Hurricane Imaging Radiometer
HIRad at a glance
The Hurricane Imaging Radiometer (HIRad) is an airborne microwave sensor that will perform surface wind speed and rain rate measurements in hurricanes and typhoons.
First Aircraft Flight
HIRad's first aircraft flight will take place during the 2009 hurricane season.
HIRad’s airborne system engineering design has been completed. Prototype subsystems are in development.
By measuring wind speed and rain rates, the HIRad instrument provides insight into the genesis, structure, and evolution of a tropical cyclone. This will enable improved forecasting of its track, wind and rain intensity and storm surge. As the plane flies through a storm, HIRad senses microwave radiation naturally emitted by foam generated on the sea surface by winds. Algorithms then determine wind speeds based on the levels of microwave radiation detected. HIRad can also determine rainfall rates within a storm system. Operational aircraft overflights with HIRad of storms will produce wide swath (10s of km) and high resolution (1 km) imagery, which will dramatically improve model initialization and the resulting forecasts.
HIRad is a multi-frequency interferometric radiometer operating from 4 GHz to 7 GHz. A one-dimensional thinned synthetic aperture array antenna is used to obtain wide-swath measurements with multiple simultaneous beams in a push-broom configuration. HIRad features software beam forming with no moving parts, internal hot, cold, and noise diode based calibration, and continuous, gap-free imaging.
The basis of the HIRad design is the Stepped Frequency Microwave Radiometer (SFMR) that has successfully measured surface wind speed and rain rate in hurricanes from the NOAA Hurricane Research Division’s (HRD) P-3 aircraft. Unlike the SFMR that views only at nadir, the HIRad provides wide-swath measurements between ± 40 degrees in incidence angle with a spot-beam spatial resolution of approximately 1-3 km. HIRad would be able to provide high resolution hurricane imaging when used on an operational hurricane surveillance aircraft such as the NOAA HRD’s Gulfstream-IV.
Chris Ruf and his team at the University of Michigan work with NASA MSFC, the University of Central Florida, RTI, and NOAA.
Engineering Feats of Note
When flying at an altitude of 35,000 ft (11 km), HIRad provides a measurement swath of 22 km and a spatial resolution of 1-3 km depending upon the operating frequency and cross-track position in the swath. This geometry provides excellent opportunity to image the high wind gradients and spiral rain bands surrounding the hurricane eye while flying the typical “butterfly” transects. This gives HIRad an obvious advantage over a profiling sensor such as SFMR. The equivalent SFMR coverage would be a single strip (one pixel wide) centered along each the aircraft track.
The HIRad also has the potential for space borne application.