CN102998682B - Data demodulation method of global positioning system (GPS) receiver for weak satellite signals - Google Patents

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

GPS is to the data demodulation method of weak satellite-signal

Technical field

The application relates to a kind of signal processing method of GPS, particularly relates to GPS by the navigation message demodulation method out in weak satellite-signal.

Background technology

Gps system is current application the most a kind of GPS (Global Position System), and GPS determines self position at the earth's surface by receiving gps satellite signal.

Gps satellite signal comprises L1 carrier wave and L2 carrier wave.L1 carrier wave raises and is shaped with C/A code (Coarse/AcquisitionCode, also thick code, catch code is claimed) and navigation message (Navigation Messages, also Data Message is claimed, numeric data code, D code), what adopt is two-phase PSK (BPSK) technology, belongs to the one of phase-modulation.When modulation signal adopts phase modulation method to be modulated to carrier wave, the speed of modulating signal phase change is commonly referred to bit rate.The bi-phase modulated signal of C/A code to be bit rate be 1.023MHz, the cycle is 1ms, and each cycle comprises 1023 (also claiming code element).The bi-phase modulated signal of navigation message to be bit rate be 50Hz, its each duration is 20ms, is equivalent to the cycle of 20 C/A codes.Navigation message is first added (i.e. XOR) mode with mould two and is added on C/A code, and the combinational code of both is modulated on L1 carrier wave again.

The work of GPS at least comprises catches and follows the tracks of two.The antenna of GPS receives the high-frequency signals such as such as L1 carrier wave, and the digital intermediate frequency obtained after down coversion, analog to digital conversion (IF, Intermediate Frequency) signal, as the input of trapping module.In order to tracking module can be allowed successfully to follow the tracks of gps satellite signal, carrier wave and the C/A coded signal of GPS inner institute initial replication must coincide to a certain extent (leading in scope at tracking module) with Received signal strength; Otherwise tracking module usually can to signal losing lock.Therefore, GPS is before commencing signal is followed the tracks of, first carrier frequency and these two parameters of C/A code phase of estimating Received signal strength are needed, then according to these Signal parameter estimation value initialization tracking modules, to help tracking to received signal, and the object of signal capture is the guestimate value in order to the carrier frequency and C/A code phase obtaining all visible gps satellite signals.

Refer to Fig. 1, this is the schematic diagram of the tracking module of existing a kind of GPS.Wherein there are two phaselocked loops.The first phaselocked loop be made up of carrier wave ring Discr., carrier wave ring wave filter, carrier wave NCO (digital controlled oscillator) etc. is called carrier wave ring, is used for following the tracks of the carrier frequency of intermediate-freuqncy signal.The second phaselocked loop be made up of code ring Discr., Loop filter, C/A code generator etc. is called a yard ring, is used for following the tracks of C/A code.Intermediate-freuqncy signal first by peeling off carrier wave with the mixing of carrier frequency, then by peeling off C/A code to relevant (Correlation) computing of instantaneous code.Correlated results i _e1, i _p, q _eLand q _pcoherent integration value I is exported respectively after integrate-dump device _e1, I _p, Q _eLand Q _pif the coherent integration time needed for tracking module is 1ms, so tracking module is while realization is followed the tracks of, and also demodulates the navigation data that speed is 1000Hz.Consider each data persistence 20ms in navigation message, then will solve the wide navigation message data bit of a 20ms from the data of every 20 1ms.Before bit synchronization, receiver does not also know which 20 1ms data in succession belongs to same navigation message data bit, a series of 1ms data will be reasonably divided into every 20 one group by data analysis by bit synchronous object exactly, then coherent integration is carried out to these 20 1ms data, by judging to solve navigation message data to the polarity of integral result; Data after demodulation carry out frame synchronization, to determine subframe reference position, thus by the word being divided into 30 group correct for demodulation data out, finally obtain navigation message.

