Types of Platforms: Ground-Based, Aerial or Satellite (Location of Sensors) (Especially for GATE-Geospatial 2022)

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Of the fundamental concepts required to understand the process that encompasses remote sensing, the first three components of this process explained — the energy source, interaction of energy with the atmosphere, and interaction of energy with the surface. The fourth component is recording of energy by the sensor by passive and active sensors.

For a sensor to collect and record energy reflected or emitted from a target or surface, it must reside on a stable platform removed from the target or surface being observed. Platforms for remote sensors may be situated on the ground, on an aircraft or balloon (or some other platform within the Earth՚s atmosphere) , or on a spacecraft or satellite outside of the Earth՚s atmosphere.

Ground-Based Platforms

Remote sensing platforms that position the sensor at the Earth՚s surface are called ground-based platforms. These systems are fixed to the Earth and the sensors are often standard tools used to measure environmental conditions such as air temperature, wind characteristics, water salinity, earthquake intensity and such. Ground-based sensors can be placed on tall structures such as towers, scaffolding, or buildings to elevate the platform.

Ground-based sensors are generally less expensive to operate and maintain than aircraft or satellite sensors, but they do not provide the aerial extent of the airborne platforms. Ground-based sensors are often used to record detailed information about the surface, which is compared with information collected from aircraft or satellite sensors.

Remote Sensing Platforms

Remote sensing platforms — at ground, in the air, space

One example of ground-based remote sensing are sensors mounted on buoys that make real-time measurements of water temperature, salinity, wind speed, and wind direction. Buoys are anchored in a body of water (the target) and they transmit the results of each measurement to receiving stations to be processed. These sensors can be used to supplement or “ground truth” measurements made from airborne or satellite sensors.

Aerial Platforms

Aerial platforms are most often sensors mounted on fixed-wing aircraft, though other airborne platforms, such as balloons, rockets, and helicopters can be used. Aircraft are often used to collect very detailed images of the Earth՚s surface and facilitate the collection of data over virtually any portion of the Earth՚s surface at any time. Aerial systems elevate the sensor above the Earth՚s surface in order to increase its aerial coverage. They also allow researchers to monitor very large area, of the surface which would be impractical with ground-based sensors or impossible or dangerous to visit. Airborne remote sensing dates back to the early 1900՚s when airplanes were used during the World Wars to conduct surveillance of the enemy. More recently, cameras mounted on aircraft have been used to monitor land use practices, locate forest fires, and produce detailed and accurate maps of remote or inaccessible locations on our planet. Weather balloons and rockets are still used by research scientists as a means for obtaining direct measurements of the properties of the upper atmosphere. These provide a less expensive and reusable alternative to aircraft and satellite systems.

Satellite Sensors V/S Aircraft Sensors

Sensors on board satellites generally can “see” a much larger area of the Earth՚s surface than would be possible from a sensor onboard an aircraft. Also, L ՚cause they are continually orbiting the Earth, it is relatively easy to collect in grey on a systematic and repetitive basis in order to monitor changes over time. The geometry of orbiting satellites with respect to the Earth can be calculated quite accurately and facilitates correction of remote sensing images to their proper geographic orientation and position. However, aircraft sensors can collect data at any time and over any portion of the Earth՚s surface (as long as conditions allow it) while satellite sensors are restricted to collecting data over only those areas and during specific times dictated by their particular orbits. It is also much more difficult to fix a sensor in space if a problem or malfunction develops!

Satellite Platforms

In space, remote sensing is sometimes conducted from the space shuttle or, more commonly, from satellites. Satellites are objects which revolve around another object - in this case, the Earth. For example, the moon is a natural satellite, whereas man-made satellites include those platforms launched for remote sensing, communication, and telemetry (location and navigation) purposes. Because of their orbits, satellites permit repetitive coverage of the Earth՚s surface on a continuing basis. Cost is often a significant factor in choosing among the various platform options.

