Editing Space-based radar (section)

== 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.