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6
Pushbroom Imagers
6.1 Introduction
We have seen in the previous chapter the limitations of an optomechanical
imaging system in generating high spatial resolution imagery from space
platforms. The limitation primarily arises from the mode of imaging on
an instantaneous geometric eld of view (IGFOV) by IGFOV basis, thereby
limiting the time to collect the radiation. The situation can be substantially
improved if data are collected for an entire strip by imaging the complete
swath at a time, since all the pixels of the strip get the integration time cor-
responding to the time taken by the satellite to move through one IGFOV.
Such a mode of operation is possible due to the development of long array
detectors.
6.2 Principle of Operation
A strip of the terrain is focused by an optical system, say a lens, onto the
linear detector. At any given instant of time, only those points on the ground
that lie in the plane dened by the optical center of the imaging system and
the line containing the sensor array (the optical center is the imaginary point
at which all the rays intersect are imaged). This plane may be referred to as
the instantaneous view plane. The radiation from the strip on the ground is
received simultaneously by every detector element of the sensor and each
detector element produces electrons proportional to the radiant ux received
by that detector element and the duration for which the detector is exposed,
also referred to as the integration time. Depending on the number of elements
in the detector, one line of information generates that many picture elements.
When such a system is mounted on a moving platform (usually such that the
array length is at a right angle to the velocity vector of the platform), as the
platform advances the view plane sweeps out a strip of the terrain that is con-
tinuously projected onto the charge-coupled device (CCD) array. Normally,
146 Building Earth Observation Cameras
the detector is exposed for a duration equal to the time taken by the subsatel-
lite point to move through one IGFOV, referred to as the dwell time. Thus, due
to the platform motion every successive exposure produces contiguous image
strips. Hence, a two-dimensional image is produced—the linear array detec-
tor producing one image line across track (XT) and successive image strips
along track (AT) by the motion of the platform (Figure 6.1). When compared to
optomechanical scanning systems, the scan mirror is avoided and instead the
detector array is used to produce one scan line of information. This mode of
scanning is referred to as pushbroom scanning. In this case, the total time taken
by the subsatellite point to move through one ground resolution element is
available for integration of the signal for all the pixels along the linear array.
The dwell time for a pushbroom scanner is given by
τ=
β
p
v
h
(6.1)
where β is IFOV, v is the velocity of the subsatellite point, and h is the satellite
height.
Swath
T
T
Optics
View plane
Linear CCD
Direction of spacecraft motion
FIGURE 6.1
Illustration of the pushbroom scanning technique. τ is the integration time.

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