Active Sensor vs Passive Sensor

This page compares Active Sensor Vs Passive Sensor and mentions difference between Active Sensor and Passive Sensor types.

A sensor is a device which comprises of optical component or system and a detector with electronic circuits used to record reflected and/or emitted energy from various objects. Active sensors and passive sensors are two main types of sensors used to gather information about the environment.

Active Sensor

Definition: Active sensors emit energy or signals into the environment and measure the response or reflection of that energy.
Operation: They actively generate and emit signals (such as radio waves, sound waves, or lasers) and then analyze the response or reflection from the target object.

The active sensors do not require an external circuitry or mechanism to power them. It directly responds to the external stimuli from its ambient environment and converts it into an output signal. For example, in a photodiode light is converted into electrical impulses.

active sensor

Example-1: communication satellite, earth observation satellite (e.g. RADARSAT-1), LISS-1 etc.
• Active sensor emits their own EM (Electromagnetic) energy which is transmitted towards the earth and receives energy reflected from the earth. The received EM energy is used for measurement purpose.
• Active sensors use their own source of energy for operation.
• Active sensors can obtain measurements anytime (Day & Night).

Example-2 : Active sonar emits sound waves underwater and detects their echoes to locate underwater objects.

Example-3 : Radar systems transmit radio waves and detect their reflections to determine the location and speed of objects.

Passive Sensor

Definition: Passive sensors do not emit their own energy; instead, they detect and measure the natural radiation or signals present in the environment.
Operation: They rely on the ambient energy or signals, such as sunlight or thermal radiation, and measure the variations in these signals without actively emitting any energy.

The passive sensors require an external mechanism to power them. The sensed properties are modulated with the sensor's inherent characteristics to generate patterns in the output of the sensor. For example, in a thermistor resistance can be detected by applying voltage difference across it or passing current through it.

passive sensor

Example-1: remote sensing satellite such as SPOT-1, LANDSAT-1 etc.
• Passive sensor receives naturally emitted EM energy within its field of view (FIV) and performs measurement using it.
• Passive sensors rely own other sources such as sun for its operation.
• Passive sensors obtain measurements only in day time.

Example-2 : Microphones pick up sound waves in the environment without emitting sound.

Example-3 : Cameras capture visible light and other wavelengths without emitting any signals themselves.

Difference between Active Sensor and Passive Sensor

The key difference between active and passive sensor type lies in how they collect and detect signals.

Parameters Active sensor Passive sensor
Emission of signals Emit signals into the environment Do not emit signals; detect existing signals.
External energy source Requires an external energy source for active components Does not require an external energy source
Operation Actively generate and emit signals, then analyze the response or reflection Rely on ambient energy or signals and measure variations without emitting signals
Range Can have a longer range due to actively emitted signals May have a limited range based on ambient signals.
Applications Used in applications where control over emitted signals is necessary. Commonly used when detecting existing signals is sufficient.
Examples Radar systems, LiDAR, active SONAR Infrared sensors, cameras, microphones

Conclusion : In summary, the choice between active and passive sensors depends on the specific requirements of the application, including the need for signal control, energy considerations, and the desired range of detection.

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