The right SNAP workflow starts with two questions: what do you want to derive? and which Sentinel-3 instrument and product level do you have? The Sentinel-3 Optical Toolbox is designed for OLCI and SLSTR data; SRAL follows a different path through the Sentinel Altimetry Toolbox. Once the instrument, level and resolution are clear, install the matching toolbox, import the product, choose an operation that fits its inputs, inspect quality flags and metadata, then save or automate the result.
Menu names below follow ESA’s documented SNAP 10.0 import instructions where noted. SNAP changes between releases, so confirm the equivalent command and current reader support in the help and release notes for the version you install.
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1. Define the result before opening SNAP
Sentinel-3 files are not interchangeable. OLCI, SLSTR and SRAL measure different quantities, and a Level-1 product does not provide the same starting information as a Level-2 product. Write down the intended output first: for example, corrected ocean-colour reflectance, an optical-water-type class, a chlorophyll-related FLH or MCI product, a stitched SLSTR image, or sea-surface temperature.
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- Product level: the level shown in the product name and metadata, such as an L1B or L2 product.
- Target variable: radiometry, a classification, a temperature field, a map for a geographic area, or a quality-controlled export.
- Resolution policy: native pixels, an explicit 500 m or 1 km SLSTR grid, or a later reprojection.
- Repeatability: one interactive inspection or a graph that can be run repeatedly.
This short inventory prevents the most common error: selecting a processor because its name sounds relevant even though its required input level or instrument is different.
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2. Match the instrument to the correct SNAP toolbox
| Instrument | What the Sentinel-3 Optical Toolbox documents | Typical decision |
|---|---|---|
| OLCI | Optical radiometry and listed ocean-colour operations, including atmospheric-correction-related processing, optical-water-type classification, Forel–Ule classification, and FLH/MCI retrieval. | Use the Optical Toolbox and select an operation whose documented inputs match your OLCI level. |
| SLSTR | 500 m and 1 km reader choices, Level-1B PDU stitching, AATSR/SLSTR regridding and an ARC sea-surface-temperature processor. | Choose the reader resolution deliberately, then select an operation suited to the SLSTR product. |
| SRAL | Not supported by the Optical Toolbox. | Install or use the Sentinel Altimetry Toolbox instead of trying to force SRAL through an optical reader. |
SNAP itself is a general processing platform. Its general features include band arithmetic, masks, reprojection and orthorectification, multithreading and the Graph Processing Framework (GPF). Those tools are useful after a product is correctly opened, but they do not make an unsuitable sensor or level suitable for a processor.
3. Install SNAP and the Optical Toolbox
ESA’s download page listed SNAP 14.0.0 when checked on 18 August 2026 at 15:15 UTC. Treat that as a dated availability point, not a permanent version number: check the current download page and release notes immediately before installing.
- Download the installer for your operating system from ESA’s SNAP distribution page.
- Include the Sentinel or Optical Toolbox during installation if the installer offers that choice.
- If it is not installed, open SNAP’s plugin manager after first launch and add the required toolbox. ESA notes that additional toolboxes can be added later.
- Restart SNAP when the plugin manager requests it, then confirm that Sentinel-3 readers and operators appear in the menus.
- Record the SNAP version and toolbox version alongside your processing notes. This matters when a product baseline changes or a reader behaves differently after an upgrade.
SNAP 14.0 Optical Toolbox release notes describe faster Sentinel-3 readers, updated Sentinel-3 DDDB handling, support for the SL_2_WST baseline 004, and fixes involving OLCI/DDDB readers. If an older installation fails to open a current product, compare its release notes and upgrade before changing the data.
4. Check the product before importing
Sentinel-3 optical products are commonly distributed as a zip archive. Do not point SNAP at an unopened archive unless your installed reader explicitly supports that workflow.
- Extract the archive into a working folder with enough free space for the complete product.
- Confirm that the extracted directory has the Sentinel-3 product structure and an
xfdumanifest.xmlfile. - Read the product name and metadata to identify the instrument, processing level, acquisition time and baseline.
