3.4.1.1. General terms

Last modified by Tuan Le on 2019/05/21 13:58

Acquisition modes

Stripmap (SM) mode acquires data with an 80 km swath at slightly better than 5 m by 5 m spatial resolution (single look). The ground swath is illuminated by a continuous sequence of pulses while the antenna beam is pointing to a fixed azimuth angle and an approximately fixed off-nadir angle (this is subject to small variations because of roll steering). SM images have continuous along track image quality at an approximately constant incidence angle.
The Interferometric Wide (IW) swath mode is the main acquisition mode over land and satisfies the majority of service requirements. It acquires data with a 250 km swath at 5 m by 20 m spatial resolution (single look). IW mode captures three sub-swaths using Terrain Observation with Progressive Scans SAR (TOPSAR). With the TOPSAR technique, in addition to steering the beam in range as in ScanSAR, the beam is also electronically steered from backward to forward in the azimuth direction for each burst, avoiding scalloping and resulting in homogeneous image quality throughout the swath.
Similar to the IW mode, the Extra Wide (EW) swath mode employs the TOPSAR technique to acquire data over a wider area than for IW mode using five sub-swaths. EW mode acquires data over a 400 km swath at 20 m by 40 m spatial resolution. The EW mode is aimed primarily for use over sea-ice, polar zones and certain maritime areas, in particular for ice, oil spill monitoring and security services. Like IW, EW mode can also be used for interferometry since it shares the same characteristics for burst synchronisation, baseline and Doppler stability.
Sentinel-1 Wave mode is similar to ERS and Evnisat wave mode imaging but with improved spatial resolution, larger vignettes and a 'leap frog' acquisition pattern as illustrated in the figure below. WV acquisitions consist of several vignettes exclusively in either VV or HH polarisation, with each vignette processed as a separate image. WV mode products can contain any number of vignettes, potentially amounting to an entire data-take. Each vignette is contained in an independent image within the product.

Processing levels

The Sentinel-1 Level-0 products consist of compressed and unfocused SAR (Synthetic Aperture Radar) raw data. Level-0 products are the basis from which all other high-level products are produced. The data includes noise, internal calibration and echo source packets as well as orbit and attitude information. The Level-0 products are not made available by ESA

Sentinel-1 Level-1 focused data are the products intended for most data users and form a baseline product from which Level-2 products are derived. The processing steps involved to produce Level-1 data products include pre-processing, Doppler centroid estimation, single look complex focusing, and image and post-processing for generation of the SLC (Single Look Complex) and GRD (Ground Range) products as well as mode specific processing for assembling of multiple sub-swath products.

  • Level-1 Single Look Complex (SLC) products consist of focused SAR data, geo-referenced using orbit and attitude data from the satellite, and provided in slant-range geometry. Slant range is the natural radar range observation coordinate, defined as the line-of-sight from the radar to each reflecting object. The products are in zero-Doppler orientation where each row of pixels represents points along a line perpendicular to the sub-satellite track.
  • Level-1 Ground Range Detected (GRD) products consist of focused SAR data that has been detected, multi-looked and projected to ground range using the Earth ellipsoid model WGS84. The ellipsoid projection of the GRD products is corrected using the terrain height specified in the product general annotation. The terrain height used varies in azimuth but is constant in range (but can be different for each IW/EW sub-swath).

Sentinel-1 Level-2 consists of geolocated geophysical products such as: Ocean (OCN) products for wind, wave and currents applications may contain the following geophysical components derived from the SAR data: Ocean Wind field (OWI), Ocean Swell spectra (OSW), Surface Radial Velocity (RVL).

  • The OWI component is a ground range gridded estimate of the surface wind speed and direction at 10 m above the surface, derived from SM, IW or EW modes.
  • The OSW component is a two-dimensional ocean surface swell spectrum and includes an estimate of wind speed and direction per swell spectrum. The OSW component provides continuity measurement of SAR swell spectra at C-band.
  • The RVL surface radial velocity component is a ground range gridded difference between the measured Level-2 Doppler grid and the Level-1 calculated geometrical Doppler. The RVL component provides continuity of the ASAR Doppler grid.