Bistatic Interferometry to Measure Clear Air Wind

The wind field can be measured either by Doppler or interferometry techniques. Weather radars such as WSR-88D measure the Doppler velocity (i.e., the radial component of wind). The proposed BIstatic radar NETwork (BINET) measures the Doppler wind component perpendicular to the surface of constant phase. NCAR's Multiple Antenna Profiler Radar (MAPR), a Spaced Antenna (SA) system, measures the cross-beam wind component (i.e., the component parallel to the baseline connecting two receivers) using interferometry. Meteorological applications of interferometry have been limited to nearly co-located transmitter/receivers, i.e., a bistatic system with extremely close transmitter receiver spacing (i.e., typically less than the transmitting antenna diameter). In clear air sensing, however, a bistatic configuration with a small scattering angle is desirable to increase radar detection of echoes from Bragg scatterers. This could allow wind measurements, especially if atmospheric plankton is absent, and possibly the depth and growth of the mixed layer. The NSF-funded Collaborative Adaptive Sensing of Atmosphere (CASA) project offers the possibility for bistatic radar deployment.

In this paper, we propose and illustrate bistatic interferometry (BI) to measure the wind components perpendicular to the scattering plane containing the transmitter and a pair of closely spaced receivers separated from the transmitter by the distance between radars in the CASA network (i.e., approximately 30 km). This could allow the measurement of mesoscale convergence throughout the mixed layer. The BI technique includes (1) Bistatic Spaced Antenna Interferometry (BSAI) and (2) Bistatic Angular Interferometry (BAI). The BSAI is based on a bistatic configuration with a pair of receiver antennas that allows interferometry signal processing (i.e., cross-correlating the two signals). In the BAI, (similar to angular interferometry (AI) with a single antenna previously reported, but in a bistatic mode), two sets of signals are collected from two overlapped bistatic radar resolution volumes and jointly processed. Both BSAI and BAI should allow reliable cross-beam wind measurements. It is expect that the interferometry technique in bistatic configuration performs better than the monostatic SA case because of the increased effective wavelength and correlation time.