CN102998682A - Data demodulation method of global positioning system (GPS) receiver for weak satellite signals - Google Patents
Data demodulation method of global positioning system (GPS) receiver for weak satellite signals Download PDFInfo
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Abstract
The invention discloses a data demodulation method of a global positioning system (GPS) receiver for weak satellite signals. The data demodulation method comprises a first step of performing 20ms coherent integration on navigation data from the initial positions of each bit of data of navigation messages in the navigation data for the bit-synchronization-finished navigation data of the weak satellite signals, and only retaining the latest K coherent integration results; a second step of finding the positions of the navigation data corresponding to frame synchronization codes of the navigation messages of the weak satellite signals by utilizing a time interval of -20ms-20ms of reception time of frame synchronization codes of navigation messages of strong satellite signals; and a third step of utilizing fast Fourier transform (FFT) to judge polarity of each bit of data after the frame synchronization codes of the navigation messages of the weak satellite signals. According to the data demodulation method, the positions of the navigation data corresponding to the frame synchronization codes of the weak satellite signals are found rapidly by utilizing N point FFT under the assistance of the strong satellite signals so as to serve as initial values, the coherent integration time is prolonged to 180ms by utilizing M point FFT, and influence of residual frequency difference is eliminated.
Description
Technical field
The application relates to a kind of signal processing method of GPS receiver, particularly relates to the GPS receiver with the navigation message demodulation method out in the weak satellite-signal.
Background technology
Gps system is the most a kind of GPS (Global Position System) of current application, and the GPS receiver is determined self position at the earth's surface;on the face of the globe by receiving gps satellite signal.
Gps satellite signal comprises L1 carrier wave and L2 carrier wave.The L1 carrier wave raises and is shaped with C/A code (Coarse/AcquisitionCode, also claim thick code, catch code) and navigation message (Navigation Messages also claims Data Message, numeric data code, D code), what adopt is two-phase PSK (BPSK) technology, belongs to a kind of of phase-modulation.When modulation signal adopted phase modulation method to be modulated to carrier wave, the speed that modulating signal phase changes was commonly referred to bit rate.The C/A code is that bit rate is the bi-phase modulated signal of 1.023MHz, and the cycle is 1ms, and per cycle comprises 1023 (also claiming code element).Navigation message is that bit rate is the bi-phase modulated signal of 50Hz, and its each duration is 20ms, is equivalent to the cycle of 20 C/A codes.Navigation message is added on the C/A code in mould two additions (being XOR) mode first, and the two combinational code is modulated on the L1 carrier wave again.
The work of GPS receiver comprises at least catches and follows the tracks of two.The antenna reception of GPS receiver arrives such as high frequency signals such as L1 carrier waves, through digital intermediate frequency (IF, the Intermediate Frequency) signal that obtains after down coversion, the analog to digital conversion, as the input of trapping module.In order to allow tracking module successfully follow the tracks of gps satellite signal, the carrier wave of the inner institute of GPS receiver initial replication and C/A coded signal must with receive signal coincide to a certain extent (leading in the scope of tracking module); Otherwise tracking module usually can be to the signal losing lock.Therefore, the GPS receiver is before commencing signal is followed the tracks of, at first need to estimate carrier frequency and these two parameters of C/A code phase that receive signal, then according to these signal parameter estimated value initialization tracking modules, helping tracking to received signal, and the purpose of signal capture is for the carrier frequency of obtaining all visible gps satellite signals and the guestimate value of C/A code phase.
See also Fig. 1, this is the schematic diagram of the tracking module of existing a kind of GPS receiver.Wherein have two phaselocked loops.By carrier wave ring Discr., carrier wave ring wave filter, carrier wave NCO(digital controlled oscillator) etc. the first phaselocked loop of formation be called the carrier wave ring, be used for following the tracks of the carrier frequency of intermediate-freuqncy signal.The second phaselocked loop that is made of code ring Discr., Loop filter, C/A code generator etc. is called the code ring, is used for following the tracks of the C/A code.Intermediate-freuqncy signal is at first by peeling off carrier wave with the mixing of carrier frequency, again by peeling off the C/A code with relevant (Correlation) computing of instantaneous code.Correlated results i
El, i
p, q
ELAnd q
pBehind integration zero clearing device, export respectively coherent integration value I
E1, I
p, Q
ELAnd Q
pIf the required coherent integration time of tracking module is 1ms, tracking module also demodulates the navigation data that speed is 1000Hz when realizing tracking so.Consider that each data in the navigation message continue 20ms, then will solve a navigation message data bit that 20ms is wide from the data of per 20 1ms.Before bit synchronization, receiver does not also know which 20 1ms data in succession belongs to same navigation message data bit, bit synchronous purpose will reasonably be divided into per 20 one group with a series of 1ms data by data analysis exactly, then these 20 1ms data are carried out coherent integration, solve the navigation message data by the polarity of integral result is judged; Data after the demodulation are carried out frame synchronization, determining the subframe reference position, thereby with demodulation data out correct be divided into 30 one group word, finally obtain navigation message.
