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SAR Extension Specification

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.

If the data has been collected by a satellite, it is strongly recommended to use the sat extension. If the data has been collected on an airborne platform it is strongly recommended to use the Instrument Fields.

To describe frame start and end times, use the Date and Time Range fields.

Item Properties

Field Name Type Description
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_frequency number The center frequency of the instrument, in gigahertz (GHz).
sar:polarizations [string] REQUIRED. Any combination of polarizations.
sar:product_type string REQUIRED. The product type, for example SSC, MGD, or SGC
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.

Note: 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.

Additional Field Information

sar:polarizations

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.

Important: In the properties of a STAC Item 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.

sar:product_type

The product type defines the type of processed data contained in the assets. A list of suggestions include:

sar:product_type Type Description
SSC complex Single-look Slant-range Complex image (standard SLC)
MGD amplitude Multilooked Ground-range Detected image
GRD amplitude Multilooked Ground-range Detected image (used by Sentinel-1)
GEC amplitude Geocoded Ellipsoid Corrected image
GTC amplitude Geocoded Terrain Corrected image
RTC amplitude Geocoded Radiometrically Terrain Corrected image
SGC complex Single-look Ground projected Complex image
SLC complex Single-look Ground projected Complex image (used by Sentinel-1)

This can vary by data provider, who all may use slightly different names. Sentinel-1 for instance uses GRD, wihch is the same as the more general MGD and SLC instead of SGC.

Common Frequency Band Names

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.

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 Date and Time Range fields. The center time of the frame should be specified with the datetime property for STAC Items.

Best Practices

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 an amplitude 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.