Research On Simulation Of GPS/BDS Broadcast Ephemerides And Atomic Clock Errors

BeiDou(COMPASS)satellite navigation system(BDS)will be built into a global system and provide global service by 2020.The constitution,interface and key technology of global system are more complex than that of regional one.Therefore it is of great significance to establish a GNSS simulation software to improve the situation of lacking comprehensive testing environment and promote the ground testing ability of BDS.This paper mainly studied the simulation methods of broadcast ephemerides and atomic clock errors based on the systematical analysis of them,which are the important parts of GNSS simulation software.The main contents are as follows:1.GPS and BDS broadcast ephemerides parameters and user algorithms are introduced,and the differences between them are analyzed.The accuracy evaluation method of broadcast ephemerides is given.Finally,the accuracy of different satellites' broadcast ephemerides of the two systems is evaluated with measured data.The results show that the accuracy of BDS GEO broadcast ephemerides is the worst,while the R direction accuracy of IGSO and MEO is about 33 cm,three dimensional one is 1.45 m,and URE is 0.67m;The R direction accuracy of all GPS satellites' broadcast ephemerides is 16 cm,three dimensional one is 1.16 m,and URE is 30cm;the accuracy of BDS broadcast ephemerides is slightly worse than that of GPS.2.Simulation algorithms of GPS broadcast ephemerides parameters are introduced,the partial derivatives of GPS satellite position on the LNAV broadcast ephemerides parameters are given,while the ones of satellite position on the CNAV broadcast ephemerides parameters are deduced,and the algorithms are implemented by C++ program.Numerical examples are shown to illustrate the algorithms can reach the URE requirement of less than 0.1m,and accord with the user algorithms' computation requirement.3.Simulation algorithms of BDS broadcast ephemerides parameters are studied,the value scale of some GEOs' broadcast parameters and singular problem of normal equations are analyzed.The simulation algorithm process of GEO satellite broadcast ephemerides is designed and implemented by C++ program.And numerical examples are presented to illustrate the algorithms' efficiency.4.The satellites' on-board atomic clocks quality assessment indicators,including frequency drift rate,frequency stability and noise type analysis are described.The quality assessment strategies of satellites' clocks are designed,then,the performances of different satellites' clocks of two systems are evaluated with measured data.The results show that the performance of BDS MEO satellites' atomic clocks has been promted,but still worse than that of GPS,especially BLOCK IIF.5.Properties and characterization of high precision frequency signals are studied.Some already existed clock models are analyzed.It is found that the systymatic part of clock error is considered carefully in essence while the stochastic part is not modeled efficiently.6.The atomic clock error simulation model “quadratic polynomial + noise” is given.Aft er that,this paper concentrated on analyzing and stud ying the simulation methods of noise,and further studying some related problems of Allan deviation method.The numerical examples show that the Allan deviation method can model atomic clock noise well based on its characterization.It is recommended that the use of original clock error data in the calculation of noise characteristics,in order to avoid the impact on noise characteristics of polynomial fitting on the clock error data;at the same time in the calculation of one-second Allan deviation,the non-negative least square algorithm is recommended in order to achieve the best simulation results.