Qualifying an L5 SBAS MOPS Ephemeris Message to Support Multiple Orbit Classes (2013)

GPS has launched its first three L5 capable satellites and is slated to achieve L5 Full Operational Capability (FOC) by the year 2019. GLONASS has returned to a full constellation of 24 operational satellites and has plans to offer CDMA signals at both the L1 and L5 frequencies. The European Galileo and Chinese Beidou constellations are currently under construction and also intend to broadcast in both the L1 and L5 (or near L5) bands. Thus, it is possible that in the next decade, there could be four constellations suitable for use in aviation with signals at L1 and L5. The Radio Technical Commission for Aeronautics (RTCA) is developing an update to the Satellite Based Augmentation System (SBAS) Minimum Operational Performance Standards (MOPS) to include the use of GPS L5. EUROCAE is similarly developing dual frequency MOPS for Galileo. The intent is that these two efforts will be merged into a single dual-frequency, multi-constellation MOPS.

New satellite orbits ranging from highly eccentric inclined geosynchronous (IGSO), medium (MEO) to low Earth (LEO) orbits are being proposed for future Global Navigation Satellite Systems (GNSS) and Satellite Based Augmentation Systems (SBAS). It is desired that in the future, the MOPS integrity message be capable of being delivered by this wide range of orbit classes. This calls for a different ephemeris and almanac message which can handle this broad spectrum of proposed orbital regimes. In the past, the MOPS message used Earth fixed Cartesian position, velocity, and acceleration (9 degrees-of-freedom (DOF)) to describe the motion of the geostationary (GEO) SBAS satellites. The proposed next generation MOPS message will have to be much more sophisticated in order to encompass potential navigation and augmentation satellites ranging from GSO to LEO. A method of optimally fitting orbital elements has been devised which allows for a wide variety of orbital elements to be employed. A new 9 DOF set of orbital elements has been selected for the ephemeris message and a 7 DOF set for the almanac which function amicably for the range of orbits in question. 

This paper evaluates the proposed L5 MOPS ephemeris message in terms of the message population in practice. A more robust algorithm is proposed and different optimization / fitting schemes are evaluated in terms of CPU cost, robustness of convergence, and performance on weeks of actual orbit data produced by the WAAS system. This qualification is based on performance of the population algorithm in terms of functionality, accuracy, fault/failure rate, as well as computing cost as the fitting algorithms must function to within the specifications and limited CPU power of the current WAAS safety computer. A real-time Kalman-Filter based orbit determination system based on the JPL-NASA GIPSY/OASIS II system is used to generate the GEO trajectories for the MOPS. In order to examine the variety of orbit classes in question, orbit trajectories of similar fidelity are required for the evaluation. Analytical Graphic’s Inc. (AGI) commercial software package Satellite Tool Kit (STK) has a built in High Precision Orbit Propagator (HPOP) which is used to generate the additional high-fidelity trajectories required for the analysis. With this, analyses are carried out for GPS, GLONASS, BeiDou, WAAS, EGNOS, QZSS, Iridium, and other proposed orbit classes such as Molniya.