Environmental Engineering Reference
In-Depth Information
Fig. 3.4
Passive and
active sensors
Active sensors are more controlled because they
do not depend upon varying illumination condi-
tions. Remote sensors collect data by detecting
the energy that is re
its orbit. These are called ascending and
descending passes. As a satellite revolves around
the Earth, the sensor sees a certain portion of the
Earth
ected from Earth. These
sensors can be on satellites or mounted on air-
craft. Remote sensors can be either passive or
active. Passive sensors respond to external
stimuli. They record radiation that is re
fl
s surface. The area imaged is referred to as
the swath. The surface directly below the satellite
is called the nadir point.
'
fl
ected
3.1.6 Satellite Sensor Characteristics
from Earth
s surface, usually from the sun.
Because of this, passive sensors can only be used
to collect data during daylight hours.
In contrast, active sensors use internal stimuli
to collect data about Earth. For example, a laser
beam remote sensing system projects a laser onto
the surface of Earth and measures the time that it
takes for the laser to re
'
The main function of most satellite sensors is to
collect information about the re
ected radiation
along a pathway, also known as the
fl
eld of view
(FOV), as the satellite orbits the Earth. The
smallest area of ground that is sampled is called
the instantaneous IFOV. The IFOV is also
described as the pixel size of the sensor. This
sampling or measurement occurs in one or many
spectral bands of the EM spectrum. The data
collected by each satellite sensor can be descri-
bed in terms of spatial, spectral and temporal
resolution.
fl
ect back to its sensor.
3.1.5 Orbits and Swaths
The path followed by a satellite is referred to as
its orbit. Satellites which view the same portion
of the Earth
s surface at all times have geosta-
tionary orbits. Weather and communication sat-
ellites commonly have these types of orbits.
Many satellites are designed to follow a north
'
Spatial Resolution
The spatial resolution (also known as ground
resolution) is the ground area imaged for the
IFOV of the sensing device. Spatial resolution
may also be described as the ground surface area
that forms one pixel in the satellite image. The
IFOV or ground resolution of the Landsat The-
matic Mapper (TM) sensor, for example, is 30 m.
The ground resolution of weather satellite sensors
is often larger than a square kilometre. There are
satellites that collect data at less than 1 m ground
-
south orbit which,
in conjunction with the
Earth
'
s rotation (west
east), allows them to cover
-
'
most of the earth
s surface over a period of time.
These are near-polar orbits. Many of these sat-
ellites orbits are also sun-synchronous such that
they cover each area of the world at a constant
local time of day. Near-polar orbits also mean
that the satellite travels northward on one side of
the earth and the southward on the second half of
