SLSTR Regridding Tool
- Former user (Deleted)
Goals
SLSTR (as well as (A)ATSR) data are acquired with a conical scanning geometry. To display the acquisitions as a raster image the raw data are transformed both for the forward and nadir view into a gridded L1 TOA product at a resolution of 1 km. This "gridding" modifies the exact pixel position and resolution. The S-3 Toolbox shall provide a “Conversion Tool” enabling the user to convert pixel position and resolution of the “re-gridded” SLSTR data for nadir and oblique view back into their original instrument geometries taking also annotated data sets into account. The tool shall also work for (A)ATSR data. This conversion tool shall have a GUI, a CLI and offer an API that allows its functionality to be called from other applications and programming languages.
Background
AATSR L1b radiometric data are regridded to a regular 1km x 1km grid. A similar approach is foreseen for Sentinel-3 L1b products. Users often overlook that the actual sampling and spatial resolution of the sensor does not match the grid resolution, which can lead to a misinterpretation of the data and to errors in retrievals. Examples of cases where such errors can occur and the improvements when the native pixel location and field of view are considered are shown below.
Figure 1: Example of AATSR Brightness Temperature in the image grid (left) and the nadir-view scan geometry (left). |
Figure 2: The same as in Figure 2‑22, but showing the forward view. |
The plots in the figure above were produced by G. Corlet from the V2.0 version of ATS_TOA_1PRUPA20100108_100657_000065272085_00451_41086_0819.N1, adjusted to reduce the geolocation and co-location errors that should be corrected in V2.1 of the data.
A similar tool has already been developed by Telespazio VEGA UK (contact Hannah Clarke) under contract to ESA (EOP-GMQ). It takes AATSR L1b gridded (GBT) products and extract pixel geolocation co-ordinates from the ungridded instrument scan. The basis for this co-ordinates transformation was the Technical Note “Instrument Pixel Co-ordinates and Measurement Times from AATSR Products” produced by the ESL team in March 2010 (see Instrument_Pixel_Coordinates_and_Measurement_Time.pdf).
Assumptions
- Adequate SLSTR L1B test data available
Requirements
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | ||||
User interaction and design
The re-gridding concept and L1b product format for the ATSR sensors were devised to account for the limitations of processing power and data storage that existed during the 1990’s. As well as the regridding, the swath edges were truncated to fit in a 512x512km frame. Although the current Envisat L1b product does allow the image data to be remapped to the original instrument reference frame, this process is not simple.
For Sentinel-3 SLSTR, the radiometric data are regridded to a regular quasi-Cartesian grid but with the ungridded data stored in an orphan pixel container. Ideally the radiometric data should be retained on the instrument grid, but a decision was made early in the program to ensure continuity with the heritage datasets. However, for SLSTR the ortho-geolocation of the original instrument pixels are now provided in the ‘Full resolution geodetic coordinates’ annotations dataset at the native detector resolution. The measured pixel data in the MDS and the Scan, Pixel and Detector number ADS are provided as a function of full-resolution image grid position, and so simple look-up technique can be used to get the measured BTs and radiances in the instrument frame. This process requires no interpolation or weighting factor so should be completely lossless and reversible. So it is very easy to map the ortho-geolocation data and other parameters on the instrument grid to the image grid.
This issue has been discussed in the AATSR Quality Working Group (QWG) several times [RD 38] and the QWG has expressed its wish to have a tool in the BEAM toolbox for converting from AATSR image grid into AATSR scan geometry. Even ideas for how this conversion tool could behave have been suggested: The user shall be able to select a region within an AATSR image that should be converted into scan geometry. The converted images shall pop-up in a new window.
Questions
Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
|---|---|
Not Doing
Implementation Effort
We will be developing the conversion tool in close collaboration with the AATSR and upcoming SLSTR QWG to get more detailed requirements on this tool. BC (R. Quast) is regular visitor of QWG meetings and consulting in questions related to user tools. The overall effort is estimated to be within one person month.