I learned about this through a classmate doing a presentation on his work for this project (at MIT).

Iirc, they used modified android devices with larger batteries. The devices mostly slept and occasionally woke up to check in so they lasted awhile. I don't remember the specifics of how they communicated unfortunately. I'm somewhat surprised this article is out. When I heard about it (~1 year ago), I thought they weren't going to release the data because it technically acquired illegally with the tracking devices.