Putting the Standardized GBAS Ionospheric Anomaly Monitors to the Test

M. Harris, T. Murphy

Abstract:	The development of GBAS continues with the introduction of an additional type of approach service intended to support approach and landing operations in CAT III conditions. This new service type, known as GBAS Approach Service Type D, or GAST D, is based on a reallocation of some responsibilities between the airborne and ground segments of GBAS. Notably, responsibility for detection and mitigation of the effects of anomalous ionospheric conditions has largely been allocated to the airborne segment and has been addressed through introduction of standardized monitors. Those monitors are defined in the latest version of the RTCA LAAS Minimum Operational Performance Standard (MOPS), DO-253C [1]. They include code carrier divergence monitoring during all phases of flight, Receiver Autonomous Integrity Monitoring (RAIM), and geometry screening that limits the projection of range measurement errors into the position domain. Additionally, the position solution computed for guidance is based on a new set of 30-second-smoothed differential corrections for comparison with a position based on the existing 100-second-smoothed corrections, both as a monitor and as a means for reduction of code-carrier divergence. This paper discusses how these airborne monitors work, how they work in conjunction with monitoring done in the ground system, and how they relate to the assessment of airborne performance that is a key part of the airworthiness certification process. The paper goes on to describe a program for validation and testing these monitors to show that their performance is adequate for the intended use. The program includes a combination of analysis and simulation to determine the effectiveness of the monitors to mitigate the effects of anomalous ionospheric conditions. The paper shows how analysis of the monitoring requirements can be combined with the aircraft geometry screening to quantify the post monitoring magnitude and probability of errors induced by ionospheric anomalies. Airframe manufacturers need such a characterization of the errors in order to be able to complete performance simulations that are a standard component of airworthiness approvals.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	1124 - 1140
Cite this article:	Harris, M., Murphy, T., "Putting the Standardized GBAS Ionospheric Anomaly Monitors to the Test," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 1124-1140.
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