Basic of Remote Sensing IV

Sources of Electromagnetic Energy

There are three main sources of electromagnetic radiation that are used in remote sensing:

•  Solar radiation (natural radiation from the sun)
•  Terrestrial radiation (natural radiation emitted by Earth's surface)
•  Artificial radiation (from a remote sensing system)

Solar Radiation

• The Sun yields a continuous spectrum of EM energy.
• This Incident radiation can be reflected from the Earth's surface.
• This process produces a large amount of short wavelength energy (from 0.4 - 0.7 µm; blue, green, and red light).
• It can also be emitted by the Earth's surface. Such emitted radiation is typically of a longer wavelength, in the middle and far infra-red wavelengths.
• Interacts with the atmosphere and surface materials (reflect, absorb). 

Since the Sun has a much higher temperature (6000 degrees K) than the Earth (303 degrees K), so the overall energy radiated by the Earth is lower and has its peak at a longer wavelength. This is why radiation emitted from the Earth is at the longer middle and far infra-red wavelengths, compared with the shorter wavelengths of reflected solar radiation.

More generally, there are 3 key concepts that underpin the way that sources of electromagnetic radiation behave:

• Radiant energy (Q) is the energy that an electromagnetic wave contains;
• Irradiance (L) is the total amount of such energy incident on a point on a surface from all directions above the surface;
• Radiant exitance (M) is the total amount of energy leaving a point on a surface to all directions above the surface.

Terrestrial Radiation

• Energy emitted from the Earth and atmosphere 
• Detectable both day and night 
• Earth's ambient temperature - 300K (26 degrees C)
• Earth radiates 160,000 times less than the sun 
• Essentially all energy is radiated at (invisible) thermal infrared wavelengths between 4 – 25 µm
• Maximum emission occurs at 9.7 µm

Artificial Radiation

Artificial radiation occurs where a remote sensing instrument sends its own energy pulse towards the Earth's surface. The characteristics of the back-scattered energy from this pulse can be recorded by a sensor onboard the same instrument. Typically, such sensors can record the time taken by the pulse to travel from the instrument to the target and back again to the instrument's sensor, as well as the strength of the echo received from the object. Remote sensing systems that make use of artificial radiation are often referred to as active remote sensing systems. Radar remote sensing systems emit an electromagnetic pulse, which is then detected as an incident radar wave on an onboard sensor.

An airborne radar remote sensing system