Understand which ephemeris is used

Ephemerides have a priority, expiration time, and enabled flag. Together, these determine which ephemeris is used for a satellite. Only one ephemeris can be active for each satellite.

The ephemeris that will be used is the highest-priority enabled ephemeris whose expiration time is in the future. A larger priority value indicates a higher priority. The available contact times returned by ListContacts are based on this ephemeris. If multiple ENABLED ephemerides have the same priority, the most recently created or updated ephemeris will be used.

If no ephemeris has been created, or if no ephemerides have ENABLED status, AWS Ground Station will use a default ephemeris for the satellite (from Space-Track), if available. This default ephemeris has priority 0.

Effect of new ephemerides on previously scheduled contacts

Use the DescribeContact API to view the effects of new ephemerides on previously scheduled contacts by returning the active visibility times.

Contacts scheduled prior to uploading a new ephemeris will retain the originally scheduled contact time, while the antenna tracking will use the active ephemeris. If the spacecraft's position, based on the active ephemeris, differs greatly from the prior ephemeris, this may result in reduced satellite contact time with the antenna due to the spacecraft operating outside the transmit/receive site mask. Therefore, we recommend that you cancel and reschedule your future contacts after you upload a new ephemeris that differs greatly from the previous ephemeris. With the DescribeContact API, you can determine the portion of your future contact that is unusable due to the spacecraft operating outside the transmit/receive site mask by comparing your scheduled contact startTime and endTime with the returned visibilityStartTime and visibilityEndTime. If you choose to cancel and reschedule your future contact(s), the contact time range must not be outside the visibility time range by more than 30 seconds. Cancelled contacts may incur costs when cancelled too close to the time of contact. For more information on cancelled contacts see: Ground Station FAQs.