Editing Space-based radar

{{short description|Use of radar systems mounted on satellites}}
{{confused|Radiolocation-satellite service}}
{{Refimprove|date=May 2021}}
[[File:ORS-2.jpg|thumb|ORS-2]]
'''Space-based radar''' or '''spaceborne radar''' is a [[radar]] operating in [[outer space]];
'''orbiting radar''' is a radar in [[orbit]] and 
'''Earth orbiting radar''' is a radar in [[geocentric orbit]].
A number of [[Earth-observing satellite]]s, such as [[RADARSAT]], have employed [[synthetic aperture radar]] (SAR) to obtain terrain and land-cover information about the [[Earth]].<ref name="variety" />

== 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]], and [[National Reconnaissance Office|NRO]] program.  The concept was to provide high-range-resolution ground [[moving target indication]] (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 [[radar]] [[satellite]]s for the [[United States Department of Defense]]. The SBR system would allow detection and tracking of [[aircraft]], ocean-going vessels (similar to the [[Soviet Union|Soviet]] [[US-A]] 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 (satellite)|Kondor]]
* Japanese [[Information Gathering Satellite]]
* German [[SAR-Lupe]]
* Chinese [[Huanjing (satellite)|Huanjing]]
* Italian COSMO - Skymed [[COSMO-SkyMed]]

== Earth-observing radars ==

Use of radar sensor for [[Earth observation]] purposes was started by [[NASA]]/[[Jet Propulsion Laboratory|JPL]]'s [[Seasat]] satellite, which carried three different radar sensors: 
* a [[synthetic aperture radar]] (SAR) for high-resolution imaging
* a [[radar altimeter]], to measure the ocean topography
* a wind [[scatterometer]] to measure wind speed and direction
[[File:Image of San Francisco, CA from Umbra space radar.tif|thumb|Image of [[San Francisco, CA|San Francisco]] from Umbra space radar]]
After [[Seasat]], 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]]-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 radar|synthetic aperture]] and/or [[Interferometric Synthetic Aperture Radar|interferometric]] 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. [[Scatterometer]]s are of high value for operational meteorology, allowing reconstruction of [[wind field]]s on a global scale. Data from [[radar altimeter]]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 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, [[Interferometric Synthetic Aperture Radar|SAR interferometry]] (both dual-pass or single-pass, as used in the [[Shuttle Radar Topography Mission|SRTM]] 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]]s, radar satellites often use [[Sun-synchronous orbit]]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]] a.k.a. US-A (SAR, Soviet Union, 1967-1988, 33 satellites)
*[[Almaz]] (Soviet Union, 1973-1977)
*[[Seasat]] (SAR, altimeter, scatterometer, US, 1978)
*[[Geosat]] (altimeter, United States, 1985-1990)
*ERS-1 and ERS-2 ([[European Remote-Sensing Satellite]]) (altimeter, combined SAR/scatterometer, 1991-2011)
*[[RADARSAT-1]] - 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]] (SAR, United States, 2000)
*[[Envisat]] (SAR, altimeter, [[European Space Agency]], 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]] (2001-2013)
** [[OSTM/Jason-2]] (2008-2019)
** [[Jason-3]] (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|SAR Lupe 1-5]] (SAR, [[German Air Force]]; 5 satellites, 2006-)
*[[RADARSAT-2]] (SAR, Canadian, 2007-)
*[[TerraSAR-X]] (SAR [[Germany]], 2007-)
*[[COSMO-SkyMed]] 1st and 2nd generation (SAR, Italy, 2007-; 6 satellites as of 2023)
*[[TecSAR]] (SAR, Israel, 2008-)
*[[TanDEM-X]] (SAR [[Germany]], 2010-)
*[[Sentinel-1]] constellation, [[European Space Agency]], SAR
** [[Sentinel-1A]] (2014-)
** [[Sentinel-1B]] (2016-2021)
** [[Sentinel-1C]] (planned to launch in 2024)
** [[Sentinel-1D]] (planned)
*[[SAOCOM]] (L band SAR constellation, [[Argentina]], 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 (satellite)|NISAR]] (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 [[Project Apollo|Apollo]] and [[Apollo Lunar Module|LEM]]) belong to two categories: imaging radars and sounders.

'''Imaging radars''': [[Synthetic aperture radar]]s are the only instruments capable of penetrating heavy cloud cover around planets such as [[Venus]], 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]]s. The [[Magellan probe]] also imaged Venus in 1990 and 1994.

The only other target of an [[imaging radar]] mission has been [[Titan (moon)|Titan]], the largest moon of [[Saturn]], in order to penetrate its opaque atmosphere. The radar of the [[Cassini probe|''Cassini'' probe]], which orbited [[Saturn]] between 2004 and 2017, provided images of [[Titan (moon)|Titan's]] surface during each fly-by of the moon. The ''Cassini'' radar was a multimode system and could operate as [[Synthetic Aperture Radar]], [[radar altimeter]], [[scatterometer]] and [[radiometer]].

'''Sounding radars''': these are low-frequency (normally, HF - 3 to 30&nbsp;MHz - or lower) ground-penetrating [[Radars]], 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 (radio)|antenna]] dimensions, the beam is very wide) and, thus, the unwanted echo from other surface objects.

The first radar sounder flown was [[ALSE]] (Apollo Lunar Sounder Experiment) on board [[Apollo 17]] in 1972.

Other sounder instruments flown (in this case around [[Mars]]), are [[MARSIS]] (Mars Advanced Radar for SubSurface and [[Ionosphere]] Sounding) on board the [[European Space Agency]]'s [[Mars Express]] probe, and [[SHARAD]] (mars SHAllow RADar sounder) on [[Jet Propulsion Laboratory|JPL]]'s [[Mars Reconnaissance Orbiter]] (MRO). Both are currently operational. A radar sounder is also used on the Japanese Moon probe [[SELENE]], launched September 14, 2007.

A similar instrument (primarily devoted to ionospheric [[Plasma (physics)|plasma]] probing) was embarked on the Japanese Martian mission [[Nozomi (probe)|Nozomi]] (launched in 1998 but lost).

==References==
{{Reflist|refs=
<ref name="variety">{{Cite Q|Q106518362}}</ref>
}}

== External links ==
* [https://www.losangeles.spaceforce.mil/ Air Force fact sheet]
* [http://www.globalsecurity.org/space/systems/sbr.htm Globalsecurity.org page]
* {{usurped|1=[https://archive.today/20130116074600/http://www.airforce-magazine.com/MagazineArchive/Pages/2002/August%202002/0802radar.aspx "The Space Based Radar Plan"]}}, John A. Tirpak, ''[[Air Force Magazine]]'', August, 2002

{{Inspace}}

[[Category:Space radars| ]]
[[Category:Earth observation satellites]]
[[Category:Synthetic aperture radar]]