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The Sen2cor Processing Handbook
This post is work in progress. You have been warned!
Introduction
The aim of me writing this document is for anyone with little prior knowledge of sentinel-2 data and its processing to learn all they need to know in order to get started working with it, and in particular the generation of an L2A product using the sen2cor processor. What that means should not be obvious to the reader. If it is, perhaps you can still gain some knowledge in skimming through the text, as I will try to explain a number of terms and concepts that are sources of confusion and are typically dropped into the litterature without much further explanation.
The text is written informally to be easy to digest, and Im hoping that in the end it will serve as the reference point that I would have liked to have put under my nose when first venturing into the space of earth observation.
To understand what we need to do in order to reach the level-2 A product starting from the raw satellite output, it is first neccessary to review what steps are involved in producing the level-1 input product from the “raw” level-0 data. We need to get some basics covered. What the sensor measures and how its ability to do those measurements is quantified is described briefly in the follwing two subsections. See these sections as building a common vocabulary for the later sections.
Resolution metrics Firstly we will briefly talk about the metrics by which we measure resolution of data obtained and derived from the satellite sensors.
Spatial resolution in the context of the sentinel instruments is the area of ground covered by a single pixel. While this may be obvious, that this resolution is wavelength dependent may not be. Additionally, each band is measured in one spatial resolution only, but can of course be up or downsampled to the users liking with the product on hand.
This term should not be confused with spectral resolution, which measures the ability of an instrument to resolve a particular part of the electromagnetic spectrum. This is also wavelength dependent, and an instrument never measures at a singular wavelength value but rather always over a bandwidth in a certain range, for a certain spatial resolution and wavelength.
Sentinel-2 is a set of two satellites - a “constellation”. While this constellation consists of sensor platforms that are similar, they are not identical in exactly what wavelengths they measure (see table below as an example for the difference between the two satellites in the sentinel 2 constellation).
We can observe that the difference in both central wavelength and bandwidth varies between bands, and band 12 at 2190 nm shows a larger deviation. However, since we are already in the short wave infrared part of the spectrum, this deviation is relatively small (about 0.8%). Having a precise measure of the central wavelength gives the ground segment a number to correct for in calculating the radiance values. More on that in the processing section, and the below section on units.
Note that certain bands are processed only to a certain spatial resolution. We will return to this in later sections where we talk about requesting specific spatial resultions to be produced by sen2cor.
Finally, there is radiometric resolution, which measures the range of brightness values that can be recorded by the satellite and is bounded by the interger type - 12 bit in the case of Sentinel-2 - used to store the data which limits it to integers between 0 to 4095. This can be thought of as the “dynamic range” in photography terms.
Units
As hinted at in the resolution section above, the radiance values (energy flux) recorded at the sensor are rescaled into a digital representation from a unit often measured in watt/(steradian/square meter). Steradian, or “square radian” here being the SI unit for volumetric angles compared to the radian being the unit for planar angles.
To make this perfectly clear: one radian unit of angle represents the angle in planar section of a circle where the circumference cut of the circle is equal to its radius. Correspondingly, the one steradian represents the volumetric element where the surface area of a cone is a circle of area radius^2. So nothing more exotic than a radian in three dimensions (how can we genereralize the radian to four, or five dimensions?).
Note that the digital number resulting in a derived radiance value is taken at sensor level - there is no separation of radiance sources (atmosphere, land objects, etc) in the recording of the data. Any diffuse medium between the land object and the sensor (a cloud for instance) will diminish the radiance contribution from that object and render it more diffusely. More on this, and other correction steps, in later sections where we discuss data processing.
Reflectance should not be confused with Radiance, in that it is the ratio of the quantity of radiation reflected from a target to the radiation incident on the target. Therefore it is unitless and depends directly on the material being observed.
These two terms - reflectance and radiance - are sometimes used interchangebly and in others with minor variations depending on the physics of the system being observed. In both cases it is worth remembering that the sensor doesnt measure either of them but rather the energy flux incident on the sensor, which can then be converted (or re-scaled) to an unsigned interger that represents the physical variable.