Space-based radar or spaceborne radar is a

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

operating in outer space; orbiting radar is a radar in

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

and Earth orbiting radar is a radar in

geocentric orbit A geocentric orbit, Earth-centered orbit, or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,21 ...

. A number of Earth-observing satellites, such as RADARSAT, have employed

synthetic aperture radar Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or 3D reconstruction, three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target regi ...

(SAR) to obtain terrain and land-cover information about the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

Military

In the United States, Discoverer II was a proposed military space-based radar program initiated in February 1998 as a joint Air Force,

DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adva ...

, and NRO program. The concept was to provide high-range-resolution ground

moving target indication Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or tree ...

(GMTI), as well as SAR imaging and high-resolution digital mapping. This program was cancelled by Congress in 2007. SBR is a less-ambitious version of Discoverer II. Space-based radar (SBR) is a proposed constellation of active

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

s for the

United States Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an United States federal executive departments, executive department of the federal government of the United States, U.S. federal government charged with coordinating and superv ...

. The SBR system would allow detection and tracking of

aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...

, ocean-going vessels (similar to the

Soviet The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

US-A Upravlyaemy Sputnik Aktivnyy ( for Controlled Active Satellite), or US-A, also known in the Western world as Radar Ocean Reconnaissance Satellite or RORSAT ( GRAU index 17F16K), was a series of 33 Soviet reconnaissance satellites. Launched betwe ...

program), and potentially land vehicles from space. This information would then be relayed to regional and national command centers, as well as E-10 MC2A airborne command posts.

Active military radar satellites

* Indian RISAT * Russian Kondor * Japanese Information Gathering Satellite * German SAR-Lupe * Chinese Huanjing * Italian COSMO - Skymed COSMO-SkyMed

Earth-observing radars

Use of radar sensor for

Earth observation Earth observation (EO) is the gathering of information about the physical, chemical, and biosphere, biological systems of the planet Earth. It can be performed via remote sensing, remote-sensing technologies (Earth observation satellites) or throu ...

purposes was started by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

/ JPL's

Seasat Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global sate ...

satellite, which carried three different radar sensors: * a

(SAR) for high-resolution imaging * a

radar altimeter A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...

, to measure the ocean topography * a wind scatterometer to measure wind speed and direction After

, SARs, altimeters and scatterometers have been flown on several other space missions. While the SAR, in principle, is similar to its airborne counterparts (with the advantage of the increased coverage and worldwide access offered by the satellite platform), the other two are specific to satellite operations. A satellite radar-altimeter is a

nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...

-looking radar with very high range resolution, which measures the ocean surface topography with an accuracy in the order of few centimeters. Additionally, analysis of the echo amplitude and shape can extract information about the wind speed and wave height, respectively. Some radar-altimeters (like CryoSat/SIRAL) employ synthetic aperture and/or

interferometric Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...

techniques: their reduced footprint allows mapping of rougher surfaces like polar ices. A wind scatterometer observes the same portion of the ocean surface from different (at least 3) angles of view as the satellite passes by, measuring the echo amplitude and the corresponding surface reflectivity. Reflectivity being affected by the ocean surface "roughness", which in turn is affected by the wind and also dependent on its direction, this instrument can determine the wind speed and direction. These three types of radar are currently used on several satellites. Scatterometers are of high value for operational meteorology, allowing reconstruction of wind fields on a global scale. Data from

s are used for the accurate determination of the geoid, monitoring of tides, ocean currents and other large-scale ocean phenomena such as

El Niño EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...

. SARs applications are many: they range from geology to crop monitoring, from measurement of sea ice to disaster monitoring to vessel traffic surveillance, not to forget the military applications (many civilian SAR satellites are, in fact, dual-use systems). SAR imaging offer the great advantage, over its optical counterparts, of not being affected by meteorological conditions such as clouds, fog, etc., making it the sensor of choice when continuity of data must be ensured. Additionally, SAR interferometry (both dual-pass or single-pass, as used in the

SRTM The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a near-global scale from 56th parallel south, 56°S to 60th parallel north, 60°N, to generate the most complete high-resol ...

mission) allows accurate 3-D Reconstruction. Other types of radars have been flown for Earth observation missions: precipitation radars such as the Tropical Rainfall Measuring Mission, or cloud radars like the one used on Cloudsat. Like other

Earth observation satellite An Earth observation satellite or Earth remote sensing satellite is a satellite used or designed for Earth observation (EO) from orbit, including spy satellites and similar ones intended for non-military uses such as environmental monitoring, me ...

s, radar satellites often use

Sun-synchronous orbit A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is ...

s so that diurnal variations of vegetation are ignored, allowing long-term variations to be more accurately measured.

