Remote sensing satellites, also known as Earth observation satellites or Earth remote sensing satellites, are used for espionage purposes or for environmental monitoring, meteorology, and mapping. The most common type is earth-imaging satellites, which capture satellite images similar to aerial photos. Some EO satellites can perform remote sensing without forming images, such as in the radio occultation of global navigation satellite systems.
Remote sensing satellites first appeared with the launch of the first artificial satellite, Sputnik 1, on October 4, 1957. It transmitted radio signals that scientists used to study the ionosphere.
In January 1958, NASA launched its first US satellite, Explorer 1. Information returned from the radiation detector led to the discovery of the Van Allen radiation belts around the Earth.
In April 1960, as part of the NASA television infrared observation satellite (TIROS) program, the TIROS-1 spacecraft transmitted the first television footage of weather patterns from space.
The instruments carried by most remote sensing satellites should operate at relatively low altitudes. They generally avoid altitudes lower than 500-600km because significant air resistance in low altitudes means they must be lifted into orbit more frequently.
The European Space Agency's Earth observation satellites ERS-1, ERS-2, and Envisat, as well as the European Meteorological Satellite Application Organization's MetOp spacecraft, all operate at around 800km. The European Space Agency's Proba-1, Proba-2, and SMOS spacecraft observe the Earth from approximately 700km. The United Arab Emirates' Earth observation satellites DubaiSat-1 and DubaiSat-2 are also placed in low Earth orbits, providing satellite images of various parts of the Earth.
To achieve global coverage at a low orbit, it must be a polar orbit or nearly a polar orbit. The orbital period of a low orbit is approximately 100 minutes. The Earth rotates about its polar axis, rotating about 25 degrees between continuous orbits, causing the ground orbit to move 25 degrees westward in longitude. Most remote sensing satellites on polar orbits are in a sun-synchronous orbit.
The sensors or instruments on the satellite use the sun as a light source or provide their own light source to measure reflected energy. Sensors powered by natural energy from the sun are passive sensors. Those that provide their own energy are called active sensors.
Passive sensors include different types of radiometers, which are used to quantitatively measure the intensity of electromagnetic radiation in selected bands, and spectrometers, which are designed to detect, measure, and examine the spectral content of reflected electromagnetic radiation. Most passive systems used for remote sensing operate in the visible, infrared, thermal infrared, and microwave portions of the electromagnetic spectrum. They measure land and sea surface temperature, vegetation characteristics, cloud and aerosol properties, and other physical properties.
Most passive sensors cannot penetrate dense cloud cover, which means they are restricted in observation areas with frequent dense cloud cover, such as in the tropics.
Active sensors include different types of radio detection and altitude sensors, radar sensors, and scatterometers. Most active sensors operate in the microwave portion of the electromagnetic spectrum, allowing them to penetrate the atmosphere in most conditions. These sensors help measure the vertical profiles of aerosols, forest structure, precipitation and wind, and sea surface topography and ice, among others.