Page 159 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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rithm development is based on a theoretical understanding of the physical
processes involved, bolstered, and refined by empirical experiments using
aircraft and satellite remote sensor observations. From this, the basic geo-
physical properties discussed earlier, such as temperatures, atmospheric
constituents, and land resources, can be distilled.

MEASUREMENT DIMENSIONS OF REMOTE SENSORS

All remote sensors observe in the context of specific spectral regions,
and within each spectral band, a sensor further defines an observation in
terms of space, time, and signal strength. The dimensional extent of a sen-
sor’s ability to observe has been defined as the swath width or field of
view (FOV). Within the FOV, a sensor’s spatial acuity is defined within
the IFOV elements (pixels). Each IFOV element is further defined in
terms of the strength of the signal identified with a specific observation or
scene location. Considering all of the IFOV elements that combine to con-
stitute a scene, the variation in signal strength produces contrast. Sensors
depend on a satellite’s orbit and the sensor’s scan pattern to determine the
precise observation time and sequence for a particular earth location.

Spatial Performance

Passive Sensor Spatial Resolution. The spatial performance of passive sen-
sors is defined by the wavelength scale dimension of the focusing aperture
(in radians) and the range between the observing sensor and the observation
area. The satellite orbit and sensor scan pattern can alter the exact dimensions
of the earth-projected sensor IFOV, but the beam projection is fundamental-
ly diffraction limited. Active sensors can go a step further in gained
improved resolution in both the range and along-track dimensions.

Active Sensor Spatial Resolution. Active sensors can improve on the
resolution in the range or beam direction by controlling the length of the
pulse of energy transmitted by the sensor. This is known as pulse ranging
and is shown in Figure 6-5.
The active sensor sends out a pulse of energy with a pulse length of z
seconds. This pulse propagates in the direction of the beam (boresight)
and illuminates the IFOV area, after which some of the energy reflects
back to the receiver. In this case, the range resolution (R,) depends on the
pulse length and the signal propagation velocity (c,) which is approxi-