What remote sensing is

Remote sensing is gathering information about objects and events with a sensor that is not in physical contact with them. The term was first used in the early 1960s. Our eyes and cameras respond to only a tiny part of the energy objects give off; remote-sensing devices respond to a much wider range of radiation, reflected or emitted by every surface warmer than 0 Kelvin (-273°C).

• It involves three things: an object surface, a recording device (sensor), and the energy waves carrying information between them.
• The eye is a natural sensor: seeing a building from across a field is a simple kind of remote sensing.

Stages in remote sensing:

• Source of energy: mainly the sun; or energy the system makes itself, like a camera flash or the beam of a radar (radio detection and ranging).
• Transmission to the earth as electromagnetic radiation (EMR), at the speed of light, 3,00,000 km per second. The electromagnetic spectrum runs from gamma rays and X-rays through ultraviolet, visible, infrared and microwaves to radio waves; remote sensing uses the visible, infrared and microwave regions.
• Interaction with the surface: energy is absorbed, transmitted, reflected or emitted, and each object responds differently, and differently in each part of the spectrum. Clear (fresh) water absorbs much red and infrared energy and looks dark; turbid water reflects more blue and green and looks light.
• Passage back through the atmosphere: gases and water molecules absorb some energy (in the middle infrared) and dust scatters blue light, so that information never reaches the sensor.
• Detection by the sensor on a satellite.
• Conversion into digital (or photographic) data, sent to an earth receiving station; India's is at Shadnagar near Hyderabad.
• Extraction of information, after correcting errors, by digital image processing or visual interpretation.
• Conversion into thematic maps and tables.

From the video: the teacher explains reflectance with snow: fresh snow reflects so much sunlight that mountaineers wear dark glasses, while a tarred road absorbs much more. A sensor records exactly these differences. He also mentions atmospheric windows, the parts of the spectrum that pass through the air with little loss, which is why sensors use them.

Example (new): on a satellite image, a muddy river carrying silt after heavy rain shows up much lighter than a clear mountain lake nearby, because the silt reflects more light back to the sensor.