# Default ephemeris data
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 By default, AWS Ground Station uses publicly available data from [Space-Track](https://www.space-track.org/), and no action is required to supply AWS Ground Station with these default ephemerides. These ephemerides are [two-line element sets (TLEs)](https://en.wikipedia.org/wiki/Two-line_element_set) associated with your satellite's [NORAD ID](https://en.wikipedia.org/wiki/Satellite_Catalog_Number). All default ephemerides have a priority of `0`. As a result, they will be overridden, always, by any non-expired, custom ephemerides uploaded via the ephemeris API, which must always have a priority of `1`, or greater. 

 Satellites without a NORAD ID must upload custom ephemeris data to AWS Ground Station. For example, satellites that have just launched or that are intentionally omitted from the [Space-Track](https://www.space-track.org/) catalog would have no NORAD ID and would need to have custom ephemerides uploaded. For more information on providing custom ephemeris data, see: [Providing Custom Ephemeris Data](providing-custom-ephemeris-data.md).