Space-Based Radar
Encyclopedia
Space-based radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 refers to space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

-borne radar systems that may have any of a variety of purposes. A number of earth-observing radar satellites, such as RadarSat, have employed synthetic aperture radar (SAR) to obtain terrain and land-cover information about the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

.

Space-Based Radar (SBR) is a proposed constellation of active radar satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s for the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 Department of Defense
United States Department of Defense
The United States Department of Defense is the U.S...

. The SBR system would allow detection and tracking of aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

, ocean-going vessels (similar to the Soviet
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 RORSAT
RORSAT
Radar Ocean Reconnaissance SATellite or RORSAT is the western name given to the Soviet Upravlyaemyj Sputnik Aktivnyj satellites. These satellites were launched between 1967 and 1988 to monitor NATO and merchant vessels using active radar...

 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
E-10 MC2A
|-See also:- External links :* *...

 airborne command posts.

Earth-observing radars

Use of radar sensor for Earth observation purposes was started by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

/JPL
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

's Seasat
Seasat
SEASAT was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board the first spaceborne synthetic aperture radar . The mission was designed to demonstrate the feasibility of global satellite monitoring of oceanographic phenomena and to help determine...

 satellite, which carried 3 different radar sensors:
  • a Synthetic aperture radar
    Synthetic aperture radar
    Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

     (SAR) for high-resolution imaging
  • a Radar altimeter
    Radar altimeter
    A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

    , to measure the ocean topography
  • a wind scatterometer
    Scatterometer
    A radar scatterometer is designed to determine the normalized radar cross section of the surface. Scatterometers operate by transmitting a pulse of microwave energy towards the Earth's surface and measuring the reflected energy. A separate measurement of the noise-only power is made and...

     to measure wind speed and direction


After Seasat
Seasat
SEASAT was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board the first spaceborne synthetic aperture radar . The mission was designed to demonstrate the feasibility of global satellite monitoring of oceanographic phenomena and to help determine...

, SARs, altimeters and scatterometers have been flown on several other space missions.

While the SAR is, in principle, 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-looking radar with very high range resolution, which allows to measure (with an accuracy in the order of few centimeters) the sea surface profile. Additionally, analysis of the echo amplitude and shape allows to extract information about the wind speed and wave height, respectively.
Some radar-altimeters (like CryoSat
CryoSat
CryoSat is an ESA programme which will monitor variations in the extent and thickness of polar ice through use of a satellite in low Earth orbit. The information provided about the behaviour of coastal glaciers that drain thinning ice sheets will be key to better predictions of future sea-level rise...

/SIRAL) employ synthetic aperture
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

 and/or interferometric
Interferometric synthetic aperture radar
Interferometric synthetic aperture radar, also abbreviated InSAR or IfSAR, is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar images to generate maps of surface deformation or digital elevation, using differences in the phase of...

 techniques: their reduced footprint allows mapping of rougher surfaces like polar ices.

A wind scatterometer
Scatterometer
A radar scatterometer is designed to determine the normalized radar cross section of the surface. Scatterometers operate by transmitting a pulse of microwave energy towards the Earth's surface and measuring the reflected energy. A separate measurement of the noise-only power is made and...

 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. Being it affected by the ocean surface "roughness", in turn affected by the wind and also dependent from its direction, it is possible to determine the wind speed and direction.

These three types of radar are currently used on several satellites. Scatterometer
Scatterometer
A radar scatterometer is designed to determine the normalized radar cross section of the surface. Scatterometers operate by transmitting a pulse of microwave energy towards the Earth's surface and measuring the reflected energy. A separate measurement of the noise-only power is made and...

s are of high value for operational meteorology, allowing reconstruction of wind fields on a global scale. Data from Radar altimeter
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

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.

SARs applications are countless: they range from geology to crop monitoring, from sea-ice mapping 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
Interferometric synthetic aperture radar
Interferometric synthetic aperture radar, also abbreviated InSAR or IfSAR, is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar images to generate maps of surface deformation or digital elevation, using differences in the phase of...

 (both dual-pass or single-pass, as used in the STRM
Shuttle Radar Topography Mission
The Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...

 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
Tropical Rainfall Measuring Mission
The Tropical Rainfall Measuring Mission is a joint space mission between NASA and the Japan Aerospace Exploration Agency designed to monitor and study tropical rainfall. The term refers to both the mission itself and the satellite that the mission uses to collect data...

, or cloud radars like the one used on Cloudsat
CloudSat
CloudSat is a NASA Earth observation satellite, which was launched on a Delta II rocket on 28 April 2006. It uses radar to measure the altitude and properties of clouds, adding to information on the relationship between clouds and climate in order to help resolve questions about global warming...

.

Like the majority of earth-observing satellites, radar satellites often use sun-synchronous orbit
Sun-synchronous orbit
A Sun-synchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time. The surface illumination angle will be nearly the same every time...

