An imaging SAR system can generally make acquisitions in different modes. Which acquisition mode to choose depends of the application but also on the desired coverage and data resolution. Even if technically all acquisitions modes can be used everywhere on the Earth’s surface, specific modes are preferred for ocean applications that are different from the ones used in land applications. The different acquisition modes can be defined either by their geometrical or by their temporal properties. The geometrical properties refer to the geometric configuration of the SAR antenna. Usually looking sideways down in a direction perpendicular to the flight direction (Stripmap mode), the antenna can also be steered around the nadir axis in order to look at a specific target for a longer time during pass-by (Spotlight mode). This configuration allows to rachieve higher azimuth resolution but reduces coverage. It is rather used for very local application where a precise information about specific targets is needed. Other geometric configurations steer the antenna around the flight direction (ScanSAR mode), yielding to a larger swath on the ground. The distance between near and far range is increased, as well as the range of incidence angles within one acquisition. Whereas it increases the area of the scene, it comes generally with a decrease of the spatial resolution in the azimuth direction. Depending on the sensors, the name of the acquisition modes as well as particular technical properties can differ. Sentinel-1 uses a TOPS configuration (Terrain observation with Progressive Scan), which combines the antenna steering properties of both ScanSAR and Spotlight modes. The temporal properties refer for specific techniques to the time interval between several acquisitions of the same area. Either these acquisitions are taken simultaneously in one pass over the area of interest (single-pass), or they are taken at different times, needing several passes over the area (repeat-pass). Specific SAR techniques such as InSAR and Tomography, while relying on those geometric and temporal properties, have additional acquisition configuration characteristics. For example, the interferometric mission TanDEM-X has three acquisition modes defined by the number of satellite emitting or receiving the signal (pursuit monostatic mode, bistatic mode, alternating bistatic mode), which allows phase referencing. Tomographic SAR uses multi-baseline observations, i.e. the antenna passes several times over an area but at different heights, allowing via different incidence angles the retrieval of structural information of specific targets.
State and explain different SAR acquisition modes
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