Vehicle Positions Update interval

[Patrick Greenwell]

I was wondering what the update interval for real time positions are and how they are reported to the MBTA's various realtime apis.

How often are the Buses & CR & Subways set to report their location data? Do the different bus types have different update intervals? Do Various routes and/or modes have different update intervals report to the MBTA? What are these intervals?

How often once reported to the MBTA how often are these updates reported to the various APIs?  Do the GTFS-Realtime .pb files get updated after every new report? consolidate reports after each second?

Do the Realtime REST APIs pull from the same place as the GTFS-Realtime files?

The GTFS-Realtime documentation states not poll the feeds more than once every 10 seconds, is this the minimum time between updates?

Clarification on these items would be great.

Thanks,

Patrick Greenwell

[Paul Harrington]

I have wondered about this too and find that when using position information you always have to consider the timestamp. The older it is, the less confident you can be in using it.

[Developer at MBTA]

Different modes do update with different frequencies and patterns. 

 - Buses send their locations in every 60 seconds, occasionally a little more often (there's a polling component and an event-based component). We do want to improve that. If/when we do make improvements it won't affect every bus at once. 

 - Commuter rail train positions are currently updated every minute as well. We're exploring plans to bring that down.

 - Subway trains update their locations when they enter a new track circuit, and the spacing of track circuits varies from place to place, but it's a few times a minute. That's not likely to change. 

The API's and GTFS-realtime are updated every 10 seconds. 

Sincerely,

developer@mbta

[Stefan Wuensch]

That's very helpful! Thanks!

What about Green Line above-ground, when it's GPS not track circuits?

Also, is there any significant change in position data latency (either greater or less) when a Green Line train makes the change from GPS to track-circuit or the reverse?

(In other words - during the period of GPS / track circuit transition is there any reduced precision or change in interval?)

Thanks!

[Developer at MBTA]

Even when the green line trains are above-ground we treat the information as if it's coming from track circuits. The reason is that the information goes into the same system that we use to track the trains when they're underground, the same system we use to track and manage the Red Orange and Blue Line trains, and that system is based around track circuits. So we divide the surface up into "virtual track circuits" and assign a train to a particular virtual track circuit based on its current and recent GPS reports. 

As for change in latency when transition between underground and surface -- what we call the "underground" tracking area actually extends onto the surface a little bit so we can briefly track a train using both the track's detection of the train and the train's GPS report simultaneously, which lets us handle handoff between the tracking zones reliably even in the event of a tracking problem affecting one of them. So there's no time at which a train has disappeared from one system and hasn't yet appeared on the other. 

Sincerely,

developer@mbta