Synthetic aperture radar in the context of "Echo"

Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth and other planets. Remote sensing is used in numerous fields, including geophysics, geography, land surveying and most Earth science disciplines (e.g. exploration geophysics, hydrology, ecology, meteorology, oceanography, glaciology, geology). It also has military, intelligence, commercial, economic, planning, and humanitarian applications, among others.

In current usage, the term remote sensing generally refers to the use of satellite- or airborne-based sensor technologies to detect and classify objects on Earth. It includes the surface and the atmosphere and oceans, based on propagated signals (e.g. electromagnetic radiation). It may be split into "active" remote sensing (when a signal is emitted by a sensor mounted on a satellite or aircraft to the object and its reflection is detected by the sensor) and "passive" remote sensing (when the reflection of sunlight is detected by the sensor).

Lacrosse or Onyx was a series of terrestrial radar imaging reconnaissance satellites operated by the United States National Reconnaissance Office (NRO). While not officially confirmed by the NRO or the Government of the United States prior to 2008, there was widespread evidence pointing to its existence, including one NASA website. In July 2008, the NRO itself declassified the existence of its synthetic aperture radar (SAR) satellite constellation.

According to former Director of Central Intelligence Admiral Stansfield Turner, Lacrosse had its origins in 1980 when a dispute between the Central Intelligence Agency and the U.S. Air Force as to whether a combined optical/radar reconnaissance satellite (the CIA proposal) or a radar-only one (the USAF proposal) should be developed was resolved in favor of the USAF.

The Spaceborne Imaging Radar (SIR) – full name 'Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR)', is a synthetic aperture radar which flew on two separate shuttle missions. Once from the Space Shuttle Endeavour in April 1994 on (STS-59) and again in October 1994 on (STS-68). The radar was run by NASA's Space Radar Laboratory. SIR utilizes 3 radar frequencies: L band (24 cm wavelength), C band (6 cm) and X band (3 cm), allowing for study of geology, hydrology, ecology and oceanography. Comparing radar images to data collected by teams of people on the ground as well as aircraft and ships using simultaneous measurements of vegetation, soil moisture, sea state, snow and weather conditions during each flight. The imaging radar was able to take images anytime regardless of clouds cover. The Radar-C system was built and operated by NASA's Jet Propulsion Laboratory (JPL). The mission was a joint work of NASA with the German and Italian space agencies. Each of the week long mission scanned about 50 million square kilometers of the Earth's surface, (19.3 million square miles).

The SIR mission revealed hidden river channels in the Sahara Desert indicating significant climate change in the past. SIR was also used for volcano research by keeping researchers a safe distance from hazardous and often inaccessible areas. The radar was also used to generate detailed three dimensional mappings of the Earth's surface.

Interferometric synthetic aperture radar, abbreviated InSAR (or deprecated IfSAR), is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar (SAR) images to generate maps of surface deformation or digital elevation, using differences in the phase of the waves returning to the satellite or aircraft. The technique can potentially measure millimetre-scale changes in deformation over spans of days to years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and in structural engineering, in particular monitoring of subsidence and structural stability.

Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted or diffracted by some material in order to characterize that object.

In physics and geosciences, the term spatial resolution refers to distance between independent measurements, or the physical dimension that represents a pixel of the image. While in some instruments, like cameras and telescopes, spatial resolution is directly connected to angular resolution, other instruments, like synthetic aperture radar or a network of weather stations, produce data whose spatial sampling layout is more related to the Earth's surface, such as in remote sensing and satellite imagery.

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 digital computer storage to record its images. In a radar image, one can see only the energy that was reflected back towards the radar antenna. The radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface in a swath, building the image as it does so.

Digital radar images are composed of many dots. Each pixel in the radar image represents the radar backscatter for that area on the ground (terrain return): brighter areas represent high backscatter, darker areas represents low backscatter.