Research On Ins Aided High Performance Navigation Software Receiver Technique

GPS(Global Positioning System)receiver technology includes satellite acquisition and tracking of signals.The signal acquisition is essentially a problem of weak signals detection,while the signal tracking is probably an estimation of weak signals parameters.A lot of researches have been done by scholars all over the world on how to acquire weak signals,so this article will focus on the problem of tracking GPS signals.As traditional commercial GPS receivers are not designed for indoor or high dynamic work environments,it is necessary to be redesigned to solve the problems mentioned above.(1)The GPS signal usually has the power of-130dBm,however when in indoor environments,this number could get much lower even to-150dBm.In this circumstance,normal tracking loop cannot work effectively.(2)When dealing with weak signals or high dynamic environments,it is more likely to get bit errors in the navigation message,so the receiver may not successfully demodulate satellite ephemeris parameters and clock parameters which is necessary for positioning calculation.In order to improve the accuracy of the receiver in harsh environments,the improving of tracking sensitivity must be improved.(3)Though reducing the loop bandwidth can reduce the noise,it could also reduce the dynamic stress tolerance of the receivers.As a result,the GPS receivers should have the function of reducing loop bandwidth as well as maintaining the ability of signal tracking.To solve these problems,many solutions have been proposed.These can be divided into two types:to design an integrated navigation system based on both GPS and INS;or to use optimal estimation technology such as a Kalman filter.Integrated navigation technology refers to a combination of two or more kinds of navigation technology in order to make up for shortcomings.The most common one is the integrated INS/GPS system.The INS system is capable of autonomous work and it has high anti-interference ability.It provides a full range of navigation information,while the accuracy of INS systems may get worse quickly with the time goes.Modern satellite navigation and positioning system has high-precision,however it could not provide a continuous positioning signal.Also when the carrier do strenuous exercise especially in severe operating environments,GPS receivers might lose GPS signal lock.In other words,the combined system can improve and compensate the accuracy of the position and velocity when GPS signals are lost.It can also evaluate a variety of errors such as gyro drift and IMU attitude error to improve the whole system function.There are many issues related to estimation when doing research.It is about how to get a best result from data with random errors by some rules in a physical system interfered by random interference.While the optimal estimation is about how to get the best estimation limited by certain rules or conditions.The process of tracking GPS signal is just a typical process of weak signal parameter estimation,so using the optimal estimation technology in GPS tracking loop to improve its sensitivity and dynamic stress tolerance is a hotspot in recent years.What can be seen in this article are researches done on tracking loop based on optimal estimation technique and INS aided tracking loop.Most of the innovation and work includes:(1)GPS tracking loop based on optimal estimation techniqueIn order to solve the GPS receiver tracking loops' unlock problem in high dynamic environments,based on the GPS satellite IF signal model,a kind of StrongTracking Kalman filter(STF)for GPS tracking loop is designed.Its measurements are just code phase errors and carrier phase errors.So that STF technique can be used to estimate the code phase errors,carrier phase errors,Doppler frequency nd Doppler frequency change rate in high dynamic environments.A simulation comparison is also done between STF Kalman and traditional PLL(Phase Lock Loop).It shows that the result gets much better in high dynamic environments especially in the case of high Doppler frequency shift.(2)A new method of INS aided tracking loopThe common INS/GPS integration methods are loose integration,tight integration and ultra-tight integration.In loose integration system,GPS and INS work separately,and only the results from both of them are coupled.The structure is simple and can solve the accuracy issue in a long time for INS.It can also provide more information than GPS.However,it is not good for high dynamic stress.In tight integration system,a virtual GPS satellite and its pseudo range is provided by INS.This proposal is better than the former one,but the performance can still not improved obviously.It will increase the complexity of the system.In ultra-tight integration system,the information from INS navigation system is used to assist GPS navigation tracking loop,this will improve the system's ability to adapt high dynamic stress.However,this kind of system is complex and it needs to modify the structure of the baseband part of a receiver.To solve the problems above,a kind of INS aided GPS receiver method is put forward.This method uses information got from INS to calculate the carrier's Doppler shift due to the rapid movement and then uses it to change GPS sampling frequency of RF front-end.This approach requires only a little changes of the GPS receiver IF sampling and it is easy to conduct.The simulate result shows that it can highly improve the system's ability to adapt high dynamic stress.Though this method has a simple structure and it has strong tolerance to high dynamic signal and is no need to modify the traditional receivers' loop,but it is unable to track a too weak signal.So a kind of(INS-Assistant Multi-Channels Coherent-Noncoherent Vector Tracking Loop Based on Nonlinear Least Squares,INS/NLSVTL)is put forward.In weak signal conditions,it can significantly improve the carrier to noise ratio of the signals,so that to improve the sensitivity of the tracking loop.(3)GPS receiver vector tracking loopsSimulate experiments are done to EKF(Extend Kalman Filter),AEKF(Adaptive Extend Kalman Filter)and IEKF(Iterated Extend Kalman Filter),to verify,analyze and compare the performance of each filter in the vector tracking loop.In order to obtain higher accuracy of fusion data,a variable estimator of noise observation is added to the traditional IEKF algorithm to form a kind of adaptive IEKF.It is also used in GPS receiver vector tracking loops.Though all visible satellites intrinsic relationship can be more fully utilized in the standard vector tracking structure and the tracking sensitivity and re-acquisition speed can be improved as well,the residual intrinsic relevance of each channel phase detector is ignored.Its improvement of the signal tracking sensitivity is also limited.Based on this,a kind of vector tracking loop structure based on the spatial domain of the joint non-coherent integration is put forward.It can make better use of residual intrinsic relevance of each channel phase detector and improve the tracking sensitivity by the method of tracking channel between the non-coherent integration.