Understand which ephemeris is used
Ephemerides have a priority, expiration time, and enabled flag. Together, these determine which ephemeris is used for tracking during a contact.
TLE and OEM ephemerides
For OEM and TLE ephemerides, 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.
Note
AWS Ground Station has a service quota on the number of ENABLED customer-provided
ephemerides per satellite (see: Service Quotas).
To upload ephemeris data after reaching this quota, delete (using DeleteEphemeris)
or disable (using UpdateEphemeris) the lowest-priority/earliest created
customer-provided ephemerides.
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
Azimuth elevation ephemerides
Azimuth elevation ephemerides work differently from OEM and TLE ephemerides. Each azimuth
elevation ephemeris is associated with a specific ground station and does not have a priority.
When you reserve a contact with azimuth elevation ephemeris, you explicitly specify which azimuth elevation
ephemeris to use through the trackingOverrides parameter.
Key differences for azimuth elevation ephemerides:
-
No priority system - you explicitly select the ephemeris for each contact
-
Ground station specific - each ephemeris is associated with a particular ground station
-
No automatic fallback - if the specified ephemeris is unavailable, the contact will fail
Note
Azimuth elevation ephemerides do not compete with OEM and TLE ephemerides. They are selected explicitly when reserving a contact and are only used when tracking overrides are specified.
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.
For OEM and TLE ephemerides, 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
For azimuth elevation ephemerides, scheduled contacts will use the specific ephemeris that was selected when the contact was reserved. If you need to update the azimuth elevation data for a scheduled contact, you can cancel and reschedule the contact with a new ephemeris.
