- Based on the source of energy: The sun provides a very convenient source of energy for remote sensing. The sun energy is either reflected, as it is for visible wavelength or absorbed and then re-emited for thermal infrared wave length.
Remote sensing systems, which measure. This naturally available energy are called passive sensons. This can only take place when the sun is illuminating the earth. There is no reflected energy available from the sun at night. Energy that is naturally emitted can be detected day or night provided that amount of energy is large enough to be recorded.
Remote sensing systems which provide their own source of energy for illumination are known as active sensors. These sensors have the advantage of obtaining data any time of day or season. Solar energy and radiant heat are the examples of passive energy sources. Synthetic Aperture Radar (SAR) is an example of active sensor.
2. Based on range electromagnetic Spectrum.
The electromagnetic spectrum is energy propagated through space between electric and magnetic fields. The electromagnetic spectrum is the extent of that energy ranging from cosmoic rays, gama rays, x-rays to ultraviolet, visible and infrared radiation including micro wave energy.
Optical Remote Sensing: Remote sensing is visible near infrared and middle infrared ranging from 0.3mm-3mm.
Thermal Remote sensing: Remote sensing of emitted radiation in the range of 3mm to 5mm and 8mm to 16mm.
Microwave Remote Sensing: Remote sensing in higher wave length from 1mm to 1m. These longer wavelength can penetrate clouds.