Remote sensing

Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"; Class 12, Ch 4 "Spatial Information Technology"

Meaning

Remote sensing means collecting and measuring information about objects and events on Earth using a recording device, called a sensor, that does not touch them. The sensor records the energy that the objects reflect (bounce back) or emit (give off, often as heat).

• It lets us study very large, distant or dangerous areas from aircraft and satellites.
• It sees energy the human eye cannot, such as infrared. This makes it the basic tool for mapping land, water, forests and disasters.

Explanation

Why it works: every object sends out energy

• The eye's limit: human eyes and ordinary cameras pick up only visible light, which is a tiny part of the energy that objects give off or reflect.
• The sensor's reach: remote sensors record a much wider range of radiation, for example infrared, thermal (heat) and microwave energy.
• Every object has an energy "signature":
• Any object warmer than 0 K (absolute zero, −273°C) reflects, emits, absorbs (takes in) and transmits (lets through) radiation.

• So water, rock, crops and cities each leave their own energy pattern, and a sensor can record it.

• Electromagnetic spectrum (EMS): the full range of electromagnetic energy. It runs from gamma rays (shortest waves) through ultraviolet, visible, infrared and microwave to radio waves (longest waves). Remote sensing depends on how energy is spread across this spectrum [2].

• Official meaning: NRSC calls it the science of getting information about the Earth's surface without touching it. You sense and record reflected or emitted energy, then process, analyse and apply that information [2].
• Origin of the term: first used in the early 1960s, by Evelyn Pruitt of the US Office of Naval Research.

Its parts: three elements (NCERT Fig. 6.1)

Element	What it is	Example
Object surface	The thing being studied	Forest, lake, crop field, city
Sensor	The device that records energy without touching the object, either as a photograph or as digital numbers	Camera on an aircraft; scanner on a satellite
Energy waves	They carry information from the object to the sensor	Sunlight reflected by a leaf; heat given off by land

How it works: the eight stages (NCERT Fig. 6.2)

1. Source of energy: usually the sun. It can also be the sensor itself (a self-emitted source). Radar, for example, sends out microwaves and records the echo.
2. Transmission to the earth's surface: energy passes down through the atmosphere. The atmosphere absorbs some of it and scatters some of it (bounces it in other directions) [2].

3. Interaction with the surface: each surface handles energy in its own way. - Healthy plants reflect strongly in near-infrared. - Clear water absorbs most near-infrared. - This difference is what lets us tell objects apart.

4. Propagation back through the atmosphere: the reflected or emitted energy travels up towards the sensor. The atmosphere affects it a second time.

5. Detection by the sensor: the sensor on an aircraft or a satellite records the energy that reaches it.
6. Conversion into data: the energy becomes a photograph (film image) or digital data (numbers arranged in a grid of cells).
7. Extraction of information: interpreters study the image, by eye or by computer, to pick out forests, water, settlements and other features.
8. Conversion into maps or tables: the results become thematic maps (maps on one theme, such as land use) or tables for planners.

• NRSC's shorter version (six stages): emission of energy (by the sun or the sensor) → transmission to the surface → interaction with the surface → transmission back to the sensor → sensor data output → data transmission, processing and analysis [2]. It is the same chain, with some steps merged.

Why it beats ground survey (conditions and advantages)

• Synoptic coverage: one image covers a very large area in a single view. For example, one IRS-1A pass covered a swath (the strip of ground imaged) about 140 km wide [3].
• Repeat coverage: the satellite passes over the same place again and again, so we can track floods, crop growth or deforestation over time.
• Access to remote areas: it can image the high Himalaya, dense forests, deserts and the open ocean.
• Beyond visible light: infrared, thermal and microwave data show things the eye misses, such as how healthy a crop is.
• Condition it depends on: a clear atmosphere for optical sensors.
• Clouds and haze block sunlight-based sensors, especially in the monsoon.
• Radar sends out its own microwave energy, so it can still work then.

Where it occurs

• Early practice (world): the first remote sensing used aerial photography, with cameras on balloons and aircraft. It recorded mainly visible light.
• Landsat 1, USA (1972):
• NASA launched it on 23 July 1972 from Vandenberg Air Force Base, California [4].
• It was first called ERTS (Earth Resources Technology Satellite). It was the first Earth-observing satellite built specifically to study and monitor land areas [4].
• It carried the Return Beam Vidicon (RBV) camera and the Multispectral Scanner (MSS). The MSS gave better data [4].

