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Active remote sensing involves systems that provide their own source of energy to illuminate the objects they observe. An active sensor emits radiation in the direction of the target to be investigated. It then detects and measures the radiation that is reflected from the target. The main advantage of active sensing is its ability to produce measurements anytime, regardless of the time of day or season.

Characteristics of Active Remote Sensing:

1. Energy Source: Active sensors have their own source of light or radiation which they emit.
2. Two-way Travel: Measurements in active remote sensing involve calculating the time taken for a signal to travel to the target and back.

Types of Active Remote Sensing:

1. Radar (Radio Detection and Ranging): Emits radio waves and measures the reflected signals. It’s used in aircraft and satellite remote sensing. The synthetic-aperture radar (SAR) is a type of high-resolution radar.
2. Lidar (Light Detection and Ranging): Uses laser light to measure distances and can be used to create high-resolution digital elevation models (DEMs) of the land surface.
3. SODAR (Sonic Detection and Ranging): Uses sound waves to measure the vertical profile of atmospheric temperature and wind speed/direction.

Applications of Active Remote Sensing:

1. Topography & Elevation: Lidar is extensively used to map terrain and features like forests in 3D.
2. Vegetation Analysis: Determine vegetation height, canopy thickness, and biomass.
3. Geology & Soil: Mapping of geological features, especially using radar which can penetrate the Earth’s surface to some extent.
4. Oceanography: Mapping ocean topography, wave heights, and sea ice.
5. Disaster Monitoring: Flood extent, oil spills, and land deformation post-earthquakes.
6. Urban Planning: 3D modeling of city landscapes using Lidar.
7. Atmospheric Studies: Analyzing components and layers of the atmosphere, especially using SODAR.

Advantages:

1. Day and Night Operations: Active sensors can operate at any time, regardless of the availability of sunlight.
2. Weather Independence: Radar, in particular, can penetrate clouds, making it possible to gather data even in adverse weather conditions.
3. Variable Distances: Some active sensors can adjust the emitted energy, allowing them to map at varying distances.
4. High Precision: Techniques like Lidar provide very high-resolution data.

Limitations:

1. Energy Requirements: Active systems require a power source to emit energy.
2. Complexity: The systems can be more complex and potentially heavier, which can be a consideration for airborne or spaceborne systems.
3. Interference: Signals from active sensors might interfere with other systems or be affected by other energy sources.

In essence, while active remote sensing systems are typically more complex and energy-intensive than their passive counterparts, they offer the advantages of all-weather, day-and-night operations and can provide very high-resolution, highly accurate data.

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