Based on Source of energy
The sun provides a very convenient source of energy for remote sensing. The sun’s energy is either reflected, as it is for visible wavelength or absorbed and then re-emitted (for thermal infrared wavelength).
Remote sensing systems, which measure this naturally available energy, are called passive sensors. 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 and night provided that the 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 examples of passive energy sources. Synthetic Aperture Radar (SAR) is an example of active sensor.
Based on Range of Electromagnetic Spectrum
Optical Remote Sensing
The optical remote sensing devices operate in the visible, near infrared, middle infrared and short wave infrared portion of the electromagnetic spectrum. These devices are sensitive to the wavelengths ranging from 300 nm to 3000 nm. Most of the remote sensors record the EMR in this range, e.g., bands of IRS P6 LISS IV sensor are in optical range of EMR.
Thermal Remote Sensing
The sensors, which operate in thermal range of electromagnetic spectrum record, the energy emitted from the earth features in the wavelength range of 3000 nm to 5000 nm and 8000 nm to 14000 nm. The previous range is related to high temperature phenomenon like forest fire, and later with the general earth features having lower temperatures. Hence thermal remote sensing is very useful for fire detection and thermal pollution. e.g., the last five bands of ASTER and band 6 of Landsat ETM+ operates in thermal range.
Microwave Remote Sensing
A microwave remote sensor records the backscattered microwaves in the wavelength range of 1 mm to 1 m of electromagnetic spectrum. Most of the microwave sensors are active sensors, having there own sources of energy, e.g, RADARSAT.
These sensors have edge over other type of sensors, as these are independent of weather and solar radiations.