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0 foreword

As to an ordinary GPS (global positioning system) The receiver comes says, it is dealt with by the hardware capture and track of the signal, mainly use ASIC (the application-specific integrated circuit) Realize. One of the main advantages of using ASIC is its significance, can deal with GPS signal effectively, but the great of its fabrication cost, but GPS technology still develops at a high speed at present, various algorithms are being still studied constantly, if realize with ASIC, obviously unrealistic. And imitate the receiving system which put up GPS with software, not only can compare some Off-The-Shelf algorithms conveniently, and can apply to reality fast. In this system, the analog signal digitization that will be input with A/ D converter, then will be transferred to the receiver of the software to deal with, not only spends less by the system that this reconstructs, but also flexible and changeable, can be to various algorithmic test, also can be suitable for GPS signal that may change in the future, the frequency substantially as including different signal transmission carriers of L5 band.

At present, the numerical MATLAB language that MathWorks Company puts out calculates the large-scale software that the function that the software has already developed into the suitable multi-disciplinary, many kinds of working platforms is powerful, its tool in numerical analysis and scientific computation can be used for carrying on algorithmic research and comparison. This text fully utilizes this characteristic of MATLAB, carry on research to the local system of GPS receiver based on software.

Characteristic of 1 GPS signal

GPS adopts the spread spectrum system of pseudo-random code. Its principle is shown in Fig. 1. Knowing by Fig. 1, the signal that GPS satellite sends to users is made up of two pieces of weight L1 and L2. The center frequency of L1 is 1 575.42 MHz, the center frequency of L2 is 1 227.6 MHz, L1 and L2 are the own wave regulating, its modulation wave is satellite navigation text D yard and pseudo random noise code P yard, C/ A yard. P yard is accurate ranging code, in order to fetch & plusmn; The pseudo-random sequence of 1, its speed is 10.23 Mbit/ s, its code length is 235.469 592 765& times; 103 code elements, cycle is about 266 d 9 h; C/ A yard is thick caught yard, in order to fetch & plusmn; C/ A one yard of pseudo-random sequences of 1, its bit rate is 1.023 Mbit/ s, its code length is 1 023 code elements, cycle is 1 ms; D yard, in order to fetch & plusmn; The navigation text code of 1, its bit rate is 50 bit/ s carrier.

The in-phase carrier component of the signal L1 is compounding yard and modulating with PoD, and orthogonal carrier component is to compound yard by C/ A D modulatingly. Only modulate P yard and D yard on the carrier signal L2. The modulating method is: First of all, P yard forms and compounds yard P D with data D one yard of modulo-2 sum, C/ A yard forms and compounds yard C/ A D with D one yard of modulo-2 sum; Then, compound yard and modulate carrier L1 with P D and c/ A D. The carrier modulates and adopts the key modulation of the phase shift, L1 adopts QPSK (the phase-shift keying of quad-phase) Modulate, 12 adopts DPSK (diphasic phase-shift keying) Modulation. These pieces of GPS weight come from a COMMON reference frequency, sees Table 1.

Realization of 2 software receivers

2.1 Key step of GPS SIGPROC signal processing

GPS receiver SIGPROC signal processing mainly includes two parts of SIGPROC signal processing and SIGPROC signal processing of intermediate frequency of radio frequency, as shown in Fig. 2.

GPS satellite signal that the SIGPROC signal processing of the radio frequency mainly refers to receiving to the aerial carries on the V Visualization of down-conversion, sampling, quantity, receive the digital intermediate frequency signal. It includes aerial, RF and A/ D converter. RF is used for changing intermediate frequency of the signal, the intermediate frequency signal is sampled and quantized digitization in A/ D converter. The intermediate frequency input is 4.31 MHz, sampling frequency is 20 MHz. This sampling frequency is most suitable for carrying on SIGPROC signal processing of intermediate frequency.

The SIGPROC signal processing of intermediate frequency mainly refers to the intermediate frequency signal of the logarithmic word and deals with relevant, catching, tracing, demodulation, after getting the navigation text data of the satellite, and then further solve and calculate the guidance information. The software herein receiver mainly deals with these problems.

2.2 catches the module

The function of catching the module is: Look for the relevant peak of maximum on the two-dimentional level of code phase of Doppler frequency of the input signal and C/ A, find the code phase of correspondent carrier frequency and C/ A, offer the rough initialization value for code phase and input signal of carrier frequency and C/ A produced here. It searches for course to be different code phases l and carrier frequency & omega; The relevant crest values of quadratic sum of I under svn situation, Q two-way relevant integral quantity compare with specified threshold value, if greater than threshold value and find out maximum value, code phases and carrier frequency values used get code phase and carrier frequency value of the input signal at this moment.

