Active vs. Passive Remote Sensing Sensors

Remote sensing involves gathering information about Earth's surface without direct physical contact. Sensors used in remote sensing are broadly classified into two types: Active and Passive.

Understanding Active Sensors

An Active Sensor carries its own energy source or illumination for the object or scene. It emits energy in the form of radiation or waves and then detects the radiation that is reflected or backscattered from the target. This allows active sensors to operate at any time and through cloud cover, as they provide their own illumination.

• Examples include RADAR (Radio Detection and Ranging) and LIDAR (Light Detection and Ranging).

Understanding Passive Sensors

A Passive Sensor detects naturally occurring electromagnetic radiation (EMR) that is reflected or emitted from the surface of the Earth. Passive sensors rely on external sources of energy, primarily the sun. They measure the amount of solar energy reflected by targets or the thermal energy emitted by them.

• Examples include cameras, radiometers, and spectroradiometers.

Analyzing the Sensor Options

Let's examine each sensor mentioned in the options:

• MODIS (Moderate Resolution Imaging Spectroradiometer): This is a passive sensor. It measures reflected solar radiation and emitted thermal radiation from Earth.
• OMI (Ozone Monitoring Instrument): This is also a passive sensor. It measures reflected and scattered light from the atmosphere, primarily for ozone and trace gas monitoring.
• CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation): This is an active sensor. It uses a LIDAR instrument to actively send laser pulses towards the Earth's surface and atmosphere and measures the backscattered signal to profile clouds and aerosols.
• VIIRS (Visible Infrared Imaging Radiometer Suite): This is a passive sensor. It collects data in visible and infrared bands, similar to MODIS, measuring reflected solar and emitted thermal radiation.

Conclusion on Active Sensor

Based on the analysis, CALIPSO is the only sensor among the options that actively emits its own energy (laser pulses) to gather information. Therefore, CALIPSO is classified as an active sensor in the field of remote sensing.