Among Fig. 1, what integration zero clearing device was exported is navigation data, and namely intermediate-freuqncy signal peels off carrier wave and C/A code signal afterwards, and speed is 1000Hz.Described integration zero clearing device is equivalent to low-pass filter, and with the high fdrequency component filtering that produces in the optical mixing process, surplus low frequency component only is in the situation that without remaining frequency difference, the result of integration zero clearing device is roughly the fixing square-wave signal of amplitude as shown in Figure 2.Bit synchronization is exactly to find the starting point of each data of navigation message from navigation data, and position polarity judges to be exactly to judge that the polarity of each data of navigation message is 1 or 0.The summation that bit synchronization and position polarity are judged is used for exactly changing the navigation data of 1000Hz into 50bps, and begins 20 continuous values are replaced with a value from the starting point of each data.Fig. 3 is the partial schematic diagram that horizontal ordinate amplifies, ordinate remains unchanged with Fig. 2.As shown in Figure 3, it is a data in the navigation message between 92～111ms, 20 data of this 20ms are added up, if 20 data sum＞0 then judge that this bit data of navigation message is 1, if 20 data sum＜0 then judge that this bit data of navigation message is 0.The said data demodulates of the application just refers to that continuous 20 the 1ms data in the navigation data are carried out a polarity to be judged.
Usually with received signal strength C/No(carrier-to-noise ratio) at 50dB-Hz to the strong satellite-signal that is called between the 32dB-Hz, with received signal strength less than weak satellite-signal of being called of 26dB-Hz.The output of integration zero clearing device shown in Figure 2 is only applicable to judge each data polarity of the navigation message in the strong satellite-signal.For weak satellite-signal, can there be frequency difference between the carrier wave that tracking module is rebuild in this locality and the carrier wave of intermediate-freuqncy signal, the existence of remaining frequency difference can affect the judgement of each data polarity of navigation message.See also Fig. 4, this is to be under the condition of 2Hz in remaining frequency difference, and the Output rusults of integration zero clearing device is roughly the square-wave signal that modulation has gradual sine wave, and the speed of square-wave signal is 1000Hz, and sinusoidal wave speed is remaining frequency difference.Owing to being subject to the impact of remaining frequency difference, begin with the starting point of each data of navigation message, calculate 20 data sums of 20ms and judge that the polarity of each data just may be wrong.
Summary of the invention
The application's technical matters to be solved provides a kind of GPS receiver at the auxiliary lower data demodulation method to weak satellite-signal of strong satellite-signal.
For solving the problems of the technologies described above, the application GPS receiver comprises the steps: the data demodulation method of weak satellite-signal
The 1st step, to weak satellite-signal, finish bit synchronous navigation data, navigation data is begun to carry out the coherent integration (coherent integration) of 20ms from the reference position of each data navigation data of navigation message, and only keep up-to-date K coherent integration result, K is 〉=9 natural number;
The 2nd step, utilize strong satellite-signal navigation message frame synchronization code time of reception-time interval of 20ms～20ms in, find the position of navigation data corresponding to the frame synchronization code of navigation message of weak satellite-signal;
In the 3rd step, utilize the FFT(Fast Fourier Transform (FFT)) judge the polarity of each data after the frame synchronization code of navigation message of weak satellite-signal.
