- Title: SAR
- Identifier: sar
- Field Name Prefix: sar
- Scope: Item
- 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.
If the data has been collected by a satellite, it is strongly recommended to use the sat extension, which in turn requires the Instrument Fields. 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.
- Examples (for example Sentinel-1 and Envisat)
- JSON Schema
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: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. |
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.
sar:product_type: The product type defines the type of procesed 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.
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 |
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.
Item Asset Object fields
| Field Name | Type | Description |
|---|---|---|
| sar:polarizations | [string] | Lists the polarizations available in the asset, in order. Any of HH, VV, HV, VH. |
The extensions page gives an overview about related extensions. Of particular relevance to SAR data:
- the Sat Extension Specification to describe SAR data collected from a satellite.