Maturation of GPS III Signal Integrity Improvements

Abstract

At the ION GNSS+ 2013 conference Lockheed Martin presented GPS III signal integrity capabilities that were being developed and had achieved a Critical Design Review level of maturity. Since that time, analysis of the spacecraft signal integrity capabilities has continued to maturation given as-built SV results and with finalization of planned development assurance activities that satisfy the requirements of RTCA/DO-254 and RTCA/DO-278. This revision to the 2013 publication provides finalized performance predictions based on the as-built spacecraft and summarizes the accomplishments of the development assurance process. 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. These requirements will allow users to define and bound potential hazards in the signal. To meet these requirements, new Space Vehicle (SV) 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 Ground Based Augmentation Systems (GBAS) and Space Based Augmentation Systems (SBAS) that provide additional integrity to users that need it. These integrity improvements along with improvements in User Range Accuracy can provide enhanced Vertical Protection Levels for aviation users that apply Advanced Receiver Autonomous Integrity Monitoring algorithms, as discussed in [6] and higher service availability, allowing aviation users to conduct more operations, in more parts of the world for greater periods of time. GPS III design features provided to meet these signal integrity requirements include more robust on-board monitoring functions that detect anomalous conditions and rapidly switch the broadcast signal to Non-Standard Code (NSC) to prevent users from tracking a potentially erroneous signal. A comprehensive set of analyses, in accordance with Society of Automotive Engineers (SAE) Aerospace Recommended Practices (ARP) 4754, have been performed to identify and quantify potential failure conditions that could lead to signal errors in excess of thresholds defined by the GPS III requirements. These analyses 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 and service history to verify compliance with the objectives of civil aviation standards, namely RTCA/DO-254 and RTCA/DO-278. In addition, audits have been conducted at the SV level to show compliance with SAE ARP4754. Signal integrity improvements have matured and come to closure for the initial GPS III satellite.