Remote data collection is the source of radiation used

Remote Sensing: Active and Passive

            Remote sensing is a means
of collecting information about an object without making physical contact with
that object. In the realm of remote sensing there are two prominent methods for
data collection: active and passive sensing. The main difference between these
two methods of data collection is the source of radiation used for imaging.
Active sensors produce their own radiation, while passive sensors rely on
radiation from the sun. It is important to note the differences in active and
passive sensing because it can have a profound effect on the mission. Both
methods of remote sensing have distinct advantages and disadvantages; one can
evaluate these to determine the most effective mode for the proposed operation.

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Active Sensing

            Satellites that use active
sensing have scanning instruments, such as radar and lasers that produce
radiation. The radiation from the sensor is used to detect terrestrial or
atmospheric characteristics. Active sensing satellites have many different
missions, they can help determine rainfall rates across the globe, detect cloud
features, record glacial movements, geography changes, ocean patterns, and even
space weather.1

One example of an active sensing satellite is
RADARSAT-2, launched in 2007 this vehicle maintains a sun-synchronous orbit and
has a wide ranging mission set. To fulfill this mission set RADARSAT-2 uses
synthetic aperture radar (SAR) which illuminates a specific area, the reflected
radiation is then measured and produces an image. RADARSAT-2 has performed
geological, ice, land, and wetlands mapping; it has also been used to observe
the arctic and Canadian coastline.2

 There are
distinct advantages to using active sensors over passive. One reason is the use
of artificial radiation; the microwaves emitted can penetrate weather features
such as clouds, rain, and snow.3
Satellites like ALOS-2 uses microwave radar to penetrate through cloud cover,
allowing it to monitor natural disasters. This powerful radar also allows
ALOS-2 the ability to monitor conditions day or night.4 Radar
from active sensors can even penetrate through plants and soil, allowing the
sensor to gather information up to a meter below the ground. Another advantage
is that they do not require light from the sun, so they can be operated during the
day or night. Active sensors use their own radiation, which means they do not
rely on the sun, however this means that a system must use a fair amount of
energy to produce an image of the target.5

Passive Sensing

            Satellites that use
passive sensing require electromagnetic waves from the sun, or infrared
radiation produced by objects on the ground to create an image. Each system is
different, one may use visible light and another will use infrared or
near-infrared light. Passive systems do not produce their own radiation; this
means the vehicle will require less power. Another advantage of the passive
sensor is limited contact with the object being observed; active sensors
project radiation and can disturb the objects they are tasked to observe.
Unfortunately, the lack of emitted radiation means that weather interference is
an issue. Clouds, dust, wind, and other poor weather conditions can disrupt
passive sensors affecting the images they produce. Also, radiation from the sun
is only provided during the day, limiting the time a passive sensing satellite
can perform its operations.6

To ensure a satellite that uses passive sensing
is working at maximum efficiency, it will have a sun-synchronous orbit. This
allows satellites like GeoEye-1 to remain in constant sunlight. Launched in
2008 GeoEye-1 provides extremely high resolution images used to map man-made
and natural features with incredible accuracy.7 Mapping
the Earth’s surface is not the only use for remote sensing satellites. The Deep
Space Climate Observatory (DSCOVR) is a passive sensing satellite that provides
data on space weather. In 2015 DSCOVR was launched from Cape Canaveral by
SpaceX, it maintains an orbit that allows for continuous observation of the sun
and sunlit side of the earth. DSCOVR measures changes in the ozone, cloud
height, climate, and provides early warning for phenomenon like solar wind.8

            Early warning systems, terrain mapping, and disaster
coverage are just a few benefits of remote sensing satellites. Active and
passive sensors have their advantages and disadvantages, but each is useful for
data collection. Future active sensors may become less intrusive and require
less power, making them superior. Extremely sensitive passive sensors could
increase performance to the point that weather and sunlight have little effect
on the image quality. For now, engineers will need to examine the mission and
determine which type of sensor is the best option.

                1. Introduction to Remote Sensing. Accessed January
25, 2018.


                2. Canadian Space Agency. RADARSAT-2. Accessed
January 25, 2018


            3. “Exploring the Benefits of Active vs.
Passive Spaceborne Systems.” Earth Imaging Journal:Remote Sensing,
Satellite Images, Satellite Imagery. September 28, 2014. Accessed January 26,


            4. “ALOS-2.” GeoImage. Accessed January
27, 2018.


            5. “Exploring the Benefits of Active vs.
Passive Spaceborne Systems.” Earth Imaging Journal

                6. “Exploring the Benefits of
Active vs. Passive Spaceborne Systems.” Earth Imaging Journal


            7.GeoEye-1 50cm Global High-Resolution Satellite
Imagery. Accessed January 27, 2018.


            8.DSCOVR – Satellite Missions – eoPortal Directory.
Accessed January 27, 2018.