Digital beamforming radar systems, capable of generating multiple antenna beams simultaneously, are now being used for high spatial resolution studies of the ocean surface and atmospheric boundary layer. This paper describes several receiver architectures applicable to digital beamforming systems and compares their relative merits. The simplest and lowest-cost architecture, comprised of a single receiver sampling a large array of antenna elements, is applicable to situations where some receiver inefficiencies can be tolerated. The second architecture described attaches a complete low noise receiver to each antenna element in the array to achieve the highest possible sensitivity. The Turbulent Eddy Profiler (TEP) utilizes this architecture to study clear air turbulence intensity and motions within the atmospheric boundary layer. A third architecture, employing a high accuracy digital receiver to sample each antenna output, is applicable to systems requiring the highest degree of antenna pattern control.
SWITCHED RECEIVER DESIGNThe Focused Phased Array Imaging Radar is a digital beamforming array radar developed by Quadrant Engineering and the University of Massachusetts for oceanographic research [l]. FOPAIR produces coherent radar images of an extended area of ocean surface (typically 60 m x 90 m) resolved to 1.5 m resolution. An X-band array of 128 elements, shown in Figure 1, provides the .25" beamwidth necessary for fine-scale imaging. All array elements share a single receiver through a high-speed switching network that samples the elements sequentially, analogous to the sequential scan of a synthetic aperture radar. For short to medium range applications, a 100 kHz PRF ensures rapid image capture (typically within 1 ms), and array scans can be repeated up to 180 scans per second. Figure 2 shows a block diagram of FOPAIR. The receiving arrays consist of linear tapered slots, which are low-cost printed circuit antennas. The receiver outputs are baseband I-and Q-channels, each with a bandwidth of up to 100 MHz. These outputs are digitized by a high-throughput sampling system capable of burst mode This work was supported by the Office of Naval Research, Army Research Office, and the Air Force Research Laboratory.Fig. 1. FOPAIR's X-band receiver array of 128 elements.sampling rates up to 400 MS/s at 12 bits.Two-dimensional radar imagery is generated from the unfocused array data, either through post-processing on a computer workstation or by an embedded DSP processor capable of real-time imaging at rates up to 72 frames/s. Radar "motion pictures" of ocean surface backscatter clearly show wave propagation and intermittent scattering events. These moving images can be viewed by opening the following web site: www.quadeng.com/ocean/fopair/fopair . html.
HIGH SENSITIVITY DIGITAL BEAMFORMING ARRAY FOR TURBULENCE IMAGINGThe Turbulent Eddy Profiler (TEP) is an example of a highly sensitive clear-air turbulence sensor requiring a dedicated receiver for each antenna [2], [3]. TEP's 90 element array provides a focused beam...
