D1 Satellite Remote Sensing

Topic

Satellite remote sensing encompasses techniques that measure—from space—electromagnetic radiation emitted or reflected by the atmosphere and the Earth's surface. Its primary advantage is providing global, systematic, and repetitive coverage that is impossible to achieve using ground-based instruments alone. Meteorological satellites primarily utilize two types of orbits, each offering distinct advantages regarding observation frequency and spatial resolution.

Geostationary satellites orbit at an altitude of approximately 36,000 km above the equator and remain seemingly stationary relative to a specific point on the surface. This allows them to continuously monitor a vast region of the planet and generate images every few minutes. However, their great distance limits spatial resolution compared to satellites in lower orbits. Low-altitude polar-orbiting satellites fly much closer to Earth, typically at altitudes of several hundred kilometers. As they move around the planet, they provide greater spatial detail; however, individual regions are observed less frequently, and global coverage is built up through successive orbits.

Geostationary satellites employ various spectral channels. Visible channels record solar radiation reflected by clouds and the surface. Thermal infrared channels detect radiation emitted by the Earth, enabling the determination of brightness temperatures for surfaces and clouds. Water vapor-sensitive channels observe specific infrared bands—particularly around 6 µm, where this gas strongly absorbs radiation—allowing for the tracking of atmospheric moisture distribution and movement, even in regions lacking clearly visible clouds.

Polar-orbiting satellites incorporate other specialized instruments. Microwave radiometers observe various frequencies and provide data on atmospheric temperature and humidity, as well as variables related to liquid water and precipitation. Scatterometers transmit microwave signals toward the ocean surface and measure backscatter; the dependence of this backscatter on sea roughness allows for the estimation of surface wind speed and direction. Radio occultation utilizes signals from navigation satellites; as these signals pass through the atmosphere, they are refracted, and analyzing this deviation enables the retrieval of vertical profiles of atmospheric properties with high vertical resolution.

All these instruments rely on common physical principles to convert recorded signals into meteorological variables. The radiative transfer equation describes how radiation is emitted, absorbed, and transmitted as it travels through the atmosphere. Consequently, the radiance that ultimately reaches the sensor contains information regarding the temperature and composition of the various layers traversed. For thermal radiation, Planck's law establishes the relationship between a body's temperature and the radiation it emits, providing the basis for converting observed radiance into brightness temperature.

Instrument calibration is required before these conversions can be performed. Detectors initially generate electrical signals that must be related to physical radiance values ​​using procedures specific to each sensor. Final accuracy therefore depends on the entire observation chain: orbital characteristics, spatial and spectral resolution, instrument sensitivity, calibration, and the algorithms used to retrieve atmospheric variables.

Geostationary satellites are notable for their high temporal resolution, enabling rapid monitoring of the evolution of clouds and weather systems, whereas polar-orbiting satellites provide observations with greater spatial detail and atmospheric profiles obtained via various sensors. The combination of both platforms, together with the physical laws that allow for the interpretation of measured radiation, makes it possible to observe the evolution of the atmosphere and the Earth's surface continuously and systematically on a global scale.

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