In Fig. 1, what integrate-dump device exported is navigation data, and namely intermediate-freuqncy signal peels off the signal after carrier wave and C/A code, and speed is 1000Hz.Described integrate-dump device is equivalent to low-pass filter, and by the high fdrequency component filtering produced in optical mixing process, only surplus low frequency component, when without remaining frequency difference, the result of integrate-dump device as shown in Figure 2, is roughly the square-wave signal that amplitude is fixing.Bit synchronization is exactly the starting point of each data finding navigation message from navigation data, and position polarity judges to judge that the polarity of each data of navigation message is 1 or 0 exactly.The summation that bit synchronization and position polarity judge, changes the navigation data of 1000Hz into 50bps with regard to being used to, and 20 continuous print values value is replaced from the starting point of each data.Fig. 3 be the horizontal ordinate of Fig. 2 is amplified, partial schematic diagram that ordinate remains unchanged.As shown in Figure 3, it is a data in navigation message between 92 ~ 111ms, 20 of this 20ms data are added up, if 20 data sum > 0, judge that this bit data of navigation message is as 1, if 20 data sum < 0, judge that this bit data of navigation message is as 0.The said data demodulates of the application just refers to that carrying out a polarity to continuous 20 the 1ms data in navigation data judges.

Usually received signal strength C/No (carrier-to-noise ratio) is called strong satellite-signal between 50dB-Hz to 32dB-Hz, what received signal strength is less than 26dB-Hz is called weak satellite-signal.The output of the integrate-dump device shown in Fig. 2 is only applicable to each data polarity judging the navigation message in strong satellite-signal.For weak satellite-signal, can there is 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.Refer to Fig. 4, this is under remaining frequency difference is the condition of 2Hz, the Output rusults of integrate-dump device, and be roughly the square-wave signal that modulation has gradual sine wave, the speed of square-wave signal is 1000Hz, and sinusoidal wave speed is remaining frequency difference.Owing to receiving the impact of remaining frequency difference, start with the starting point of each data of navigation message, 20 the data sums calculating 20ms judge that the polarity of each data may be just wrong.

Summary of the invention

Technical problems to be solved in this application be to provide a kind of GPS strong satellite-signal auxiliary under data demodulation method to weak satellite-signal.

For solving the problems of the technologies described above, the data demodulation method of the application's GPS to weak satellite-signal comprises the steps:

1st step, to weak satellite-signal, complete bit synchronous navigation data, from reference position in navigation data of each data of navigation message to the coherent integration (coherent integration) of carrying out 20ms navigation data, and only retain a up-to-date K coherent integration results, K is the natural number of >=9;

2nd step, utilizes in the time interval of-20ms ~ 20ms of the time of reception of the frame synchronization code of the navigation message of strong satellite-signal, the position of the navigation data finding the frame synchronization code of the navigation message of weak satellite-signal corresponding;

3rd step, utilizes FFT (Fast Fourier Transform (FFT)) to judge the polarity of each data after the frame synchronization code of the navigation message of weak satellite-signal.

Described method the 2nd step specifically comprises:

2.1st step, when GPS demodulates the frame synchronization code in the navigation message of any strong satellite-signal, immediately front 8 coherent integration results (8 the up-to-date coherent integration results namely except the coherent integration results of current 20ms) by the end of the coherent integration results of a upper 20ms in K coherent integration results in the 1st step are multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 respectively according to by experienced order newly, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the first amplitude;

2.2nd step, also first 8 (8 namely up-to-date coherent integration results) by the end of the correlation integral result of current 20ms in K coherent integration results in the 1st step are multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 respectively according to by experienced order newly, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the second amplitude.

