GPS III Signal Integrity Improvements

S. Shaw, A.J. Katronick

Abstract:	GPS III, the next generation of satellites in the GPS constellation, has been designed to meet explicit signal integrity requirements defined by the GPS Directorate to support all classes of GPS users. Signal integrity is the degree of trust that a user of the GPS signal can place in the correctness of received signal and associated data. For GPS III, those requirements are defined in terms of signal anomaly type (e.g., a user range error), an anomaly magnitude and a probability of occurrence. These requirements will allow users to define and bound potential hazards in the signal. To meet these requirements new space vehicle design features and safety analyses have been included as part of GPS III program development. These efforts will result in improved signal integrity and a solid foundation for assumptions on basic GPS III satellite performance that are used in the development of user equipment and for GBAS and SBAS that provide additional integrity to users that need it. GPS III design features provided to meet these signal integrity requirements include more robust on-board monitoring functions that will detect anomalous conditions and trigger a switch to Non-Standard Code to protect users. A comprehensive set of analyses, in accordance with SAE ARP-4754, have been performed to identify and quantify potential failure conditions that could lead to signal errors in excess of error thresholds defined by the requirements. These include Functional Hazard Analysis (FHA), Fault Tree Analysis (FTA), Common Cause Analysis (CCA), Failure Mode Effects and Criticality Analysis (FMECA), and other specific safety analyses. The results of these analyses provide predictions for the rate of occurrence (probability per hour) of signal faults of different types as well as the rate of signal outages. Hardware and software development processes on GPS III have also been subjected to development assurance audits to comply with the requirements of RTCA DO-254 and RTCA DO-278, respectively. The intent of this paper is to provide an overview of the GPS III integrity requirements, space vehicle safety design, and implementation of the system safety development process. An overview of the GPS III safety development process will be provided, including defined hazard categories, resulting Development Assurance Levels (DAL), and process audit activities. An overview of the SAE ARP-4754 methodologies as applied on GPS III will be provided and is of particular interest since application of these methodologies to space vehicle development has never been done before. Analysis results using these methodologies and which incorporate space vehicle integrity design features will be provided, including major branches of the fault tree and results of performance predictions for each type of anomaly defined in the GPS III design specification. This includes predictions for each of the major classes of potential SV anomalies that could result in a signal error. Predictions for SV outage rates will also be provided, including categories of hardware and software faults that could result in an unscheduled signal outage. Integrity design features included in the GPS III design will be discussed. The GPS III design includes monitoring functions intended to detect anomalous conditions that have the potential to result in signal errors in excess of defined integrity requirements and remove the signal under these conditions. These functions include: • Clock phase monitoring, • Space vehicle attitude monitoring to detect dynamic conditions that could result in an erroneous signal • Circuit fault detection within the signal generation functions • Space vehicle processor fault detection.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	936 - 945
Cite this article:	Shaw, S., Katronick, A.J., "GPS III Signal Integrity Improvements," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 936-945.
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