A contrast-based phase calibration algorithm for digital beamforming remote sensing radars using three contrast metrics is presented. The algorithm corrects time-varying antenna array phase errors that defocus digital beamforming remote sensing radar imagery. Amplitude errors are treated by equalizing the received powers in all elements. As such, the algorithm does not produce an absolute (or radiometric) calibration vector for the array. The performance of the algorithm is studied using a combination of simulated and real radar data under various conditions and is compared with a clutter-based calibration algorithm. An analytical proof showing that maximizing the expected value of the 4-norm metric is equivalent to phase-calibrating the image, except for a linear phase offset, is provided. We find that the clutter calibration algorithm performs best for statistically homogeneous scenes but that the contrast-calibration algorithms perform better with scenes with larger contrast ratios.

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In this work, the accuracy of the Doppler beam-swinging (DBS) technique for wind measurements is studied using an imaging radar-the turbulent eddy profiler (TEP) developed by the University of Massachusetts, with data collected in summer 2003. With up to 64 independent receivers, and using coherent radar imaging (CRI), several hundred partially independent beams can be formed simultaneously within the volume defined by the transmit beam. By selecting a subset of these beams, an unprecedented number of DBS configurations with varying zenith angle, azimuth angle, and number of beams can be investigated. The angular distributions of echo power and radial velocity obtained by CRI provide a unique opportunity to validate the inherent assumption in the DBS method of homogeneity across the region defined by the beam directions. Through comparison with a reference wind field, calculated as the optimal uniform wind field derived from all CRI beams with sufficient signal-to-noise ratio (SNR), the accuracy of the wind estimates for various DBS configurations is statistically analyzed. It is shown that for a three-beam DBS configuration, although the validity of the homogeneity assumption is enhanced at smaller zenith angles, the rootmean-square (RMS) error increases because of the ill-conditioned matrix in the DBS algorithm. As expected, inhomogeneities in the wind field produce large bias for the three-beam DBS configuration for large zenith angles. An optimal zenith angle, in terms of RMS error, of approximately 9°-10° was estimated. It is further shown that RMS error can be significantly reduced by increasing the number of off-vertical beams used for the DBS processing.@en

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 three-dimensional 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 ±12.5° 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 regious. 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.@en

A contrast-based phase calibration algorithm for digital-beamforming radars using three contrast metrics is presented. The performance of the algorithm is studied using a combination of simulated and real radar data. We find that the entropy metric yields the smallest calibration vector, but that the M ₃ metric appears more robust in low SNR conditions.

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For many years, spatial and temporal inhomogeneities in precipitation fields have been studied using scanning radars, cloud radars, and disdrometers, for example. Each measurement technique has its own advantages and disadvantages. Conventional profiling radars point vertically and collect data while the atmosphere advects across the field of view. Invoking Taylor's frozen turbulence hypothesis, it is possible to construct time-history data, which are used to study the structure and dynamics of the atmosphere. In the present work, coherent radar imaging is used to estimate the true three-dimensional structure of the atmosphere within the field of view of the radar. The 915-MHz turbulent eddy profiler radar is well suited for imaging studies and was used in June 2003 to investigate the effects of turbulence on the formation of rain. The Capon adaptive algorithm was implemented for imaging and clutter rejection purposes. In the past several years, work by the authors and others has proven the Capon method to be effective in this regard and to possess minimal computational burden. A simple but robust filtering procedure is presented whereby echoes from precipitation and clear-air turbulence can be separated, facilitating the study of their interaction. By exploiting the three-dimensional views provided by this imaging radar, it is shown that boundary layer turbulence can have either a constructive or destructive effect on the formation of precipitation. Evidence is also provided that shows that this effect can be enhanced by updrafts in the wind field.