2.3rd step, when newly entering the coherent integration results of a up-to-date 20ms in K coherent integration results described in the 1st step, first 8 (8 namely up-to-date coherent integration results) by the end of the coherent integration results of the up-to-date 20ms entered in K coherent integration results in 1st step are multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 respectively according to by experienced order newly, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the 3rd amplitude;

Described N is the power of 2, and N >=8;

2.4th step, find 8 groups of coherent integration results corresponding to maximum that in the first amplitude, the second amplitude, the 3rd amplitude, 8 bit data of navigation message corresponding after these 8 groups of coherent integration results demodulation are exactly 8 frame synchronization codes of the navigation message coming from weak satellite-signal.

Described method the 3rd step specifically comprises:

3.1st step, for the navigation message of described weak satellite-signal 8 frame synchronization codes after each data polarity judge, all to wait that 8 coherent integration results of 8 bit data judged before the data bit of polarity are multiplied by symbol corresponding to this 8 bit data respectively according to by experienced order newly, treat and judge that 20ms coherent integration results corresponding to the data bit of polarity is multiplied by 1, supplement M-9 individual 0 again, then be M point FFT, maximum amplitude calculated to FFT result and is called the 4th amplitude;

3.2nd step, to wait that 8 coherent integration results of 8 bit data judged before the data bit of polarity are multiplied by symbol corresponding to this 8 bit data respectively according to by experienced order newly again, treat and judge that 20ms coherent integration results corresponding to the data bit of polarity is multiplied by-1, supplement M-9 individual 0 again, then be M point FFT, maximum amplitude calculated to FFT result and is called the 5th amplitude;

In the 3.1st step and the 3.2nd step, if described in wait to judge that 8 bit data before the data bit of polarity are 1, then the symbol of its correspondence is 1; If be 0, then the symbol of its correspondence is-1; Described M is the power of 2, and M > 9;

3.3rd step, if the 4th amplitude >=the 5th amplitude, then judges to wait to judge that the data bit of polarity is as 1;

If the 4th amplitude < the 5th amplitude, then judge to wait to judge that the data bit of polarity is as 0.

The application achieve strong satellite-signal auxiliary under, the position of the navigation data utilizing N point FFT to find the frame synchronization code of weak satellite-signal corresponding fast, in this, as initial value, coherent integration time has been extended to 180ms by recycling M point FFT, thus improve the accuracy of each data of the navigation message of weak satellite-signal being carried out to polarity judgement, eliminate the impact of remaining frequency difference.

Accompanying drawing explanation

Fig. 1 is the structural representation of the tracking module in GPS;

Fig. 2 is the output waveform (in strong satellite-signal situation) of integrate-dump device in 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 (under having remaining frequency difference situation, in order to display directly perceived, for strong satellite-signal) of integrate-dump device in Fig. 1;

Fig. 5 is the earth and gps satellite track schematic diagram;

Fig. 6 is the process flow diagram of the application's GPS to the data demodulation method of weak satellite-signal.

Embodiment

Navigation message is with frame (frame) for unit outwards sends, and each frame length is 1500 (bit).The navigation message of complete set is made up of 25 frames, amounts to 37500.Each frame is made up of 5 subframes (subframe) again, and each subframe lengths is 300.First 8 always 10001001 of each subframe, are called frame synchronization code.Emission rate due to navigation message is 50bps, and therefore frame synchronization code occurred every the 6s cycle.

If be decimal number 1 by the symbol definition of binary number " 1 ", the symbol definition of binary number " 0 " is decimal number-1, and so the symbol of frame synchronization code is exactly 1 ,-1 ,-1 ,-1,1 ,-1,1,1.

Refer to Fig. 5, wherein O point represents the earth's core, and A point represents GPS position, and S point represents the gps satellite being positioned at same level with GPS, and B point represents the gps satellite being positioned at the zenith of GPS.According to general elliposoidal earth model, the distance between S point and A point is 25785km, and the transmission time of gps satellite signal is 85.95ms.Distance between B point and A point is 20192km, and the transmission time of gps satellite signal is 67.31ms.This shows that the maximum time difference that same GPS receives different gps satellite signals is 18.64ms.Therefore, after the frame synchronization code of navigation message obtaining strong satellite-signal, only need to calibrate weak satellite-signal in the time range of-20ms ~ 20ms, just can obtain the position of the navigation data of weak satellite-signal corresponding to the frame synchronization code of navigation message.

