Observations made from space divide into two main categories. In the first, some of Earth’s natural phenomena produce infinitely small variations in the radio emissions of molecules that exist on our planet, emissions that can be measured by sensitive satellite-mounted sensors. In the second, space offers an ideal location from which to carry out global imaging of the planet.
When it comes to observing natural phenomena on Earth, satellites equipped with radio instruments (radars, altimeters or passive sensors) designed to measure geophysical parameters can now be used for different types of observation:
- climatology, understanding the atmosphere, or meteorology: measurements can be used to study a range of different geophysical phenomena (salinity, humidity or temperature profiles) affecting the oceans or land masses, helping to identify, for example, areas of drought susceptible to outbreaks of forest fires, or to add to our knowledge of cyclones;
- altimetry: sensors record the altitude of the oceans and inland waters, and even of the ground, revealing information such as the impact of earthquakes on ground topography or on tectonic plates;
- detection of electric and magnetic disturbances of the Earth, often linked to earthquakes. /li>
The spectrum required for these observations is dictated by the physical characteristics of the phenomena to be observed. Because these phenomena are natural in origin, the required frequency bands are unique, and cannot be open to reorganisation.
In the field of imaging, satellites are ideal platforms for acquiring images of the Earth. There are many optical imaging satellites currently in orbit and they are naturally not dependent on radio frequencies to gather their images, since they rely on visible light or infrared. These satellites are inoperative, however, in cloudy conditions or at night. Equipping them with synthetic aperture radar, however, enables them to carry out imaging irrespective of meteorological conditions.
Imaging applications, both optical and radar, also contribute to Earth observation, for example by monitoring changes in sea and lake ice, or possible marine pollution incidents, whether accidental or deliberate. They can also encourage better use of agricultural resources by observing agricultural fragmentation or deforestation.
Downloading the data gathered, while not a scientific application as such (simply a data transfer) is essential for the data to be used and is thus a major issue in the operation of Earth observation systems. These applications can be divided into three types:
- data collection platforms (beacons in systems such as ARGOS), which are used to gather scientific information such as readings of temperature, pressure, humidity or water levels, as measured by instruments deployed across the Earth’s surface, transmitted directly to satellites, which then retransmit the data to ground stations for processing;
- direct links between satellites and ground stations, so that data collected on board the satellites can be downloaded to laboratories for interpretation;
- data relay systems: geostationary satellites communicate with non-geostationary Earth observation satellites, which transmit their observations to the geostationary satellites by radio or laser; the geostationary satellites then download the data to Earth. These systems offer the advantage of more frequent downloads of the data collected, without having to wait for the non-geostationary satellite’s next pass over the data collection station. The United States of America and the Russian Federation have deployed such systems since the 1990s. Thanks to a public-private partnership between ESA and Airbus Defence and Space, a comparable European infrastructure, known as the European Data Relay System or EDRS, is in the process of deployment.
Unlike the spectrum used for observation of natural phenomena or for imaging, frequency bands used for the transmission of scientific data have no particular specific physical characteristics and are much more coveted by other applications.
At the European level, Earth observation applications are grouped under a structural programme known as COPERNICUS (for further details, please visit the project’s official website or the ESA website).
Radio astronomy can also be used to study the physical and chemical properties of the Earth’s atmosphere; when used for this purpose, it is referred to as aeronomy, the study of those planetary atmospheric regions in which the phenomena of ionisation and dissociation, mostly triggered by solar radiation, take place. Aeronomy is particularly useful to our understanding of holes in the ozone layer, the greenhouse effect or the magnetic storms that can disrupt telecommunications systems. Whereas meteorology focuses its attention primarily on atmospheric dynamics, aeronomy pays more attention to the physical and chemical structure of the atmosphere, using the measurement techniques of radio astronomy in dedicated frequency bands. The 22.21-22 GHz band is currently the one most commonly used, due to the presence of one of the spectral lines of water vapour at 22.235 GHz. Radio astronomers mainly use radiometers for these measurements, because their cooled receivers are highly sensitive. Unlike radio telescopes, radiometers offer much lower antenna gains and are much smaller (in size and weight), making them movable.