25.

[Cover] [Contents] [Index]

Page 12

pages that provide links to international and national space agencies and projects. The Committee for Earth Observation Satellites (CEOS) also maintains an information site.

Details of the operation of the main types of sensor carried by remote sensing satellites can be found in textbooks (e.g. Lillesand and Keifer, 2000; Mather, 1999a) and readers who may be unfamiliar with these details are referred to one of these sources.

1.3 Atmospheric correction

Electromagnetic energy detected by remote sensing instruments (especially those which operate in the optical region of the spectrum) consists of a mixture of energy reflected from or emitted by the ground surface and energy that has been scattered within or emitted by the atmosphere. The magnitude of the electromagnetic energy in the visible and near-infrared region of the spectrum that is detected by a sensor above the atmosphere is dependent on the magnitude of incoming solar energy (irradiance), which is attenuated by the process of atmospheric absorption, and by the reflectance characteristics of the ground surface. Hence, energy received by the sensor is a function of incident energy (irradiance), target reflectance, atmospherically scattered energy (path radiance), and atmospheric absorption. Interpretation and analysis of remotely sensed images in the optical region of the spectrum is based on the assumption that the values associated with the image pixels accurately represent the spatial distribution of ground surface reflectance, and that the magnitude of such reflectance is related to the physical, chemical, or biological properties of the ground surface. Clearly, this is not the case unless corrections are applied to take account of variations in solar irradiance and in the magnitude of atmospheric absorption and scattering, as well as in the sensitivity of the detectors used in the remote sensing instrument. The response of these detectors to a uniform input tends to change over time. Correction for these effects is vital if thematic images of a given area are to be compared over time, for example over a crop-growing season.

The necessity for atmospheric correction depends on the objectives of the analysis. In general, land cover identification exercises that are based on single-date images do not require atmospheric correction if it can be assumed that all pixels in the image are equally affected by atmospheric processes, as the pixels are being compared with other pixels within the image. The validity of this statement depends also on the quality of the image (for example, an image displaying severe haze effects, spatially varying haze phenomena, or cloud/cloud shadow effects may be unsuitable for classification). Atmospheric correction and sensor calibration are necessary when multisensor or multidate images are being classified, or where the aim of pattern recognition is to identify land cover change over time, in

[Cover] [Contents] [Index]