Described the 2nd step of method specifically comprises:
The 2.1st step, when the GPS receiver demodulates frame synchronization code in the navigation message of any strong satellite-signal, immediately with among K coherent integration result in the 1st step by the end of front 8 coherent integration results (i.e. 8 up-to-date coherent integration results except the coherent integration result of current 20ms) of the coherent integration result of a upper 20ms according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the first amplitude;
The 2.2nd step, also with among K coherent integration result in the 1st step by the end of the correlation integral result's of current 20ms front 8 (they being 8 up-to-date coherent integration results) according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the second amplitude.
The 2.3rd step, when the coherent integration of newly advancing a up-to-date 20ms among the 1st step described K coherent integration result as a result the time, with among K coherent integration result in the 1st step by the end of the coherent integration result's of the up-to-date 20ms that enters front 8 (they being 8 up-to-date coherent integration results) according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the 3rd amplitude;
Described N is 2 power, and N 〉=8;
The 2.4th step, find those maximum in the first amplitude, the second amplitude, the 3rd amplitude corresponding 8 groups of coherent integration results, 8 bit data of corresponding navigation message are exactly 8 frame synchronization codes that come from the navigation message of weak satellite-signal after these 8 groups of as a result demodulation of coherent integration.
Described the 3rd step of method specifically comprises:
The 3.1st step, polarity for each data after 8 frame synchronization codes of the navigation message of described weak satellite-signal is judged, all with 8 coherent integration results of 8 bit data before the data bit of polarity to be judged according to multiply by respectively symbol corresponding to this 8 bit data by experienced new order, treat 20ms coherent integration result corresponding to data bit who judges polarity and multiply by 1, replenish again M-9 individual 0, then be M point FFT, FFT result calculated maximum amplitude be called the 4th amplitude;
The 3.2nd step, 8 coherent integration results of 8 bit data before the data bit that again will polarity be judged are according to multiply by respectively symbol corresponding to this 8 bit data by experienced new order, treat 20ms coherent integration result corresponding to data bit who judges polarity and multiply by-1, replenish again M-9 individual 0, then be M point FFT, FFT result calculated maximum amplitude be called the 5th amplitude;
If 8 bit data before the data bit of polarity described to be judged are 1, then its corresponding symbol is 1; If be 0, then its corresponding symbol is-1;
Described M is 2 power, and M＞9;
The 3.3rd step is if the 4th amplitude 〉=the 5th amplitude judges that then the data bit of polarity to be judged is 1;
If the 4th amplitude＜the 5th amplitude judges that then the data bit of polarity to be judged is 0.
The application has realized assisting of strong satellite-signal lower, utilize N point FFT to find fast the position of navigation data corresponding to the frame synchronization code of weak satellite-signal, with this as initial value, recycling M point FFT has arrived 180ms with the coherent integration time lengthening, thereby improved each data of the navigation message of weak satellite-signal are carried out the polarity Accuracy of Judgement, eliminated the impact of remaining frequency difference.
Description of drawings
Fig. 1 is the structural representation of the tracking module in the GPS receiver;
Fig. 2 is the output waveform (in the strong satellite-signal situation) of integration zero clearing device among Fig. 1;
Fig. 3 is the partial schematic diagram that horizontal ordinate amplifies, ordinate is constant of Fig. 2;
Fig. 4 is the output waveform (having in the remaining frequency difference situation, for demonstration directly perceived, take strong satellite-signal as example) of integration zero clearing device among Fig. 1;
Fig. 5 is the earth and gps satellite track schematic diagram;
Fig. 6 is that the application GPS receiver is to the process flow diagram of the data demodulation method of weak satellite-signal.
Embodiment
Navigation message outwards sends take frame (frame) as unit, and each frame length is 1500 (bit).The complete navigation message of one cover is comprised of 25 frames, amounts to 37500.Each frame is comprised of 5 subframes (subframe) again, and each subframe lengths is 300.Front 8 always 10001001 of each subframe are called frame synchronization code.Because the emission rate of navigation message is 50bps, so frame synchronization code occurred every the 6s cycle.
If be decimal number 1 with the symbol definition of binary number " 1 ", the symbol definition of binary number " 0 " is decimal number-1, and the symbol of frame synchronization code is exactly 1 ,-1 ,-1 ,-1,1 ,-1,1,1 so.
