Radiometer-based Estimation of the Atmospheric Optical Thickness
Vassilia Karathanassi(1), Demitrius Rokos(2), Vassilios Andronis(2) and Alex Papayannis(2)
(1) National Technical University of Athens, Heroon Polytechniou 9, 15780 Athens, Greece
(2) National Technical University of Athens, Heroon Polytechniou 9, 15780, Greece
Atmospheric optical thickness affects the quality of satellite imagery, especially over urban areas where aerosol concentrations (sulfates, soot, mineral dust, etc.) are high. Optical thickness is usually provided by protometer or Lidar ground station measurements and introduced in empirical atmospheric correction models. But Lidar wavelengths don’t correspond - in number and value - to those used by satellite sensors, therefore introducing drawbacks in atmospheric correction methods.
Thus, in this paper a methodology was developed for estimating atmospheric optical thickness by the use of satellite images and ground radiometer (GER 1500). Lidar estimations of the optical thickness at two wavelengths: 355 nm (infrared) and 532 nm, served for validation purposes. Within this framework, two major issues are investigated. The first concerns the most appropriate target in an urban environment that yields the most accurate atmospheric optical thickness value. The second deals with the appropriate surrounding area of the target.
Experiments were carried out in Athens, Greece. The study area has been selected because it presents high concentrations of air pollutants. .Radiometric and Lidar measurements simultaneous to SPOT XS satellite pass were implemented. Thus, radiance and reflectance of seven targets: grass (approximately 4cm high), bare soil, asphalt, bush (approximately 1m high), limestone, white pine, and olive leaves were measured. For each target, the extinction coefficient has been estimated on the basis of GER 1500 measurements and satellite image digital values and then compared to coefficient values provided by Lidar system measurements. Experiments have been repeated several times. Each time the targets have been placed in a different surrounding environment consisting of natural or artificial materials.
Evaluation of results showed that using the methodology developed, targets of “white pine” within a white artificial area produce the most accurate atmospheric optical thickness. Validation of the method using other test areas that present various atmospheric compositions will be the objectives of next research work.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,