Engineering Challenges of the HIAPER Cloud Radar: Real-time Active Platform Stabilization and Pressure/Temperature Control

The airborne HIAPER Cloud Radar (HCR) has participated in two test flight programs since 2013 and was recently deployed on its first rapid-response deployment in February 2015. Various engineering efforts were put in to mitigate the environmental challenges identified during initial test program. The new real-time, active platform stabilization system was implemented to achieve a real-time, nadir (or zenith) pointing cloud radar system.

To mitigate the velocity uncertainty due to horizontal winds contaminating nadir or zenith pointing observations, a platform-stabilized reflector system and algorithm was designed for HCR. With the real-time adjustment of reflector position, the antenna beam remains at the desired position. This design simplifies the post-flight correction needed to obtain true vertical velocity and minimizes the contamination from horizontal winds. It also brings a series of engineering challenges – accurate mechanical design, high rate inertia and position reporting. The most accurate velocity corrections are obtained with nadir-pointing and the ground echo is detected and can be utilized as a 0 m/s reference.

The large altitude range that HIAPER covers introduces additional challenges to HCR: receiver gain fluctuations due to temperature and steep pressure differential between exterior and system pressure vessel. The key components of the receiver have been temperature stabilized to minimize fluctuations. A nitrogen supplemental system is designed to provide additional pressure to the system pressure vessel. With this supplemental system, HCR can operate approximately 80 hours at 40,000 feet altitude with the current leak rate.

This paper describes the engineering implementations of the antenna stabilization system and the supplemental pressure system, the performance of the temperature stabilized receiver and the engineering challenges in maintaining a nadir-pointing beam during it's first rapid deployment, Nor'easter, in February 2015.