See also Fig. 5, wherein the O point represents the earth's core, A point expression GPS receiver position, and the expression of S point and GPS receiver are positioned at the gps satellite of same level, and the expression of B point is positioned at the gps satellite of the zenith of GPS receiver.According to general elliposoidal earth model, the distance between S point and the A point is 25785km, and the transmission time of gps satellite signal is 85.95ms.Distance between B point and the A point is 20192km, and the transmission time of gps satellite signal is 67.31ms.This maximum time difference that shows the gps satellite signal that same GPS receiver reception is different is 18.64ms.Therefore, after the frame synchronization code of the navigation message that obtains strong satellite-signal, only need in the time range of-20ms～20ms, calibrate weak satellite-signal, just can obtain the position of the navigation data of weak satellite-signal corresponding to the frame synchronization code of navigation message.
See also Fig. 6, the application GPS receiver comprises the steps: the data demodulation method of weak satellite-signal
Initial situation: the GPS receiver is by the weak satellite-signal of antenna reception to high frequency, by its down coversion and/or sampling are obtained intermediate-freuqncy signal, by catching, follow the tracks of the navigation data that obtains 1000Hz, then obtain the starting point of each the navigation message data in the navigation data by bit synchronization again.The application will do be exactly to come from weak satellite-signal, finish bit synchronous navigation data and carry out a polarity and judge.
The 1st step, to come from weak satellite-signal, finish bit synchronous navigation data, navigation data is begun to carry out the coherent integration of 20ms from the reference position of each data navigation data of navigation message, and only keep 9 up-to-date coherent integration results.Numerical value 9 is also extendible to be＞9 random natural number.Be concise explanation, the present embodiment is take preferred value 9 as example.
The computing formula of the coherent integration of described 20ms is:
The 2nd step, utilize strong satellite-signal navigation message frame synchronization code time of reception-time interval of 20ms～20ms in, find the position of navigation data corresponding to the frame synchronization code of navigation message of weak satellite-signal, each frame synchronization code of navigation message is corresponding to the navigation data of 20ms length.According to shown in Figure 5, in fact can the time of reception of the frame synchronization code of the navigation message of strong satellite-signal ± time range of 18.64ms in the frame synchronization code of the navigation message of satellite-signal a little less than the location.Because the time of each data of navigation message is kept 20ms, for ease of calculating, therefore be set as ± 20ms.
In the 3rd step, utilize the FFT(Fast Fourier Transform (FFT)) judge the polarity of each data after the frame synchronization code of navigation message of weak satellite-signal.
Described the 2nd step of method specifically comprises:
The 2.1st step, when the GPS receiver passes through bit synchronization, position polarity judgement, frame synchronization success to the navigation data that comes from any strong satellite-signal after, when again demodulating the frame synchronization code in the navigation message, immediately with among 9 coherent integration results in the 1st step older 8 according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the first amplitude.
The 2.2nd step, also with among 9 coherent integration results in the 1st step newer 8 according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 0 after the multiplied result, then be N point FFT, FFT result is calculated maximum amplitude be called the second amplitude.
Above-mentioned the 2.1st step, the 2.2nd step can carry out simultaneously, need two FFT computing units this moment; Also can successively carry out, only need a FFT computing unit this moment.The N value is preferably 64, also can be other 〉=82 power.The value of N can have influence on the frequency resolution of FFT, and N is larger, and frequency resolution is higher, and then the frequency interval is less, and result of calculation is subjected to the impact of remaining frequency difference less.When N 〉=32, frequency interval≤1.5625Hz.When N 〉=64, frequency interval≤0.78125Hz.
Above-mentioned symbol will be older or newer 8 coherent integration results and frame synchronization code multiplies each other successively, replenishes 0 again and is N point FFT, can be with removing frequency difference.This is that near the FFT result's amplitude frequency of signal place is the highest, can be understood as again to have carried out mixing and integration because the effect of FFT is exactly the frequency that finds signal.
The 2.3rd step, when the 1st step newly advance among described 9 coherent integration results up-to-date one (if 9 memory blocks are full, also mean and delete the oldest one) time, with among 9 coherent integration results in the 1st step newer 8 according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the 3rd amplitude.
The 2.4th step, find those maximum in the first amplitude, the second amplitude, the 3rd amplitude corresponding 8 groups of coherent integration results, 8 bit data of corresponding navigation message are exactly 8 frame synchronization codes that come from the navigation message of weak satellite-signal after these 8 groups of as a result demodulation of coherent integration.
