Design Considerations for Developing Airborne Dual-Polarization Dual-Doppler Radar

Recently, ground-based phased array radar (PAR) demonstrated the swift estimation of accurate Doppler velocity and reflectivity of precipitation and clouds when compared to mechanically scanning radar. PAR uses electronic scanning (e-scan) to rapidly collect radar measurements. Since an airborne radar has only a limited amount of time to collect measurements over a specified sample volume, the e-scan will significantly enhance temporal and spatial resolutions of airborne radar observations. At present, airborne weather radars, such as NCAR's Electra Doppler radar (ELDORA), use mechanical scan and they are not compatible for collecting dual-polarization radar measurements.

The ELDORA is a slotted waveguide array with a dual-transmitter, dual-beam, rapid scan and a step-chirped waveform. These attributes significantly improved the spatial scale from >1 km to 300m (Hildebrand et al. 1996, Bluestein and Wakimoto, 2003). To date ELDORA is the most sensitive scanning airborne radar while also collecting measurements at the highest possible spatial and temporal resolutions. However, ELDORA's X-band frequency limits penetration into heavy precipitation owing to attenuation, and it is not designed, nor can it be modified, to collect polarimetric microphysical measurements. Signal attenuation in precipitation at X-band is about a factor of five to seven times larger when compared to C-band (Bringi et al, 2001).

NCAR/EOL is investigating potential configurations for the next generation of airborne radar that is capable of retrieving dynamic and microphysical characteristics of clouds and precipitation (Loew et al. 2007, Moore et al. 2007). NCAR maintains a C-130 aircraft in its fleet for airborne atmospheric measurements, including dropsonde, in-situ sampling and remote sensing of clouds, chemistry and aerosols. Therefore, the addition of a high resolution precipitation radar to the NSF/NCAR C-130 platform will produce transformational enhancement in its mission to benefit of cloud and aerosol physics, precipitation physics, chemistry and convective dynamics communities, among others.

This paper presents the concept, and preliminary design of a C-band, airborne, dual-polarization, dual-Doppler precipitation radar. EOL proposes to develop a novel airborne phased array radar (APAR) to be operational on NSF/NCAR C-130 aircraft with improved spatial resolution and polarimetric measurement capability. Preliminary design specification of the APAR that may replace the ELDORA are described in this paper. Design specifications of PAR are more stringent than a ground-based PAR.