Page 153 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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Sensor Scanning. In most cases, a remote sensor would ideally cover as
much of the observable area (field of view) as possible. To do this with an
IFOV smaller than the FOV, the sensor must shift its concentration in
some way, as a function of time, over the FOV. One important point to
keep in mind is that the satellite (in all cases except for a geostationary
satellite platform) is itself in motion over the planetary surface.
There are many methods that can be used to change the direction of the
sensor’s IFOV, including mechanically moving a telescope, mirror, or anten-
na; electronically shifting an antenna beam or optical path; and/or arranging
a line or array of sensors with respect to the satellite’s orbital motion. The
scan can also follow a variety of patterns, some of which are illustrated in
Figure 6-2. Many sensors scan from side to side across (normal to) the satel-
lite nadir track. This type of pattern is known as a cross-truck scan. Since the
grazing angle and range are constantly changing as the scan gets farther from
nadir, the GIFOV also constantly changes in the range direction.
Some sensors, such as microwave radiometers, hold a constant nadir
(look) angle and then scan the sensor around the satellite nadir in a circu-
lar pattern known as a conical scan. Or a sensor can be held stationary and
simply use the satellite motion to move the field of view across the sur-
face in a technique called a pushbroom scan. Several modern optical sen-
sors employ thousands of cross-track detectors in the sensor focal plane,
and use the satellite’s orbital translation to create such a scan. These and
other variations on these themes are used to permit the selective IFOV of
the sensor (representing the limit of a sensor’s spatial acuity) to collect
information over a broader range of the satellite field of view.
If a contiguous scan coverage is required, the scan rate (time for the
sensor to complete one full sweep of the desired areas) is set so as to allow
each scanned area to overlap slightly with previously scanned areas so
that no spots are missed. Many sensors do not require a contiguous scan
and instead may use a sampling scan to achieve their measurement goals.

Example Problem:

A remote sensing satellite is in a circular orbit at an altitude of 1,OOO
km. The sensor is a radiometer with its beam aligned normal to the