Page 162 - Intro to Space Sciences Spacecraft Applications

Spectral (Radiometric) Performance Remote Sensing 149

The spectral performance of a sensor depends on the frequency (or wave-
length) of the radiation which the sensor is designed to detect. The wave-
length(s) used to make a measurement is selected based on the radiative
(radiometric) properties of the target and/or on other considerations, such
as interaction with or avoidance of the atmosphere or ionosphere. For
instance, in clouds, water occurs as aerosol-sized particles of liquid rather
than vapor. Clouds absorb and scatter electromagnetic radiation at wave-
lengths less than about 0.3 cm. Sensors are designed to take advantage of
this effect to measure atmospheric characteristics. Only radiation of
microwave and longer wavelengths is capable of penetrating clouds without
being significantly scattered, reflected, or absorbed. These (radar) fkequen-
cies are commonly used when information below cloud layers is desired.
The spectral performance of a system is a measure of how well the sys-
tem uses these frequencies (or combinations of frequency ranges) to
extract information on a desired target. Much of the success of a remote
sensor depends on knowledge of the radiometric properties of the target.
These targethcene characteristics determine the relative strength or inten-
sity of observed signals based on their reflectance, absorption, or other
properties for the particular frequencies involved.

Temporal Performance

The temporal performance of a remote sensor is a function of the
repeated observations of a phenomenon based on the expected rate of
changes in that phenomenon. Global winds change on an hourly or daily
basis while changes in the earth’s geodetic shape or magnetic field may
take years or millennia to occur. The observation rate of a specific phe-
nomenon or area is a function of the orbit in which the satellite is placed
and the sensor FOV produced by the scan pattern. An orbit and scan pat-
tern can be preferentially designed to create a revisit sequence and local
time observation tailored to a particular application.

Early remote sensing satellites were technology and lift capacity con-
strained, resulting in relatively simple sensors designed to observe singular
regions of the electromagnetic spectrum. Modem satellites tend to maximize