Synergistic Effects of Doppler Radar/GPS Navigation Integration and the Development of an Advanced Navigation System for Helicopter Applications

Developments are underway to embed GPS into Doppler radar navigation systems for various military applications. The advantages of combining GPS and Doppler into a single system are numerous, the most powerful involving the synergistic effects of continuous, accurate navigation during long duration, hostile missions. Enhanced “fly-through” capability is achieved by using the Doppler radar when the GPS is jammed or operating with a reduced satellite constellation. The Doppler radar navigation systematic errors are reduced through the use of calibration algorithms in the sensor fusion processor. An important application of this technology is the large fleet of Army utility helicopters that already have Doppler navigation systems, and need GPS to enhance their capability. This application requires minimum impact on helicopter installation and associated costs, and minimal changes in current system operating procedures to avoid extensive pilot training. This paper discusses the approach taken to integrate GPS into the AN/ASN-128 helicopter Doppler Navigation System that is currently under development for the U.S. Army. The system architecture and hardware subsystems are described, including the performance characteristics of the Doppler radar and embedded GPS receiver. The expanded operational features of the combined system are discussed, with concentration on low-impact GPS-compatible human interfaces, avionics retrofit and enhanced operational features. Critical features covered include: data loading of waypoints and almanac data, automatic GPS updating to Doppler, Doppler and Baro- Altimeter aiding to GPS under reduced satellite coverage conditions, vertical navigation for reduced minimums landing capability and mode control. This paper briefly examines candidate mixing techniques for combining GPS and Doppler radar sensor information. A combined GPS/Doppler error model is analyzed to determine performance enhancements during navigation periods without GPS. This analysis shows the overall system will continue to provide critical mission data for up to 30 minutes. This paper shows that a Doppler radar navigation system with an embedded GPS receiver achieves the goals of retrofitting modern helicopter platform avionics: performance improvements, minimal impact on aircraft installations, and minimal impact on the crew.