Phased-Array Design for Biological Clutter Rejection: Simulation and Experimental Validation

Abstract

This paper highlights recent results obtained with the Turbulent Eddy Profiler (TEP), which was developed by the University of Massachusetts. This unique 915-MHz radar has up to 64 spatially separated receiving elements, each with an independent receiver. The calibrated raw data provided by this array could be processed using sophisticated imaging algorithms to resolve the horizontal structures within each range gate. After collecting all of the closely spaced horizontal slices, the TEP radar can produce threedimensional images of echo power, radial velocity, and spectral width. From the radial velocity measurements, it is possible to estimate the three-dimensional wind with high horizontal and vertical resolution. Given the flexibility of the TEP system, various array configurations are possible. In the present work exploitation of the flexibility of TEP is attempted to enhance the rejection of clutter from unwanted biological targets. From statistical studies, most biological clutter results from targets outside the main imaging field of view, that is, the sidelobes and grating lobes (if they exist) of the receiving beam. Because the TEP array's minimum receiver separation exceeds the spatial Nyquist sampling requirement, substantial possibilities for grating-lobe clutter exist and are observed in actual array data. When imaging over the transmit beam volume, the receiving array main lobe is scanned over a Ϯ12.5°region. This scanning also sweeps the grating lobes over a wide angular region, virtually guaranteeing that a biological scatterer outside of the main beam will appear somewhere in the imaged volume. This paper focuses on suppressing pointlike targets in the grating-lobe regions. With a subtle change to the standard TEP array hardware configuration, it is shown via simulations and actual experimental observations (collected in June 2003) that adaptive beamforming methods can subsequently be used to significantly suppress the effects of point targets on the wind field estimates. These pointlike targets can be birds or planes with strong reflectivity. By pointlike the authors mean its appearance is a distinct point (up to the imaging resolution) in the images. The pointlike strong reflectivity signature exploits the capability of adaptive beamforming to suppress the interference using the new array configuration. It should be noted that this same array configuration does not exhibit this beneficial effect when standard Fourier beamforming is employed.

Effects of Wind Field Inhomogeneities on Doppler Beam Swinging Revealed by an Imaging Radar

“…Because of the flexible and simultaneous beams provided through CRI, the TEP radar is ideal for the study of various DBS configurations with different pointing angles and numbers of beams. In addition, images of echo power and radial velocity across the region of across the DBS beams are obtained through the socalled pulse-pair beam-forming (PPB) process after successful phase calibration (Cheong et al 2004). The PPB is an efficient joint implementation of beam forming and the pulse-pair processor, which can be used to estimate the spectral moments without the need of synthesizing time series for each beam position.…”

“…Coherent radar imaging (CRI) was developed to reconstruct a two-dimensional, angular distribution of the atmospheric structure within the volume illuminated by the transmitted beam (e.g., Kudeki and Sürücü 1991;Palmer et al 1998;Yu et al 2000;Hélal et al 2001). In addition, the angular distribution of radial velocity can be obtained by an efficient algorithm (Cheong et al 2004). CRI can be thought of as a beam-forming technique, in which a number of receiving beams can be synthesized simultaneously by coherently combining signals from spatially separated receivers.…”

“…TEP is a unique 915-MHz boundary layer imaging radar with an array of up to 64 independent receivers. Using adaptive array processing, threedimensional volumetric images can be constructed within the field of view with a temporal resolution of approximately 2.5 s (Lopez-Dekker and Frasier 2004;Cheong et al 2004). In addition to imaging clear-air turbulence, the TEP radar has been used for studies of the interaction of precipitation with turbulence (Palmer et al 2005).…”

New Observations by Wind Profiling Radars

“…Using CRI, moving clutter is able to be suppressed. From the CRI measurement using TEP, Cheong et al (2006) succeeded in separating clear-air echoes and the biological scattering which was moving in the grating-lobe region, and demonstrated that the separation of biological scattering greatly reduced the error of wind velocity estimates. Chen et al (2007) also applied CRI to data measured by multiple antenna profiler radar (MAPR) of National Center for Atmospheric Research (NCAR) in order to mitigate effects of bird contamination in wind velocity estimates.…”

Advanced intermittent clutter filtering for radar wind profiler: signal separation through a Gabor frame expansion and its statistics

“…It mostly affects L-band and UHF-systems, that is Boundary Layer profilers and Tropospheric profilers, as discussed in Wilczak et al (1995). Intermittent clutter is an issue for the standard Doppler-beam swinging radars as well as for spaced antenna and imaging radar systems, where new mitigation techniques like adaptive beamforming have recently been proposed (Cheong et al, 2006;Chen et al, 2007). We mention in passing that other remote sensing instruments used in Meteorology are also affected by migrating birds (Mastrantonio et al, 1999;Gauthreaux and Belser, 1998;Gauthreaux et al, 1998;Zhang et al, 2005;.…”