Sensors

A satellite is composed of modular units, each of which is equipped with a set of sensors.
Another possibility to classify earth observation satellites is according to the sensors used:
- passive sensors which measure the reflected sunlight or thermal radiation(Optical)
- active sensors which make use of their own source of radiation (Radar)
Most of the remote sensing satellite platforms today are in near-polar orbits. The satellite travels northwards on one side of the Earth and then toward the southern pole on the second half of its orbit.
These are called ascending (ANX) and descending (DNX) passes, respectively.
Passive sensors recording reflected solar energy only image the surface on a descending pass, when solar illumination is available. Active sensors which provide their own illumination can also image the surface on ascending passes.