CN103439720A - GPS L2C signal capture method - Google Patents

Abstract

The invention provides a GPS L2C signal capture method. The method comprises: first of all performing coarse capturing on GPS L2C signals with a low signal-to-noise ratio, wherein the coarse capture comprises carrier wave phase synchronization and CM code generation and synchronization; putting fine capture into service after the coarse capture, and performing a maximum likelihood estimation on captured CM code energy signals so as to obtain CM code delay signals in optimum coupling; and putting CL code capture into service according to the CM code delay signals, realizing CL long code capture by using the fixed phase relationship between CM short codes and CL long codes, and finally realizing the rapid capture of L2C signals. According to the invention, an energy-based search strategy is utilized over a frequency search dimension so that data jumping effects are overcome; a noncoherent integration technology and a data pilot frequency binary channel combination capture technology are combined in the capture method so that the noise influence is further minimized; and the GPS L2C signal capture method provides possibility for the application of GPS L2C signals in weak signal conditions and the calculating complexity is lowered.

Description

A kind of GPS L2C signal acquisition methods

Technical field

The invention belongs to wireless communication technology field, especially relate to a kind of GPSL2C signal acquisition methods.

Background technology

The operation of entering the orbit on September 26th, 2005 along with first GPSIIR-M satellite, the GPSIIR-M satellite provides a new civilian pseudo-noise code (L2C code) for global user, the L2C signal has adopted special civilian moderate-length code (CM code) and civilian long code (CL code) to carry out time division multiplex, compare traditional C/A code, L2C code code length is longer, make the cross correlation value between signal less, and then the interference between signal is also less, also make long coherent integration become possibility, have more advantage in the low signal-to-noise ratio environment, make the L2C signal can realize indoor, alameda, application under the weak signal conditions such as tunnel.Up to the present had 8 modernization satellites to start to launch the L2C test signal, official estimates can realize full constellation (24 star) operation at GPSL2C signal in 2016.

But the L2C signal is effectively caught very difficult, if still adopt the catching method of original L1C/A code to be caught, calculated amount and computing time are all that present hardware condition can't meet.And because the tracking error of code and carrier wave can directly have influence on the positioning precision of receiver, therefore must solve along with improving constantly civilian gps signal performance requirement, under more weak signal conditioning or disturb under larger condition, can effectively reduce calculated amount, shorten capture time and do not lose the high sensitivity Trapped problems of acquisition performance simultaneously.

Catching time-multiplexed L2C code can realize by two kinds of patterns.The first pattern is to utilize the auxiliary CL code of CM code to catch, and in acquisition procedure, at first the CM code is hunted down, and then utilizes the phase relation between CM code and CL code to catch the CL code.The second pattern is directly caught the CL code, i.e. CL code Direct Acquisition.This GPSL2C signal acquisition methods that the present invention announces is based on the catching method under the first pattern, utilizes the auxiliary CL code of CM code to catch.

And existing acquisition algorithm based on the first pattern is not satisfactory to the processing of unknown navigation data bits saltus step in CM code acquisition procedure, perhaps directly ignore the impact of data bit saltus step, complicacy and calculated amount that the technology that overcomes the data bit saltus step that perhaps adopted is introduced increase greatly, and the impact of seeking to overcome the navigation data bits saltus step only increases the algorithm of appropriate complicacy and calculated amount and the key issue that catching method becomes this technical field research simultaneously.

Summary of the invention

Technical matters to be solved by this invention is to overcome the deficiencies in the prior art, the present invention proposes a kind of GPSL2C signal acquisition methods.Described method is taken into account simultaneously and is caught calculated amount and acquisition sensitivity, can effectively overcome the impact of data jump on catching, noise can be carried out again to appropriateness average, reduce mistake and catch rate, realize that the high sensitivity of receiver GPSL2C signal under Low SNR is caught.

For solving the problems of the technologies described above, the technical solution used in the present invention is as follows:

A kind of GPSL2C signal acquisition methods, comprise the steps:

Steps A, adopt the time-domain parallel fft algorithm slightly to catch low signal-to-noise ratio GPSL2C signal, comprise that carrier phase synchronization, CM code occur and synchronize, adopt carrier phase synchronization to realize carrier synchronization, the generation of employing CM code realizes that the CM code peels off and obtain CM coded signal energy approximation value with synchronizeing;

Step B, essence is caught CM code energy signal, and will catch the CM code energy signal obtained and do maximal possibility estimation, obtains the CM code delay signal under optimum matching;

Step C, the CL code is caught: according to CM code delay signal, utilize CM short code and the fixing phase relation of CL long code, be to comprise 75 CM code cycles in each CL code cycle, by 75 CM code relative phase interval locations, realize catching of CL long code, thereby realize the fast Acquisition of GPSL2C signal.

