SAR imagery (Radar satellite data)

Radar imaging is indispensable compared with other observation methods in difficult weather: it can acquire images at any time of day and in any weather, measure coordinates and geometric characteristics of objects with high accuracy, and observe and detect objects invisible in the optical or infrared bands of the electromagnetic spectrum, hidden by snow or vegetation, or located beneath the Earth's surface.

Initially the tasks of radar imaging were limited to studying the ocean surface. Now the range of radar-imaging tasks has expanded, and radar imaging makes it possible to obtain data on millimetre-scale displacements of objects on the Earth's surface, track oil pollution and ship traffic in water areas, and also create digital terrain models (the well-known SRTM DTM was created from radar data processing).

SAR processing

SAR processing is a demanding task because a raw radar image looks unfamiliar to the human eye. Specialised software is used, and specialists must have proper experience in creating secondary products of radar space imagery. Even so, the advantages of SAR make radar imagery highly sought after.

Three degrees of radar data processing can be distinguished:

• Single Look Complex (SLC) image — information obtained by focusing the transmitter signal. At this SAR processing level the radar image includes two channels: an amplitude ("panchromatic") grayscale image and a phase image of the signal, used to create digital elevation models and displacement maps of the Earth's surface
• Geocoded image. The radar image has a spatial reference in a given coordinate system. Radar images at this SAR processing level are typically delivered in GeoTIFF format.
• Orthorectified image. A high-quality radar image without geometric distortions from the terrain. Radar satellite imagery at this SAR processing level is used to create cartographic materials and for spatial analysis.

Use of radar imaging

The tasks of radar imaging expand every year. Innovative SAR processing opens ever newer applications of radar imagery:

• Creation and updating of digital topographic maps. The ability of radar imaging to acquire images regardless of weather makes it a real breakthrough for mapping the Russian Far East and other countries where snow hinders surveys in winter and the rainy season brings continuous cloud cover throughout the imaging season. Another advantage of SAR for mapping is the relatively high spatial resolution of radar images, which enables maps to be created and updated up to a scale of 1:50,000.
• With phase information from the radar signal, digital elevation and terrain models can be created by interferometric processing. They in turn can be used in many areas of society.
• Multi-temporal radar images provide information on displacements of the Earth's surface. This helps detect subsidence at mineral deposits, deviations of infrastructure objects from their normal position and track landslide processes
• Radar imaging of water surfaces makes it possible to detect oil spills. Because of the structure of the substance they have a smoothed surface and a different radar reflectivity compared with natural water. This makes an oil spill relatively easy to identify on a radar image.
• Radar imaging also makes it possible to find ships whose tracking signal has been switched off deliberately or after an accident. The signal on a radar image is well reflected from metal surfaces and enables ships to be detected automatically even on medium-resolution radar images. It is also important that radar images can be obtained just an hour after acquisition. This timeliness helps the relevant services locate the vessel more accurately.
• The varying reflectivity of radar imaging makes it possible to detect burn scars and clear-cuts in forests and to monitor the condition of agricultural crops.

Advantages of SAR (radar imaging)

• Ability to perform radar imaging in any weather (including heavy cloud cover) and at any time of day (including night);
• High level of detail due to the high spatial resolution of the radar image;
• A cost-effective alternative to GPS monitoring of ground displacements.

The purpose of space radar imaging is to obtain timely, up-to-date information about the terrain and water surface needed to solve numerous tasks in various fields; the main ones are:

• creation and updating of topographic maps and plans, as well as other cartographic products;
• creation of engineering-geological maps during geological surveys
• land-management and cadastral work;
• creation of digital terrain models for planning and development of telecommunication networks;
• agricultural research, detection of plant diseases, determination of the vegetation phase;
• monitoring of emergencies and their consequences, control of emergency restoration work;
• forest inventory (forest accounting, determination of age, height and diameter of growing trees, timber stock, annual increment, etc.);
• detection of emergency oil spills;
• detection of fuel dumping from ships;
• information support for the prevention and elimination of oil and petroleum-product spills;
• monitoring of deformation and displacement of the Earth's surface at mineral deposits;
• monitoring of structural deformations in cities;
• monitoring of critical industrial facilities;
• pipeline monitoring;
• monitoring of roads and railways and other objects;
• monitoring of ice conditions and ship movements;
•  monitoring of hazardous natural and natural-technogenic processes, including earthquakes, landslides and collapses, avalanches, mudflows, loess subsidence, flooding, karst, suffosion, river erosion, and reworking of sea and reservoir shores.

Initially the tasks of radar imaging were limited to studying the ocean surface. Now the range of radar-imaging tasks has expanded, and radar imaging makes it possible to obtain data on millimetre-scale displacements of objects on the Earth's surface, track oil pollution and ship traffic in water areas, and also create digital terrain models (the well-known SRTM DTM was created from radar data processing).

Advantages of using radar images

Radar imaging has a number of advantages over other observation methods: high accuracy of measuring coordinates and geometric characteristics of objects, and the ability to obtain radar images at any time of day and in any weather (microwave radiation passes freely through clouds and does not require daylight, unlike images in the visible and IR bands).

• Ability to perform radar imaging in any weather (including heavy cloud cover) and at any time of day (including night);
• High level of detail due to the high spatial resolution of the radar image;
• A cost-effective alternative to GPS monitoring of ground displacements.

Innovative processing opens ever newer applications of radar imagery:

• The ability of radar imaging to acquire images regardless of weather makes it a real rescue for mapping the Russian Far East and other countries where snow hinders surveys in winter and the rainy season brings continuous cloud cover throughout the imaging season. Another advantage of SAR for mapping is the relatively high spatial resolution of radar images, which enables maps to be created and updated up to a scale of 1:50,000.
• With phase information from the radar signal, digital elevation and terrain models can be created by interferometric processing. They in turn can be used in many areas of society.
• Multi-temporal radar images provide information on displacements of the Earth's surface. This helps detect subsidence at mineral deposits, deviations of infrastructure objects from their normal position and track landslide processes
• Radar imaging of water surfaces makes it possible to detect oil spills. Because of the structure of the substance they have a smoothed surface and a different radar reflectivity compared with natural water. This makes an oil spill relatively easy to identify on a radar image.
• Radar imaging also makes it possible to find ships whose tracking signal has been switched off deliberately or after an accident. The signal on a radar image is well reflected from metal surfaces and enables ships to be detected automatically even on medium-resolution radar images. It is also important that radar images can be obtained just an hour after acquisition. This timeliness helps the relevant services locate the vessel more accurately.
• The varying reflectivity of radar imaging makes it possible to detect burn scars and clear-cuts in forests and to monitor the condition of agricultural crops.

Radar imaging provides radar images in any weather and at any time of day, ensures high measurement accuracy and is used for mapping, interferometry, monitoring of oil spills, ice, ships, forests and agricultural crops.