Active Sensor vs. Passive Sensor: Key Differences Explained

This article explains the difference between active and passive sensors. A sensor is a device comprised of an 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 and Operation

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. 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.

Examples:

• Example 1: Communication satellites, earth observation satellites (e.g., RADARSAT-1), LISS-1, etc.
  • Active sensors emit 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 purposes.
  • 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 and Operation

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.

Examples:

• Example 1: Remote sensing satellite such as SPOT-1, LANDSAT-1, etc.
  • Passive sensors receive naturally emitted EM energy within its field of view (FIV) and performs measurement using it.
  • Passive sensors rely on other sources such as the 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.

Active Sensor vs. Passive Sensor: Key Differences

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

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.