In steps A, described time-domain parallel fft algorithm comprises that step is as follows:

Steps A-1, adopt carrier phase synchronization ,-90 of the carrier signal of described low signal-to-noise ratio GPSL2C signal and this locality generation is spent to skew and carry out computing cross-correlation, and whether differentiate low signal-to-noise ratio GPSL2C signal synchronous with the kurtosis of the carrier signal of this locality generation; The local code phase place of sliding as asynchronous, a clock period of carrier shift; Finally obtain carrier synchronization L2C signal;

Steps A-2, carry out computing cross-correlation by the coded signal of above-mentioned carrier synchronization L2C signal and the generation of local CM code generator, and whether differentiation carrier synchronization L2C signal synchronizes with the kurtosis of local CM coded signal; The local code phase place of sliding as asynchronous, a clock period of deduction local code generator; Synchronously by the IFFT conversion, obtain effective CM code delay signal;

Steps A-3, ask for effective CM code delay signal energy, obtains CM code energy signal.

In step B, described essence is caught CM code energy signal, its process is: at first adopt the local energy computing method, obtain the CM code delay signal that not affected by data jump, adopt again non-coherent integration to obtain the GPSL2C signal energy, finally adopt data and pilot tone binary channels combination catching method to average the noise in the GPSL2C signal energy.

The invention has the beneficial effects as follows: the present invention proposes a kind of GPSL2C signal acquisition methods, at first described method slightly catches low signal-to-noise ratio GPSL2C signal, and it comprises that carrier phase synchronization and CM code occur and synchronize; The CM code is enabled essence again and is caught after slightly catching, and will catch the CM code energy signal obtained and do maximal possibility estimation, obtains the CM code delay signal under optimum matching; According to CM code delay signal, enable the CL code and catch, utilize CM short code and the fixing phase relation of CL long code to realize catching of CL long code, finally realize the fast Acquisition of L2C signal.The inventive method adopts the search strategy based on energy on the frequency search dimension, has overcome the impact of data jump; In catching method, in conjunction with adopting non-coherent integration technology and data pilot binary channels combination capture technique, noise effect is further reduced; For the GPSL2C signal provides possibility in the application under weak signal conditions, and computation complexity is little.

The accompanying drawing explanation

Fig. 1 is whole module of the present invention and connects block diagram.

Fig. 2 is middle CM code generation and the synchronization program block diagram of slightly catching of the present invention.

Fig. 3 is the middle delay local code clock control block diagram of slightly catching of the present invention.

Fig. 4 is the comparative graph that essence of the present invention is caught middle employing energy search.

Fig. 5 is non-coherent integration schematic diagram in the present invention.

Fig. 6 is CM code and CL code phase graph of a relation in the present invention.

Embodiment

Below in conjunction with accompanying drawing, a kind of GPSL2C signal acquisition methods that the present invention is proposed is elaborated:

As shown in Figure 1, at first signal receiver slightly catches low signal-to-noise ratio GPSL2C signal, and it comprises that carrier phase synchronization and CM code occur and synchronize; The CM code can be enabled essence after slightly catching and catch, the upper search strategy based on calculating energy of proportion search dimension, this policies using the problem that reduces of the acquisition performance that in the acquisition procedure, unknown navigation data saltus step brings.And will catch the CM code energy signal obtained and do maximal possibility estimation, obtain the CM code delay signal under optimum matching; According to CM code delay signal, can enable the CL code and catch, utilize CM short code and the fixing phase relation of CL long code to realize catching of CL long code, finally realize the fast Acquisition of L2C signal.

