Measuring Directional Wave Spectra, Mean Square Slope and Rain Rate with the NOAA Wide Swath Radar Altimeter

The Wide Swath Radar Altimeter (WSRA) of the NOAA/ESRL/Physical Sciences Division is an airborne instrument that produces a real-time topographic map of the sea surface. WSRA is installed on NOAA’s WP-3D aircraft, and over the past decade has routinely operated during reconnaissance flights into hurricanes. From the sea surface topographic maps, WSRA processing software estimates the following data products: directional wave spectra, the mean square slope of the sea surface, and rain rate. These data products are calculated and transmitted in real-time to the National Hurricane Center. The data are also made available on a public website during and after the flights.

The WSRA is a digital beamforming radar with an antenna comprised of 64 narrow microstrip subarrays oriented in the along-track direction, and spaced at half wavelength intervals in the cross-track direction. The radar returns from sequential transmissions on each of the 62 active array elements are collected and coherently combined to produce 80 narrow beams spread over ±30º from the antenna boresight, covering a swath of about 3.46 km at the typical 3 km (10,000 feet) aircraft altitude.

The WSRA ocean directional wave spectra, important information about the air-sea interface, are produced at a 50-second rate. That makes the WSRA a unique instrument that routinely documents the rapid spatial variation of sea surface inside the evolving hurricane.