AWAKEN/Instrumentation/ScanningRadar

From Open Energy Information




Scanning Radar

Description

Like LiDAR, radar exploits the Doppler effect: a short pulse of energy is emitted from a radar, and when the pulse strikes an object, the backscattered energy is measured to identify the object, its location, and how it is moving along the direction of the radar beam. A network of Next Generation Radar (NEXRAD) systems provides precipitation and wind observations across the United States, but the resolution of the winds and accuracy of the wind retrievals do not offer the same benefits as typical instrumentation like radar wind profilers (Fast et al. 2007) and are not generally considered useful for wind energy applications.

However, other types of radars do offer substantial advantages for observations for wind energy applications. Texas Tech University (TTU) maintains research-grade mobile Ka-band (35 gigahertz GHz) Doppler radar systems that provide an along-beam range resolution of 15 m and a half-power beam width of 0.49 °(Hirth et al. 2012). In one test case, a complete set of 10 scans (at 10 different elevation angles) across a 30° sector required 45 s. The TTUKa radars can provide continuous radial velocity measurements by relying on distributed meteorological targets (e.g., water droplets and ice crystals) to scatter their signal. Clear air signals can sometimes be returned, but with intermittent coverage in low-humidity environments. Comparisons of dual-Doppler retrievals of wind speed and direction from the TTUKa radars with tower-based anemometers suggest that retrievals of mean wind speed and direction profiles in both clear-air (nonprecipitating) and precipitating environments agree well at altitudes above approximately 50 m. At lower altitudes, radar-based estimates were slightly higher than in situ measurements. Further, bulk turbulence parameters were also slightly underestimated by the dual-Doppler Ka-band radar retrievals (Gunter et al. 2015).

Recent developments in the X-band (8-12 GHz) radar, which is similar to the Ka-band radar, are promising. The main objective for the development of the X-band technology was to improve clear air (i.e., nonprecipitating) data availability, and early tests demonstrate a substantial increase in data availability in every atmospheric condition, whereas the X-band technology provides for data from a larger maximum range (J. Schroeder, personal communication, Nov. 2016). The change in wavelength and increase in sensitivity of the receiver chain enables the X-band systems to rely on Bragg scattering from temperature gradients in the atmosphere. The range resolution is as small as 9 m, and the maximum range can be up to 37.5 km, assuming a pulse repetition frequency of 4,000 Hz. Scanning is very fast: for a 65-degree-sector volume with 14 elevation tilts, a measurement volume is acquired every minute. Validation efforts are underway as of 2017.

Quantities Measured

Resolution

Accuracy

Limitations

Platform

TRL

Scanning Radar are generally considered to have a TRL of X.

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