Refer to Fig. 6, the data demodulation method of the application's GPS to weak satellite-signal comprises the steps:

Initial situation: GPS receives the weak satellite-signal of high frequency by antenna, by obtaining intermediate-freuqncy signal to its down coversion and/or sampling, again by catching, following the tracks of the navigation data obtaining 1000Hz, then obtained the starting point of each the navigation message data in navigation data by bit synchronization.What the application will do is exactly to come from weak satellite-signal, complete bit synchronous navigation data and carry out polarity and judge.

1st step, to come from weak satellite-signal, complete bit synchronous navigation data, from reference position in navigation data of each data of navigation message to the coherent integration of carrying out 20ms navigation data, and only retain 9 up-to-date coherent integration results.Numerical value 9 is also extendible is the random natural number of > 9.For concise explanation, the present embodiment is for preferred value 9.

The computing formula of the coherent integration of described 20ms is:

and wherein k is the reference position of navigation message a data in navigation data, I _p(i) and Q _pi () carries out the value after 1ms integration to correlated results, the I that namely in Fig. 1, the integrate-dump device of tracking module exports _pand Q _p.

2nd step, utilize in the time interval of-20ms ~ 20ms of the time of reception of the frame synchronization code of the navigation message of strong satellite-signal, the position of the navigation data finding the frame synchronization code of the navigation message of weak satellite-signal corresponding, each frame synchronization code of navigation message corresponds to the navigation data of 20ms length.According to Fig. 5, in fact can the time of reception of the frame synchronization code of the navigation message of strong satellite-signal ± frame synchronization code of the navigation message of the weak satellite-signal of time range inner position of 18.64ms.Because the time of each data of navigation message maintains 20ms, for ease of calculating, therefore be set as ± 20ms.

3rd step, utilizes FFT (Fast Fourier Transform (FFT)) to judge the polarity of each data after the frame synchronization code of the navigation message of weak satellite-signal.

Described method the 2nd step specifically comprises:

2.1st step, when GPS is judged by bit synchronization, position polarity the navigation data coming from any strong satellite-signal, after frame synchronization success, when again demodulating the frame synchronization code in navigation message, 1 ,-1 ,-1 ,-1,1 ,-1,1,1 is multiplied by respectively according to by experienced order newly immediately by 8 older in 9 coherent integration results in the 1st step, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the first amplitude.

2.2nd step, also be multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by 8 newer in 9 coherent integration results in the 1st step respectively according to by experienced order newly, to supplementing N-8 individual 0 after multiplied result, being then N point FFT, maximum amplitude being calculated to FFT result and is called the second amplitude.

Above-mentioned 2.1st step, the 2.2nd step can be carried out simultaneously, now need two FFT computing units; Also successively can carry out, now only need a FFT computing unit.N value be 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, then frequency interval is less, and result of calculation is less by the impact of remaining frequency difference.When N >=32, frequency interval≤1.5625Hz.When N >=64, frequency interval≤0.78125Hz.

Above-mentioned older or 8 newer coherent integration results to be multiplied successively with the symbol of frame synchronization code, then to supplement 0 and be N point FFT, can with removing frequency difference.This is that FFT result amplitude near the frequency of signal place is the highest, can be understood as and has again carried out mixing because the effect of FFT is exactly the frequency finding signal, and integration.

2.3rd step, when newly enter in 9 coherent integration results described in the 1st step up-to-date one (if 9 memory blocks are full, also mean and delete the oldest one) time, 1 ,-1 ,-1 ,-1,1 ,-1,1,1 is multiplied by respectively according to by experienced order newly by 8 newer in 9 coherent integration results in the 1st step, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the 3rd amplitude.

