@prefix ns0: . @prefix skos: . ns0:altLabel [ ] ; skos:definition """Data Source: The eight channel sun-photometer named SXM-2 (440, 522, 613, 672, 781, 871 and 1030 nm with 10 nm FWHM) was used on the ground. It follows the sun automatically using a 4 quadrant detector. Spatial Resolution: Spatial resolution is a function of the solar zenith angle and as such is not pertinent to measurements by a sunphotometer. For very accurate measurements however, it can be estimated to be approximately 10 * sin(zenith angle) in km. Thus for 30 degrees, it is 5 km. Meteorological parameters determine the effective spatial resolution. The time of optical thickness data acquisition was in September, 1990. SXM-2 instrument cycles through a set of 8 filters in about 20 seconds and the frequency of sampling can be varied from once every 4 minutes to one-half minute. Instrument Description: Mission Objectives - To measure the aerosol optical thickness using a multi-channel sunphotometer. This is part of the Correction and Calibration effort for FED, and will allow the estimation of the atmospheric effects on the transmitted and reflected radiation so that the appropriate correction schemes can be employed to infer reflectivity of the ground from aircraft radiometric data. Principles of Operation - The SXM-2 automatically tracks the sun. The detector is a silicon photodiode detector that is kept at a constant tempera- ture. The instrument has 1.5 degree field-of-view. Data can be acquired once every 1/2 min or 4 min onto data cassettes or a computer. 4.4 Instrument Measurement Geometry. The field-of-view (FOV) for SXM-2 is 1.5 degrees. Additional information available at "http://forest.gsfc.nasa.gov/html/fedmac/opt_thic/opt_thic.html" [Summary provided by NASA]"""@en ; skos:prefLabel "SXM-2"@en ; skos:inScheme ; skos:broader ; a skos:Concept . skos:prefLabel "Photometers"@en ; a skos:Concept ; skos:narrower .