The radar wind profiler/radio acoustic sounding system (RWP/RASS), available in 915-MHz (for U.S. deployments) and 1290-MHz (for foreign deployments), measures wind profiles and backscattered signal strength between (nominally) 0.1 km and 5 km and virtual temperature profiles between 0.1 km and 2.5 km. However, ARM no longer operates the RASS portion.
The RWP transmits electromagnetic energy into the atmosphere in as many as five directions (four tilted in opposing vertical planes and on vertical) and measures the strength and frequency of backscattered energy. Calculation of the Doppler shift of the returned signal allows calculation of the atmospheric wind profile.
Windy Radar
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Under a memorandum of understanding signed in 2014 and building off of the successful Interagency Field Test & Evaluation program radar mitigation testing campaigns, a consortium of federal agencies composed of the U.S. Department of Defense, the U.S. Department of Energy, the Federal Aviation Administration, and the National Oceanic and Atmospheric Administration established the Wind Turbine Radar Interference Mitigation Working Group to address these conflicts.
An L-band radar wind profiler was established at National Atmospheric Research Laboratory, Gadanki, India (13.5N, 79.2E), to provide continuous high-resolution wind measurements in the lower atmosphere. This system utilizes a fully active array and passive beam-forming network. It operates at 1280 MHz with peak output power of 1.2 kW. The active array comprises a 16 16 array of microstrip patch antenna elements fed by dedicated solid-state transceiver modules. A 2D modified Butler beam-forming network is employed to feed the active array. The combination of active array and passive beam-forming network results in enhanced signal-to-noise ratio and simple beam steering. This system also comprises a direct intermediate frequency (IF) digital receiver and pulse compression scheme, which result in more flexibility and enhanced height coverage. The scientific objectives of this profiler are to study the atmospheric boundary layer dynamics and precipitation. Observations made by this profiler have been validated using a collocated GPS sonde. This paper presents the detailed system description, including sample observations for clear-air and precipitation cases.
In recent years, radar wind profilers (RWPs) operating close to the 1-GHz band have been extensively used for atmospheric research and operational meteorology. It is also referred as the lower-atmospheric wind profiler (LAWP) because it is used to probe the lower part of the atmosphere (up to about 5 km). Many LAWPs have been developed in the recent past by research and commercial groups for meteorological applications, in particular to understand the dynamics of the atmospheric boundary layer (ABL), air quality studies, and precipitation (Balsley and Gage 1982; Rogers et al. 1993; Gage et al. 1994; Ralph 1995; Fabry and Zawadzki 1995; Rao et al. 2008). The pioneering work in making the RWPs practical is accomplished in 1980s at National Oceanic and Atmospheric Administration (Carter et al. 1995). Initial LAWPs (Ecklund et al. 1988, 1990; Hashiguchi et al. 1995) are configured with either dish antenna or passive array for simplicity and commercial viability. Advances in data acquisition and processing systems and radar hardware led to the development of more complex and advanced systems in later years for a better understanding of boundary layer turbulence, wind variability, and synoptic systems (Mead et al. 1998; May et al. 2002; Law et al. 2002; Hashiguchi et al. 2004; Imai et al. 2007).
LAWP operates at 1280 MHz and employs the Doppler beam-swinging (DBS) technique for measuring the wind vector. The functional block diagram of the system is shown in Fig. 1. It consists of an active array, a 2D Butler BFN, an exciter and receiver unit, and a direct IF digital receiver (DRx), which also acts as radar controller. Active array (antenna elements with integral transceiver modules) and BFN, which are distributed in nature, have been mounted at the roof level of the instrumentation room. The centralized subsystems, like the exciter and receiver DRx, are housed in an instrumentation rack located at the ground level. Exciter generates the local oscillator (LO), IF, and radio frequency transmitter (Tx-RF) signals with reference to a high stable oven-controlled crystal oscillator (OCXO). The Tx-RF pulse is fed to the 2D BFN through the Tx/Rx switch. BFN distributes the signal among the output ports with appropriate phase gradient and feeds the antenna array elements via transceiver modules (TMs). In Rx mode, the signals received by the antenna array are amplified in the front-end section of TMs, passed through the BFN, and fed to the downconverter via the Tx/Rx switch. The 70-MHz Rx IF output from the downconverter is amplified, band limited, and then fed to the direct IF digital receiver, which performs the digital quadrature detection, baseband-matched filtering, coherent averaging, and raw data transfer to the host PC. The host PC performs the data cleaning, fast Fourier transform (FFT), incoherent integration, and computation of spectral moments and wind vector. Specifications of the profiler are shown in Table 1. The following subsections present a detailed description of the design, realization, and functioning of the various subsystems of LAWP.
Active array consists of a planar array in which the antenna (ANT) elements are fed directly by dedicated solid-state TMs. The planar array consists of 256 microstrip patch ANT elements arranged in a 16 16 matrix over an area of 2.8 m 2.8 m. Figure 2 shows different phases of array construction and installation. The patch elements are fabricated using 125-mil RT/Duroid 5870 substrate. The patch element, designed for linear polarization, is rectangular in shape with dimensions of 73.3 mm 73.1 mm with a ground plane of 92 mm 92 mm, and is incorporated with mounting holes at the four corners in order to be fitted onto an aluminum ground panel. Designed with a coaxial probe feed, the antenna elements are found to have return loss better than 15 dB over the bandwidth of 15 MHz. The cross-polarization level of the patch elements is measured to be better than 23 dB. Gain of the elements is measured to be 6.5 dB with a half-power beamwidth (BW) of more than 70 in cardinal planes. Sixteen elements, fitted onto an aluminum ground panel along the H plane, form the basic linear array panel. A preliminary radome, fabricated with a fiber reinforced plastic (FRP), is fitted on to the linear array panel for environmental protection. Figure 2a shows the picture of the patch element (left) and the 16-element linear array panel with (right top) and without (right bottom) preliminary radome. The effect of the FRP radome is found to be negligible on the radiation characteristics. An interelement spacing of 0.73, where is the operating wavelength, is chosen to have an optimal compromise between the required minimum beamwidth and maximum grating-free steer angle. Sixteen such linear panels are laid along the E plane to construct the full planar array. Dummy narrow aluminum panels are used as spacers between the linear antenna panels in order to realize the same interelement spacing along the E plane. The roof of the instrumentation room is extruded and incorporated with inverted reinforced cement concrete (RCC) beams and a square iron frame over which the planar array is installed. Figure 2b shows the picture of the extruded roof, and Fig. 2c shows the picture of the planar array partially filled with linear array panels. The bottom side of the linear array panels and dummy panels, which are contiguously laid, acts as a firm composite ground plane for the entire planar array. Figure 2d shows the picture of the entire planar array covered by a secondary FRP radome (left) and surrounded by the grounded aluminum clutter suppression fence (right). The mutual coupling between the closest antenna elements is measured to be better than 27 dB. After installation, the array axes are found to have azimuthally rotated 11 clockwise with respect to the principal geographical directions.
The BFN is built with passive networks instead of using active phase shifters to reduce the cost and complexity of the system. In addition to performing the beam formation, this network also acts as a distribution network for the active array. It is realized with a 2D 256-port modified Butler matrix and beam selection switch to feed the elements of the 16 16 active array. In this scheme, the traditional Butler matrix is modified to generate a zenith (broadside) beam, which is essential for a wind profiler for measuring the vertical wind component and studying precipitation. The 2D Butler network is built by connecting two sets of linear (1D) networks arranged in orthogonal directions. 152ee80cbc
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