2.4th step, find 8 groups of coherent integration results corresponding to maximum that in the first amplitude, the second amplitude, the 3rd amplitude, 8 bit data of navigation message corresponding after these 8 groups of coherent integration results demodulation are exactly 8 frame synchronization codes of the navigation message coming from weak satellite-signal.

The frame synchronization code 10001001 of the navigation message of each gps satellite sends simultaneously, when GPS receives last of frame synchronization code in the navigation message of certain moment strong satellite-signal, according to Fig. 5, at this moment GPS should receive last position of weak satellite-signal frame synchronization code at front 20ms or current 20ms or next 20ms.First amplitude, the second amplitude, the 3rd amplitude just correspond respectively to front 20ms, current 20ms, next 20ms.

Suppose that last position of the frame synchronization code of weak satellite-signal reaches receiver at current 20ms, now the data of receiver 20ms coherent integration are I _{p_20}(8), Q _{p_20}(8), next 20ms is about to the data of the 20ms coherent integration newly entered is I _{p_20}(9), Q _{p_20}(9).Suppose that remaining frequency difference is 0, then two formulas below should be had 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 method the 3rd step specifically comprises:

3.1st step, for the navigation message of described weak satellite-signal 8 frame synchronization codes after each data polarity judge, all to wait that 8 coherent integration results of 8 bit data judged before the data bit of polarity are multiplied by symbol corresponding to this 8 bit data respectively according to by experienced order newly, treat and judge that the coherent integration results of the 20ms that the data bit of polarity is corresponding is multiplied by 1, supplement M-9 individual 0 again, then be M point FFT, maximum amplitude calculated to FFT result and is called the 4th amplitude;

3.2nd step, to wait that 8 coherent integration results of 8 bit data judged before the data bit of polarity are multiplied by symbol corresponding to this 8 bit data respectively according to by experienced order newly again, treat and judge that the coherent integration results of the 20ms that the data bit of polarity is corresponding is multiplied by 1, supplement M-9 individual 0 again, then be M point FFT, maximum amplitude calculated to FFT result and is called the 5th amplitude;

Described 8 bit data waiting to judge before the data bit of polarity, if there are data to be binary number 1, then the symbol of its correspondence is ten systems several 1; If there are data to be binary number 0, then the symbol of its correspondence is decimal number-1.

Above-mentioned 3.1st step, the 3.2nd step can be carried out simultaneously, now need two FFT computing units; Also successively can carry out, now only need a FFT computing unit.M value is preferably 64, also can be the power of 2 of other > 9.The value of M can have influence on the frequency resolution of FFT, and M is larger, and frequency resolution is higher, then frequency interval is less, and result of calculation is less by the impact of remaining frequency difference.When M >=32, frequency interval≤1.5625Hz.When M >=64, frequency interval≤0.78125Hz.

3.3rd step, if the 4th amplitude >=the 5th amplitude, then judges to wait to judge that the data bit of polarity is as 1;

If the 4th amplitude < the 5th amplitude, then judge to wait to judge that the data bit of polarity is as 0.

If there is no remaining frequency difference, then the maximum amplitude of FFT result is in 0 frequency position.If there is remaining frequency difference, then the maximum amplitude of FFT result is from the nearest frequency place of remaining frequency difference.Be only 0 illustrate the principle judged like this with remaining frequency difference below: suppose that 9 navigation message data bit that gps satellite sends are respectively 1,1,1,0,1,1,0,1,1, the polarity of first 8 is judged, and last 1 is wait to judge polarity position.The coherent integration results of 9 20ms calculated 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 vertical just like ShiShimonoseki,

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))|

Such as, after 8 frame synchronization codes of the navigation message coming from weak satellite-signal can be judged like this followed by the polarity of first data bit:

First, corresponding 8 coherent integration results are multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 according to by experienced order newly by 8 frame synchronization codes respectively that come from the navigation message of weak satellite-signal, 1 is multiplied by the coherent integration results of 20ms corresponding to the data bit of the back to back navigation message after these 8 frame synchronization codes, supplement 23 0 again, then be 64 FFT, maximum amplitude calculated to FFT result and is called the 4th amplitude.