- Keep the original extracted product unchanged. Work from copies or write derived products to a separate output directory.
If extraction produces a directory ending in .SEN3, keep that folder intact. Moving individual measurement files out of it can break relative paths expected by the reader.
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5. Open an OLCI or SLSTR product
The following menu path is documented for SNAP help version 10.0. Labels can move in later releases.
- Launch SNAP.
- Choose File / Import / Optical Sensors / Sentinel-3.
- Select the product’s
xfdumanifest.xmlfile. - Wait for the product to appear in Product Explorer, then expand the bands, tie-point grids, metadata and quality information.
SNAP also documents two shortcuts: choose File / Open Product… and select the manifest, or drag the *.SEN3 folder or its manifest into Product Explorer. If those choices are absent, use the integrated help for your installed version rather than assuming the old menu path is still identical.
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SLSTR reader choices
When opening SLSTR data, the documented reader presents three resolution-handling approaches:
- 500 m: opens bands supplied at the 500 m resolution.
- 1 km: resamples all data to a 1 km resolution.
- Generic reader: leaves bands at their native resolutions.
These are not interchangeable labels. Selecting 1 km changes the grid before later processing; selecting the 500 m option does not mean every source measurement was originally acquired at 500 m; and the generic reader preserves mixed native resolutions that may require explicit handling in later operators. Write the choice into your processing record.
6. Pick an operation that matches the objective and input level
ESA lists several Sentinel-3-specific operators, but the feature list is not a universal, validated chain. Check each operator’s help for required bands, product level, masks and parameters before running it.
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OLCI radiometry and atmospheric corrections
The OLCI radiometry processor includes corrections such as gas absorption and Rayleigh correction. Use it when your objective requires corrected radiometric quantities and your product level supplies the inputs the operator expects. Inspect the resulting bands and metadata rather than assuming that opening an OLCI product has already applied every correction.
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Optical-water-type classification
The OWT operation is based on atmospherically corrected reflectances. It therefore belongs after the required correction stage, not as a blind first step on any OLCI file. Confirm that the reflectance bands and quality information required by the operator are present.
Forel–Ule classification
Forel–Ule processing produces a water-colour class. The class is meaningful only when the input reflectances and masking are appropriate to the product and scene. Treat cloud, land and other quality flags as part of the classification workflow.
FLH and MCI retrievals
ESA lists fluorescence line height (FLH) and maximum chlorophyll index (MCI) retrievals among the OLCI capabilities. Select the retrieval that matches your scientific variable and verify the expected corrected-reflectance inputs before execution.
SLSTR Level-1B PDU stitching
SLSTR Level-1B data can contain separate product dissemination units (PDUs). The listed stitching operation is specifically an L1B operation; it should not be presented as a generic step for every SLSTR level.
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Regridding and sea-surface temperature
SNAP lists AATSR/SLSTR regridding and an ARC sea-surface-temperature processor. Regridding changes the spatial grid, while the SST processor addresses a temperature product. Keep those purposes separate, and document the source grid and any resampling introduced before analysis.
7. A practical interactive workflow
- Open and identify: confirm instrument, level, acquisition and baseline in the Product Explorer and metadata.
- Inspect available bands: expand the band list and verify that the wavelengths or temperature channels required by your objective exist.
- Inspect masks and flags: look for cloud, land, invalid, saturated and other quality indicators supplied with the product.
- Choose the operator: open its help, check required inputs and select parameters appropriate to the scene and product level.
- Run a small check: process a representative subset or scene first when the operation is computationally expensive. Compare the output dimensions, units and metadata with the operator documentation.
- Review the result visually: use band displays, colour scales, masks and statistics to find obvious stripes, empty tiles, bad geolocation or unmasked clouds.
- Save a derived product: write it to a new directory and retain the processing parameters and SNAP version.
