Kevin D. Vanderwerf

Performance of Honeywell's Inertial/GPS Hybrid (HIGH) for RNP Operations

Honeywell has developed an algorithm that tightly integrates GPS and IRS into a hybrid navigation solution adequate to achieve 100% worldwide availability of RNP 0.1 without the use of differential corrections. This algorithm, referred to as Honeywell Inertial GPS Hybrid (HIGH), improves all four of the navigation characteristics critical for RNP operations - accuracy, integrity, continuity and availability. With its increased level of availability, HIGH can support RNP operations that are lower than can be achieved with stand-alone GPS. RNP operations require the navigation system provide an integrity bound called Horizontal Integrity Limit (HIL). An RNP operation can proceed as long as the HIL remains below the threshold required for that operational level. On average, stand-alone HILs are at least 50% worse than HIGH HILs under good satellite geometries, and become significantly degraded or unavailable when in RAIM holes or during GPS outages. HIGH enhances system continuity by coasting on the inertial when no satellite measurements are available. Honeywell has applied the HIGH technology on both military and commercial aircraft platforms. Operational benefits of HIGH are examined and contrasted with stand-alone GPS based solutions. HIGH is based on Honeywells solution separation method to provide the integrity level and the Fault Detection and Exclusion (FDE) capability. With this method, a bank of Kalman filters is used to provide multiple hybrid solutions, each excluding different combinations of zero, one or two satellites. The algorithm detects and isolates a satellite failure by comparing the various solutions. Since one of the solutions will not contain the effects of the satellite error, an uncorrupted solution is always available. Appendix R of RTCA/DO-229C provides the requirements and test procedures for tightly integrated GPS/Inertial systems. Honeywells HIGH implementations have demonstrated compliance to Appendix R. Compliance to Appendix R is critical to ensure a consistent basis for certification for these types of systems. The paper provides results from flight tests and simulations implementing the HIGH technology. These results support the claimed enhanced performance for integrity, continuity and availability and are contrasted with stand-alone GPS systems. The results clearly show that HIGH can support lower RNP operations such as RNP 0.1 as well as future lower RNP levels. Plans for enhancing the performance in future applications are also examined. I. INTRODUCTION The required navigation performance (RNP) concept allows aircraft to operate in a defined airspace based on the

Schuler pumping of inertial velocity errors due to gravity anomalies along a popular North Pacific airway

Gravity anomalies have long been a major error source for high precision inertial navigation, but were thought to have a minimal effect on the performance of the less precise inertial navigation systems used for commercial air transport. For the most part that is true. However, there are isolated areas where gravity anomalies are so significant that they become the dominant error source even for a commercial air transport inertial system. One such area, the Kuril trench, lies directly along a popular North Pacific airway. The anomalies along this airway not only are very large but vary in such a way as to Schuler pump the inertial velocity errors when flying at cruising speed. This paper describes the results of a recent night test program which led to the discovery of the unique variations along this airway.