• It was built to last 1 year but worked until 6 January 1978, imaging about 75% of the Earth's surface [4].

• IRS-1A, India (1988):

• India's first home-built remote sensing satellite, launched on 17 March 1988 from Baikonur Cosmodrome (Kazakhstan, then USSR) on a Soviet Vostok rocket [3].
• It was placed in a polar sun-synchronous orbit (a north–south orbit over the poles that passes each place at the same local time, so the lighting stays similar between images) [3].
• It weighed 975 kg and carried LISS-I (73 m resolution) and LISS-II (36.25 m resolution) [3]. Resolution is the size of the smallest ground area one picture cell shows. A smaller number means finer detail.

• It worked for 8 years and 4 months, until July 1996 [3].

• How India uses it:

• Mapping land use, forests, water bodies and glaciers in hard-to-reach areas such as the Himalaya.

• Mapping floods, cyclones and landslides by comparing "before" and "after" images.

• National Geospatial Policy, 2022 (Government of India):

• Notified on 28 December 2022 [5][6].
• Its vision runs to 2035. It opens up access to geospatial data (data tied to a place on Earth, such as maps, satellite images and coordinates) for government, business and universities [6].
• Its targets are high-resolution topographical survey and mapping by 2030 and an accurate Digital Elevation Model (DEM: a digital 3-D model of land height) for the whole country [5][6].

Don't confuse with

• Ground survey: the surveyor is in contact with the land and covers a small area at a time. Remote sensing works without contact and covers a large area in one view, again and again.
• Aerial photography vs satellite remote sensing: aerial photography is the earliest form of remote sensing. It used aircraft or balloon cameras and recorded mainly visible light. Satellites like Landsat 1 (1972) added multispectral scanners and repeat coverage.
• Passive vs active remote sensing: both are remote sensing. Passive sensors record energy from the sun (reflected) or from the object itself (emitted). Active sensors such as radar send out their own energy and record the echo, so they can see through clouds.
• GIS (Geographic Information System): GIS is a computer system that stores, analyses and displays place-based data. Remote sensing collects the data. Remote sensing images often feed into a GIS.

Prelims Hooks

• Three elements of remote sensing: object surface, sensor, energy waves. "Ground control station" or "GPS receiver" is a trap. Neither is one of the three.
• NCERT gives 8 stages. They start with the source of energy and end with conversion into maps or tables. Detection by the sensor is stage 5, not the last stage. NRSC merges them into 6 stages [2].
• The term was first used in the early 1960s by Evelyn Pruitt (US Office of Naval Research). The energy source can be the sun or the sensor itself (for example, radar).
• Any object above 0 K (−273°C) reflects, emits, absorbs and transmits radiation. In practice, that means every real object.
• Landsat 1 = ERTS, launched 23 July 1972 from Vandenberg (California). It was the first satellite built specifically to monitor Earth's land areas [4].
• IRS-1A: 17 March 1988, Baikonur, Vostok rocket, polar sun-synchronous orbit, LISS-I (73 m) and LISS-II (36.25 m) [3]. National Geospatial Policy: notified 28 December 2022, vision to 2035, high-resolution mapping by 2030 [5][6].

Mains Points

• Better than ground survey (GS-I, maps and resources):
• It gives a wide view (synoptic coverage) and repeat coverage, reaches remote areas, and records beyond visible light.

• This makes it the basic tool for mapping land use, forests, water bodies and Himalayan glaciers that survey teams cannot easily reach.

• Disaster management and its limits (GS-III):

• Comparing "before" and "after" images lets planners map floods, cyclones and landslides quickly, without sending people into danger zones.

• Stages 2 and 4 depend on the atmosphere, and monsoon clouds block optical sensors. So India needs radar, which sends its own microwave energy, and images must be checked against ground data.

• Indigenous capability and open data (GS-III, science and technology):

• India moved from depending on foreign data to its own satellite, IRS-1A (1988) [3], and then to opening up geospatial data under the National Geospatial Policy 2022 [5].
• This supports planning for agriculture and cities, and helps private geospatial businesses grow.

Related concepts

• Stages in remote sensing

Read more

• What remote sensing is and its stages