List of Earth-observing radar satellites

Earth-observing radar satellites and satellite constellations include the following, with operational dates.

Inactive

RORSAT Upravlyaemy Sputnik Aktivnyy ( for Controlled Active Satellite), or US-A, also known in the Western world as Radar Ocean Reconnaissance Satellite or RORSAT (GRAU index 17F16K), was a series of 33 Soviet reconnaissance satellites. Launched between ...

a.k.a. US-A (SAR, Soviet Union, 1967-1988, 33 satellites) *

Almaz The Almaz () program was a highly secret Soviet Union, Soviet military space station program, begun in the early 1960s. Three crewed military reconnaissance stations were launched between 1973 and 1976: Salyut 2, Salyut 3 and Salyut 5. To co ...

(Soviet Union, 1973-1977) *

(SAR, altimeter, scatterometer, US, 1978) * Geosat (altimeter, United States, 1985-1990) *ERS-1 and ERS-2 (

European Remote-Sensing Satellite European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of two satellites, ERS-1 and ERS-2, with ERS-1 being launched in 1991. ERS-1 ERS-1 launched ...

) (altimeter, combined SAR/scatterometer, 1991-2011) *

RADARSAT-1 RADARSAT-1 was Canada's first commercial Earth observation satellite. It utilized synthetic aperture radar (SAR) to obtain images of the Earth's surface to manage natural resources and monitor global climate change. As of March 2013, the sate ...

- natural resources and climate change (SAR, Canadian, 1995-2013) * Tropical Rainfall Measuring Mission (precipitation radar, 1997-2015) * JERS-1 (SAR, Japan, 1998-2001) * QuikSCAT (scatterometer, United States, 1999-2008) *Shuttle Imaging Radar on the

Shuttle Radar Topography Mission The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a near-global scale from 56th parallel south, 56°S to 60th parallel north, 60°N, to generate the most complete high-resol ...

(SAR, United States, 2000) *

Envisat Envisat ("Environmental Satellite") is a large Earth-observing satellite which has been inactive since 2012. It is still in orbit and considered space debris. Operated by the European Space Agency (ESA), it was the world's largest civilian Ear ...

(SAR, altimeter,

European Space Agency The European Space Agency (ESA) is a 23-member International organization, international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 ...

, 2002-2012) * RISAT-1 (SAR, ISRO India, 2012-2016)

Active

* Jason satellite series - joint US-France oceanographic altimeter missions ** TOPEX/Poseidon (1992-2006) **

Jason-1 Jason-1 was a satellite altimeter oceanography mission. It sought to monitor global ocean circulation, study the ties between the ocean and the atmosphere, improve global climate forecasts and predictions, and monitor events such as El Niño ...

(2001-2013) ** OSTM/Jason-2 (2008-2019) **

Jason-3 Jason-3 is a satellite altimeter created by a partnership of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and National Aeronautic and Space Administration (NASA), and is an international cooperative mi ...

(2016-) ** Sentinel-6 Michael Freilich (2020-) ** Sentinel-6B, planned for launch in 2025 * CloudSat (cloud radar, United States, 2006-; daytime only since 2011) * MetOp (scatterometer, European, 3 satellites, 2006-) with planned successor MetOp-SG * SAR Lupe 1-5 (SAR,

German Air Force The German Air Force (, ) is the aerial warfare branch of the , the armed forces of Germany. The German Air Force (as part of the ) was founded in 1956 during the era of the Cold War as the aerial warfare branch of the armed forces of West Ger ...

; 5 satellites, 2006-) * RADARSAT-2 (SAR, Canadian, 2007-) *

TerraSAR-X TerraSAR-X is an imaging radar Earth observation satellite, a joint venture being carried out under a public-private-partnership between the German Aerospace Center (DLR) and Astrium, EADS Astrium. The exclusive commercial exploitation rights ar ...

(SAR

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

, 2007-) * COSMO-SkyMed 1st and 2nd generation (SAR, Italy, 2007-; 6 satellites as of 2023) *

TecSAR TecSAR-1, also known as TechSAR, Polaris and Ofeq-8, is an Israeli reconnaissance satellite, equipped with a synthetic-aperture radar (SAR) developed by Elta Systems. It was successfully launched at 03:45 UTC on 21 January 2008, by PSLV ...

