Sentinel-1 Acquisition Modes
Sentinel-1’s C-SAR observes in four different acquisition modes: * Stripmap (SM) * Interferometric Wide Swath (IW) * Extra Wide swath (EW) * Wave (WV)
These modes are schematically shown in Figure 1.
Figure 1: The Sentinel-1 acquisition modes (from Torres et al., 2012).
Stripmap
The Stripmap mode is used for consistency with the heritage Earth Remote Sensing (ERS) and Environmental Satellite (Envisat) missions. When put into this mode, the instrument provides constant coverage, using a geometric resolution of 5 \(\times\) 5 m2 at a 80 km swath width. There are six swaths, with each one having an antenna capable of generating a beam with fixed azimuth and elevation pointing. This kind of elevation beamforming is used to suppress range ambiguity. There is an option to select one of six beams by changing the incidence angle and the elevation beamwidth. These six overlapping swaths cover a total range of 375 km.
Interferometric Wide Swath (IW)
The Interferometric Wide Swath mode (IW) combines a 250 km swath at a moderate resolution of 5 \(\times\) 20 m2 and appears to be the most widely used to detect a surface target. It is the main acquisition mode over land and implements a newly formulated ScanSAR mode ‘Terrain Observation with Progressive Scans’ (TOPS). With TOPSAR, the radar beam is additionally steered from backward to forward in the azimuth direction per burst, which results in homogeneous image quality throughout the swath. See Figure 2 for a schematic presentation of the IW mode.
Figure 2: Schematic presentation of the TOPSAR acquisition mode.
Extra Wide Swath (EW)
This mode is similar to the IW mode, except that it uses TOPSAR techniques, which help cover a wider area of up to 400 km by using five sub-swaths. This unit only images at a coarser resolution of ~20 \(\times\) 40 m2. The EW mode is mostly applicable for maritime applications, as well as over ice and polar zones, where large coverages and short revisit times are necessary.
Wave (WV)
The Wave mode resembles the ERS and Envisat scanning, but at better spatial resolution. The Wave mode data product is composed of single strip map operations with an alternating elevation beam between 23o and 36.5o incidence angle. This results in the generation of 20 \(\times\) 20 km2 vignettes with a 5 \(\times\) 5 m2 ground resolution, exclusively in VV or HH polarisation and at regular intervals of 100 km.