As shown in Figure 2, because the L2CM code has good auto-correlation and their cross correlation, when two group codes do not have phase differential, correlated results just has kurtosis to occur, thereby judges that two group codes are synchronous.If it is not synchronous, the local code phase place of sliding, by a clock period of clock control circuit deduction local code generator, make local code postpone a code element, and continue to carry out relevant to the signal received, until synchronous, count the phase differential that just can obtain local code and receiving code by the code element number to being delayed;

As shown in Figure 3, be the middle delay local code clock control block diagram of slightly catching of the present invention, in figure, CLK is global clock, and L2C is the L2CM code of input, and L2OUTC is the synchronous L2CM signal of output; When above-mentioned clock control circuit is initial, U1, U2 be all in " 0 " state, and the QN of U2 end guarantees with door U3 as often opening, L2C can be smoothly by with door, thereby triggering for generating L2CM code.When local L2CM code is ahead of the L2CM code received, cf inputs a high level pulse, the state turnover of trigger U1 is to " 1 ", thereby changed the trigger condition of J2K trigger U2, next L2C clock result and door U3 while, thereafter along impelling U2 to turn to " 1 ", the QN end of U2 is " 0 ", Chang Kai and door U3 are closed, the Q of trigger U2 end removes again to control the R end of U1 simultaneously, make the U1 reset, changed so again the trigger condition of U2, when the next pulse of L2C clock arrives, because close and can not pass through with door U3, its rear edge makes again the U2 upset for " 0 ".The QN end of trigger U2 makes to reopen with door.Such circuit has guaranteed once to control and can deduct accurately a pulse of L2C original clock, thereby reaches the purpose that postpones local code generation.LD is set in addition, FRN termination VCC.

After slightly having caught, adopt the local energy computing method, bandwidth is a chosen in advance value, and local energy is defined as follows:

$\mathrm{\ϵ}(\stackrel{\‾}{\mathrm{\τ}},{\stackrel{\‾}{f}}_d)=\frac{1}{{2L}_e+1}\underset{i={-L}_e}{\overset{L_e}{\mathrm{\Σ}}}{S}^2(\stackrel{\‾}{\mathrm{\τ}}{,\stackrel{\‾}{f}}_d+\mathrm{i\Δf})$

Wherein,

for local energy,

for local carrier frequency,

for local code postpones, L _efor the frequency of calculating local energy, to count concentrate on each Frequency point on the frequency search dimension near,

be to calculate intersection ambiguity function (CrossAmbiguityFunction-CAF), Δ f is the frequency search step-length, and i means summation operation step-length ordinal number.Therefore the zone that bandwidth comprises is

energy capture can, on the search volume calculated based on FFT, use moving average filter to realize the calculating of portion of energy to each code delay point, thereby can obtain a curve that is not subject to the impact of data jump in actual applications.

As shown in Figure 4, at Doppler shift f _d=1kHz, code delay time τ=1.8ms, carrier-to-noise ratio is 37dBHz ^-1condition under.Its dotted line is the frequency spectrum developed curve when having data jump to occur, can see the impact due to data jump, be that division has occurred the 1kHz time-frequency spectrum in frequency displacement, now use the maximal possibility estimation criterion to estimate to obtain wrong frequency displacement position, thereby reduce capture rate.The curve that solid line obtains for starting the energy capture module, the energy capture bandwidth B _ithe calculated rate of getting is counted.Can be observed from figure, adopt the method can overcome the impact of data jump on catching, noise can be carried out again to appropriateness on average, estimate thereby obtain correct code delay.

The effect of coherent integration as shown in Figure 5 is remove carrier wave and spreading code and generate cumulative signal, and the data of utilization different length are carried out coherent integration and can be obtained different gain process.The non-coherent integration technology is added up to the result after coherent integration, for data jump, GPSL2C signal frequently occurs, due to satellite navigation message phase overturn and the restriction that has frequency loss, the coherent integration time can not be long, and non-coherent integration is not subject to the restriction of navigation message phase overturn.Therefore can adopt non-coherent integration to increase capture-data length, the impact of attenuating noise signal for weak signal.

The specific embodiments of non-coherent integration technology is described as follows:

For weak signal, can adopt non-coherent integration to increase capture-data length, noise effect is further reduced, and, along with the increase of non-coherent integration times M, acquisition performance is significantly improved, its signal energy

computing formula can be expressed as:

${\mathrm{\ϵ}}_M(\stackrel{\‾}{\mathrm{\τ}},{\stackrel{\‾}{f}}_d)=\frac{1}{M}\underset{k=1}{\overset{M}{\mathrm{\Σ}}}{\mathrm{\ϵ}}_k(\stackrel{\‾}{\mathrm{\τ}},{\stackrel{\‾}{f}}_d)$

Wherein

local energy while being the k time integration value.