1 ,-1 ,-1 ,-1,1 ,-1,1,1 is multiplied by respectively according to by experienced order newly again with 8 coherent integration results corresponding to 8 of the navigation message of weak satellite-signal frame synchronization codes,-1 is multiplied by the coherent integration results of 20ms corresponding to the data bit of the navigation message after these 8 frame synchronization codes, supplement 23 0 again, then be 64 FFT, maximum amplitude calculated to FFT result and is called the 5th amplitude.

If the 4th amplitude >=the 5th amplitude, then judge that back to back data bit after 8 frame synchronization codes of the navigation message of weak satellite-signal is as 1.

If the 4th amplitude < the 5th amplitude, then judge that back to back data bit after 8 frame synchronization codes of the navigation message of weak satellite-signal is as 0.

The application achieve strong satellite-signal auxiliary under, the position of the navigation data utilizing N point FFT to find the frame synchronization code of weak satellite-signal corresponding fast, in this, as initial value, recycling M point FFT method extends the coherent integration time of weak satellite-signal, thus improve the accuracy of each data of the navigation message of weak satellite-signal being carried out to polarity judgement, and eliminate the impact of remaining frequency difference.Because FFT counts seldom, so it is few to implement computational resource that is 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, then need the coherent integration being performed for more than 20ms just can solve the polarity of each data of navigation message.

N point FFT formula is:

$F\_\mathrm{data}\left(n\right)=\underset{m=0}{\overset{N-1}{\mathrm{\&Sigma;}}}x\left(m\right)e^{-j(2\&CenterDot;\mathrm{\&pi;}\&CenterDot;n\&CenterDot;m/N)},n=\mathrm{0,1},...,N-1$

From formula, this is one and first takes advantage of the process then added up, and for n=0, above formula frequently carries out mixing to signal 0 and add up, and for n=1, above formula carries out mixing at Fs/N frequency place to signal and adds up.Formula be above exactly by signal respectively frequency 0, Fs/N, 2Fs/N ..., place's mixing is also cumulative.The result of calculation of FFT is maximum in which frequency place amplitude, then signal frequency is just near which frequency, so just can eliminate the impact of remaining frequency difference.

The application have employed the input of 9 data as FFT in the 3rd step, and each data are the results after 20ms integration, are equivalent to the coherent integration being 180ms after carrying out FFT computing, are specially adapted to the data demodulates to weak satellite-signal.

The method that the application proposes also can be used for the demodulation for weak satellite-signal in other Global Positioning System (GPS), in the receiver system as GLONASS, the Big Dipper, Galileo etc.

These are only the preferred embodiment of the application, and be not used in restriction the application.For a person skilled in the art, the application can have various modifications and variations.Within all spirit in the application and principle, any amendment done, equivalent replacement, improvement etc., within the protection domain that all should be included in the application.

Claims (8)

1. GPS is to a data demodulation method for weak satellite-signal, comprises the steps:

1st step, to weak satellite-signal, complete bit synchronous navigation data, from each data of navigation message in navigation data reference position to the coherent integration of carrying out 20ms navigation data;

It is characterized in that, in the 1st step, only retain a up-to-date K coherent integration results, K is the natural number of >=9; Also comprise the steps:

2nd step, utilizes in the time interval of-20ms ~ 20ms of the time of reception of the frame synchronization code of the navigation message of strong satellite-signal, the position of the navigation data finding the frame synchronization code of the navigation message of weak satellite-signal corresponding;

3rd step, utilizes FFT to judge the polarity of each data after the frame synchronization code of the navigation message of weak satellite-signal.

