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Bangalore, India

Rajarajan D.,Space Navigation Group | Babu R.,Space Navigation Group | Saha S.,Space Navigation Group | Rathnakara S.C.,Space Navigation Group
Indian Journal of Radio and Space Physics | Year: 2013

Radio occultation (RO) is the Global Navigation Satellite System (GNSS) based remote sensing of Earth's atmosphere for atmospheric parameter retrieval and total electron content (TEC) computation. Several RO missions have been launched by ISRO, viz. Meghatropiques and Oceansat-2, for atmospheric studies. Current implemented methods are able to provide the parameters only in post processed mode and hence, are available after some duration. This is due to the limitation of availability of precise satellite orbit for GNSS and LEO satellites which serves as one of the fundamental input to the process. However, a faster turnaround time is possible if the parameters are computed in near real time from the on-board solutions. This paper highlights the work that has resulted in optimizing the atmospheric parameter retrieval and TEC computation using the radio occultation technique.

Neetha T.,Space Navigation Group | Kartik A.,Space Navigation Group | Ratnakar S.C.,Space Navigation Group | Ganeshan A.S.,Space Navigation Group
Journal of Spacecraft Technology | Year: 2011

The Indian Regional Navigation Satellite System (IRNSS) is planned to be a constellation of 7 satellites - 3 in GEO and 4 in GSO orbit. This constellation is expected to provide position accuracies similar to GPS in a region centered on India with a coverage extending up to 1500 km around India. The IRNSS users continuously receive navigation information from the IRNSS satellites in the form of data bits modulated on the navigation signals. This information is computed and controlled by the IRNSS Navigation Center(INC), includes the satellite's time, its clock correction and ephemeris parameters, almanacs and health for all other IRNSS satellites in the constellation, ionospheric delay parameters and text messages. From this information, the users compute the satellite's precise position and clock offset and less precise positions and clock offsets of other satellites in the constellation. The IRNSS navigation message design process included numerous trade-off studies which weighed various representations and algorithms against variables such as message size, accuracy, update frequency and user computational requirements. Other factors such as parameters similar to GPS legacy, graceful degradation and future user requirements were also considered. A new format called IRNSS Broadcast Data Structure (Hybrid Data Structure) is designed that is a blend of fixed format of Sub frame structure for accuracy related parameters (time, ephemeris and clock) and flexible format of messages for value added services like: ionospheric grid parameters, text messages, and differential corrections etc. Finally, upon selecting the appropriate design structure, the design was fine tuned to its final form and user algorithm implementation trade-offs were performed. The representation algorithms and user algorithms were jointly tested using a simulated satellite ephemeris trajectory and satellite clock model. The results of these tests demonstrate that the user models represent the simulated ephemeris to within 0.07m with precise parameters, and to within 0.1m with truncated parameters.

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