The aim of this paper is to describe a strategy to provide improved accuracy of primary navigation parameters to the Indian regional Navigational satellites. The broadcast parameters of the Navic satellites can be classified under two category, one is the validity period of 7200s called long period normal sets and second one is the validity period of 900s called short period contingency sets of primary navigation parameters. The Navic satellite constellation works with minimum number of synchronous satellites to provide the best user solution within India and boundaries. Thus the outage of any one or more satellites will increase the Dilution of Precision (DOP) and degrades the user solution for any sudden anomalous behaviour of the measurements. This anomalous behaviour (called events) arises due to on-board frequency variations, frequent planned Station Keeping (SK) operations because of synchronous orbits, IRNSS system time scale switch-over or the combination of events. In loop back the broadcast parameters accuracy was continuously monitored through Line of Sight (LOS) range error from the observed IRNSS ground reference stations range measurements. In begin conditions the Least square (Lsq) based solutions were accurate and the observed LOS error were as expected. But post the occurrence of any event, the accumulation of batch data starts freshly for least square solution. The obtained Lsq based solution was inaccurate due to lack of data under many circumstances. During the events occurrence to minimize the outage duration of the satellite contingency sets were generated using continuously running Kalman Filter based near real time estimation using one-way measurements. But the filter estimated state may not be optimum at the particular epoch. Hence a strategy of forward and backward approach (FBF) was adopted just before parameter uplink through sequential (KF) for adaptation of real behaviours of the measurements to providing an improved optimal solution.

1. Introduction
2. Principles of Orbit Improvement
3. Dynamical System Modelling
4. Parameter Estimation and Accuracy Improvement
    A. Batch Least Square Process (OCEB)
    B. Sequential Kalman Filter Process (OCEK)
    C. Primary Parameter Generation Dynamical System
5. Smoothening and its improvements
6. User Equivalent Range Error (UERE)
7. Summary
8. Acknowledgements
References

• Prasanta Mula(Space Navigation Group, ISRO Satellite Center) Corresponding Author
• Babu R(Space Navigation Group, ISRO Satellite Center)
• Nirmala S(Space Navigation Group, ISRO Satellite Center)