2. GPS according to claim 1 is to the data demodulation method of weak satellite-signal, it is characterized in that, described method the 2nd step specifically comprises:

2.1st step, when GPS demodulates the frame synchronization code in the navigation message of any strong satellite-signal, immediately up-to-date 8 in K coherent integration results in the 1st step except the coherent integration results of current 20ms are multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 respectively according to by experienced order newly, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the first amplitude;

2.2nd step, also be multiplied by 1 ,-1 ,-1 ,-1,1 ,-1,1,1 by 8 up-to-date in K coherent integration results in the 1st step respectively according to by experienced order newly, to supplementing N-8 individual 0 after multiplied result, being then N point FFT, maximum amplitude being calculated to FFT result and is called the second amplitude.

2.3rd step, when newly entering up-to-date one in K coherent integration results described in the 1st step, 1 ,-1 ,-1 ,-1,1 ,-1,1,1 is multiplied by respectively according to by experienced order newly by 8 up-to-date in K coherent integration results in the 1st step, to supplementing N-8 individual 0 after multiplied result, then be N point FFT, maximum amplitude calculated to FFT result and is called the 3rd amplitude;

Described N is the power of 2, and N >=8;

2.4th step, find 8 groups of coherent integration results corresponding to maximum that in the first amplitude, the second amplitude, the 3rd amplitude, 8 bit data of navigation message corresponding after these 8 groups of coherent integration results demodulation are exactly 8 frame synchronization codes of the navigation message coming from weak satellite-signal.

3. GPS according to claim 1 is to the data demodulation method of weak satellite-signal, it is characterized in that, described method the 3rd step specifically comprises:

3.1st step, for the navigation message of described weak satellite-signal 8 frame synchronization codes after each data polarity judge, all to wait that 8 coherent integration results of 8 bit data judged before the data bit of polarity are multiplied by symbol corresponding to this 8 bit data respectively according to by experienced order newly, treat and judge that 20ms coherent integration results corresponding to the data bit of polarity is multiplied by 1, supplement M-9 individual 0 again, then be M point FFT, maximum amplitude calculated to FFT result and is called the 4th amplitude;

3.2nd step, to wait that 8 coherent integration results of 8 bit data judged before the data bit of polarity are multiplied by symbol corresponding to this 8 bit data respectively according to by experienced order newly again, treat and judge that 20ms coherent integration results corresponding to the data bit of polarity is multiplied by-1, supplement M-9 individual 0 again, then be M point FFT, maximum amplitude calculated to FFT result and is called the 5th amplitude;

In the 3.1st step and the 3.2nd step, if described in wait to judge that 8 bit data before the data bit of polarity are 1, then the symbol of its correspondence is 1; If be 0, then the symbol of its correspondence is-1; Described M is the power of 2, and M > 9;

3.3rd step, if the 4th amplitude >=the 5th amplitude, then judges to wait to judge that the data bit of polarity is as 1;

If the 4th amplitude < the 5th amplitude, then judge to wait to judge that the data bit of polarity is as 0.

4. GPS according to claim 1 is to the data demodulation method of weak satellite-signal, it is characterized in that, K=9.

5. GPS according to claim 2 is to the data demodulation method of weak satellite-signal, it is characterized in that, the value of N is larger, and frequency resolution is higher, and result of calculation is less by the impact of remaining frequency difference.

6. GPS according to claim 5 is to the data demodulation method of weak satellite-signal, it is characterized in that, N=32 or 64.

7. GPS according to claim 3 is to the data demodulation method of weak satellite-signal, it is characterized in that, the value of M is larger, and frequency resolution is higher, and result of calculation is less by the impact of remaining frequency difference.

8. GPS according to claim 7 is to the data demodulation method of weak satellite-signal, it is characterized in that, M=32 or 64.