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SAR Extension Specification (sar)

Extension Maturity Classification: Proposal

This document explains the fields of the STAC Synthetic-Aperture Radar (SAR) Extension to a STAC Item. 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.

It is not necessary, but recommended to use the Commons extension (see chapter "Placing common fields in Collections").

Item fields

Note: In the following 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:platform string REQUIRED. Unique name of the specific platform the instrument is attached to. For satellites this would be the name of the satellite (e.g., landsat-8, sentinel-2A), whereas for drones this would be a unique name for the drone.
sar:constellation string Name of the constellation that the platform belongs to. See below for details.
sar:instrument string REQUIRED. Name of the sensor used, although for Items which contain data from multiple sensors this could also name multiple sensors.
sar:instrument_mode string REQUIRED. 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 REQUIRED. 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_wavelength number The center wavelength of the instrument, in centimeters (cm).
sar:center_frequency number The center frequency of the instrument, in gigahertz (GHz).
sar:polarization [string] REQUIRED. A single polarization or a polarization combinations specified as array. See below for more details.
sar:bands [Band Object] This is a list of the available bands where each item is a Band Object. See section "Band Object" for details.
sar:pass_direction string|null REQUIRED. Direction of the orbit, either ascending, descending or null if not relevant.
sar:type string REQUIRED. The product type, for example RAW, GRD, OCN or SLC for Sentinel-1.
sar:resolution [number] The maximum ability to distinguish two adjacent targets, in meters (m). The first element of the array is the range resolution, the second element is the azimuth resolution.
sar:pixel_spacing [number] The distance between adjacent pixels, in meters (m). The first element of the array is the range pixel spacing, the second element is the azimuth pixel spacing. Strongly RECOMMENDED to be specified for products of type GRD.
sar:looks [number] The number of groups of signal samples (looks). The first element of the array must be the number of range looks, the second element must be the number of azimuth looks, the optional third element is the equivalent number of looks (ENL).
sar:absolute_orbit [number|[number]] A list of absolute orbit numbers. See below for details.
sar:off_nadir [number|[number]] Viewing angle(s). Measured in degrees (0-90). See below for details.

sar:absolute_orbit lists absolute orbit numbers. Usually corresponds to the orbit count within the orbit cycle (e.g. ALOS, ERS-1/2, JERS-1, and RADARSAT-1, Sentinel-1). For UAVSAR it is the Flight ID. A range can be specified as two element array in the array, e.g. [25101, [25131, 25140]] would be 25101 and 25131 to 25140.

sar:constellation is the name of the group of satellites that have similar payloads and have their orbits arranged in a way to increase the temporal resolution of acquisitions of data with similar geometric and radiometric characteristics. Examples are the Sentinel-1 constellation, which has S1A, S1B, S1C and S1D and RADARSAT, which has RADARSAT-1 and RADARSAT-2. This field allows users to search for Sentinel-1 data, for example, without needing to specify which specific platform the data came from.

sar:off_nadir is the angle from the sensor between nadir (straight down) and the scene center. Measured in degrees (0-90). A range can be specified as two element array in the array, e.g. [20.1, [24.5, 30]] would be 20.1 and 24.5 to 30.

sar:polarization specifies a single polarization or a polarization combination. For single polarized radars one of HH, VV, HV or VH must be set. Fully polarimetric radars add all four polarizations to the array. Dual polarized radars and alternating polarization add the corresponding polarizations to the array, for instance for HH+HV add both HH and HV.

Common Frequency Band Names

The sar:freuency_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.

Common Name Wavelength Range (cm) Frequency Range (GHz) Satellites
P 30 - 120 0.25 - 1
L 15 - 30 1 - 2 ALOS, JERS, NISAR, SOACOM
S 7.5 - 15 2 - 4 HJ-1C
C 3.8 - 7.5 4 - 8 EnviSat, ERS, Radarsat, Risat-1, Sentinel-1
X 2.4 - 3.8 8 - 12.5 Cosmo-SkyMed, TerraSAR-X, RanDEM-X, PAZ, KOMPSat-5
Ku 1.7 - 2.4 12.5 - 18
K 1.1 - 1.7 18 - 26.5
Ka 0.75 - 1.1 26.5 - 40

Date and Time

In SAR, you usually have frame start and end time. To describe this information it is recommended to use the Datetime Range Extension Specification. The center time of the frame should be specified with the datetime property for STAC ITems.

Band Object

The bands contained in SAR image are dependent on the sar:type. For example, single look complex (SLC) data contain both phase and amplitude information of the signal. This may be provided for instance in form of complex number components (i and q bands) for each available polarization. Multilooked data (for example GRD in case of Sentinel-1) contain only amplitude and intensity bands for each polarization. Geocoded data contain radiometrically calibrated and terrain corrected data such as sigma0 or flattening gamma and may also contain angular information such as projected local incidence angle. Details about each band and the respective processing applied is given in its description.

Field Name Type Description
name string The name of the band.
description string Description to fully explain the band, should include processing information. CommonMark 0.28 syntax MAY be used for rich text representation.
data_type string Specifies the type of the data contained in the band, for example amplitude, intensity, phase, angle, sigma0, gamma0.
unit string The unit of measurement for the data, preferably the symbols from SI or UDUNITS.
polarization string|null The polarization of the band, either HH, VV, HV, VH or null if not applicable.

Associating assets with bands

Asset definitions that contain band data should reference the band index. Each asset should provide a sar:bands property that is an array of 0 based indexes to the correct Band Objects.

Item Asset Object fields

Field Name Type Description
sar:bands [number] Lists the band names available in the asset.

Extensions

The extensions page gives an overview about related extensions, for example:

Placing common fields in Collections

A lot of SAR data will have common metadata across many Items. It is not necessary, but recommended to use the Commons extension in combination with STAC Collections. The exact metadata that would appear in a STAC Collection record will vary depending on the dataset.

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