The GPSL2C signal is to have data, the twin-channel signal of pilot tone, can adopt the binary channels capture technique, further improves capture rate, realizes that the high sensitivity of L2C signal is caught, and its computing formula can be expressed as:

${\mathrm{\ϵ}}_M(\stackrel{\‾}{\mathrm{\τ}},{\stackrel{\‾}{f}}_d)=\frac{1}{2M}\underset{k=1}{\overset{M}{\mathrm{\Σ}}}{\mathrm{\ϵ}}_{k,d}(\stackrel{\‾}{\mathrm{\τ}},{\stackrel{\‾}{f}}_d)+\frac{1}{2M}\underset{k=1}{\overset{M}{\mathrm{\Σ}}}{\mathrm{\ϵ}}_{k,p}(\stackrel{\‾}{\mathrm{\τ}},{\stackrel{\‾}{f}}_d)$

Wherein, subscript d, p is representative data passage and pilot channel respectively,

the local energy of data channel signal while being the k time integration,

the local energy of pilot channel signal while being the k time integration.

As shown in Figure 6, describedly utilize CM code delay signal capture CL coded signal, once, after the CM code catches and follow the tracks of successfully, by the phase relation between CM and CL code, realize CL fast Acquisition and tracking.Each CL comprises 75 CM cycles in the cycle, can utilize the chip estimated value of CM by preserving 75 CL code relative phase interval locations, obtains the CL code sequence of 75 different initial positions, and initial position is:

Ce+(i-1)×10230，i＝1,2,...,75

The chip estimated value that Ce is the highest relevant peaks place, directly utilize these 75 CL code sequences to be correlated with input signal, and obtaining in 75 correlations maximum is exactly the corresponding code phase of CL.Then the CL code is caught in these positions of L2C searcher receiver, thereby realizes catching of CL coded signal.

Claims (3)

1. a GPS L2C signal acquisition methods, is characterized in that, comprises the steps:

Steps A, adopt the time-domain parallel fft algorithm slightly to catch low signal-to-noise ratio GPS L2C signal, comprise that carrier phase synchronization, CM code occur and synchronize, adopt carrier phase synchronization to realize carrier synchronization, the generation of employing CM code realizes that the CM code peels off and obtain CM coded signal energy approximation value with synchronizeing;

Step B, essence is caught CM code energy signal, and will catch the CM code energy signal obtained and do maximal possibility estimation, obtains the CM code delay signal under optimum matching;

Step C, the CL code is caught: according to CM code delay signal, utilize CM short code and the fixing phase relation of CL long code, be to comprise 75 CM code cycles in each CL code cycle, by 75 CM code relative phase interval locations, realize catching of CL long code, thereby realize the fast Acquisition of GPS L2C signal.

2. a kind of GPS L2C signal acquisition methods according to claim 1, is characterized in that, in steps A, described time-domain parallel fft algorithm comprises that step is as follows:

Steps A-1, adopt carrier phase synchronization,-90 of the carrier signal of described low signal-to-noise ratio GPS L2C signal and this locality generation is spent to skew and carry out computing cross-correlation, and whether differentiate low signal-to-noise ratio GPS L2C signal synchronous with the kurtosis of the carrier signal of this locality generation; The local code phase place of sliding as asynchronous, a clock period of carrier shift; Finally obtain carrier synchronization L2C signal;

Steps A-2, carry out computing cross-correlation by the coded signal of above-mentioned carrier synchronization L2C signal and the generation of local CM code generator, and whether differentiation carrier synchronization L2C signal synchronizes with the kurtosis of local CM coded signal; The local code phase place of sliding as asynchronous, a clock period of deduction local code generator; Synchronously by the IFFT conversion, obtain effective CM code delay signal;

Steps A-3, ask for effective CM code delay signal energy, obtains CM code energy signal.

3. a kind of GPS L2C signal acquisition methods according to claim 1, it is characterized in that, in step B, described essence is caught CM code energy signal, its process is: at first adopt the local energy computing method, obtain the CM code delay signal that not affected by data jump, then adopt non-coherent integration to obtain GPS L2C signal energy, finally adopt data and pilot tone binary channels combination catching method to average the noise in GPS L2C signal energy.

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