Stripmap is an acquisition mode of Synthetic Aperture Radar (SAR) data. It is the most simple, common acquisition mode of the SAR satellite sensors. In this mode, the antenna of the radar system is pointed in a fixed direction related to the flight direction. The displacement of the illuminated footprint corresponds to the displacement of the sensor along the orbit. This results in a continuous acquisition strip parallel to the flight direction. The ground coverage and resolution varies depending on the considered sensor and technical requirements. For X-band spaceborne sensors, a spatial resolution of 3 m can be achieved with a swath width in range direction of 30 km, e.g. for TerraSAR-X. In C-band, a spatial resolution up to 5 m is achieved e.g. by Sentinel-1 with a swath width of 80 km. For L-band spaceborne sensors, the spatial resolution achievable in stripmap mode varies between 3 and 10 m, with a swath width of 50-70 km, e.g. ALOS PALSAR2.
Contrary to other acquisition modes, no antenna steering is needed in azimuth direction and the elevation beam is fixed in a specific range direction. This allows for an uninterrupted coverage along the flight direction.
Stripmap data show high resolution with sufficient coverage for regional applications and can therefore be used for e.g. detailed land cover analysis at regional scale such as the mapping of urban footprints. Furthermore, it can be used for the mapping of small island or to support emergency actions.
Spotlight is a SAR acquisition mode that allows increasing the illumination time of a particular area of interest by steering the antenna beam in azimuth direction. In this mode, the beam elevation is fixed, but the antenna is steered in azimuth direction, increasing therefore the length of the synthetic aperture. The rotation center of the antenna for steering is situated behind the scene at far range. The antenna footprint slides slightly forward over the scene in the azimuth direction during acquisition, but slower than in Stripmap mode, due to the antenna steering. The longest illumination time in azimuth direction results in an azimuth resolution that is highly enhanced compared to e.g. the Stripmap or the ScanSAR acquisition modes. However, this improvement is done to the detriment of the coverage. As for the other acquisition modes, the ground coverage and resolution depends on the considered sensor. For TerraSAR-X, a minimum coverage of 10 km in range and 5 km in azimuth direction is achieved in the Spotlight mode, with and azimuth resolution of about 1 m. The L-Band sensor Alos 2 also allow Spotlight acquisition mode, with a coverage of 25 km in both directions and a resolution of 1 m in azimuth direction, and down to 3 m in range direction.
Due to the very high resolution achieved in both directions, this acquisition mode is particularly usefull for urban area analysis as it allows for the detection of small objects. Therefore, Spotlight data are often used for the detection and recognition of man-made structures and objects, such as roads, buildings and even vehicles.
In the ScanSAR acquisition mode, the antenna beam is successively steered to different elevation angles. This results in adjacent, slightly overlapping stripes, or sub-swaths along the range direction, parallel to the azimuth direction, each stripe having a different incidence angle at its center. During antenna steering in elevation, transmitter and receiver are off. Therefore, each stripe is illuminated for a shorter time as for the StripMap mode, leading to a degradation of the azimuth resolution. However, ScanSAR allow a larger coverage in range direction than the other imaging modes. Each sub-swath is illuminated for a shorter time than in the Stripmap case. The timing is adjusted though, such that the time-varying antenna footprint repeat cyclically. Similar to the other acquisition modes, the achievable resolution and coverage of ScanSAR products depends on the considered sensor and its properties. For X-Band, e.g. for TerraSAR-X, a total swath width of 100 km in range direction can be achieved using four adjacent sub-swaths or, using a Wide ScanSAR mode with six adjacent sub-swaths, a swath width up to 270 km can be achieved. A Wide ScanSAR scene shows incidence angles ranging from 15.6° in near to 49° in far range. The azimuth resolution varies between 18.5 m and 40 m, for ScanSAR and WideScan SAR modes respectively. For the L-Band sensor ALOS-PALSAR 2, a swath width up to 40 km can be achieved, with incidence angles ranging from 8° to 70° and an azimuth resolution of 60 m.
The ScanSAR mode is well suited for large-area monitoring, e.g. for sea ice or glacier monitoring, as well as for mapping large-scale disasters, such as oil slick, or areas devastated by forest fires. Using interferometry, topography mapping and deformation monitoring is also possible.
The Staring Spotlight mode is only available for a few sensors. It follows the same principle of antenna steering in azimuth direction as the standard Spotlight mode, except that the rotation center of the antenna for steering is situated at a nearer range position, within the illuminated scene. This induces that the illuminated antenna footprint stays almost the same during the whole acquisition. Contrarily to the Spotlight mode, the antenna footprint does not slide along the azimuth direction during the SAR acquisition. Additionally, the steering angle is higher for the Staring Spotlight mode than for the standard Spotlight mode, increasing therefore the length of the synthetic aperture and leading to an even higher resolution in azimuth direction.
The Staring Spotlight mode is implemented on the X-Band sensor TerraSAR-X since 2013 and achieves an azimuth resolution up to 0.25 m. Similar to the standard Sportlight mode, this happens to the detriment of the coverage. The scene size is highly dependent of the incidence angle and varies from 7.5 km to 4 km in range and from 2.5 to 2.7 km in azimuth direction. A larger coverage is obtained for smaller incidence angles.
Due to their extremely high resolution, staring spotlight acquisitions are principally used for the observation and/or monitoring of small scale objects and phenomena, e.g. small landslides, or for tomographic analysis.