Earth observation satellite

• 1. Introduction

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  An Earth observation satellite is designed to watch the planet from orbit. By 2008, more than 150 of them were doing exactly that, pulling in over ten terabits of data every single day. By 2021, that fleet had grown past 950 spacecraft, and the largest number of them belonged to a single American company, Planet Labs. How did a technology built for spying and weather forecasts turn into a fleet numbering in the hundreds? What decides the exact orbit a satellite flies, and what can these instruments actually see once they're up there?

• 2. Cold War Origins

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  Herman Potočnik imagined orbiting spacecraft watching the ground below in his 1928 book, The Problem of Space Travel. He described how the unusual conditions of space could support scientific experiments. The book also outlined geostationary satellites, an idea first put forward by Konstantin Tsiolkovsky, and discussed radio contact between such satellites and the ground. It stopped short, though, of proposing that satellites could relay mass broadcasts or telecommunications.

  The Soviet Union launched Sputnik 1 on the 4th of October 1957, the first artificial satellite and the starting point for satellite remote sensing. It sent back radio signals that scientists used to study the ionosphere. The United States Army Ballistic Missile Agency followed by launching Explorer 1 for NASA's Jet Propulsion Laboratory on the 31st of January 1958. Readings from its radiation detector led to the discovery of the Van Allen radiation belts around Earth. NASA's TIROS-1 spacecraft launched on the 1st of April 1960 as part of the Television Infrared Observation Satellite program. It sent back the first television footage of weather patterns ever taken from space.

  The U-2 incident in 1960 exposed the risks of manned aerial spying and pushed the United States to accelerate satellite surveillance programs such as CORONA. The wider Cold War had already been driving faster development of satellite launch systems and camera technology. Governments wanted the ability to gather intelligence on enemy military infrastructure and gauge nuclear posture. After 1960, satellites largely replaced aircraft overflights for surveillance work.

  That shift from spy planes to orbiting cameras created a new engineering problem for the decades ahead. Engineers needed orbits that could watch the same ground at the same time, every day.

• 3. Orbits That Make It Work

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  Most Earth observation satellites carry instruments that work best at altitudes above 500-600 kilometers. Lower orbits face enough air drag to force frequent reboost maneuvers. The European Space Agency operates ERS-1, ERS-2 and Envisat, along with EUMETSAT's MetOp spacecraft, at about 800 kilometers. ESA's Proba-1, Proba-2 and SMOS spacecraft fly lower, at roughly 700 kilometers. The United Arab Emirates operates DubaiSat-1 and DubaiSat-2 in low Earth orbit as well, supplying imagery of various regions of the planet.

  A satellite in a low polar orbit completes a full loop roughly every 100 minutes. During that time, the Earth turns about 25 degrees on its polar axis. That rotation shifts the satellite's ground track about 25 degrees west with each pass, exposing a new stretch of the globe on every orbit. Most of these satellites fly in Sun-synchronous orbits, crossing each point on the ground at the same time of day. That consistency makes it easier to compare observations from one pass to the next.

  A geostationary orbit sits far higher, at about 36,000 kilometers. There, a satellite's orbital period matches Earth's 24-hour rotation, letting it hover over one fixed spot. Hovering over the same point lets it watch more than a third of the Earth without interruption. Three such satellites, spaced 120 degrees apart, can together cover the entire globe. Meteorological satellites are the main users of this arrangement.

  These orbital choices, low and fast or high and fixed, decide what job each satellite ends up doing once it reaches space.

• 4. Eyes On The Weather

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  Mount St. Helens once sent a volcanic ash cloud into the sky, and weather satellite images helped track its spread. The same imagery has tracked activity from other volcanoes, such as Mount Etna. Smoke from fires in the western United States, including blazes in Colorado and Utah, has also been monitored from orbit this way.

  Weather satellites see far more than clouds and cloud systems. City lights, fires, the effects of pollution, and auroras all show up in this data. So do sand and dust storms, snow cover, ice mapping, the boundaries of ocean currents, and shifts in energy flow.

  Watching ash and smoke plumes is one kind of task. Tracking a slow-moving oil slick or a retreating ice field calls for a different set of instruments entirely.

• 5. Tracking A Changing Earth

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  In 2002, an oil spill off the northwest coast of Spain was tracked closely by the European satellite ENVISAT. Though ENVISAT is not a weather satellite, it carries an instrument called ASAR that can detect changes on the sea surface.

  Tropospheric nitrogen dioxide and sulfur dioxide readings let scientists track emissions produced by human activity. The same satellites monitor changes in vegetation, atmospheric trace gases, sea state, ocean color, and ice fields. Comparing a region's current vegetation to its long-term average lets scientists catch a drought as it develops.

  Environmental satellites almost always operate in Sun-synchronous orbits paired with what engineers call a frozen orbit. A frozen orbit is the closest possible approach to a circular path that stays undisturbed by the Earth's oblateness. It also resists the gravitational pull of the Sun and Moon, solar radiation pressure, and air drag.

  Radarsat-1 and TerraSAR-X are among the satellites used to map terrain from orbit.

  None of these measurements reach the ground without a radio signal. Which frequency that signal can use, and how far it can travel, is governed by an entirely different set of rules.

• 6. Rules Of The Radio Spectrum

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  Article 1.51 of the International Telecommunication Union's Radio Regulations defines what counts as an Earth exploration-satellite service. It covers a radiocommunication service between earth stations and one or more space stations, including links between the space stations themselves. That service gathers information about the Earth's characteristics and natural phenomena, including the state of the environment, from passive or active sensors on satellites. Similar information can be collected from airborne or Earth-based platforms and distributed to earth stations within the system. The service can also include platform interrogation and the feeder links needed to operate it.

  Article 1 of the ITU Radio Regulations sorts this service into several categories. These include fixed service, fixed-satellite service, inter-satellite service, Earth exploration-satellite service, and meteorological-satellite service.

  Article 5 of the 2012 edition of the ITU Radio Regulations sets out which frequencies each service can use. National administrations then fold most of these allocations into their own Tables of Frequency Allocations and Utilisations. An allocation can be primary, secondary, exclusive, or shared, with primary allocations written in capital letters and secondary ones in small letters. Even military use of a frequency band that also carries civilian traffic must follow the ITU's rules.

  Between 401-402 megahertz, the allocation covers meteorological aids, space operation, Earth exploration-satellite use, and meteorological-satellite use. It also covers fixed and mobile services, other than aeronautical mobile. Between 13.4-13.75 gigahertz, the band is allocated to active Earth exploration-satellite use, radiolocation, and space research. The same band also carries standard frequency and time signal-satellite transmissions from Earth to space.

  Because these allocations apply everywhere the ITU has authority, they hold across every continent a satellite passes over. The same 13.4-13.75 gigahertz band it uses to scan one storm is shared, by treaty, with the next.