The types of sensing payloads (also known as “sensing instruments”) used in satellites for measurement can be broken down into two categories:
- Active sensors generate their own illumination in the form of light or electromagnetic waves, similar to using a flash on a camera. The synthetic aperture radar (SAR) aboard the satellite launched in the joint NASA-Indian Space Research Organization (NISAR) ISRO mission in 2025 is an example of an active sensor.
- Passive sensors sense illumination from other sources, similar to taking a picture outside where the sun is providing the light. An example of a passive sensor is a multispectral imager such as those used on the Sentinel 2 mission from the European Space Agency (ESA).