The frame synchronization code 10001001 of the navigation message of each gps satellite sends simultaneously, when the GPS receiver receives certain constantly during last of the frame synchronization code in the navigation message of strong satellite-signal, according to Fig. 5 as can be known, at this moment the GPS receiver should receive at front 20ms or current 20ms or next 20ms last position of weak satellite-signal frame synchronization code.The first amplitude, the second amplitude, the 3rd amplitude just correspond respectively to front 20ms, current 20ms, next 20ms.
Last position of supposing the frame synchronization code of weak satellite-signal has arrived receiver at current 20ms, and this moment, the data of receiver 20ms coherent integration were I
P_20(8), Q
P_20(8), the data of the 20ms coherent integration that soon newly enters of next 20ms are I
P_20(9), Q
P_20(9).Suppose that remaining frequency difference is 0, then should have following two formulas to set up:
Mag2＞Mag1, Mag2＞Mag3, wherein
Mag1=|I
P_20(0)-I
P_20(1)-I
P_20(2)-I
P_20(3)+I
P_20(4)-I
P_20(5)+I
P_20(6)+I
P_20(7)+j(Q
P_20(0)-Q
P_20(1)-Q
P_20(2)-Q
P_20(3)+Q
P_20(4)-Q
P_20(5)+Q
P_20(6)+Q
P_20(7))|
Mag2=|I
P_20(1)-I
P_20(2)-I
P_20(3)-I
P_20(4)+I
P_20(5)-I
P_20(6)+I
P_20(7)+I
P_20(8)+j(Q
P_20(1)-Q
P_20(2)-Q
P_20(3)-Q
P_20(4)+Q
P_20(5)-Q
P_20(6)+Q
P_20(7)+Q
P_20(8))|
Mag3=|I
P_20(2)-I
P_20(3)-I
P_20(4)-I
P_20(5)+I
P_20(6)-I
P_20(7)+I
P_20(8)+I
P_20(9)+j(Q
P_20(2)-Q
P_20(3)-Q
P_20(4)-Q
P_20(5)+Q
P_20(6)-Q
P_20(7)+Q
P_20(8)+Q
P_20(9))|
I.e. the second amplitude＞first amplitude, the second amplitude＞the 3rd amplitude.
Described the 3rd step of method specifically comprises:
The 3.1st step, polarity for each data after 8 frame synchronization codes of the navigation message of described weak satellite-signal is judged, all with 8 coherent integration results of 8 bit data before the data bit of polarity to be judged according to multiply by respectively symbol corresponding to this 8 bit data by experienced new order, the coherent integration result who treats 20ms corresponding to the data bit of judging polarity multiply by 1, replenish again M-9 individual 0, then be M point FFT, FFT result calculated maximum amplitude be called the 4th amplitude;
The 3.2nd step, 8 coherent integration results of 8 bit data before the data bit that again will polarity be judged are according to multiply by respectively symbol corresponding to this 8 bit data by experienced new order, the coherent integration result who treats 20ms corresponding to the data bit of judging polarity multiply by-1, replenish again M-9 individual 0, then be M point FFT, FFT result calculated maximum amplitude be called the 5th amplitude;
8 bit data before the data bit of polarity described to be judged are binary number 1 if data are arranged, and then its corresponding symbol is decimal number 1; If it is binary number 0 that data are arranged, then its corresponding symbol is decimal number-1.
Above-mentioned the 3.1st step, the 3.2nd step can carry out simultaneously, need two FFT computing units this moment; Also can successively carry out, only need a FFT computing unit this moment.The M value is preferably 64, also can be other＞92 power.The value of M can have influence on the frequency resolution of FFT, and M is larger, and frequency resolution is higher, and then the frequency interval is less, and result of calculation is subjected to the impact of remaining frequency difference less.When M 〉=32, frequency interval≤1.5625Hz.When M 〉=64, frequency interval≤0.78125Hz.
The 3.3rd step is if the 4th amplitude 〉=the 5th amplitude judges that then the data bit of polarity to be judged is 1;
If the 4th amplitude＜the 5th amplitude judges that then the data bit of polarity to be judged is 0.
