Beidou Satellite Broadcast Ephemerisand Almanac Parameters Fitting Algorithms

Broadcast ephemeris and almanac, as satellite navigation message, are two importantelements, playing a very important role in the real-time navigation positioning. Fornavigation users, precise orbital information is the basis of precise positioning. However,compared to the high precision afterwards precise ephemeris,broadcast ephemeris updatecycle in real-time and data aspects have greater advantage. Broadcast ephemeris andalmanac parameters are obtained by fitting the forecast track, and the fitting algorithmusually adoptOrbital elements fittingor State vector integration, which are typical of GPSand GLONASS broadcast ephemeris parameters fitting method. While, due to the orbitalelements fitting methodhaving a simple user algorithm, highfitting precision, strongextrapolation ability,it iswidely used in GPS, BDS, GALILEO system. However, due to thelayout of the different constellations, the applicability oforbital elements fitting on differentnavigation system has become a hot topic for current research scholars.Currently, the BDS regional navigation system has begun to provide services, but theBDS global navigation system is still in construction. Regional system usedthe orbitalelements fitting methodto get the broadcast ephemeris parameters, which is similar to GPS’s16parameters model. And for three kinds of different, the satellites broadcast ephemerisfitting accuracy all better than10cm.Howeverthe accuracy indicator in BDS the globalsystem is superior to the regional BDS system, in which broadcast ephemeris fittingaccuracy required to achieve5cm. This article will use18parametersbroadcast ephemerisinBDS global systems, analyzing the fitting accuracy of the model and it’s applicability ofthe BDS satellites. Meanwhile, based on the analysis of medium accuracy almanac andsimplicity almanac in GPS, considering the small inclination angle problem of the GEOsatellites, designed BDS’s almanac model, and used the orbital elements fitting almanacparameters, assessedthefitting almanac accuracy. Specific research contents are as follows:（1）Introducethe theory of navigation satellites broadcast ephemeris and almanac andfitting algorithm, analyze the characteristics of GPS and GLONASS broadcast ephemerisand almanac model. （2）Considering the GEO satellite’s characteristicsin BDS, adopting orbital rotationmethod and GIVENS orthogonal transformation to solve the broadcast ephemeris Fittingfailure,which is caused by small inclination and small eccentricity.（3）Derive the partial derivative of16and18parameters, and analysis the impact ofreconciling items related to partial derivative, the experiment proved the influenceof thereconciling items keepingin10-5magnitude.（4）Applying orbital elements fitting algorithm, according to BDS precise ephemeris,respectivelyfitting16and18broadcast ephemeris parameters, and analyzing the twomodels’precision about the fitting parameters, the results show that16parameters broadcastephemeris’ averagefitting user range error (URE) is3cm, the maximum is not exceeding8cm,18broadcast ephemeris parameters averagefitting user range error is5mm, themaximum does not exceed3cm.（5）Analysed the fitting accuracy effects of different arcs to two broadcast ephemerismodel, the results showed that，with the increasing of the arc，both16parameters and18parameters’ fitting accuracydeclined,when fitted arc greater than4h, accuracy divergencevelocityrapidly increased. So to ensure the accuracy of the track, it is recommended thatfitting arcs is better less than4hours.（6）For the particularity of the big dipper constellation, design of a suitable BDS systemalmanac parameters and fitting algorithm, it could also apply to GEO, IGSO and MEOsatellites, the experiment validate the loss of accuracy of the fittingalmanac are less than1000m, completely meetingthe satellite quick searchrequirements.