ILLUMINATION AND OBSERVATION GEOMETRY FOR NOAA-AVHRR IMAGES

It is very important to consider the illumination and observation geometry when satellite data are processed, i.e. to take into account all the angles in the radiation path. The importance of modifications introduced in the radiation and the geometric distortions of the image depends on the range of values of the angles. The angles related to the illumination geometry depend on the Sun's position relative to the sensor and the pixel being observed, while those related to the observation geometry depend on the radiometer characteristics. NOAA satellites carry the AVHRR sensor with an observation angle of +/- 56 degrees and their scanning line is approximately 2800 km wide. For these satellites, the angular values involved in the illumination and observation geometry show an important gradient along the scanning line. The purpose of the present paper is to analyse in detail both the illumination and observation geometry. The results of such analysis were applied as an example using raw data of the NOAA-11-AVHRR images over the Rio de In Plata and its ocean front region. As a part of the analysis and to improve the visual interpretation of the results, one-band synthetic images of the different illumination parameters were obtained. The results show that the large sensor field of view reduces the availability of the swath in after a few hundred kilometers. On the other hand, high latitudes should be excluded from the analysis, especially at winter solstice because of the large values of solar zenith angles. Different types of sporadic natural events that happened in the southern region of South America are also discussed, considering the results of the analysis and the purpose of enhancing the usefulness of AVHRR data in monitoring hazards. The phenomena considered were fires, volcano eruptions, and red tide blooms.