For OLCI Level-2 data, SNAP generates recommended flags and its import help advises considering them. Use those flags in displays and calculations where appropriate. They are a practical quality-control aid, not a complete validation method for every science objective.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.8. Resolution, reprojection and resampling decisions
Resolution errors often look like algorithm errors. With SLSTR, decide at import whether you need the documented 500 m option, the 1 km all-data resampling option or native-resolution bands from the generic reader. Later reprojection or resampling is a separate operation and should be recorded separately.
- Use a common grid only when the downstream comparison requires one.
- Do not describe a resampled 1 km product as native 1 km measurements without checking the source bands.
- When combining bands at different native resolutions, inspect the operator’s resampling policy and output metadata.
- Keep geolocation and tie-point information with the derived product; exporting only a visual image can discard information needed for analysis.
9. Save results and make the workflow repeatable
Desktop processing
The desktop interface is suitable for exploring a scene, testing parameters and checking masks. Save the product after each meaningful stage when you need an audit trail, and use descriptive names that include instrument, level, operation and date.
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SNAP’s Graph Processing Framework lets you represent a chain of operators and run it repeatedly, including from the command line. A graph is useful when the same corrections, masks, reprojection or export steps must be applied to many scenes. Build and test it interactively first, then confirm that every node receives the level and bands it expects.
Python and Java APIs
ESA documents Java and Python APIs for plugin development. Python support was substantially revised in SNAP 12 and later, so follow the current esa_snappy instructions for the exact SNAP version installed. Do not copy an older Python setup recipe into a newer installation without checking compatibility.
Automation should preserve the same decisions as an interactive run: reader selection, SLSTR resolution policy, quality masks, resampling, output format and software version. The available documentation does not establish one output format or one graph that is correct for every Sentinel-3 objective.
10. Troubleshoot common failures
The product does not open
- Confirm that the archive was extracted and that you selected
xfdumanifest.xmlor the intact.SEN3folder. - Check that the Optical Toolbox is installed and enabled.
- Compare your SNAP version with current release notes, especially when the product uses a newer baseline.
- Look for a damaged or incomplete download by checking whether expected measurement files are missing.
SRAL is missing from the Sentinel-3 import options
This is expected for the Optical Toolbox. SRAL is outside its documented scope; use the Sentinel Altimetry Toolbox.
The SLSTR image has an unexpected size or alignment
Review whether you selected 500 m, 1 km or the generic native-resolution reader. A 1 km choice resamples all data, while the generic reader preserves native resolutions.
An operator is unavailable or fails immediately
- Verify that the operation supports your instrument and product level.
- Check whether it requires atmospherically corrected reflectances, Level-1B PDUs or another intermediate product.
- Confirm that required bands and quality information were imported.
- Read the operator help for your installed release; names and supported baselines can change.
The output contains clouds, land or invalid values
Inspect and apply the supplied masks and recommended OLCI Level-2 flags. Recheck the mask expression and display the quality bands themselves before interpreting the science output.
11. A decision checklist
- Have you stated the variable or map you need?
- Is the file OLCI, SLSTR or SRAL?
- What product level and processing baseline does its metadata show?
- Does the selected operator document that instrument and level?
- For SLSTR, have you intentionally chosen 500 m, 1 km or native resolution?
- Did you inspect bands, metadata, masks, flags and geolocation before trusting the output?
- Are corrections, stitching, regridding and retrieval steps recorded in order?
- Will a GPF graph or current
esa_snappysetup reproduce the run? - Did you record the SNAP and toolbox versions used?
12. What a defensible SNAP workflow looks like
A defensible workflow is not “open Sentinel-3 and run every processor.” It is a documented chain connecting an instrument and product level to a specific scientific objective. Install a current toolbox, import the manifest or intact product folder, choose SLSTR resolution consciously, run only operators whose inputs match the data, use quality information, and retain enough metadata to reproduce the result. When compatibility is uncertain, consult the release notes and version-specific help before altering the product or blaming the algorithm.
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