Model Design Of Broadcast Ephemerides For LEO Augmentation Navigation Satellites

Broadcast ephemerides are the core of satellite navigation messages,which directly determine the communication efficiency and the accuracy of user navigation and positioning.Therefore,the design of broadcast ephemeris models is the key technology of navigation systems.The traditional broadcast ephemeris models were mainly designed for the MEO/GEO satellites,and could not be directly applied to LEO satellites with more complex perturbations.LEO satellites are important parts of augmentation navigation systems.As the improving of the precision of LEO determination,higher requirements are put forward on the accuracy of the broadcast ephemeris.It's necessary to design suitable broadcast ephemerides for LEO satellites according to its orbital characteristics.The design of broadcast ephemeris mainly includes three parts: the representation of the short-time dynamics(parameter design and fitting algorithm);the user algorithm for calculating satellite positions;the design of ephemeris interface.In this paper,models of orbital elements ephemerides and orbital list ephemerides were studied;ephemerides models of different parameters were designed for LEO satellites at altitudes between 300 and 1500km;user algorithm were given for calculating satellite positions,and the ephemeris interface were designed.The main research contents and conclusions are as follows:(1)Orbital list ephemerides of 19/22/25 parameters based on the ECEF coordinate system were designed for LEO satellites.Through the analysis of the dynamic characteristics for LEO satellites in ECEF coordinate system,three kinds of broadcast ephemerides models were designed by using the quadratic polynomial and periodic terms to fit the empirical force.This paper analyzes the influence of the factors such as the fitting parameters,the fitting time,the data interval,the orbit altitude and so on.The fitting experiments of measured data of GRACE-A satellite show that the average URE of the 20 min ephemeris fitting is 8.4 cm,and the average URE of the 10 min ephemeris fitting is 0.7cm.(2)Orbital list ephemerides of 15/16/18 parameters based on the RTN coordinate system were designed for LEO satellites.In consideration of the along-track feature of the atmospheric drag.The constant and periodic terms of the empirical force were decomposed to R/T/N directions.The advantage is that the parameters could be reduced according to the magnitude and variation characteristics of the perturbation components,which enhanced the scalability of the designed ephemerides.From users' point of view,the maximum visible time of the satellites was taken as the fitting time,which ensured the accuracy of the ephemeris and reduces the update frequency of users.The fitting experiments of the 16 parameters models show that,for the satellite at 300~1500km altitude,fitting errors of R/T/N directions are less than 11/10/3 cm,RMS of URE are less than 10 cm.The fitting results of the 18 parameters model show that fitting error of R/T/N three directions are less than 5/4/3 cm,and the RMS of URE are less than 4 cm.(3)The adaptive reconstruction of LEO satellite model with GPS16/18 parameters was studied.In order to solve the problem of small eccentricity,the fitting algorithm was improved: first,parameters based on the first nonsingular orbital elements were calculated;then,they were converted to standard broadcast ephemeris.The fitting experiments show that,for the satellite at 300~1500km altitude,the 16 parameters models' fitting errors in R/T/N directions are less than 7/17/3 cm,RMS of URE are less than 10 cm.the 18 parameters models' fitting errors of R/T/N directions are less than 3/7/1 cm,RMS of URE are less than 4 cm.(4)Interfaces of the designed 16/18 parameters ephemerides were designed.The orbital list ephemerides of 16/18 parameters take up 280/326 bits,which are reduced by 62/100 compared with the classical GPS16/18 ephemeris.The orbital elements ephemerides of 16/18 parameter type 314/360 bits,which are reduced by 28/66 bits compared with the classical GPS16/18 ephemeris.Orbital list ephemerides have a great advantage in the number of bits,and the parameters are more stable.Orbital elements ephemerides have a great advantage in the universality,and could be integrated with the existing GPS/BDS/GALILEO ephemerides.