SLC is an abbreviation and stands for Single Look Complex. SLC data are one so called radar product. Like all radar products they have been derived from SAR raw data, often called Level 0 products, downloaded from the SAR satellite by the satellite operators. They apply a software called a processor to transform SAR raw data into formats that can be used by users for different applications. SLC data are often referred to as Level 1 products and are the first SAR product derived from the raw data to be made available to users.
As the name suggests, SLC data only contain one single look, which means that the azimuth compression has been carried out using the full azimuth bandwidth of the SAR sensor leading to the highest spatial resolution in azimuth direction. But as a consequence, SLC data suffers from maximum speckle.
The word “complex” in SLC means that the data are stored as complex numbers with a real and an imaginary part. In this way, SLC data contain both – phase stored in the real part and amplitude information stored in the imaginary part of the complex number for one resolution cell.
SLC data are given in slant-range geometry and appears to be distorted. The is due to the fact that the spacing between pixels in the slant range direction is directly proportional to the signal travel time or time interval between backscattered and received radar pulses. And this time interval in again is directly proportional to the slant range distance between the sensor and the imaged objects at the Earth’s surface and not to the horizontal ground distance between the nadir and the imaged object. Therefore, SLC images appear distorted, which means that they look compressed in near range (close to the nadir) and getting ever more expanded in towards the far range.
SLC data are the basis for further SAR products generated and are required for interferometric analysis methods, which rely on phase and amplitude information.
From the Single Look Complex (SLC) product the Multi-look Detected/Multi-looke (MLD/MLI) can be generated. It is produced by multi-looking, i.e., averaging, over range and/or azimuth resolution cells.
Precision Images (PRI) are the Multi-look Detected/Multi-looked Intensity (MLD/MLI) images that have been resampled into square pixels, rotated to account for the view direction of the instrument and warped by some predefined operation that the projected image pixels are georeferenced onto a specified geographical coordinate system.
Ground Range Detected (GRD) radar imagery is a Level-1 product that has been derived from Level 0 (raw data) SLC SAR data by a Processing Facility via the application of a processing software. GRD products usually consist of focused SAR data that has been detected, multi-looked and projected to ground range using an Earth ellipsoid model.
Focused SAR data are generated in a raw data processing step. During focusing, the two-dimensional signal energy of a point target that is spread in range and azimuth direction is aggregated and put into a single image pixel in the output data set.
Detected means that the complex numbers representing phase and amplitude values in the original data set have been converted to real numbers by taking their absolute square (or complex conjugate). In the resulting image data, the phase information is not present any longer and only amplitude information remains as the pixel value.
The SAR imagery in GRD radar data is given in ground range geometry, which differs from the slant geometry of the SLC data. In ground range geometry, the spacing between the image objects at the Earth’s surface is in direct proportion to their real distance along a hypothetical flat ground surface. Here, image coordinates are oriented along ground range and flight direction. This means that they do not show the distorted appearance of an SLC image.