Remote Sensing Technologies: Earth Observation Methods

Main Mechanisms of Space Remote Sensing

Orbital Satellites for Earth Observation

Orbital satellites are fundamental to remote sensing and are primarily of two types:

• Geostationary Orbit: Their movement is synchronized with the Earth's rotation, allowing them to always observe the same area. Due to their high altitude, they cover large regions, making them useful for studying global phenomena.
• Polar Orbit: These satellites follow a circular orbit perpendicular to the Equator. They observe different areas with each pass, and being at a lower altitude, they cover a smaller area but with significantly more detail.

Multispectral Scanning Sensors

These are among the most common remote sensing instruments. They function as a scanner, meticulously tracking each parcel of land and collecting the reflected visible and infrared radiation. This data is crucial for analyzing vegetation health, land cover, and other surface characteristics.

Microwave Sensors

Microwave sensors operate by detecting or emitting microwave radiation. They are categorized into two main types:

• Passive (Microwave Radiometer): These sensors detect naturally emitted microwave radiation. They are very useful for applications like detecting the movement of icebergs and mapping the extent of polar ice, as microwave emission increases with decreasing surface temperature.
• Active (Radar): Active sensors emit their own microwaves and then collect and analyze the return signal. This allows them to penetrate clouds and darkness, providing valuable data regardless of weather conditions or time of day.

Images from microwave sensors often appear somewhat distorted and show various shades of gray, though they can later be colorized. Generally, a smoother surface reflects more radiation away from the sensor, causing it to appear dark or black in the image. Conversely, dry and rough surfaces tend to scatter more radiation back to the sensor, making them appear brighter.

Stereoscopic Images

Utilizing sensors on aircraft or satellites, stereoscopic imaging often combines multi-band imaging and radar simultaneously. This technique, commonly associated with aerial photography, involves a pair of photos taken with slightly different incidence angles and overlapping. When viewed through a stereoscope, these images produce a strong sense of three-dimensional relief, aiding in topographic analysis.

Radar Altimetry

Radar altimetry is a technique that creates a topographic representation of the ground. It uses colored bands to indicate different elevation intervals, providing precise measurements of terrain height and surface variations.

Anaglyph Images

Anaglyph images result from the superposition of two images of the same area, typically one red and one cyan (or blue). When viewed with special red-cyan glasses, these images produce a compelling sense of three-dimensional relief, making them popular for visualizing terrain and geological features.

Lidar Sensors

Lidar (Light Detection and Ranging) sensors emit pulses of laser light (visible or infrared) that interact with atmospheric pollutants and dust, scattering and returning to the sensor. Lidar is extensively used to detect air pollution and can differentiate various gaseous pollutants based on the precise time delay between pulse emission and return signal, as well as the signal strength. This technology also excels in creating highly detailed 3D topographic maps.