Remote Sensing

Remote sensing is the science of making inference about objects from measurements, made at a distance, without coming into physical contact with the object under study. This can be used to gather information about an object without actually coming in contact with it. Human apply remote sensing in their day-to-day business, through vision, hearing and sense of smell. United Nation defined as “Remote sensing means sensing of the earth’s surface from space by making use of properties of electromagnetic wave emitted, reflected or diffracted by the sensed objects, for the purpose of improving natural resources management, land use and the protection of the environment.”

A remote sensing system consists of a source – Sun or source of electromagnetic radiation; a sensor – to collect radiation; and a platform – an aircraft, balloon, rocket, satellite or even a ground-based sensor-supporting stand. The instruments used to measure the radiation reflected/emitted by the target under study are usually referred to as Remote Sensors.

Sensors:

Sensors or instruments, on board satellites and aircraft use the sun as a source of illumination or provide their own source of illumination, measuring the energy that is reflected back. Sensors has two main types i.e. active sensor and passive sensor. Sensor that use natural energy from the sun are called passive sensors. Those that provide their own source of energy are called active sensors.

The remote sensing application most used passive systems to operate in the visible, infrared, thermal infrared, and microwave portions of the electromagnetic spectrum. These sensors measure the land and sea surface temperature, vegetation properties, cloud and aerosol properties, and other physical properties.

Active sensors include different types of radio detection and ranging (radar) sensors, altimeters, and scatterometers. These sensors operate in the microwave band of the electromagnetic spectrum. Theses sensors most useful for measuring the vertical profiles of aerosols, forest structure, precipitation and winds, sea surface topography, and ice, etc.

Source : earthdata.nasa.gov
Orbits :

The satellite follows a generally elliptical orbit around the earth. The time taken to complete one revolution of the orbit is called the orbital period. Remote sensing satellites are often launched into special orbits such that the satellite repeats its path after a fixed time interval. This time intervals are called the repeat cycle of the satellite.

There are three main types of orbits in which satellites reside : Polar; non-polar, low-Earth orbit, and geostationary.

A Polar orbit travels north-south over the poles and take approximately an hour and a half for a full rotation. As the satellite is in orbit, the Earth is rotating beneath it. As a result, a satellite can observe the entire Earth’s surface in the time span of 24 hours. Polar-orbiting satellites are in an orbital plane that is inclined at nearly 90 degree to the equatorial plane. This inclination allows the satellite to sense the entire globe, including the polar regions, providing observations of locations that are difficult to reach via the ground.

Non-Polar, low-Earth orbits are at an altitude of typically less than 2,000 km above the Earth’s surface. These orbits do not provide global coverage but instead cover only a partial range of latitude. The Global Precipitation Mission is an example of this.

Geostationary satellite follows the Earth’s rotation and travel at the same rate of the rotation; because of this, the satellite appear to an observer on Earth to be fixed in one location.  Satellites in the geostationary orbits are located at a high altitude of 36,000 km.  The geostationary orbits are commonly used by meteorological satellites. E.g. Weather satellites.

Source: crisp.nus.edu.sg, earthdata.nasa.gov, NPTL, gisgeography.

MPSC : Mains-GS I Paper – Topic – Fundamental of Remote Sensing

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