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94
GHz Doppler Cloud Radar: Galileo
Galileo, the 94 GHz (3.2 mm wavelength) cloud radar has been operated
at the Chilbolton Observatory since 1994. The radar is a fully coherent,
W band, single polarisation Doppler radar. The radar is designed to
complement the 35 GHz (9 mm) Copernicus cloud radar currently operating
at the Observatory for observations of cloud microphysics and climatology.
The radar provides real-time measurement and display of the full Doppler
spectrum, and the moments Z, v, and w. The latest results from Galileo
can be viewed on the Chilbolton
Weather Web.
Millimetre-wave cloud radars exploit the fact that the echo intensity
of Rayleigh scatter increases with the inverse-fourth power of the wavelength.
These radars normally operate in the windows in the atmospheric absorption
spectrum at 35 GHz and 94 GHz.
The shorter-wavelength radar (94 GHz) is sensitive to a small amount
of Mie scattering from the largest cloud particles. The 35 GHz backscatter,
however, is overwhelmingly due to the Rayleigh mechanism. A comparison of measurements at the 2 frequencies, given by the dual-wavelength ratio (DWR) provides a measure of the size of the scattering particles.
Together with measurements of radar reflectivity, this allows ice water
content to be derived in cirrus clouds.
The radar is currently on the ground within 100 m of the 25 m steerable
antenna. Copernicus, the 35 GHz cloud radar is also located within 5 m of Galileo. Galileo is installed
in a positioning unit that can be manually adjusted in azimuth and elevation.
The most common mode of operation is vertically pointing, but the radar
can be pointed at lower elevations for calibration purposes, and for
operation along the same path as the radars on the 25 m antenna.
Details of the Radar
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