Studies on Precise Orbit Determination for the Regional Navigation Satellite

A mixed navigation constellation is employed in the regional satellite navigation system.The GEO(Geostationary Orbit) satellite plays a significant role in the system,and the POD(Precise Orbit Determination) is the key technology in the regional satellite navigation system.Therefore,how to employ several kinds of tracking techniques to achieve multi-mode and multi-level precise orbit determination for navigation satellites is worthy of in-depth study in regional satellite navigation system. Based on the framework of regional satellite navigation system and multi-technology and multi-mode,POD for navigation satellites is studied in this paper.Several GEO satellite POD strategies are put forward,so are the POD strategies based on time synchronization framework. Based on the short-arc tracking,rapid orbit recovery has been achieved.Summarily,the main contents are as following: (1)The basic theory of navigation systems is introduced,and definitions of the clock error and the pseudo-range are put forward.Measurement models and error correction models for different kinds of measurements are analyzed and discussed. (2)Based on the UTOPIA software,a POD software for navigation satellites has been accomplished, which can deal with many types of measurements,including pseudo-range data,transfer ranging data,satellite laser ranging(SLR) data,and uplink/downlink measurements of satellite-ground time synchronization.In this software,clock offsets of satellites and stations are modeled flexibly and effectively,and POD for navigation satellites has been achieved combining different kinds of tracking data. (3)Two approaches to calibrate ranging biases of the system are proposed,namely the colocation comparison of two tracking systems and the combined POD method,with calibration accuracies estimated to be 0.5 ns and better than 1 ns respectively.The calibration problem for time delays has been solved. (4)A POD strategy based on the combination of SLR and C-band transfer ranging for GEO is presented.Using data from a C-band tracking network in China,POD experiments indicate that meter-level POD accuracy is achievable for GEO.Root-mean-square(RMS) of the post-fit C-band ranging data is about 0.205 m.The radial component errors of POD are evaluated with SLR data from a station in Beijing,with residual RMS of 0.133 m.Orbital overlapping experiments show the total orbit error is a few meters.Computations of SLR residuals also suggest that for 2-hour prediction,the predicted radial error is about 0.373 m.The POD for GEO has been achieved. (5)Another GEO combined orbit determination strategy is studied in this paper,which combines pseudo-range data and C-band biased ranging data.This strategy successfully overcomes the C-band ranging data deficiency problem caused by limited stations and tracking time available. With the combination of biased ranging and psudorange data,clock offsets between the GEO and stations may be estimated simultaneously along with orbital parameters,maintaining self-consistency between the satellite ephemeris and clock offsets parameters.Using data collected for the GEO satellite of Regional satellite navigation system with 3 C-band biased ranging stations and 4 L-band pseudo-range tracking stations,POD experiments indicate that meter-level accuracy is achievable.Root-mean-square(RMS) of the post-fit C-band ranging data is about 0.203 m,and the RMS of the post-fit pseudo-range is 0.408m.Radial component errors of the POD experiments are independently evaluated with SLR data from a station in Beijing,with the residual RMS of 0.076 m.SLR validation also suggests that for 2-hour orbital predication,the predicted radial error is about 0.404 m,the clock offset error is better than 1.5 ns.Even for the combination of one C-band biased ranging station and 4 pseudo-range stations,POD is able to achieve reasonable accuracy with radial error of 0.280 m and 2 h predicted radial error of 0.888 m.Clock synchronization between the GEO and tracking stations is achieved with an estimated accuracy better than 1.6 ns. (6)By employing simulation data,the issue of GEO POD is studies based on several kinds of measurements,including"transfer ranging and pseudo-range","SLR and pseudo-range", and"SLR and transfer ranging".The POD strategies and accuracy are discussed based on the three different modes.Effects of the quadratic term of the clock offset and the effect of the pseudo-range stations on POD accuracy are discussed.Clock offsets of the satellite and stations are estimated simultaneously,maintaining self-consistency between the satellite ephemeris and clock offsets parameters.Effects of systematic errors on POD accuracy are analyzed and discussed based on different POD modes.POD has been achieved based on a constrain from a single transfer ranging station.Systematic errors are calibrated in the POD process combining the SLR and transfer ranging data. (7)POD features of GEO satellites are discussed based on the undependant time synchronization framework in Regional satellite navigation,including the satellite-ground and ground-ground time synchronization modes,the single satellite-ground time synchronization mode,and the single ground-ground time synchronization mode.By employing simulation data,the POD possibilities are probed based on the three modes.Results show that systematic errors including clock offsets and time delays are the main factor for the GEO POD accuracy,and that 2 ns accuracy is required to achieve POD with the accuracy of better than 10 m.For the mode of ground-ground time synchronization,two time transfer stations are required at least,and time synchronization for satellite-ground and ground-ground of all stations could be achieved.But POD accuracy of this mode is not stable,with estimated parameters of highly relevant.In view of accuracy of the satellite orbit and clock offsets,the mode of satellite-ground and ground-ground time synchronization is much better than other two modes.The quadratic term of the clock offset and the clock offsets of both the satellite and stations are required to estimate for different POD modes. (8)Three kinematic orbit determination approaches are brought forward to achieve rapid orbit recovery based on short-arc tracking,they are polynomial fitting approach based on the single point position determination,polynomial fitting approach based on pseudo-range,and ephemeris parameters fitting approach.These approaches overcome divergence of dynamical orbit determination approach,and obtain satellite velocity information,therefore POD is achieved based on the short-arc tracking.POD tests based on short-arc kinematic orbit determination approaches are carried out,using measurements from the MEO and GEO satellites with a 10 minutes tracking arc.Results show that:for the MEO satellite,the position accuracy is better than 10 m,and the velocity accuracy is 2 cm/s,and 5-min orbit prediction accuracy is 15.02 m; for the GEO satellite,the position accuracy is better than 19 m,and the velocity accuracy is 4 mm/s which is better than the MEO satellite,and 5-min orbit prediction accuracy is 17.76 m, 10-minute orbit prediction accuracy is 18.168 m.