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

Some of the former and current earth-observing radar satellites are:
  • RORSAT
    RORSAT
    Radar Ocean Reconnaissance SATellite or RORSAT is the western name given to the Soviet Upravlyaemyj Sputnik Aktivnyj satellites. These satellites were launched between 1967 and 1988 to monitor NATO and merchant vessels using active radar...

     (SAR, Soviet Union, 1967-1988)
  • Seasat
    Seasat
    SEASAT was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board the first spaceborne synthetic aperture radar . The mission was designed to demonstrate the feasibility of global satellite monitoring of oceanographic phenomena and to help determine...

     (SAR, altimeter, scatterometer, US, 1978)
  • RADARSAT-1
    RADARSAT-1
    Radarsat-1 is Canada's first commercial Earth observation satellite.-Mission:It was launched at 14h22 UTC on November 4, 1995 from Vandenberg AFB in California, into a sun-synchronous orbit above the Earth with an altitude of 798 kilometers and inclination of 98.6 degrees...

     (SAR, Canadian, 1995)
  • RADARSAT-2
    RADARSAT-2
    Radarsat-2 is an Earth observation satellite that was successfully launched December 14, 2007 for the Canadian Space Agency by Starsem, using a Soyuz FG launch vehicle, from Kazakhstan's Baikonur Cosmodrome....

     (SAR, Canadian, 2007)
  • SAR Lupe 1-5 (SAR satellites of the German
    Germany
    Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

     Luftwaffe
    Luftwaffe
    Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

    )
  • TerraSAR-X
    TerraSAR-X
    TerraSAR-X, a German Earth observation satellite, is a joint venture being carried out under a public-private-partnership between the German Aerospace Center DLR and EADS Astrium GmbH; the exclusive commercial exploitation rights are held by the geo-information service provider Infoterra GmbH....

     (SAR Germany
    Germany
    Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

    , 2007)
  • TanDEM-X
    TanDEM-X
    TanDEM-X is the name of TerraSAR-X's twin satellite, a German Earth observation satellite using SAR - a modern radar imaging technology. It is a second, almost identical spacecraft to TerraSAR-X...

     (SAR Germany
    Germany
    Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

    , 2010)
  • COSMO-SkyMed
    COSMO-SkyMed
    COSMO-SkyMed is an Earth observation satellite system funded by the Italian Ministry of Research and Ministry of Defence and...

     (SAR, Italy, 2007)
  • TecSAR
    TecSAR
    TecSAR, also known as TechSAR, Polaris and Ofek-8, is an Israeli reconnaissance satellite, equipped with synthetic aperture radar developed by Elta Systems...

     (SAR, Israeli, 2008)

  • TOPEX/Poseidon
    TOPEX/Poseidon
    Launched in 1992, TOPEX/Poseidon was a joint satellite mission between NASA, the U.S. space agency, and CNES, the French space agency, to map ocean surface topography. The first major oceanographic research vessel to sail into space, TOPEX/Poseidon helped revolutionize oceanography by proving the...

     (altimeter)
  • Jason 1
    Jason 1
    Jason-1 is a satellite oceanography mission 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 and ocean eddies....

     / Jason 2 (altimeter)
  • Shuttle Imaging Radar (see Shuttle Radar Topography Mission
    Shuttle Radar Topography Mission
    The Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...

    ) (SAR)
  • JERS-1
    JERS-1
    Japanese Earth Resources Satellite 1 was a satellite launched in 1992 by the Japan Aerospace Exploration Agency...

     (SAR)
  • Geosat
    Geosat
    The GEOSAT was a U.S. Navy Earth observation satellite, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with an orbital period of 23.07 days and a 330 pass orbit. The satellite carried a radar altimeter capable of measuring the distance from the satellite to sea surface...

     (altimeter)
  • ERS-1 & ERS-2 (European Remote-Sensing Satellite
    European Remote-Sensing Satellite
    European remote sensing satellite was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.-Instruments:...

    ) (altimeter, combined SAR/scatterometer)
  • Envisat
    Envisat
    Envisat is an Earth-observing satellite. It was launched on 1 March 2002 aboard an Ariane 5 from the Guyana Space Centre in Kourou, French Guyana into a Sun synchronous polar orbit at an altitude of...

     (SAR, altimeter)
  • Tropical Rainfall Measuring Mission
    Tropical Rainfall Measuring Mission
    The Tropical Rainfall Measuring Mission is a joint space mission between NASA and the Japan Aerospace Exploration Agency designed to monitor and study tropical rainfall. The term refers to both the mission itself and the satellite that the mission uses to collect data...

     (Precipitation Radar)
  • Cloudsat
    CloudSat
    CloudSat is a NASA Earth observation satellite, which was launched on a Delta II rocket on 28 April 2006. It uses radar to measure the altitude and properties of clouds, adding to information on the relationship between clouds and climate in order to help resolve questions about global warming...

