Remote sensing involves the interaction of electromagnetic radiation with targets of interest from a distance. There are seven key elements in the remote sensing process: (1) an energy source, (2) interaction with the atmosphere, (3) interaction with the target, (4) recording by a sensor, (5) transmission and processing, (6) interpretation and analysis, and (7) application of the information gathered. The quality of data collected depends on factors like the sensor-target distance and spatial resolution, which varies across the sensor's field of view. Remote sensing provides large-scale coverage efficiently but requires specialized training and has some limitations compared to field data collection.

1. Remote Sensing

2. ◆ Remote sensing the process involves an
interaction between incident radiation and the
targets of interest. This is exemplified by the use
of imaging systems where the following seven
elements are involved. Note, however that remote
sensing also involves the sensing of emitted
energy and the use of non-imaging sensors.
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4. 1. Energy Source or Illumination (A) - the first requirement for remote sensing is to have an
energy source which illuminates or provides electromagnetic energy to the target of interest.
2. Radiation and the Atmosphere (B) - as the energy travels from its source to the target, it will
come in contact with and interact with the atmosphere it passes through. This interaction may
take place a second time as the energy travels from the target to the sensor.
3. Interaction with the Target (C) - once the energy makes its way to the target through the
atmosphere, it interacts with the target depending on the properties of both the target and the
radiation.
4. Recording of Energy by the Sensor (D) - after the energy has been scattered by, or emitted
from the target, we require a sensor (remote - not in contact with the target) to collect and
record the electromagnetic radiation.
5. Transmission, Reception, and Processing (E) - the energy recorded by the sensor has to be
transmitted, often in electronic form, to a receiving and processing station where the data are
processed into an image (hardcopy and/or digital).
6. Interpretation and Analysis (F) - the processed image is interpreted, visually and/or digitally or
electronically, to extract information about the target which was illuminated.
7. Application (G) - the final element of the remote sensing process is achieved when we apply
the information we have been able to extract from the imagery about the target in order to
better understand it, reveal some new information, or assist in solving a particular problem.

5. ◆ For remote sensing instruments, the distance
between the sensor and the target plays a large
role in the amount and quality of data produced.
Also, due to the factors defined below, the spatial
resolution will vary from image center to the edge
of the swath.
◆ Instantaneous Field of View : The angular cone
of visibility of a remote sensor. It determines the
area on the Earth's surface sensed at a particular
moment in time.
◆ Nadir : The point on the Earth's surface directly
below the satellite. Spatial Resolution is optimal
here. Also called the sub-satellite point.
◆ Limb : The edge of the apparent disk of a
celestial body. Also used to refer to either edge of
the sensor's swath, as shown in the graphic at
right. Since the sensor IFOV is distorted at the
limb, spatial resolution will be lower here.

6. The sensor mechanisms are very variable, and each has a distinct set of characteristics, but the main points
are:
a) The sensor can be an active system (where the satellite or the aircraft provides the source of
illumination, this technique is used when no suitable natural source of radiation exists), or be
a passive system (the source of the object illumination is independent of the sensor and it is a natural
source).
b) A variety of different parts of the electromagnetic spectrum can be used including:
Visible wavelengths and reflected infrared (imaging spectrometer): Remote sensing in the visible and near
infrared (VNIR) wavelengths usually falls into the passive category. Here the sun is the source of the
irradiance on the object being observed. The sensor collects the solar radiation which is reflected by the
object. Active remote sensing occurs at these wavelengths only in the rare case where an aircraft
carries a laser as the source of illumination.
Blue, green, and red are the primary colors or wavelengths of the visible spectrum. They are defined as such
because no single primary color can be created from the other two, but all other colors can be formed
by combining blue, green, and red in various proportions.
Thermal sensors: Remote sensing in the thermal-infrared wavelengths usually falls into the passive
category, but in this case, the source of the radiation is the object itself. There is no irradiance and the
sensor detects radiation which has been emitted by the object.
Microwaves (radar): These are used in the active remote sensing systems. The satellite or aircraft carries an
antenna which emits a microwave signal. This signal is reflected by the ground and the return signal is
detected again by the antenna.

