Radar propagation near the sea surface depends on meteorological conditions and sea surface roughness. Often strong gradients of humidity and temperature can be observed close to the air-water interface leading to abnormal propagation effects such as ducting. For shipborne radars operating at frequencies above L-band, the evaporation duct is the dominant propagation mechanism affecting the maximum detection range of horizon-search radars. Ducting can also increase sea clutter return within and beyond the normal horizon, and surface-based ducts can enhance land clutter return from extended ranges.
During sea trials in the Skagerrak and the Baltic Sea in 2003 and 2004, FWG was responsible for environmental characterization of the boundary layer. In-situ measurements included recordings of atmospheric and sea surface parameters. Investigations with multi-sensor buoys and with radiosondes were performed on board the German research vessel PLANET respectively on FGS HELMSAND. The drift buoys developed by FWG provided unperturbed, time resolved information on air-sea interaction processes. In addition to meteorological parameters sea state, sea surface roughness, and sea surface temperature were measured. Refractivity profiles were determined based on data sets gathered by measurements of pressure, humidity and temperature from the sea surface up to 1 km altitude.
Simultaneously to atmospheric measurements radar propagation investigations were performed by FGAN-FHR (Research Institute for High Frequency Physics and Radar Techniques). PLANET, FGS STOLLERGRUND were illuminated by a radar operating at X-, Ka- and W-band. The radar system was located at the land-based test site Hirtshals, Denmark during the trials in 2003 and at the land-based test site Surendorf, Germany during the experiment in 2004. Radar propagation characteristics at X-band were measured on board the ships with two omnidirectional antennas mounted in two different altitudes above sea surface. Results of refractivity variability in the marine boundary layer are presented in conjunction with radar propagation data and model outputs.