Environmental Satellites

In the early 1960՚s researchers started mounting sensors on satellites placed into orbit over the Earth and ushered in a new era of environmental remote sensing that continues to grow at a rapid pace today. The vantage point of space allows researchers to observe and measure phenomena on a time and spatial scale that was previously impossible. Today, satellites provide us with views of the Earth that allow us to monitor global change and understand our planet.

This wealth of data comes with a price, however. To build a satellite and place it into orbit is a very difficult and expensive endeavour. Satellites must be operated remotely from the ground and data from the satellite sensors must be transmitted to the surface. The communications technologies in remote sensing satellites can be very complex and expensive to engineer and maintain. A number of satellites have failed to reach orbit, or failed to operate once in orbit around the earth, which is a testament to the incredible complexity involved in designing, building, and operating a satellite.

All these difficulties notwithstanding, environmental satellites have contributed greatly to our understanding of the Earth՚s environment and continue to be used extensively for remote sensing research. For example, weather satellite technology, one of the first practical applications of satellite remote sensing, has vastly expanded our understanding of the Earth՚s weather by providing a synoptic (large scale) view of our weather systems that was previously impossible. It was only after the advent of satellites that weather patterns such as hurricanes and mid-latitude cyclones were fully understood. Prior to satellites, any knowledge of these storms was collected through ground level observations that unfortunately did not provide the information necessary to adequately understand them. The contribution of satellites to our understanding of dangerous weather events has saved countless numbers of lives since the early 1960՚s.

Weather Satellites Are Geostationary

Weather Satellites Are Geostationary

Most of the images we see on television weather forecasts are from geostationary satellites. This is because they provide broad coverage of the weather and cloud patterns on continental scales. Meteorologists (weather forecasters) use these images to help them determine in which direction the weather patterns are likely to go. The high repeat coverage capability of satellites with geostationary orbits allows them to collect several images daily to allow these patterns to be closely monitored.

Communications and Data Collection

Data collected from a remote sensing system must be retrieved and delivered to the end users. Often, this must be done quickly for the data to be of any use, such as in the case of severe thunderstorm forecasting where storms develop into severe storms within minutes. Thus the transmission, reception, processing, and distribution of data from a satellite sensor must be carefully designed to meet the users՚ needs.

Ground-Based vs. Satellite Platforms

  • Ground-based remote sensing platforms can transmit data using ground-based communication systems, such as radio and microwave transmissions or computer networks. Some systems can store data on the platform, allowing researchers to manually collect the data from the platform. Data collected in an aircraft can be stored on board and retrieved once the aircraft lands. Satellite data, however, is very difficult to obtain since the satellite remains in space during its entire operational lifetime. This data must be transmitted back to the Earth to a ground receiving station, which can receive the data and process it for distribution to the end user.
  • Data collected from a satellite platform can be transmitted to Earth in a variety of ways. A satellite can transmit data directly to a ground receiving station that is within its line of sight. When the satellite is not in sight of a ground station, it can store its data on board and “dump” the data later, when it is back in sight of a ground station. Finally, for immediate transmission, a satellite can relay data to the ground receiving station through a series of communications satellites in orbit around the Earth, transferring data from one satellite to the next until it can reach the ground receiving station desired. The data received at the ground station are in a raw digital format. They may then, if required, be processed to correct systematic, geometric and atmospheric distortions to the imagery, and be translated into a standardized format. The data are written to some form of storage medium such as tape, disk, or CD. The data are typically archived at most receiving and processing stations, and full libraries of data are managed by government agencies as well as commercial companies responsible for each sensor՚s archives.

Ground-Based vs. Satellite vs. Aerial Platforms

Table Supporting: Groundbased_vs_Satellite_vs_Aerial_Platforms
Geostationary Satellite36000 kmFixed point observationINSAT, NOAA
Polar orbiting satellite (Earth observation)500 - 1000 kmRegular observationIRS, Landsat,

SPOT etc.

Space shuttle240 - 350 kmIrregular observation, space experimentSkylab
Airplane1 - 10 kmReconnaissance, aero surveys, various investigations, data collectionAerial photograph՚s
Hang balloon50 m - 1000 mVarious investigations, reconnaissance, archeological investigations
Ground based platformsUp to 30 mGround truth, archeological investigations

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