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A combination of pulse pair processing and beamforming techniques referred as pulse beamforming (PPB) for estimating velocity and spectral width using beamforming techniques was discussed. A statistical investigation into the applicability of Capron and fourier PPB techniques for high-resolution estimation of the wind and reflectivity field maps was also carried out. The pulse pair processing was extended to the beamformed imaging systems by using reflectivity derived weight vectors to estimate the pulse pair parameters for both velocity and spectral width estimates. By numerical simulation, the new technique provided robust and computationally efficient estimates of the spectral moments.@en

An implementation of a radio acoustic sounding system (RASS) using a UHF volume imaging radar is presented. The volume-imaging ability of the radar permits the study the spatial structure of the RASS echo observing both the diffraction pattern on a receiving antenna array and the beamformed images of the RASS intensity. Fine space and time resolution RASS observations of a developing diurnal boundary layer are considered, where observed structures are consistent with theoretical predictions presented in the RASS literature. The RASS amplitude signature is highly variable in space and time, hindering the estimation of complete instantaneous fields of virtual temperature. However, it is possible to obtain spatial statistics. Structure functions computed explicitly show r2/3 behavior typical of isotropic turbulence and vertical profiles of the structure function parameter show a z-4/3 behavior, although they are significantly larger than expected based on surface measurements.@en

The calibration of large antenna arrays in the absence of a beamforming point source is a common problem in beamforming radars and interferometric radiometers. In this paper the fundamentals of the redundant space calibration (RSC) method for phase and amplitude are reviewed, pointing out the parallelism between the active and passive cases, the technique is then applied in a general and systematic way to two cases of interest: hexagonal planar and Y-shaped arrays, which are known to be the optimum periodic two-dimensional configurations. In both cases, the system of equations is determined taking into account the available redundancies and symmetries of these structures. The performance of the RSC method is analysed in terms of the propagation of errors in reference phase/amplitude. The technique is then considered for two hexagonal array systems: the Turbulent Eddy Profiler (TEP), a volume-imaging radar of the lower atmosphere developed at the University of Massachusetts, and the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), an L-band interferometric radiometer to be launched in 2005 in the Earth Explorer Opportunity Mission-Soil Moisture and Ocean Salinity Mission (SMOS) of the European Space Agency (ESA). In the TEP case the proposed scheme stabilizes an otherwise ill-posed inversion problem, and in the MIRAS case it presents an alternative calibration method to the noise-injection one, without any additional hardware required.@en

A minimum entropy based calibration algorithm for digitally beamformed imaging radars is presented. The algorithm is derived from work in SAR which corrects phase errors due to platform motion. The performance of the algorithm is compared to traditional techniques through both simulations and data from two imaging radars. The algorithm performs well with similar sidelobe levels for a point target. The results show that the minimum phase entropy algorithm produces a calibration vector without the need for a calibration target or for stringent scene statistics.@en

The Turbulent Eddy Profiler (TEP) is a volume-imaging 915 MHz radar designed for atmospheric boundary-layer turbulence studies. TEP is a pulsed, phased-array radar using digital beamforming techniques to provide four-dimensional images of refractive index fluctuations and wind vectors on grid scales comparable to those of large-eddy simulations. During October 1999, TEP was deployed during the Cooperative Atmospheric Surface Exchange Study (CASES'99) held near Wichita, KS. In this paper we present observations from this experiment which focused on the evolution and dynamics of the stably-stratified nocturnal boundary layer. Unlike the convective boundary layer, radar echoes are characterized by intermittent bursts of shear-induced turbulence. The scales of turbulence features are generally smaller than the resolution of the UHF radar. This is demonstrated by comparison with a co-located S-band FMCW radar. Estimation of turbulence intensity is also complicated by the presence of significant bird and insect scatter.

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The University of Massachusetts Microwave Remote Sensing Laboratory (MIRSL) has augmented the Turbulent Eddy Profiler (TEP) with an acoustic source to implement a Radio Acoustic Sounding System (RASS). TEP is a volume-imaging 915 MHz radar wind profiler. It can be thought of as a densely packed array of 915 MHz wind profilers sharing a common transmitter. Using digital beamforming techniques, TEP simultaneously generates over 40 contiguous beams within a 25 degree field of view. As a result, TEP provides a four-dimensional view (3D volume and time) of atmospheric turbulence structure within a volume of the boundary layer. The addition of RASS capabilities to TEP allows it to measure virtual temperature. In principle, the TEP-RASS system is capable of measuring three-dimensional temperature fields. However, the horizontal resolution is limited in practice by the size of the reflected and acoustically-focused electromagnetic radiation incident on the receiving array. In this paper we present the TEP-RASS system implementation and an assessment of the feasibility of measuring horizontal temperature variations via RASS technique.

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