Active and passive remote sensing differ primarily in their source of radiation. Active remote sensing utilizes its own source of energy, while passive remote sensing relies on naturally occurring energy, such as reflected sunlight.
Active Remote Sensing Explained
Active remote sensing systems generate their own energy or light and then measure the radiation reflected back from the Earth’s surface.
Key Characteristics:
- Own Energy Source: The system has its own energy source, meaning it does not depend on external sources like the sun.
- Controlled Emission: The sensor emits a specific kind of energy, usually in the form of electromagnetic radiation.
- Time Independence: Active sensors can work day and night because they don’t rely on sunlight.
- Examples:
- Radar (Radio Detection and Ranging): Uses microwaves to image Earth.
- Lidar (Light Detection and Ranging): Employs laser pulses to measure distances and create 3D models.
Passive Remote Sensing Explained
Passive remote sensing instruments detect and measure the electromagnetic radiation that is naturally emitted or reflected by objects or surfaces.
Key Characteristics:
- Natural Energy Source: Relies on external sources of energy, primarily sunlight.
- No Emission: The sensor does not emit any energy of its own.
- Sunlight Dependency: Limited to daytime hours, although some sensors can operate at night by measuring thermal emissions.
- Examples:
- Multispectral Scanners: Capture images at different wavelengths in the visible, near-infrared (NIR), and mid-infrared (MIR) portions of the electromagnetic spectrum.
- Radiometers: Measure the intensity of electromagnetic radiation.
- Optical Cameras: Capture images in the visible light spectrum.
Key Differences Summarized
| Feature | Active Remote Sensing | Passive Remote Sensing |
|---|---|---|
| Energy Source | Own, controlled emission | External, typically solar radiation |
| Operation Time | Day and night | Primarily daytime, though some thermal sensors work at night |
| Dependency | Independent of natural sources | Dependent on natural sources |
| Examples | Radar, Lidar | Multispectral Scanners, Radiometers, Optical Cameras |
| Wavelengths | Uses both short (visible, NIR, MIR) and long (microwave) wavelengths | Primarily uses short (visible, NIR, MIR) wavelengths. |
Practical Insights
- Weather Penetration: Active systems like radar can penetrate clouds and are less affected by weather conditions. Passive systems can be limited by clouds.
- Data Complexity: Data from active sensors can be more complex to interpret than that of passive sensors.
- Spatial Resolution: Both active and passive remote sensors can achieve various levels of spatial resolution, but active sensors often provide more accurate height data.
In summary, the crucial distinction lies in whether the sensor uses its own energy source (active) or relies on external sources (passive), which impacts their operational capabilities and applications. The use of different wavelengths of radiation, including short (visible, NIR, MIR) and long (microwave), is also a key feature that determines the type of information each sensor can collect.
