Rain rate measurement with an airborne scanning radar altimeter

The NASA Scanning Radar Altimeter (SRA) sweeps a radar beam of 1° half-power width (two-way) across the aircraft ground track within ±22° of nadir, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 points spaced across the swath at 0.7° incidence angle intervals. Flights were made into Hurricane Humberto on 23 and 24 September 2001, aboard a WP-3D hurricane research aircraft of the NOAA Aircraft Operations Center. The ranges produce raster lines of sea surface topography at a 10 Hz rate. The SRA was primarily designed to produce sea surface directional wave spectra, but the backscattered power measurements can be used to determine path integrated rain rate below the aircraft.