Dear Student,
Active remote sensing have its own source of light or illumination. In particular, it actively sends a pulse and measures the backscatter reflected back to the sensor. The sensor emits radiation which is directed towards the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active remote sensing include the ability to obtain measurements anytime, regardless of the time of day or season. Active remote sensing can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated.
Passive remote sensing measure reflected sunlight emitted from the sun. When the sun shines, passive sensors measure this energy. Passive remote sensing can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. There is no reflected energy available from the sun at night.
Regards
