Remote sensing of radio refractivity by lidar

Deterministic modelling of tropospheric radio propagation and exploitation of variations of microwave radar propagation require high quality, high resolution refractivity profiles in the first 1-2 km above the Earth's surface. Currently, routine measurements of refractivity are based on radiosonde measurements (via in-situ measurement of temperature and water vapour). Unfortunately, radiosondes have many drawbacks, including limited time sampling, limited accuracy and limited vertical resolution. Refractometer cavities offer an alternative, high quality, means of measuring radio refractive index, but the measurements are again in-situ, and profiles are very expensive to collect. A lidar developed at RAL has been installed at the Chilbolton Observatory for the measurement of water vapour and temperature profiles in the troposphere. The wavelength to be transmitted, 355 nm, generated by a frequency-tripled Nd:YAG laser, has been chosen after detailed consideration of factors affecting both safety and performance.