Page 149 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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Many sensors make primary use of the spectral characteristics of the
response or the strength of the response to determine the context and con-
tent of the measurement and are referred to as spectrometers or rudiome-
ters. Whatever the primary method or measurement, all sensors have per-
formance values described in terms of spectral (frequency-related), spatial
(geometric), and radiometric (radiative-properties) dimensions. Moving
or scanning sensors also provide a temporal (time-based) pattern of obser-
vation, sometimes referred to as the revisit interval. Current research and
development activities lead the way to eventual space-based evaluation of
ocean and land subsurface physical and biological characteristics includ-
ing soil moisture and bathymetric measurements. Finer spectral and spa-
tial observations of land areas are on the horizon, and even surface pres-
sure monitoring from space appears to be possible in the future.

Classification of Remote Sensors

Earlier in this chapter, we identified passive remote sensors that collect
electromagnetic energy. The camera (without lighting aids such as a flash
unit) is the classic example of a passive sensor: Incoming light impinges
on sensitive film which retains the image. Radiometers are devices that
replace the film with detectors that are sensitive to the incoming radiation
(which can be in other portions of the electromagnetic spectrum besides
the visible frequencies) and convert the information to electronic signals
that can be transmitted to and evaluated by users on the ground. Collec-
tion devices include lenses and antennas that focus the energy on detec-
tors such as solid state materials, charge coupled devices (CCDs), and
other spectrally unique and energy-sensitive conversion elements.
Sensors that create their own source of target or scene illumination are
referred to as active sensors. These systems create and transmit energy
that propagates from the satellite to the target, interacts with the target,
and returns to the sensor. Active sensors include radars (altimeters, scat-
terometers, real aperture radar (RAR) imagers, synthetic aperture radar
(SAR) imagers) and lasers (laser altimeters, scatterometers, and imaging
devices). These instruments have the benefit of being able to control the
timelfrequency content, direction, and energy level of the illumination.
Among other things, this characteristic permits active sensors to greatly
improve on the spatial (geometric) performance of the system using meth-
ods which will be described later.