Understanding AIS Data Volume and Downsampling

Note on the volume and latency of AIS messages and downsampling.

Satellite vs. Terrestrial Feeds

Spire satellites spend most of their time over open ocean listening for AIS messages. These messages are published shortly after our satellites pass over a Spire ground station to download them. Given this, satellite-received AIS (S-AIS) messages are often published in "bursts". The primary advantage of S-AIS is global coverage.

Terrestrial AIS message (T-AIS) collection generally occurs near coastlines and takes place after a ship transmits them. Given that, terrestrially-received AIS messages are immediately made available to you. The primary advantage of T-AIS is extremely low latency.


For Terrestrial Feed

Due to the sheer number of T-AIS messages Spire receives, we do not by default provide all of Terrestrial AIS messages within the Spire Sense platform. T-AIS messages are downsampled within 5-minute periods based on MMSI and message type. For example, if we receive 2 position messages with a particular MMSI 3 minutes apart from terrestrial sources, only the first of the two position messages would get published. If these messages were 7 minutes apart, both would get published.

Note though that there is an option for clients to receive full (non downsampled) terrestrial AIS data. 

As an example of data volumes 2021-06-22. A client receiving Spire satellite AIS and down sampled terrestrial AIS would receive up to about 80 million AIS messages per day. If they were to receive full (non down sampled) terrestrial AIS then the number would jump to over 300 million AIS messages per day.

Satellite AIS 

Satellite AIS messages are not downsampled.

Dynamic AIS 

Dynamic AIS messages are not downsampled. For more information about Dynamic AIS see here 

Virtual Downsampling through Vessels API

If a client wishes to receives updates at a different frequency, perhaps every 10 or 15 minutes, hourly or even daily, then this can be achieved by using Vessels API and calling it at the required frequency for updates. IE calling the Vessels API hourly with give you the latest updates at an hourly interval. This is virtual downsampling controlled by the client.

Combined Satellite & Terrestrial Feeds

Given that T-AIS is published immediately and there's often some delay between the collection and download of an S-AIS message, it will often be the case that AIS messages are published within our services out of timestamp order. (We don't store our AIS messages in a buffer and sort them before publishing them.) This is to give you access to our AIS messages as soon as they're downloaded.

Satellite AIS Message Download Priority

Spire has many satellites in a variety of orbits and a number of global ground stations to collect and download AIS data. It’s not always realistic for us to get all of the AIS messages off of a satellite on the initial pass of a ground station. In order to get the most important data down first, we prioritize the most recent message for each MMSI and get all of those down first before moving on to the older messages either during that same contact or during subsequent ones with other ground stations.

What that means in terms of how we push data to our AIS production services is that you’ll get the “freshest” data for an MMSI from each satellite quickly but then in some cases get the “backfill” a little bit later.

Spire Sense Maintenance Periods & Outages

We do our best to minimize customer impacts during maintenance windows, but sometimes, brief outages are required to improve your longterm experience. In some these cases, data in Spire Sense may be unavailable.

Please subscribe to our Status Page to stay informed of when these maintenance windows (or unexpected outages) occur.