- Title: Satellite
- Identifier:https://stac-extensions.github.io/sat/v1.2.0/schema.json
- Field Name Prefix: sat
- Scope: Item, Collection
- Extension Maturity Classification: Stable
- Owner: @emmanuelmathot
- History: Prior to March 2, 2021
This document explains the Satellite Extension to the SpatioTemporal Asset Catalog (STAC) specification. It adds metadata related to a satellite that carries an instrument for collecting data. It will often be combined with other extensions that describe the actual data, such as the EO or SAR extensions.
- Examples:
- example-landsat8.json: Shows the basic usage of the extension in a STAC Item
- example-sentinel1.json: Shows the more advanced usage of the extension in a STAC Collection
- JSON Schema
- Changelog
| Field Name | Type | Description |
|---|---|---|
| sat:platform_international_designator | string | The International Designator, also known as COSPAR ID, and NSSDCA ID |
| sat:orbit_state | string | The state of the orbit. Either ascending or descending for polar orbiting satellites, geostationary for geosynchronous satellites, or crossing when the acquisition spans a polar crossing. |
| sat:absolute_orbit | integer | The absolute orbit number at the start of acquisition. |
| sat:relative_orbit | integer | The relative orbit number at the start of acquisition. |
| sat:orbit_cycle | integer | The number of repeat cycle done by the satellite at the time of the acquisition. Repeat cycle is the time between two successive identical orbits. |
| sat:orbit_state_vectors | Map<string, [number]> | The state vectors of the satellite at the time of acquisition. |
| sat:anx_datetime | string | The Ascending Node Crossing (ANX) time, in UTC. It is formatted according to RFC 3339, section 5.6. |
| sat:acquisition_station | string | The acquisition station (ground station) where the satellite data was downlinked. Equivalent to eop:acquisition_station in OGC 10-157r4 and OGC 17-003r2. |
At least one of the fields must be specified.
The International Designator, also known as COSPAR ID, and NSSDCA ID and is an international identifier assigned to artificial objects in space.
Indicates the type and current state of orbit. Satellites are either geosynchronous in which case they have one state:
geostationary, or they are sun synchronous (i.e., polar orbiting satellites) in which case they are either ascending or
descending. For sun synchronous satellites it is daytime during one of these states, and nighttime during the other.
If the acquisition spans a polar crossing (i.e., the satellite transitions between ascending and descending during the
acquisition), the value crossing shall be used.
A count of orbits from 1 to the number of orbits made in the total satellite lifecycle. In mission planning and tasking, the absolute orbit may be used as a reference in the non systematic acquisition missions. The resulting Item can be tagged with the absolute orbit and thus searchable as such. In the case of orbital changes during the mission modifying the ground track and thus the repeat cycle and thus the number or relative orbits, the combination of cycle and relative orbit is not sufficient to derive an absolute orbit.
When the orbit changes during data acquisition (i.e., the acquisition spans multiple orbits), the convention is to use the absolute orbit number at the start of the acquisition. This situation can occur, for example, in consolidated altimetry products such as Sentinel-3.
A count of orbits from 1 to the number of orbits contained in a repeat cycle, where relative orbit 1 starts from a specific reference location of the sub-satellite point (the point on the earth directly below the satellite). It resets to 1 when the sub-satellite point revisits the reference location.
When the orbit changes during data acquisition (i.e., the acquisition spans multiple orbits), the convention is to use the relative orbit number at the start of the acquisition. This situation can occur, for example, in consolidated altimetry products such as Sentinel-3.
The UTC time when the satellite crosses the Ascending Node. For geocentric and heliocentric orbits, the ascending node (or north node) is where the orbiting object moves north through the plane of reference. Used to quickly compute orbital parameters without having to download the product. For instance to compute on the fly a baseline between 2 satellite acquisition, to find the best candidate in the archive from a post-disaster event (e.g earthquake) scene acquisition for a DInSAR processing.
The repeat cycle of the satellite. The repeat cycle is the number of orbits required for the satellite to return to the same position in its orbit. It is used to determine the relative orbit number. For instance, a satellite with a 16-day repeat cycle will have 16 relative orbits.
The state vectors of the satellite at the time of acquisition. The state vectors are a set of parameters that describe the position and velocity of the satellite at a given time. The state vectors are used to compute the position of the satellite at any time during the acquisition. The state vectors are usually provided in the ECI frame. The field is a map where the key is the time of the state vector and the value is an array of 6 elements (optional 9). The first 3 elements are the position of the satellite in meters and the last 3 elements are the velocity of the satellite in meters per second. The optional 3 additional elements are the acceleration of the satellite in meters per second squared.
Example:
{
"sat:orbit_state_vectors": {
"2015-03-05T05:19:40.103Z": [
5082939.4189831074,
1595651.7369050409,
4648033.5588545678,
5173.8779329387489,
-269.5432365485845,
-5550.246440359806
],
"2015-03-05T05:19:45.449Z": [
5110519.0376543682,
1594174.328112772,
4618284.7202579351,
5143.1200754073143,
-283.12343661734587,
-5578.2160610647188
],
"2015-03-05T05:19:50.449Z": [
5136162.3887853986,
1592727.024757518,
4590328.5904560406,
5114.1961953273549,
-295.79384304587683,
-5604.2108611458289
]
}
}It is recommended that the state vectors are provided in a separate file and referenced as a link with relationship type sat:osv.
The file should correspond to the example above.
The ground station where the satellite data was downlinked. This information is crucial for data lineage and provenance tracking,
as it identifies the first point of data reception on Earth. In the context of Earth Observation missions, this corresponds to the
eop:acquisitionStation in OGC standards (10-157r4 and 17-003r2). This field can be used for filtering datasets based on the
ground station used for downlinking, which can be relevant for certain processing workflows or quality assessments.
The following types should be used as applicable rel types in the
Link Object.
| Type | Description |
|---|---|
| sat:osv | Link to a file containing the orbit state vectors. |
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:
npm installThen to check markdown formatting and test the examples against the JSON schema, you can run:
npm testThis 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:
npm run format-examples