Performance parameters |

### Sentinel-3 Readers Options

##### SLSTR

- Read Sentinel-3 SLSTR L1B products with per-pixel geo-codings instead of using tie-points
  Sentinel-3 SLSTR L1B products contain tie point grids and, for each of the eight different grid types, per-pixel geo-codings. Spatial locations per pixel can be derived from both, however, the result might vary slightly. When using tie-point geo-coding, the product is opened quicker and the pixel locations are more similar across different grids, using per-pixel geo-codings will lead to more precise results.
- Load orphan pixels of Sentinel-3 SLSTR L1B products
  If selected, orphaned SLSTR L1B pixels will be read as extra bands. In the product explorer, they are found in the *orphan* subfolder.
- Read Sentinel-3 SLSTR L1B product with custom calibration
  This option provides a way to set scaling factors, offsets, and adjustment factors to SLSTR L1B bands. The three values will be combined like this:

  `value = ((raw_value * factor) + offset) * adjustment_factor`

  When switched on, the OPTTBX will consider factors, offsets and adjustment factors given in the opttbx.properties file.
  Using the EDIT button next to the checkbox, calibration coefficients can be written to this file from within SNAP.
  Please see the note below on where to find this file.
  Factors and offsets can be set to each band.
  To set a factor, add a line like this:

  `opttbx.reader.slstrl1b.xx.calibration.factor = 2.0`

  Likewise, offsets are set like this:

  `opttbx.reader.slstrl1b.xx.calibration.offset = 0.5`

  and adjustment factors like this:

  `opttbx.reader.slstrl1b.xx.calibration.adjustment_factor = 1.2`

  Where xx is to be replaced by any band name in lowercase. Note that possibly pre-existing scaling factors or offsets, defined by the Sentinel-3 product will be replaced.
- Apply S3MPC recommendation to adjust SLSTR L1B S5 and S6 radiometric calibration
  When this option is set, the bands S5 and S6 from a SLSTR L1B product are re-calibrated with the following adjustment factors:
  - S5 for Nadir View: 1.12
  - S6 for Nadir View: 1.2
  - S5 for Oblique View: 1.15
  - S6 for Oblique View: 1.26
  These values are the adjustment factors recommended by the S3MPC. To use other factors, uncheck this box and define others using SLSTR L1B custom calibration.
- Read Sentinel-3 SLSTR L2C products with per-pixel geo-codings instead of using tie-points
  Sentinel-3 SLSTR L2C products contain tie point grids and, for each of the two different grid types, per-pixel geo-codings. Spatial locations per pixel can be derived from both, however, the result might vary slightly. When using tie-point geo-coding, the product is opened quicker and the pixel locations are more similar across the different grids, using per-pixel geo-codings will lead to more precise results.

##### OLCI

- Read Sentinel-3 OLCI products with per-pixel geo-coding instead of using tie-points
  Sentinel-3 OLCI products contain tie point grids and a per-pixel geo-coding. Spatial locations per pixel can be derived from both, however, the result might vary slightly. When using tie-point geo-coding, the product is opened quicker, using the per-pixel geo-coding will lead to more precise results.
- Read Sentinel-3 OLCI L1 product with custom calibration
  This option provides a way to set scaling factors and offsets to OLCI L1 bands. When switched on, the OPTTBX will consider factors and offsets given in the opttbx.properties file.
  Using the EDIT button next to the checkbox, calibration coefficients can be written to this file from within SNAP.
  Please see the note below on where to find this file.
  Factors and offsets can be set to any band.
  To set a factor, add a line like this:

  `opttbx.reader.olcil1.xx.calibration.factor = 2.0`

  where xx is to be replaced by the name of the band in lowercase. Likewise, offsets are set like this:

  `opttbx.reader.olcil1.xx.calibration.offset = 0.5`

  Note that possibly pre-existing scaling factors or offsets, defined by the Sentinel-3 product will be replaced.

##### MERIS

- Read ENVISAT MERIS products in SAFE format with per-pixel geo-coding instead of using tie-points
  ENVISAT MERIS products in SAFE format contain tie point grids and a per-pixel geo-coding. Spatial locations per pixel can be derived from both, however, the result might vary slightly. When using tie-point geo-coding, the product is opened quicker, using the per-pixel geo-coding will lead to more precise results.

#### NOTE: opttbx.properties file

The opttbx.properties file let's users specify special properties for the Optical Toolbox. There are two locations where this file can be defined. At both places, if the file does not yet exist it can simply be created as a plain text file.

One is located within the SNAP installation directory in the etc folder and defines the properties for the installed SNAP instance. Here you need usually admin right to modify the file.
The other is located in the .snap/etc folder within the user's home directory. The second, in the user directory defines the settings per user and overrules the ones in the installation directory.

Depending on the operating system and your settings the .snap folder might not be visible to you. You can do the following to make it visible:

OS | Action
--- | ---
Windows | Nothing to do. The directory is visible by default.
Linux | On the command line you nee to type `ls -la` to see the directory. In a file explorer like the one of Ubuntu you need to make sure that in the settings the display of hidden files is enabled.
Mac | Since macOS Sierra (10.12) you can press `<CMD> + <SHIFT> + <.>` in finder to make the directory visible. Prior this version you can nivagate to the user directory and then press `<CMD> + <SHIFT> + <G>`. This brings up an input dialog where you can enter the folder name .snap. On enter you will be navigated to this directory.