Precise Orbit Determination for NavIC Satellites

Precise Orbit Determination (POD) is an important process for navigation applications like Precise Point Positioning (PPP) and Real Time Kinematics (RTK). Earlier researchers have carried out extensive research for GNSS constellations placed in Medium Earth Orbits (MEO) having dynamic behaviour different from Geo-Synchronous Orbits (GSO) satellites. In this paper an attempt is made to study the challenges that have to be addressed for implementing POD along with the corresponding ground segment. The Navigation with Indian Constellation (NavIC) is a satellite-based navigation system offering an independent positioning and timing service over India and neighbouring regions using constellation of 3 near Geosynchronous and 4 Inclined Geosynchronous (IGSO) Satellites[1]. Presently orbit estimation of these satellites is being carried-out based on statistical orbit determination techniques using measurements from a network of 16 IRNSS Range and Integrity Monitoring Stations (IRIMS) and 4 IRNSS CDMA Ranging Stations (IRCDR) at regular intervals. Satellite laser ranging data provided by a few stations of the International Laser Ranging Service (ILRS) is also being utilized to assess the quality of Signal in-Space Range Error (SISRE). Currently the NavIC system is able to provide a user position and velocity accuracy of around 10 meter and 1 cm/sec (3σ) using least square batch processing orbit determination methods. In order to meet the Real Time Kinematics (RTK)[3] and Precise Point Position Products (PPP)[4] wherein the user has to determine his position within sub-meter accuracy, it is necessary to incorporate Precise Orbit Determination (POD)[2] in the process which uses Extended Kalman Filter with smoother function. Along with this, by locating optimal number of International GNSS Service stations within the coverage area would further aid in improving the POD accuracy. The precise ephemeris for the NavIC constellation can be generated using measurements from a wide baseline of IGS stations. These measurements need to be processed by a precise orbit determination engine in order that very accurate estimates of the orbit and clock are generated in near real time. This paper addresses the implementation of such an engine for deriving the precise ephemeris of the NavIC constellation and thus support PPP and RTK services through NavIC.