The thinking of the algorithm is as follows. In different frequency, carry on two-dimentionally and search for looking for the relevant peak of maximum with C/ A code phase. Make local carrier generator fix when being desired frequent a bit at first, then make C/ A one yard of generators that 1 bring into operation correlator 1 ms time from the code phase, keep and clear out I, Q two-way integral quantity, ask the quadratic sum of I, Q two-way integral quantity as the relevant crest value. Make C/ A one more generator, bring into operation l ms time, make relevant crest value from the code phase 2, until obtaining 1 ms temporal correlation peak value that the phase place brings into operation from 1023. Make the frequency of the local carrier generator increase the step length of a frequency, such as 500 Hz, repeat the above-mentioned course, until finish searching for all possible frequency domain and all code phases. Look for the value of the relevant peak of maximum and correspondent frequency and code phase value in searching for the two-dimentional level that is got. If the correlation value of maximum is greater than the average correlation value of relevant level by 16 times, can be thought to include signal of this satellite in the received signal, has already detected its peak of signal correlation, it is for the subsequent procedure to decode tracing to note correspondent frequency and code phase. Capture time can be confirmed by the point size of relevant crest value of the two-dimentional level under the worst situation. Static behavior fixed Doppler's frequency swing until - 5 kHz- +5 kHz, regarding 500 Hz as the ferquency interval needs searching for on 21 frequency points, each frequency point needs search of 1 023 code phases, the worst capture time is about 21 s, namely

2.3 following modular

The function of the following modular is: Realize code phase and frequency, C/ A code phase of the phase place and input signal and the ridiculing rate, phase synchronization of the carrier of the carrier of local C/ A, output the integrated signal of Middle Road of I way for decoding the message signal of base band. The thinking of the algorithm is as follows. Regard one yard of tracing and carrier tracking as two pieces of course turned over alone. Yard of tracing uses and delays the feedback control C/ A phase place that one yard of generators adjusts yard of the ring leadingly. Among them exceed future (E) , middle way (P) Delay the way (L) Half a chip of every phase difference, the representation signal ahead of time that differentiate, and lag behind and reaching on time. After synthesizing, signal these respectively represent input signal and associativity between the first yard here. The signal with highest correlation value is chosen and kept, namely can confirm the middle way has already aligned with code phase of the input signal when the energy of ultra future and energy of delaying the way are the same.

Carrier tracking is to use FLL (frequency locking is surrounded) first Tracking frequency, after making the frequency jitter wane in some time in tracing, give the frequency locked PLL (the phase locked loop) As the initialization value, use PLL to carry on tracing to the phase velocity of the signal, for some time after tracing is steady, the result of the accumulator can be used for decoding the text data of base band. I of carrier tracking, Q way integral quantity use I of the middle way, Q integral quantity. Regard it as the initialization value of FLL to catch the rough frequency received first, use I, Q integral quantity to try to get frequency difference to adjust tracking frequency, can treat the segment of tracing fixed for a long time after confirming the frequency has already been very close to actual frequency, will lock the frequency and give PLL the initialization value as, will use PLL to track to frequency and phase place of the signal, frequency and phase place of local carrier and input signal will be locked after treating tracing steady in some time, the result of the accumulator can be used for decoding the text data of base band. The schematic diagram which synthesizes one yard of tracing and carrier tracking is shown in Fig. 3.

Demodulation of 2.4 texts of navigating, positioning the module

Publish the text of navigating for demodulation, need further dispelling the phase error of the carrier. The demodulation module of text of navigating dispels the error with the following algorithms, and by me " (k)Provide and navigate the data bit of the text at present.

In the type: I(k) And Q(k) I, the intersection of Q and branched k accumulated value once immediately along separate routes; θ K is the carrier phase error estimate, & theta; k =θ k1-1+ K-1, and & theta; 0 =0,Among them θ K is the estimation residual error of the phase error of the carrier, and θ k =sign(I" (K))Q" (k).

3 design verification and simulation result

In order to verify the design model, this text uses GPS intermediate frequency signal that GPS signal simulator produces as and loses the signals of people, and observe the corresponding module output waveform in several models are clicked.

The signal simulator can be realized by MATLAB, and can adjust the signal to noise ratio, Doppler frequency, code phase isoparameter of the signal.

The intermediate frequency signal center frequency that this text uses is 16.8 MHz. The simulation result is shown in Fig. 4 - Fig. 6.

4 conclusions

This text uses the local system of MATLAB software implementation GPS software receiver, and paid proving in designing the module through MATLAB software. Prove the result shows, this design is enough to realize the treatment to GPS signal, including functions such as signal catching, one yard of tracing, carrier tracking and navigation text demodulation,etc., have not merely offered the fundamental reference basis for the fact that GPS software receiver is designed, it can be visual as the design, assessment of the new GNSS high-performance signal processing method of various studies the platform.