Experts use two types of remote sensing instruments to monitor or measure phenomena found in the Earth's lithosphere, biosphere, hydrosphere, and atmosphere.
The two primary types include active sensors and passive sensors.
Active Sensors
The majority of active sensors operate in the microwave portion of the electromagnetic spectrum, which makes them able to penetrate the atmosphere under most conditions. An active technique views the target from either end of a baseline of known length. The change in apparent view direction (parallax) is related to the absolute distance between the instrument and target.
Examples:
Laser altimeter
Lidar
Radar
Ranging Instrument
Scatterometer
Sounder
Passive Sensors
Passive sensors include different types of radiometers and spectrometers. Most passive systems used in remote sensing applications operate in the visible, infrared, thermal infrared, and microwave portions of the electromagnetic spectrum. Passive remote sensors include the following:
Accelerometer
Hyperspectral radiometer
Imaging radiometer
Radiometer
Sounder
Spectrometer
Spectroradiometer
For further reading, see links below:
earthdata.nasa.gov
www.physicalgeography.net
earthobservatory.nasa.gov