Data acquisition Platforms

Data Acquisition: Platforms

Platforms

Platforms refer to the structures or vehicles on which remote sensing
instruments are mounted. The platform on which a particular sensor is
housed determines a number of attributes, which may dictate the use
of particular sensors. These attributes include:

• distance the sensor is from the object of interest
• periodicity of image acquisition
• timing of image acquisition and
• location and extent of coverage

There are three broad categories of remote sensing platforms: ground
based, airborne, and spaceborne.

1. Ground based -- A wide variety of ground based platforms are
used in remote sensing. Some of the more common ones are hand
held devices, tripods, towers and cranes. Instruments that are
ground-based are often used to measure the quantity and quality
of light coming from the sun or for close range characterization
of objects. For example, to study properties of a single plant or a
small patch of grass, it would make sense to use a ground based
instrument.

Laboratory instruments are used almost exclusively for research,
sensor calibration, and quality control. Much of what is learned from
laboratory work is used to understand how remote sensing can be
better utilized to identify different materials. This contributes to the
development of new sensors that improve on existing technologies.

Field instruments are also largely used for research purposes. This
type of remote sensing instrument is often hand-held or mounted on a
tripod or other similar support.

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Permanent ground platforms are typically used for monitoring
atmospheric phenomenon although they are also used for long-term
monitoring of terrestrial features.

Towers and cranes are often used to support research projects where
a reasonably stable, long-term platform is necessary. Towers can be
built on site and can be tall enough to project through a forest canopy
so that a range of measurements can be taken from the forest floor,
through the canopy and from above the canopy.

Example:

The BOREAS (BOReal Ecosystem-Atmosphere Study) field experiment
was conducted to gain knowledge about relationships between the
boreal forest and Earth's atmosphere.

2. Airborne -- Airborne platforms were the sole non-ground-based
platforms for early remote sensing work. The first aerial images
were acquired with a camera carried aloft by a balloon in 1859.
Balloons are rarely used today because they are not very stable
and the course of flight is not always predictable, although small
balloons carrying expendable probes are still used for some
meteorological research.

At present, airplanes are the most common airborne platform. Nearly
the whole spectrum of civilian and military aircraft are used for
remote sensing applications. When altitude and stability requirements
for a sensor are not too demanding, simple, low-cost aircraft can be
used as platforms. However, as requirements for greater instrument
stability or higher altitudes become necessary, more sophisticated
aircraft must be used.

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In this section, aircraft are divided into three categories (low, mid,
and high) based on their altitude restrictions.

In general, the higher an aircraft can fly, the more stable a platform it
is, but correspondingly more costly to operate and maintain.

Low altitude aircraft typically fly below altitudes where supplemental
oxygen or pressurization are needed (12,500 feet above sea level).
They are good for acquiring high spatial resolution data limited to a
relatively small area. Included in this class are the common fixed-wing,
propeller driven planes used by private pilots, such as the Cessna 172
or 182, and Piper Cherokee. This class of aircraft is inexpensive to fly
and can be found throughout the world. Some of these airplanes are
specially outfitted for mounting remote sensing instruments in the
underside of the plane, however, many times instruments are simply
hung out the door using simple mounts.

Helicopters are usually used for low altitude applications where the
ability to hover is required. Helicopters are quite expensive to operate
and they are typically used only when needed. Ultra light aircraft are a
class of aircraft that is gaining popularity. The Federal Aviation
Authority (FAA) defines an ultra light as a single seat powered flying
machine that weighs less than 254 pounds, has a top speed of 55 knots
(63 mph), stalls at 24 knots (28 mph) or less and carries no more than
5 gal. of fuel. These small, often portable, aircraft are inexpensive and
are able to take off and land where larger aircraft cannot. They are
limited to flying at lower elevations and at slow speeds. If the demands
of the remote sensing requirement are not too strict, ultra light
aircraft may be a reasonable alternative to larger aircraft.

Mid altitude aircraft have an altitude limit less than 30,000 feet above
sea level. This includes a number of turbo-prop aircraft. Often at
higher altitudes, there is less turbulence so stability is better. This
class of airplane is used when stability is more important and when it is

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necessary or desired to acquire imagery from a greater distance than
available from low altitude aircraft. These aircraft can obtain greater
areal coverage more quickly than low altitude platforms. An example of
this class is the C-130 cargo plane and the Cessna C402.

High altitude aircraft can fly at altitudes greater than 30,000 feet
above sea level. This class of airplane is usually powered by jet engines
and is used for specialized tasks, such as atmospheric studies,
research to simulate satellite platforms, and other applications where
a high altitude platform is required. High altitude aircraft are good for
acquiring large areal coverage with typically lower spatial resolutions.

Another class of aircraft that has been in use for many years is
remote control aircraft, or drones. Remotely controlled aircraft are
often used for conditions when it may be too hazardous to fly. They
have been used extensively by the military.

3. Spaceborne -- The most stable platform aloft is a satellite,
which is spaceborne. The first remote sensing satellite was
launched in 1960 for meteorology purposes. Now, over a hundred
remote sensing satellites have been launched and more are being
launched every year. The Space Shuttle is a unique spacecraft
that functions as a remote sensing satellite and can be reused for
a number of missions.

Satellites can be classified by their orbital geometry and timing. Three
orbits commonly used for remote sensing satellites are geostationary,
equatorial and Sun synchronous. A geostationary satellite has a period
of rotation equal to that of Earth (24 hours) so the satellite always
stays over the same location on Earth. Communications and weather
satellites often use geostationary orbits with many of them located
over the equator. In an equatorial orbit, a satellite circles Earth at a
low inclination (the angle between the orbital plane and the equatorial

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plane). The Space Shuttle uses an equatorial orbit with an inclination of
57 degrees.

Sun synchronous satellites have orbits with high inclination angles,
passing nearly over the poles. Orbits are timed so that the satellite
always passes over the equator at the same local sun time. In this way
the satellites maintain the same relative position with the sun for all of
its orbits. Many remote sensing satellites are Sun synchronous which
ensures repeatable sun illumination conditions during specific seasons.
Because a Sun synchronous orbit does not pass directly over the poles,
it is not always possible to acquire data for the extreme Polar Regions.
The frequency at which a satellite sensor can acquire data of the
entire Earth depends on sensor and orbital characteristics. For most
remote sensing satellites the total coverage frequency ranges from
twice a day to once every 16 days.

Another orbital characteristic is altitude. The Space Shuttle has a low
orbital altitude of 300 km whereas other common remote sensing
satellites typically maintain higher orbits ranging from 600 to 1000 km.

Most remote sensing satellites have been designed to transmit data to
ground receiving stations located throughout the world. To receive
data directly from a satellite, the receiving station must have a line of
sight to the satellite. If there are not sufficient designated receiving
stations around the world, any given satellite may not readily get a
direct view to a station, leading to potential problems of data
discontinuity. To work around this problem, data can be temporarily
stored onboard the satellite and then later downloaded upon acquiring
contact with the receiving station. Another alternative is to relay data
through TDRSS (Tracking and Data Relay Satellite System), a network
of geosynchronous (geostationary) communications satellites deployed
to relay data from satellites to ground stations.

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The payload for remote sensing satellites can include photographic
systems, electro-optical sensors, and microwave or LiDAR systems. For
applications benefiting from simultaneous coverage by different
sensors, more than one sensing system can be mounted on a single
satellite. In addition to sensor systems, there are often devices for
recording, preprocessing and transmitting the data.

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