Research On GPS Software Receiver Technology For Weak Signal

With the popularity of smart devices,the need of navigation applications has been digged,and the services based on user's location have slowly become a part of people's lives.Meanwhile,the varieties and complexities brings a challenge for the performance of the navigation equipment.As the most important method of outdoor navigation,the performance of GNSS is significant to the development of navigation.In the complex environment,such as tall buildings,canyons,forest and so on,the transmission,refraction and reflection of signal or other factors will lead to the energy attenuation,which badly impacts the performance of signal's acquisition and tracking.Finally the accuracy and consecutiveness of position becomes very worsen.So the high sensitivity GPS receiver is urgently required.The acquisition is the first step of signal synchronization which directly influences the performance of the receiver.For the problem of parameter's configuration and optimization,an evaluation method based on Monte-Carlo simulation is designed after the analyzing the factors that influence weak signal's acquisition.For the problem of cross-correlation interfere in the weak signal environment,the characteristics of interfere is analyzed quantitatively based on the theory and experiment,which provides the basis for the inhibition of the interference.When the signal energy is attenuated,the conventional bit synchronization method is not applicable any more.For this problem,the histogram method and bit energy method is analyzed and compared based on simulation.And the performance of bit energy method is analyzed under the different bit inverse probability.The signal's tracking is the process of estimating the signal's parameter finely and the receiver's ability to adapt to the complex environment is mostly determined by the level of weak signal's tracking.The conventional discriminator integral time is restricted by bit inverse.For this problem,a FFT discriminator is designed which is more applicable to weak signal.Generally,there is contradictory between dynamic and sensitivity in the tracking loop.In order to improve the sensitivity,the INS is integrated in the software receiver.Considering that oscillator error is an important factor affects tracking sensitivity,a method is proposed based on the PLL to analyze the error.Meanwhile the necessary condition is illuminated on which the error should be feedback to the NCO.In the weak signal environment,it is hard to realize frame's synchronization,obtain the signal's transmission time and assemble the pseudorange.To overcome this drawback,a new method of frame's synchronization is proposed based on the signal's matching.Finally,a complete set of software receiver system is implemented.The quantitative test of the simulator indicates that,with the auxiliary of doppler and code phase,the detect probability of 750ms integral time is 90%when the C/NO is 22dB-Hz.When the positioning error is 100 m(or 60 m),the tracking sensitivity is as low as 11 dB-Hz(or 14 dB-Hz)in the static environment and 15 dB-Hz(or 16dB-Hz)in the dynamic environment bellow 0.75g.And the sensitivity is 12 dB-Hz(or 14 dB-Hz)if the tracking loop is aided by INS.The vehicle's test indicates that software receiver developed by this thesis has strong robustness on the signal's acquisition and tracking and is close to U-Blox LEA-6T in terms of overall effect of position.