Research And Realization Of The Multi-frequency GPS Signal Simulation And Time-interval Measurement System

With the improvement of positioning accuracy requirements,single-frequency GPS signal simulator can not meet the development needs of GPS receivers.The development of the multi-frequency GPS signal simulator can facilitate the differential positioning of the GPS receiver to improve the positioning accuracy,and also improve the function of the GPS signal simulator.In the signal simulator,the accuracy of the output PPS pulse signal is a very important indicator.In order to measure the simulator PPS pulse signal,it is necessary to design a time interval measurement system.With these problems and needs,this thesis designs a multi-frequency GPS signal simulator and time interval measurement system,and implements it on the hardware platform.The main research results and innovations are as follows:(1)In-depth study of the GPS L2 C signal system.For the problem that the CM code and the CL code are time-division multiplexed and only the CM code modulates the data code,the simulation and testing are strictly performed.According to the result of the simulation tool,the internal timing of the FPGA is controlled so that the starting moments of the CM code,the data code,the carrier,and the PPS pulse signal are completely identical.The pseudo code duration is strictly controlled,so that the CM code and the CL code are timedivision multiplexed and alternately output.The L2 C IF signal acquisition result collected by the acquisition card shows that the CM code is successfully captured,and there is no leakage or multiple capture.(2)Design a multi-frequency GPS signal simulator,which can be started by sending the scene file using the host computer software.The positioning test of the output signal of the GPS signal simulator shows that the horizontal positioning accuracy of the simulator signal has reached the standard of general commercial signal simulator.The attenuation test of the output signal power of the GPS signal simulator shows that the linear attenuation interval of the output signal power is 21-41 dBm.(3)According to the measurement requirements of the simulator PPS pulse signal,based on the research of the clock phase interpolation method,a continuous time interval measurement method is designed.Through the transmission of data across the clock domain and the alternate use of the open signal and the closing signal(the incoming pulse signal is used as the closing signal of the current measurement as the opening signal for the next measurement),and the simulation tool is used to continuously adjust the data transmission timing,and the completion is completed.Continuous time interval measurement system design.Tests have shown that the system is capable of measuring continuous input pulse signals and continuously outputting time interval measurements.(4)For the problem of large output error and low stability using crystal frequency,the use of time interval measurement system and standard receiver PPS pulse signal to control and tame the crystal oscillator.The measured value obtained by the time interval measurement system is compared with the actual value,and the difference between the two is converted into a crystal pressure control value output,and then converted into an analog voltage by the DAC 7512 to voltage control the crystal oscillator.Until the measured value is the same as the actual value,the output frequency of the crystal oscillator is used as the frequency of the whole system,and during this period,the voltage control tame action is maintained.The final test results show that the accuracy of the measured values obtained by this method is better than 4.03 ns,and it is very stable,and the measurement accuracy is better and more stable than when the voltage controlled taming crystal oscillator is not used.(5)The time interval measurement system is designed,which consists of clock signal generation module,crystal pressure control and tame module,time interval measurement module,serial communication module,pulse signal generation module and main control module.The upper computer software can display the final measurement value.And the time interval measurement system can be controlled.