i.e. the signal period is lengthened to 2ms, then the fundamental frequency of FFT transform
Figure FDA0003532449550000044
Namely, the capture resolution of the carrier Doppler dimension is 500Hz, and the step length is omega multiplied by L which is 4000 Hz; in the weak satellite mode, firstly, performing coherent integration on the partial sum generated by the short-time correlation matched filtering, obtaining L partial sums after further accumulation every 4ms, and then sending the partial sums into an FFT module; coherent integration only folds the sampling points with the same phase in the code period, and the frequency resolution and the step length at the moment are consistent with those in the strong star mode.
CN202210209207.4A 2022-03-04 2022-03-04 High-dynamic high-sensitivity Beidou D1 signal capturing method Pending CN114563803A (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115276712A (en) * 2022-07-22 2022-11-01 山东航天电子技术研究所 Low-complexity burst spread spectrum signal capturing method
CN115657093A (en) * 2022-12-29 2023-01-31 成都奇芯微电子有限公司 Method based on captured data storage

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115276712A (en) * 2022-07-22 2022-11-01 山东航天电子技术研究所 Low-complexity burst spread spectrum signal capturing method
CN115657093A (en) * 2022-12-29 2023-01-31 成都奇芯微电子有限公司 Method based on captured data storage

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