Research On Key Technology Of Basedband Signal Processing For Missile-born GNSS Software Receivers

Precision-guided munitions are playing an increasingly important role in modern high-tech local wars,and are important symbols of information wars.Due to its futures of all-weather operations,low cost,et al.,satellite guidance has been a major guidance mode for guided munitions.Compared with conventional GNSS receivers,the received satellite signals of missile-born GNSS receivers have higher dynamics and lower carrier-to-noise ratio(CNR).The signals may even be interrupted.Thus,higher requirements are put forward for core technology of receivers,namely baseband signal processing.For signal acquisition,due to the fact that the flight time of a shell is just dozens of seconds,the acquisition speed is very important.The acquisition precision affects the time for tracking loops to reach a stable state,thus it also affects the time-to-first-fix for a receiver.Therefore,the precision is also one of the important acquisition factors.For signal tracking,high dynamics require the loop to increase bandwidth,which will allow more thermal noise.Thus,a difficulty for carrier tracking under conditions of high dynamics and low CNR is to balance the dynamic performance and noise performance.With the design of new satellite navigation signals and the improvement of receiver's baseband signal processing algorithms,the receiver must be update accordingly.Software receivers can update algorithm functions conveniently without changing the hardware structure.Software receivers have been a development tendency of satellite navigation receivers.Based on the application background for precision-guided munitions and the development tendency for satellite navigation receivers,in this dissertation,a research has been made on related technologies of baseband signal processing for missile-born GNSS receiver.The main works and research findings of this dissertation are as follows.(1)The work of designing digital intermediate frequency signal source for software receivers is carried out,so as to provide the subsequent baseband signal processing algorithms with controllable signal source.Specifically,three aspects of work are done.1)The signal structure of four existing satellite navigation systems in the world are studied.The similarities and differences between them are analyzed.Results are obtained that despite of the differences between the signal structure for different systems,there exist some similarities for baseband signal processing.Thus,the work of this dissertation are based on GPS L1 C/A signal.2)Based on the Matlab/Simulink platform,a GPS L1 C/A software simulator is developed.3)A principle prototype for GPS pseudolite is developed.Test results show that the parameters are controllable for the developed signal sources,meeting the requirements.(2)Based on the application background for precision-guided shell,a signal acquisition algorithm taking into account both the precision and speed are proposed.This algorithm predicts the carrier frequency shift and code phase using auxiliary information during the initialization process of the shell before launching,so as to narrow the search range and increase the acquisition speed.Basing on the predicted value,the frequency is refined via Chirp z-transform,so as to improve the frequency resolution.To improve the algorithm efficiency,original data is down-sampled inspired by the idea of coherent integration and dump in baseband signal processing for satellite navigation receivers.The computation load is decreased.Consequentially,the acquisition efficiency is improved.Results indicate that the acquisition speed is markedly increased,while remaining the high frequency resolution.To furtherly improve the precision and sensitivity,the method of increasing the coherent integration time combing down-sampling is proposed.Results indicate that,signals with CNR of 22 dB-Hz can be acquired using 10 ms of coherent integration time.For signals with CNR of 25dB-Hz,the frequency error is only 5.9 Hz.(3)The research work of vector tracking loop for missile-born GNSS receivers are carried out with an emphasis on the application of vector tracking loop in high-dynamic signal tracking.A simulation model for generating missile-born receiver's digital intermediate frequency signal are established,based on which the frequency tracking performance of three tracking loops are compared,i.e.traditional loops with fixed bandwidth,Kalman filter based phase lock loop and vector tracking loop.Results show that vector tracking loop has better dynamic and noise performances.To address the problem of tracking performance degradation and loop failure caused by the decrease of visible satellites number and navigation solution precision in vector tracking loops,a vector frequency tracking algorithm assisted by expected trajectory is proposed.The error divergence during short signal interruption is avoided,and the loop can track the interrupted signal once it recovers.(4)Temperature compensated crystal oscillators(TCXOs)with small size are typically used for missile-born GNSS receivers because of the narrow space of a shell.The frequency accuracy and stability of TCXOs are usually poor.Considering the fact that the oscillator has important effects on receiver's baseband signal processing,the research of these effects are carried out in this dissertation.To provide a software receiver with signal source with high fidelity,the phase noise of a reference clock should be generated.Thus,a low-cost method for measuring an oscillator's phase noise is proposed,which meets the requirements of the precision.This method is based on a self-developed pseudolite transmitter and receiver system.The carrier-to-noise ratio is controllable.The thermal noise effect can be reduced effectively with a high carrier-to-noise ratio,so that the loop bandwidth can be enlarged as much as possible.The short-term frequency stability with the averaging time of 1 ms is achieved using this method with an error of 2.22%.The effect of phase noise for phase lock loop,receiver's clock error prediction and code phase prediction is analyzed using the simulated signal source.Besides,considering the high density of components of the printed circuit board of a missile-born GNSS receiver which easily produces heat and then affects the frequency stability of the oscillator,the effect of temperature variation for carrier loop is studied experimentally.The research on key technology of baseband signal processing for satellite navigation receivers carried out in this thesis has certain theoretical significance and application value for the engineering realization of missile-born GNSS software receivers.