     (cloud radar)
  • Metop
    MetOp
    MetOp is a series of polar orbiting meteorological satellites operated by the European Organisation for the Exploitation of Meteorological Satellites. The satellites are all part af the EUMETSAT Polar System. It is intended to replace the soon to be retired TIROS network...

     (scatterometer)
  • QuickScat (scatterometer)

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
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...

 and LEM
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...

) belong to two categories: imaging radars and sounders.

Imaging radars: Synthetic aperture radar
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

s are the only instruments capable of penetrating heavy cloud cover around planets such as Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, which was the first target for such missions. Two Soviet spacecraft (Venera 15
Venera 15
Venera 15 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems...

 and Venera 16
Venera 16
Venera 16 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems...

) imaged the planet in 1983 and 1984 using SAR and Radar altimeter
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

s. The Magellan probe
Magellan probe
The Magellan spacecraft, also referred to as the Venus Radar Mapper, was a 1,035-kilogram robotic space probe launched by NASA on May 4, 1989, to map the surface of Venus using Synthetic Aperture Radar and measure the planetary gravity...

 also imaged Venus in 1990 and 1994.

The only other target of an imaging radar
Imaging radar
Traditional radar sends directional pulses of electromagnetic energy and detects the presence, position and motion of an object by analyzing the portion of the energy reflected from the object back to the radar station. Imaging radar attempts to form a picture of the object as well, by mapping...

 mission has been Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, the largest moon of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, in order to penetrate its opaque atmosphere. The radar of the Cassini probe, in orbit around Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, is currently providing images of Titan surface at each fly-by of the moon. The Cassini radar is a multimode system and can operate as Synthetic Aperture Radar
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

, radar altimeter
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

, scatterometer
Scatterometer
A radar scatterometer is designed to determine the normalized radar cross section of the surface. Scatterometers operate by transmitting a pulse of microwave energy towards the Earth's surface and measuring the reflected energy. A separate measurement of the noise-only power is made and...

 and radiometer
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....

.

Sounding radars: these are low-frequency (normally, HF - 3 to 30 MHz - or lower) ground-penetrating Radars, used to acquire data about the planet sub-surface structure. Thanks to their low operating frequency they can penetrate for hundred 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
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

 dimensions, the beam is very wide) and, thus, the unwanted echo from other surface objects.

The first radar sounder flown was ALSE
ALSE
The ALSE was a ground-penetrating radar experiment that flew on the Apollo 17 mission.-Mission and Science:...

 (Apollo Lunar Sounder Experiment) on board Apollo 17
Apollo 17
Apollo 17 was the eleventh and final manned mission in the American Apollo space program. Launched at 12:33 a.m. EST on December 7, 1972, with a three-member crew consisting of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, Apollo 17 remains the...

 in 1972.

Other sounder instruments flown (in this case around Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

), are MARSIS
MARSIS
MARSIS is a low frequency, pulse-limited radar sounder and altimeter used on the ESA Mars Express mission...

 (Mars Advanced Radar for SubSurface and Ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 Sounding) on board the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

's Mars Express
Mars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...

 probe, and SHARAD
SHARAD
SHARAD is a subsurface sounding radar embarked on the Mars Reconnaissance Orbiter probe. It complements the MARSIS instrument on Mars Express, providing lower penetration capabilities but much finer resolution .SHARAD is developed under the responsibility of the Italian Space Agency SHARAD (Mars...

 (mars SHAllow RADar sounder) on JPL
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

's Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...

 (MRO). Both are currently operational. A radar sounder is also used on the Japanese moon probe SELENE
SELENE
SELENE , better known in Japan by its nickname after the legendary Japanese moon princess, was the second Japanese lunar orbiter spacecraft. Produced by the Institute of Space and Astronautical Science and the National Space Development Agency , both now part of the Japan Aerospace Exploration...

, launched September 14, 2007.

A similar instrument (primarily devoted to ionospheric plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 probing) was embarked on the Japanese martian mission Nozomi (launched in 1998 but lost).

Defense radars

  • Lacrosse (satellite)
    Lacrosse (satellite)
    For the 2009 Lunar Impactor Mission designed to crash into the moon, see LCROSS.Lacrosse and Onyx are the code names for the United States' National Reconnaissance Office terrestrial radar imaging reconnaissance satellite. While not officially confirmed by the NRO or anybody in the U.S...

  • SAR Lupe


Discoverer II was a proposed military space-based radar program initiated in February 1998 as a joint Air Force, DARPA, and NRO
National Reconnaissance Office
The National Reconnaissance Office , located in Chantilly, Virginia, is one of the 16 U.S. intelligence agencies. It designs, builds, and operates the spy satellites of the United States government.-Mission:...

 program. The concept was to provide high-range-resolution ground moving target indication
Moving target indication
Moving target indication is a mode of operation of a radar to discriminate a target against clutter. In contrast to another mode, stationary target indication, it takes an advantage of the fact that the target moves with respect to stationary clutter. The most common approach takes advantage of...

 (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.

External links

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