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CEOS-ARD for Radar

This extension specifies how to create STAC Items that comply to the CEOS-ARD product family specifications (PFS) for radar sensors. The PFS for radar sensors is the Combined CEOS-ARD for Synthetic Aperture Radar, version 1.0 and includes the specifications for:

  • Normalised Radar Backscatter (NRB)
  • Polarimetric Radar (POL)
  • Ocean Radar Backscatter (ORB)
  • Geocoded Single-Look Complex (GSLC)

Note

This document is a draft and not finalized yet. We welcome feedback from early adopters.

Additional resources:

  • Examples (ToDo)
  • JSON Schema: not provided

Table of Contents:

Document Structure

In general, the fields required in this extension are required to either meet the minimum requirements (threshold) by the CEOS-ARD specification or are required fields in STAC. Any additional optional field provided may lead to a higher percentage for the CEOS-ARD desired requirements (target/goal).

The column Field Name refers to the STAC field names. The column Req. refers to the requirement number in the CEOS-ARD specification.

CEOS-ARD requirements 1.1, 1.2, and 2.1 are generally covered by implementing and publishing STAC metadata for the data.

STAC Extensions

This profile doesn't define new STAC fields, it's just a profile that uses existing STAC extensions to map and fulfill the CEOS-ARD requirements.

As the identifier for this profile is just a collection of existing extensions and only defines required fields, you get a good bit of validation already without providing the identifier/schema of this profile in stac_extensions. If your metadata is already compliant to this profile, you can omit the identifier for this profile in stac_extensions to avoid costly regeneration of the Items. You won't get validation whether all required fields are present, but this could be checked manually in the CEOS assessment/review.

The following STAC extensions are relevant for this profile:

Name Schema URI for stac_extensions
CEOS-ARD https://stac-extensions.github.io/ceos-ard/v0.2.0/optical/schema.json
Processing https://stac-extensions.github.io/processing/v1.1.0/schema.json
Projection https://stac-extensions.github.io/projection/v1.0.0/schema.json
Raster https://stac-extensions.github.io/raster/v2.0.0/schema.json
SAR https://stac-extensions.github.io/sar/v1.0.0/schema.json

For details about the CEOS-ARD extension, please see the separate CEOS-ARD extension README. The CEOS-ARD extension fields can either be provided in the Collection or in the Items.

STAC Collections

CEOS-ARD lists a lot of requirements (and fields) that have common values across all generated STAC Items and assets. Thus, it is recommended to provide a STAC Collection for the Items and put common fields (in the STAC Item properties) into Collection Summaries. While the STAC Item fields still need to be in the Item, too, you can de-duplicate links and assets by putting common links once into the STAC Collection links. Also, common assets can be just put once into the STAC Collection using Collection Assets. All this is still CEOS-ARD compliant as it doesn't require all information to be in a single file.

STAC Items (ARD Product)

STAC Items must always be valid, but not all STAC Item requirements are covered here, only additional requirements and mappings to fulfill the CEOS-ARD requirements are listed here:

Field Name Req. Description
stac_extensions n/a REQUIRED. Must contain all implemented STAC extensions.
geometry 1.4 REQUIRED. The geometry of the acquisition in WGS84.
bbox 1.7.8 REQUIRED. The bounding box of the acquisition in WGS84.

Generally, CEOS-ARD requests to provide DOIs for various resources. For STAC, DOIs (e.g. 10.1109/5.771073) must be converted to URLs (e.g. https://doi.org/10.1109/5.771073). See Resolve a DOI name for details.

Extensions

The following STAC extensions are relevant for the product Items:

Name Required
CEOS-ARD ✓
Processing ✗
Projection ✓
Raster ✗
SAR ✓

Providing the CEOS-ARD fields and links fulfills req. 1.3 and 1.4. The fields can be provided either in the Collection or in each Item.

Common Metadata

Field Name Req. Description
start_datetime 1.5 REQUIRED. The datetime of the earliest start datetime of all relevant acquisitions.
end_datetime 1.5 REQUIRED. The datetime of the latest end datetime of all relevant acquisitions.
attribution 1.3 Attribution / Copyright information if required by the data provider.

Projection

The metadata is required to specify the coordinate reference system through either proj:code or one of the alternatives.

Field Name Req. Description
proj:code / proj:wkt2 / proj:projjson 1.7.11 REQUIRED. One of the fields is required to be provided, recommended is proj:code. If there's no suitable CRS code, set proj:code to null and add either proj:wkt2 or proj:projjson.
proj:bbox 1.7.7 REQUIRED. The bounding box of the acquisition in the projection defined in proj:code / proj:wkt2 / proj:projjson.

Links

All links can be provided either in the Collection or in the Item, unless the links are specific for each Item and/or differ between the Items (such as derived_from). If that's the case, they must be provided per Item.

In addition to the links defined in specific extensions above, the following relation types are relevant:

Relation Type Req. Description
derived_from 1.5 REQUIRED. URL to the source data acquisitions, ideally as STAC metadata. The source metadata files must comply to the requirements below. The number of links with this relation type identify the number of source data acquisitions.

STAC Items (Source Data)

The specification for STAC Items that cover the source data, please see the "CEOS ARD for Radar - Source Data" specification

Notes / ToDo

  • Req. 1.5 / 1.6 / 2.8: It is not clear how to specify the Acquisition ID Image as the derived_from links don't contain a numeric indice. See also #26.