Products and Algorithms - Sentinel-1 SAR Technical Guide - Sentinel Online
Products and Algorithms
The SENTINEL-1 C-band radar antenna beam illuminates the ground to the right side of the satellite. Due to satellite motion and the along track (azimuth) beam width of the antenna, each target element only stays inside the illumination beam for a short time. As part of the on-ground processing, the complex echo signals received during this time are added coherently. In this way a long antenna is synthesised, with the synthetic aperture length being equal to the distance the satellite travelled during the integration time. In principle, the along track resolution obtainable with SAR is approximately half the real antenna length. To enhance the radiometric resolution, multi-look azimuth processing can be employed and consequently, the along track resolution will be reduced by a factor equal to the look number. The across track or range resolution is a function of the transmitted radar bandwidth.
The fact that the system works coherently from end-to-end means that both the amplitude and the phase relationships between the complex transmitted and received signals are maintained throughout the instruments and the processing chain. This facilitates aperture synthesis as well as multi-pass radar interferometry using pairs of images taken over the same area at different times.
The SENTINEL-1 Synthetic Aperture Radar (SAR) instrument may acquire data in four exclusive modes:
- Stripmap (SM). This is a standard SAR stripmap imaging mode where the ground swath is illuminated with a continuous sequence of pulses and with the antenna beam pointing to a fixed azimuth and elevation angle.
- Interferometric Wide swath (IW). Data is acquired in three swaths using the Terrain Observation with Progressive Scanning SAR (TOPSAR) imaging technique. In IW mode, bursts are synchronised from pass to pass to ensure alignment of interferometric pairs.
- Extra Wide swath (EW). Data is acquired in five swaths using the TOPSAR imaging technique. EW mode provides very large swath coverage at the expense of spatial resolution.
- Wave (WV). Data is acquired in small stripmap scenes called 'vignettes', situated at regular intervals of 100 km along track. The vignettes are acquired by alternating; acquiring one vignette at a near range incidence angle while the next vignette is acquired at a far range incidence angle
SM, IW and EW Level-0 products are generated as Product Slices and systematically processed to Level-1 and Level-2 slice products.
This algorithms and products section of the SENTINEL-1 technical guide covers: