Measurement of Net Radiation from Shipboard Sensors

Abstract

A new method for the measurement of the net flux of radiation from shipborne sensors is described. In this method two matched Funk net pyrradiometers are mounted, one on each side of the ship. Each sensor is gimbal mounted in a rotating hemispherical shield so that it views half of the sky and the surface of the water. Since each sensor views opposite halves of the sky as well as the surface, the sum of the two signals represents the hemispherical net radiation. For comparison, net radiation is also measured with a single Funk net pyrradiometer mounted on a gimbal at the end of a boom which extends from the bow of the ship. The net radiation measured using the two sensor halves is slightly higher during the daytime than that measured using the single sensor. The lower radiation at the single sensor is due to the screening of a part of the incoming radiation by the ship's superstructure and the surface boom. The validity of the two net radiation measurements was assessed from the radiation balance defined as the total incoming radiation of all wavelengths minus the reflected and emitted by the surface minus the net radiation. The hourly radiation balance which should be zero theoretically is computed for both sets of net radiation measurements using the five independently measured irradiances obtained throughout the GATE experiment. The standard deviation of the hourly radiation balance, obtained from the measurements of the two sensor-halves, is much lower than that resulting from the single sensor. These results show the superiority of the new method for the measurement of net radiation at sea. Further, no special ship maneuvers such as pointing the sensors in the direction of the sun, etc., are required.