If there is no remaining frequency difference, then FFT result's maximum amplitude is in 0 position frequently.If there is remaining frequency difference, then FFT result's maximum amplitude is from the nearest frequency place of remaining frequency difference.The below is only take remaining frequency difference as 0 principle that illustrates such judgement: suppose that 9 navigation message data bit that gps satellite sends are respectively 1,1,1,0,1,1,0,1,1, front 8 polarity is judged, and last 1 is polarity to be judged position.The coherent integration result of 9 20ms that calculate in the receiver corresponding with these 9 navigation message data bit is I
P-20(0)～I
P-20(8), Q
P-20(0)～Q
P-20(8), then should be tied to form just like ShiShimonoseki vertical,
Mag4＞Mag5, wherein
Mag4=|I
P_20(0)+I
P_20(1)+I
P_20(2)-I
P_20(3)+I
P_20(4)+I
P_20(5)-I
P_20(6)+I
P_20(7)+I
P_20(8)+j(Q
P_20(0)+Q
P_20(1)+Q
P_20(2)-Q
P_20(3)+Q
P_20(4)+Q
P_20(5)-Q
P_20(6)+Q
P_20(7)+Q
P_20(8))|
Mag5=|I
P_20(0)+I
P_20(1)+I
P_20(2)-I
P_20(3)+I
P_20(4)+I
P_20(5)-I
P_20(6)+I
P_20(7)-I
P_20(8)+j(Q
P_20(0)+Q
P_20(1)+Q
P_20(2)-Q
P_20(3)+Q
P_20(4)+Q
P_20(5)-Q
P_20(6)+Q
P_20(7)-Q
P_20(8))|
For example, can judge like this after 8 frame synchronization codes of the navigation message that comes from weak satellite-signal followed by the polarity of first data bit:
At first, 8 frame synchronization codes of navigation message that come from weak satellite-signal with corresponding 8 coherent integration results according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, coherent integration result to 20ms corresponding to the data bit of the back to back navigation message after these 8 frame synchronization codes multiply by 1, replenish 23 0 again, then be 64 FFT, FFT result calculated maximum amplitude be called the 4th amplitude.
Again with corresponding 8 the coherent integration results of 8 frame synchronization codes of the navigation message of weak satellite-signal according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, coherent integration result to 20ms corresponding to the data bit of the navigation message after these 8 frame synchronization codes multiply by-1, replenish 23 0 again, then be 64 FFT, FFT result calculated maximum amplitude be called the 5th amplitude.
If the 4th amplitude 〉=the 5th amplitude judges that then 8 frame synchronization codes back to back data bit afterwards of the navigation message of weak satellite-signal is 1.
If the 4th amplitude＜the 5th amplitude judges that then 8 frame synchronization codes back to back data bit afterwards of the navigation message of weak satellite-signal is 0.
The application has realized assisting of strong satellite-signal lower, utilize N point FFT to find fast the position of navigation data corresponding to the frame synchronization code of weak satellite-signal, with this as initial value, recycling M point FFT method prolongs the coherent integration time of weak satellite-signal, thereby improved each data of the navigation message of weak satellite-signal are carried out the polarity Accuracy of Judgement, and eliminated the impact of remaining frequency difference.Because FFT counts seldom, so it is few to implement computational resource simple, that use.
For strong satellite-signal, the coherent integration of carrying out 10～20ms just can demodulate the polarity of each data of navigation message from navigation data.For weak satellite-signal, the coherent integration that then needs to surpass 20ms just can solve the polarity of each data of navigation message.
N point FFT formula is:
From formula, this is one and takes advantage of first then cumulative process, and take n=0 as example, following formula is frequently signal to be carried out mixing and cumulative 0, and take n=1 as example, following formula is at Fs/N frequency place signal to be carried out mixing and cumulative.Above formula be exactly with signal respectively frequency 0, Fs/N, 2Fs/N ..., locate mixing and cumulative.The result of calculation of FFT is maximum in which frequency place amplitude, and then signal frequency just near which frequency, so just can be eliminated the impact of remaining frequency difference.
The application has adopted 9 data as the input of FFT in the 3rd step, and each data is the results behind the 20ms integration, carries out being equivalent to do after the FFT computing coherent integration of 180ms, is specially adapted to the data demodulates to weak satellite-signal.
The method that the application proposes also can be used in other Global Positioning System (GPS) the demodulation for weak satellite-signal, in the receiver system such as GLONASS, the Big Dipper, Galileo etc.
