The ability to selectively examine portions of the spectrum enables satellite sensors to
perform specific tasks, such as providing visual images, monitoring temperatures, and
detecting reflected or emitted radiation.
Sensors monitor or 1.) natural radiation coming from the Earth or 2.) reflected
radiation resulting from having sent energy to Earth that is reflected back to the satel-
lite. Sensors are categorized as either active or passive instrumentation. Active instru-
ments transmit their own radiation to detect an object or area for observation and
receive the reflected or transmitted radiation. Active instruments include both imaging
and non-imaging sensors. Imaging sensors include real and synthetic aperture radars;
non-imaging sensors include altimeters and scatter-ometers. Active altimeters use either
radar pulses or lasers for measuring altitude. Scatterometers use radar to determine
wind speed and direction.
Passive instruments sense only radiation emitted by the object or reflected by the
object from another source. The two types of passive sensors are imagers and
sounders. Imagers measure and map sea-surface temperature, cloud temperature, and
land temperature. Imager data are converted into pictures. Sounders are a special type
of radiometer (instrument that quantitatively measure electro-magnetic radiation)
which measure changes in atmospheric radiation relative to height (ground processing
of this information produces temperature information), and changes in water vapor
content of the air at various levels.