Depending on the source of electromagnetic energy, remote sensing can be classified as passive or active remote sensing. Passive and active remote sensing are two main approaches used to acquire information about the Earth's surface and atmosphere using remote sensing technology. Here's an overview of passive and active remote sensing:

1. PASSIVE REMOTE SENSING

   • In passive remote sensing, sensors detect natural electromagnetic radiation emitted, reflected, or scattered by objects on the Earth's surface and atmosphere. Passive sensors do not emit their own radiation but rely on the ambient energy sources, such as sunlight or thermal radiation, to illuminate the Earth's surface.
   • Examples of passive remote sensing include:
     • Optical sensors: These sensors capture sunlight reflected or scattered by objects on the Earth's surface in various spectral bands, such as visible, near-infrared, and thermal infrared wavelengths. Examples include multispectral and hyperspectral imaging sensors.
     • Passive microwave sensors: These sensors detect natural microwave radiation emitted by the Earth's surface and atmosphere, such as thermal emissions from the Earth's surface and atmospheric water vapor and precipitation.
   • Passive remote sensing is commonly used for applications such as land cover mapping, vegetation monitoring, oceanography, atmospheric studies, and climate research.
2. ACTIVE REMOTE SENSING
   
   • In active remote sensing, sensors emit their own electromagnetic radiation and measure the reflected or backscattered energy from objects on the Earth's surface and atmosphere. Active sensors actively illuminate the target with pulses of energy and measure the time it takes for the energy to return to the sensor.
   • Examples of active remote sensing include:
     • Radar (Radio Detection and Ranging): Radar sensors emit microwave or radio waves towards the Earth's surface and measure the strength and timing of the reflected signals to create images and maps of surface features. Different radar techniques, such as synthetic aperture radar (SAR) and interferometric SAR (InSAR), are used for various applications, including topographic mapping, terrain analysis, and monitoring of land deformation.
     • LiDAR (Light Detection and Ranging): LiDAR sensors emit laser pulses towards the Earth's surface and measure the time it takes for the pulses to return after reflecting off objects on the ground. LiDAR data provides highly accurate three-dimensional information about the Earth's surface, including terrain elevation, vegetation structure, and land cover.
   • Active remote sensing is widely used for applications such as topographic mapping, terrain modeling, forest inventory, urban planning, infrastructure monitoring, and disaster management

Both passive and active remote sensing techniques have their advantages and limitations, and the choice between them depends on factors such as the desired spatial resolution, spectral coverage, temporal frequency, and specific application requirements. By combining passive and active remote sensing approaches, researchers and practitioners can obtain complementary information and gain a comprehensive understanding of the Earth's surface and atmosphere.