Be the application's preferred embodiment only below, and be not used in restriction the application.For a person skilled in the art, the application can have various modifications and variations.All within the application's spirit and principle, any modification of doing, be equal to replacement, improvement etc., all should be included within the application's the protection domain.
Claims (8)
1. a GPS receiver is characterized in that the data demodulation method of weak satellite-signal, comprises the steps:
The 1st step, to weak satellite-signal, finish bit synchronous navigation data, from each data of navigation message navigation data reference position navigation data is begun to carry out the coherent integration of 20ms, and only keep up-to-date K coherent integration result, K is 〉=9 natural number;
The 2nd step, utilize strong satellite-signal navigation message frame synchronization code time of reception-time interval of 20ms～20ms in, find the position of navigation data corresponding to the frame synchronization code of navigation message of weak satellite-signal;
In the 3rd step, utilize FFT to judge the polarity of each data after the frame synchronization code of navigation message of weak satellite-signal.
2. GPS receiver according to claim 1 is characterized in that the data demodulation method of weak satellite-signal, and described the 2nd step of method specifically comprises:
The 2.1st step, when the GPS receiver demodulates frame synchronization code in the navigation message of any strong satellite-signal, immediately with among K coherent integration result in the 1st step except the coherent integration result of current 20ms up-to-date 8 according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the first amplitude;
The 2.2nd step, also with among K coherent integration result in the 1st step up-to-date 8 according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 0 after the multiplied result, then be N point FFT, FFT result is calculated maximum amplitude be called the second amplitude.
The 2.3rd step, when newly advancing up-to-date one among the 1st step described K coherent integration result, with among K coherent integration result in the 1st step up-to-date 8 according to multiply by respectively 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by experienced new order, to replenishing N-8 individual 0 after the multiplied result, then be N point FFT, FFT result calculated maximum amplitude be called the 3rd amplitude;
Described N is 2 power, and N 〉=8;
The 2.4th step, find those maximum in the first amplitude, the second amplitude, the 3rd amplitude corresponding 8 groups of coherent integration results, 8 bit data of corresponding navigation message are exactly 8 frame synchronization codes that come from the navigation message of weak satellite-signal after these 8 groups of as a result demodulation of coherent integration.
3. GPS receiver according to claim 1 is characterized in that the data demodulation method of weak satellite-signal, and described the 3rd step of method specifically comprises:
The 3.1st step, polarity for each data after 8 frame synchronization codes of the navigation message of described weak satellite-signal is judged, all with 8 coherent integration results of 8 bit data before the data bit of polarity to be judged according to multiply by respectively symbol corresponding to this 8 bit data by experienced new order, treat 20ms coherent integration result corresponding to data bit who judges polarity and multiply by 1, replenish again M-9 individual 0, then be M point FFT, FFT result calculated maximum amplitude be called the 4th amplitude;
The 3.2nd step, 8 coherent integration results of 8 bit data before the data bit that again will polarity be judged are according to multiply by respectively symbol corresponding to this 8 bit data by experienced new order, treat 20ms coherent integration result corresponding to data bit who judges polarity and multiply by-1, replenish again M-9 individual 0, then be M point FFT, FFT result calculated maximum amplitude be called the 5th amplitude;
If 8 bit data before the data bit of polarity described to be judged are 1, then its corresponding symbol is 1; If be 0, then its corresponding symbol is-1;
Described M is 2 power, and M＞9;
The 3.3rd step is if the 4th amplitude 〉=the 5th amplitude judges that then the data bit of polarity to be judged is 1;
If the 4th amplitude＜the 5th amplitude judges that then the data bit of polarity to be judged is 0.
4. GPS receiver according to claim 1 is characterized in that K=9 to the data demodulation method of weak satellite-signal.
5. GPS receiver according to claim 2 is characterized in that the data demodulation method of weak satellite-signal, and the value of N is larger, and frequency resolution is higher, and result of calculation is subjected to the impact of remaining frequency difference less.
6. GPS receiver according to claim 5 is characterized in that the data demodulation method of weak satellite-signal, N=32 or 64.
7. GPS receiver according to claim 3 is characterized in that the data demodulation method of weak satellite-signal, and the value of M is larger, and frequency resolution is higher, and result of calculation is subjected to the impact of remaining frequency difference less.
8. GPS receiver according to claim 7 is characterized in that the data demodulation method of weak satellite-signal, M=32 or 64.
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