Managing Long Time Constant and Variable Rate Carrier Smoothing for DFMC GBAS

The International Civil Aviation Organization (ICAO) Ground-Based Augmentation System (GBAS) Working Group (GWG) is working on the development of standards for Dual-Frequency Multi-Constellation (DFMC) GBAS. At the time of this writing, this standards work is well underway but not yet complete. Architecture trade studies were conducted in order to select the overall architecture and to develop the concept of operation (ConOps) for DFMC GBAS. A baseline architecture is emerging and additional trade studies are underway to refine and mature the architecture. A previous paper discussed an alternative architecture candidate proposal for consideration for the future DFMC GBAS based on uplinking raw measurements in addition to the differential corrections that are already uplinked to support legacy GBAS service types. A second paper extended the work to provide greater clarity regarding proposed ionospheric gradient monitoring techniques. One of the advantages of the alternative architecture is that it enables the use of Divergence Free Smoothing (DFS) and longer smoothing time constants (e.g., 300 to 600 seconds versus 100 seconds). Raw measurements are uplinked to the aircraft and the smoothing algorithms are applied in the airborne equipment. This enables the airborne equipment to use a variety of smoothing types and smoothing intervals depending on the situation. This paper gives information about how the various smoothing types can be managed and how variable rate smoothing can be applied to converge smoothing filters in a timely manner. This will be particularly important in equatorial regions where scintillation may cause frequent loss of lock. The alternative architecture includes a mechanism to initialize airborne smoothing of the ground system measurements using information computed on the ground. This paper gives greater detail on this initialization scheme and how it can enable faster convergence even with variable rate smoothing. Finally, the paper explores how appropriate error characterization parameters (often referred to as “standard deviations” or “sigmas”) can be computed for the outputs of variable rate smoothing filters and fixed interval smoothing filters during convergence. This error characterization will enable the smoothed pseudorange measurements to be appropriately weighted in the position solution and integrity monitoring algorithms even before the smoothing fully converges to the target smoothing interval. The alternative architecture also includes a mechanism for initializing the ground component of the ionospheric gradient monitoring detection statistic. This paper describes the mechanism and describes the algorithm for the application of the uplinked data in the airborne equipment.