I learned about this through a classmate doing a presentation on his work for this project (at MIT).<p>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&#x27;t remember the specifics of how they communicated unfortunately. I&#x27;m somewhat surprised this article is out. When I heard about it (~1 year ago), I thought they weren&#x27;t going to release the data because it technically acquired illegally with the tracking devices.