For SAR data, usually three processing levels are distinguished, ranging from level 0 (less processed) to level 2 (higher processed).
Level 0 products consist of compressed and unfocussed raw data and are the basis for the processing of higher level products. Level 0 data are principally used for research in the topic of signal processing. As for optical data, level 0 product are annotated with several metadata, such as calibration and orbit information, and acquisition time and date.
Level 1 data can be separated in two distinct product types, depending if the full complex information is used (amplitude and phase) or only the amplitude information. The product denomination depends on the sensor type; for Sentinel 1 the names Single Look Complex (SLC) and Ground range detected (GRD) are used, respectively. Both products can be generated from the Level 0 data. Level 1 data are the products that are used by most scientific users. The processing toward Level-1 data includes Doppler centroid estimation and data focusing. The Level 1 SLC product consists of the real and imaginary part of focused complex SAR data in slant range geometry, from which the phase and amplitude information can be retrieved. This is available for all acquired polarisations. Additional orbit information for georeferencing is provided with the data. The Level 1 GRD data consist of focused and multi-looked SAR data that have been projected to ground range geometry. GRD data only contain amplitude information, therefore the phase information is lost. The multi-looking step is particular for GRD data and allows both speckle reduction and square pixel resolution. As for the SLC data, the GRD data are annotated with orbit information for georeferencing. The Level-1 products are not calibrated, they include however information about calibration constants, which are sensor dependent. Further processing is needed in order to obtain calibrated radar cross section information from the original data intensity values.
Level 2 products describe geolocated derivated geophysical products such as ocean wind field or surface radial velocity. Such products are for example available for download on the Sentinel-1 Copernicus Hub. Further Level- 2 data are for example differential interferograms or change maps, which can be processed on different online platforms (e.g. Hyp3) and provide information about surface deformation or more generally changes between several acquisitions.
The denomination of the product types on the different levels may differ from sensor to sensor, but the processing steps stay almost the same, depending additionally on the considered acquisition modes. For example, GRD products are also called for other sensors Multi-Looked Detected (MLD) products.