This document explains the fields of the Synthetic-Aperture Radar (SAR) Extension to the SpatioTemporal Asset Catalog (STAC) specification.
SAR data is considered to be data that represents a snapshot of the earth for a single date and time taken by a synthetic-aperture radar system such as Sentinel-1, RADARSAT or EnviSAT.
If the data has been collected by a satellite, it is recommended to use the Satellite extension. If the data has been collected on an airborne platform it is recommended to use the Instrument Fields.
To describe frame start and end times, use the Date and Time Range fields.
To describe the product type, use the Product extension.
The fields in the table below can be used in these parts of STAC documents:
Important
In this specification range values are meant to be measured perpendicular to the flight path and azimuth values are meant to be measured parallel to the flight path.
Field Name Type Description sar:polarizations [string] RECOMMENDED. Any combination of polarizations. sar:instrument_mode string RECOMMENDED. The name of the sensor acquisition mode that is commonly used. This should be the short name, if available. For example,WV for "Wave mode" of Sentinel-1 and Envisat ASAR satellites. sar:frequency_band string RECOMMENDED. The common name for the frequency band to make it easier to search for bands across instruments. See section "Common Frequency Band Names" for a list of accepted names. sar:center_frequency number RECOMMENDED. The center frequency of the radar signal transmitted by the instrument, in gigahertz (GHz). sar:bandwidth number The range bandwidth of the SAR instrument, in gigahertz (GHz), representing the full bandwidth used in range compression and image formation. sar:resolution_range number The range resolution, which is the maximum ability to distinguish two adjacent targets perpendicular to the flight path, in meters (m). sar:resolution_azimuth number The azimuth resolution, which is the maximum ability to distinguish two adjacent targets parallel to the flight path, in meters (m). sar:pixel_spacing_range number The range pixel spacing, which is the distance between adjacent pixels perpendicular to the flight path, in meters (m). Strongly RECOMMENDED to be specified for products of type GRD. sar:pixel_spacing_azimuth number The azimuth pixel spacing, which is the distance between adjacent pixels parallel to the flight path, in meters (m). Strongly RECOMMENDED to be specified for products of type GRD. sar:looks_range number Number of range looks, which is the number of groups of signal samples (looks) perpendicular to the flight path. sar:looks_azimuth number Number of azimuth looks, which is the number of groups of signal samples (looks) parallel to the flight path. sar:looks_equivalent_number number The equivalent number of looks (ENL). sar:observation_direction string Antenna pointing direction relative to the flight trajectory of the satellite, either left or right. sar:relative_burst number Identification number that uniquely identifies a burst cycle within each repeat cycle. sar:beam_ids [string] Composition of the swath of the SAR acquision referencing the beam identifiers. sar:product_type string DEPRECATED in favor of product:type. See Product type.
Caution
sar:product_type.sar:product_type and it's not required any longer, but product:type is strongly recommended.sar:product_type.Specifies a single polarization or a polarization combination. Allowed values: HH, VV, HV, VH, LH, LV, RH, RV, CH or CV For single polarized radars, one the allowed values must be set. For dual polarized radars and alternating polarization, add the corresponding polarizations to the array. For fully polarimetric radars, add all polarizations to the array. For instance, for HH+HV, add both HH and HV.
Important
In Item Properties and Collections, sar:polarizations must be a set with unique elements. In assets sar:polarizations can contain duplicate elements and, if possible, the polarizations must appear in the same order as in the file.
The sar:beam_ids field is used to reference the beam identifiers of the SAR acquisition. According to the mission and the sensor mode, the beam identifiers can be used to identify the composition of the swath of the SAR acquisition. The beam identifiers are usually provided in the metadata of the SAR data.
The sar:frequency_band is the name that is commonly used to refer to that band's spectral properties. The table below shows the common name based on the wavelength and frequency ranges for several SAR satellites.
It is RECOMMENDED to include the product:type field. The product type for SAR data defines the type of processed data contained in the assets. A list of suggestions for product:type include:
This can vary by data provider, who all may use slightly different names. Sentinel-1 for instance uses GRD, which is the same as the more general MGD and SLC instead of SGC.
In SAR, you usually have frame start and end time. To describe this information it is recommended to use the Date and Time Range fields. The center time of the frame should be specified with the datetime property for STAC Items.
One of the emerging best practices is to use Asset Roles to provide clients with more information about the assets in an item. The following list includes a shared vocabulary for some common SAR assets. This list should not be considered definitive, and implementors are welcome to use other asset roles. If consensus and tooling consolidates around these role names then they will be specified in the future as more standard than just 'best practices'.
Role Name Description local-incidence-angle Points to the local incidence angle file. ellipsoid-incidence-angle Points to the ellipsoid incidence angle file. noise-power Points to the noise power file. amplitude Points to the intensity file with focused SAR data that has been ground range detected (e.g. GRD). magnitude Points to the intensity file where data are represented as complex numbers containing amplitude and phase information (e.g SLC). sigma0 Points to the radar backscatter file where data is referenced in ground surface. It is often derived from anamplitude or a magnitude role asset. beta0 Points to the radar backscatter file where data is referenced in the slant range plane and is radiometrically calibrated. It is often derived from an amplitude or a magnitude role asset. gamma0 Points to the radar backscatter file where data is referenced in the plane perpendicular to the line of sight. It is often derived from an amplitude or a magnitude role asset. date-offset Points to the date-offset file. covmat Points to the Points to the Normalized Polarimetric Radar Covariance Matrix (CovMat) file. prd Points to the Polarimetric Radar Decomposition (PRD) file.
All contributions are subject to the STAC Specification Code of Conduct. For contributions, please follow the STAC specification contributing guide Instructions for running tests are copied here for convenience.
The same checks that run as checks on PR's are part of the repository and can be run locally to verify that changes are valid. To run tests locally, you'll need npm, which is a standard part of any node.js installation.
First you'll need to install everything with npm once. Just navigate to the root of this repository and on your command line run:
Then to check markdown formatting and test the examples against the JSON schema, you can run:
This will spit out the same texts that you see online, and you can then go and fix your markdown or examples.
If the tests reveal formatting problems with the examples, you can fix them with:
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