A description of AIS message ingestion behavior and the timing of their availability.
Though vessel data will often be made available in timestamp order, this should not be an expectation, especially if you ingest AIS messages from both satellite and terrestrial sources.
Satellite Contact Opportunities
The following scenario sometimes occurs between different Spire satellites:
- Satellite 1 decodes a message from a vessel
- Satellite 2 decodes a newer message from the same vessel 5 minutes later
- Satellite 2 makes ground station contact 5 minutes later; the message with the newer timestamp is downloaded first
- Satellite 1 makes ground station contact 15 minutes after Satellite 2; the message with the older timestamp is downloaded second
This scenario becomes even more extreme when comparing timestamps between messages received from a satellite and a terrestrial source since the latter should be available to you within seconds of receipt.
Satellite AIS Message Download Priority
In order to offer you the most up-to-date information we have from each observed ship as quickly as possible, Spire satellites prioritize downloading the most recently collected AIS message from each ship they recently heard from. Given this, in some cases, the older messages are intentionally downloaded after as backfill.