7. ◆ Each part of the spectrum has different characteristics and
gives rather different information about earth’s surface. In
addition, different surface covers (vegetation, water, soil,
etc) absorb and reflect differently in different parts of the
spectrum. Different wavebands in the electromagnetic
spectrum therefore tend to be useful for different purposes.
◆ c) The sensor may be sensitive to a single portion of the
electromagnetic spectrum (e.g. the visible part of the
spectrum, like panchromatic film which is sensitive to the
same wavebands as our eyes). Alternatively, it may be able
to detect several parts of the spectrum simultaneously. This
latter process is called multispectral sensing.
◆ d) Sensor equipment takes many shapes and forms, such
as cameras, scanners, radar

8. Active versus Passive sensors

9. ADVANTAGES OF REMOTE SENSING
◆ Large area coverage: Remote sensing allows coverage of very large areas which
enables regional surveys on a variety of themes and identification of extremely large
features.
◆ Remote sensing allows repetitive coverage which comes in handy when collecting
data on dynamic themes such as water, agricultural fields and so on.
◆ Remote sensing allows for easy collection of data over a variety of scales and
resolutions.
◆ A single image captured through remote sensing can be analyzed and interpreted
for use in various applications and purposes. There is no limitation on the extent of
information that can be gathered from a single remotely sensed image.
◆ Remotely sensed data can easily be processed and analyzed fast using a computer
and the data utilized for various purposes.
◆ Remote sensing is unobstructive especially if the sensor is passively recording the
electromagnetic energy reflected from or emitted by the phenomena of interest. This
means that passive remote sensing does not disturb the object or the area of
interest.
◆ Data collected through remote sensing is analyzed at the laboratory which
minimizes the work that needs to be done on the field.

10. ◆ Remote sensing allows for map revision at a small to
medium scale which makes it a bit cheaper and faster.
◆ Color composite can be obtained or produced from three
separate band images which ensure the details of the
area are far much more defined than when only a single
band image or aerial photograph is being reproduced.
◆ It is easier to locate floods or forest fire that has spread
over a large region which makes it easier to plan a
rescue mission easily and fast.
◆ Remote sensing is a relatively cheap and constructive
method reconstructing a base map in the absence of
detailed land survey methods.

11. DISADVANTAGES OF REMOTE SENSING
◆ Remote sensing is a fairly expensive method of analysis especially when
measuring or analyzing smaller areas.
◆ Remote sensing requires a special kind of training to analyze the images. It
is therefore expensive in the long run to use remote sensing technology
since extra training must be accorded to the users of the technology.
◆ It is expensive to analyze repetitive photographs if there is need to analyze
different aspects of the photography features.
◆ It is humans who select what sensor needs to be used to collect the data,
specify the resolution of the data and calibration of the sensor, select the
platform that will carry the sensor and determine when the data will be
collected. Because of this, it is easier to introduce human error in this kind
of analysis.
◆ Powerful active remote sensing systems such as radars that emit their own
electromagnetic radiation can be intrusive and affect the phenomenon
being investigated.

12. ◆ The instruments used in remote sensing may sometimes be un-calibrated
which may lead to un-calibrated remote sensing data.
◆ Sometimes different phenomena being analyzed may look the same during
measurement which may lead to classification error.
◆ The image being analyzed may sometimes be interfered by other
phenomena that are not being measured and this should also be accounted
for during analysis.
◆ Remote sensing technology is sometimes oversold to the point where it
feels like it is a panacea that will provide all the solution and information for
conducting physical, biological or scientific research.
◆ The information provided by remote sensing data may not be complete and
may be temporary.
◆ Sometimes large scale engineering maps cannot be prepared from satellite
data which makes remote sensing data collection incomplete.