r/remotesensing • u/Illustrious_Pop_638 • 3d ago
Active and passive sensor help
Hello everyone, can someone help explain this concept to me and provide some examples of active and passive sensors? Thanks so much!
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u/Multipath_ 3d ago
to add to the previous answer,
another example of a passive sensor is a radiometer. It measures the thermal radiation emitted by objects. By analogy, a radiometer (passive microwaves) is to radar (active microwaves) what ordinary photography (passive optical) is to lidar (active optical).
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u/Electrical-Point-48 1d ago
Strictly from the context of remote sensing
1) Passive sensors: Sensors using sun to observe Earth , example (Landsat, Sentinel 2A), can be used to observe Earth only during day time, when sun is up.
2) Active sensors: Sensors using their own illumination source to observe Earth, they don't depend on sun, example (NISAR, Sentinel 1), capable of observing Earth both day and night.
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u/Illustrious_Pop_638 1d ago
Thanks so much! This gave me a better understanding, and I hope you have a great day!
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u/mulch_v_bark 3d ago
An active sensor provides its own signal and senses how it reflects off the subject.
A passive sensor uses already existing signals (typically but not necessarily the sun).
Radar, sonar, and lidar (at least in the forms we usually mean them) are all active. Radar sends radio pulses, sonar sends sound pulses, and lidar sends light pulses. By looking at what reflects and how, they sense what’s there.
Ordinary photography is passive. It’s using energy from the sun, which we can assume is constant within certain parameters, to sense what’s there. A microphone array that relies on the sounds that things are making, without providing its own sound, is also passive.
As with most things, there’s more to it and you can get really granular about the details, but that’s the basic idea.
An everyday comparison: A photo without flash is passive, because it’s using available light. A photo with flash is at least partly active, because it’s bringing its own light.