(SAR, Israel, 2008-) *

TanDEM-X TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is a German twin satellite mission using interferometric synthetic aperture radar (InSAR). It is developed in a public-private partnership between the German Aerospace centre (DLR In ...

(SAR

, 2010-) * Sentinel-1 constellation,

, SAR ** Sentinel-1A (2014-) ** Sentinel-1B (2016-2021) ** Sentinel-1C (2024-) ** Sentinel-1D (planned) * SAOCOM (L band SAR constellation,

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

, 2018-) * Tomorrow-R1, the first commercial radar satellite, operated by Tomorrow.io (United States, 2023-) * Umbra-SAR - operated by American firm ''Umbra Space''

Planned

NISAR The NASA-ISRO Synthetic Aperture Radar (NISAR) mission is a joint project between NASA and ISRO to co-develop and launch a dual-frequency Synthetic Aperture Radar (SAR) on an Earth observation satellite in 2025. It will be the first radar im ...

(SAR, joint United States-India, planned for launch in 2025)

Planetary radars

Most of the radars flown as payload in planetary missions (i.e., not considering avionics radar, such as docking and landing radars used in

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

and LEM) belong to two categories: imaging radars and sounders. Imaging radars:

Synthetic aperture radar Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or 3D reconstruction, three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target regi ...

s are the only instruments capable of penetrating heavy cloud cover around planets such as

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

, which was the first target for such missions. Two Soviet spacecraft ( Venera 15 and Venera 16) imaged the planet in 1983 and 1984 using SAR and

Radar altimeter A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...

s. The Magellan probe also imaged Venus in 1990 and 1994. The only other target of an

imaging radar Imaging radar is an application of radar which is used to create two-dimensional images, typically of landscapes. Imaging radar provides its light to illuminate an area on the ground and take a picture at radio wavelengths. It uses an antenna and ...

mission has been

Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

, the largest moon of

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

, in order to penetrate its opaque atmosphere. The radar of the ''Cassini'' probe, which orbited

between 2004 and 2017, provided images of Titan's surface during each fly-by of the moon. The ''Cassini'' radar was a multimode system and could operate as

Synthetic Aperture Radar Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or 3D reconstruction, three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target regi ...

, scatterometer and

radiometer A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. Microwave radiometers operate in the micro ...

. Sounding radars: these are low-frequency (normally, HF - 3 to 30 MHz - or lower) ground-penetrating

Radars Radar is a system that uses radio waves to determine the distance ('' ranging''), direction (azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircr ...

, used to acquire data about the planet sub-surface structure. Their low operating frequency allow them to penetrate hundreds of meters, or even kilometers, below the surface. Synthetic aperture techniques are normally exploited to reduce the ground footprint (due to the low operating frequency and the small allowable antenna dimensions, the beam is very wide) and, thus, the unwanted echo from other surface objects. The first radar sounder flown was

ALSE The ALSE (Apollo Lunar Sounder Experiment) (also known as Scientific Experiment S-209, according to NASA designations) was a ground-penetrating radar (subsurface sounder) experiment that flew on the Apollo 17 mission. Mission and science This expe ...

(Apollo Lunar Sounder Experiment) on board

Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...

in 1972. Other sounder instruments flown (in this case around

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

), are MARSIS (Mars Advanced Radar for SubSurface and

Ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

Sounding) on board the

Mars Express ''Mars Express'' is a space exploration mission by the European Space Agency, European Space Agency (ESA) exploring the planet Mars and its moons since 2003, and the first planetary mission attempted by ESA. ''Mars Express'' consisted of two ...

probe, and SHARAD (mars SHAllow RADar sounder) on JPL's

Mars Reconnaissance Orbiter The ''Mars Reconnaissance Orbiter'' (''MRO'') is a spacecraft designed to search for the existence of water on Mars and provide support for missions to Mars, as part of NASA's Mars Exploration Program. It was launched from Cape Canaveral on Au ...

(MRO). Both are currently operational. A radar sounder is also used on the Japanese Moon probe

SELENE In ancient Greek mythology and Ancient Greek religion, religion, Selene (; , meaning "Moon")''A Greek–English Lexicon's.v. σελήνη is the goddess and personification of the Moon. Also known as Mene (), she is traditionally the daughter ...

, launched September 14, 2007. A similar instrument (primarily devoted to ionospheric plasma probing) was embarked on the Japanese Martian mission Nozomi (launched in 1998 but lost).

References

External links

Air Force fact sheet

* , John A. Tirpak, '' Air Force Magazine'', August, 2002 {{Inspace Earth observation satellites Synthetic aperture radar