Page 57 - Digital Analysis of Remotely Sensed Imagery

30 Cha pte r T w o

differing from nadir, and seven geophysical parameters. Free access to
these data and the results processed from them is granted to approved
users only.
SeaWiFS data have a narrow and focused application area,
namely, the study of ocean color on the global scale, which is critical
to studying the concentration of microscopic marine plants (e.g.,
phytoplankton) and ocean biogeochemical properties. In conjunction
with ancillary data, SeaWiFS data enable retrieval of meaningful
biologic parameters such as photosynthesis rates.

2.3 Earth Resources Satellite Data
There are several satellites in this category, all of which share the same
characteristics of capturing radiation in the visible light, and NIR
spectrum at a medium spatial resolution and a return period of around
20 days. Introduced in this section are six of the lead satellites/sensors:
Landsat, Le Systeme Pour l’Observation de la Terre (SPOT, or Earth
Observation System), Indian Remote Sensing (IRS), Advanced
Spaceborne Thermal Emission and Reflection Radiometer (ASTER),
Moderate Resolution Imaging Spectroradiometer (MODIS), and
Advanced Land Observing Satellite (ALOS).

2.3.1 Landsat Data
The launch of the first Landsat satellite by the National Aeronautics and
Space Administration (NASA) on June 23, 1972, ushered remote sensing
into the space era and aroused an enormous interest in digital image
processing. During the course of the NASA space program, Landsat
images have evolved toward a higher spatial resolution and a finer spectral
resolution. Although some early satellites in this series are no longer in
service, they did collect a tremendous quantity of data that are indispen-
sable in long-term monitoring applications. These historic data are also
essential in studying changes in land cover and the environment.
Initially called the Earth Resources Technology Satellite and later
renamed Landsat, the Landsat program represents the first unmanned
satellite designed specifically to acquire medium resolution,
multispectral data about the Earth on a systematic and repetitive basis.
The satellite has a circular, sun-synchronous orbit that is inclined at 99°
(Table 2.5). At a height of 915 km and an orbital period of 103 minutes,
the satellite is able to complete 14 revolutions a day around the globe.
A distance of 2760 km is shifted at the equator between two consecutive
orbits on the same day. The same orbit travels by 160 km from one day
to the next, resulting in a maximum overlap of only 14 percent between
images recorded in successive orbits at the equator. Thus, it is impossible
to establish three-dimensional (3D) viewing for most of the Earth’s
surface from Landsat imagery. Eighteen days later the orbit returns to
where it starts. These parameters have not changed for the first three
satellites to maintain consistency in the data acquired.