DEFINITION OF REMOTE SENSING = A study of Earth and environment from a distance through emitted or reflected electromagnetic energy.
Most technology developed for intelligence purposes - previously done through balloons and on pigeons or planes.
Provides long term imagery - show changes in land surface i.e. in vegetation, development, Ozone layer coverage etc - can see damage after a hazard.
Development has enabled wide spread use and vast coverage of the Earth.
Remote Sensing Components:
- Earth Surface: A variety of targets.
- Satellite/Plane: The platform that carries the remote sensing device (sensor).
- Sun: Source of EMR (so-called ‘passive’ remote sensing).
- Atmosphere: The medium the EMR passes through between source and sensor.
Electromagnetic radiation (EMR)
Produced whenever an electric charge (e.g. an electron) changes its velocity (i.e. accelerated or decelerated).
All particles in matter undergo random thermal motion, colliding and thus emitting EMR.
Electromagnetic radiation consists of :
- An electrical field and a magnetic field
- Amplitude of both waves travel at the speed of light c. 300 million metres per second
- Wavelength λ (distance between wave crests) usually expressed as fractions of a meter in either microns (µm = 10-6 m) or nanometers (nm=10-9 m)
- Frequency (v) is a measure of how many wave crests pass a fixed point per sec (Hz)
- Expression as an equation: radiation light (c) = v x λ where c = 3 x 10 8 ms-1 and remains constant in any medium
- Therefore as v increases, λ decreases (and visa versa)
Particle (quantum) theory suggests that EM radiation is composed of many discrete units called photons or quanta. The energy of a quantum is given as:
Q = h v
(where Q = energy of a quantum (Joules - J) – radiant energy, h = Planks constant, (6.626 x 10-34 J/sec) and v = frequency)
Combining the wave and particle theories for EM radiation gives the equation:
Shows that the energy of a quantum is inversely proportional to its wavelength. Therefore the longer the wavelength of EM radiation, the lower its energy content.
The electromagnetic spectrum shows the range of all possible EMR frequencies.
Spatial resolution controlled by:
- field of view (FOV) (narrow swath sensors have small FOV’s and vice versa e.g. Landsat only a few degrees = 185km swath)
- total FOV (FOV times number of pixels)
- flying/ orbit altitude
LandSat = 30x30m resolution - each satelltie has different resolutions depending on what they were designed to examine. Need to choose satellite imagery carefully as not all show every feature of the Earth.
- Measure of the frequency of coverage (i.e. how often is any particular location observed).
- Satellite-based sensors = a function of the satellite orbit and the sensor swath…