I'm trying to think of way to make a timing system like Freelap: https://www.freelap.com

The way I understand it, Freelap uses transmitters at the start and finish of a track, that emit electromagnetic fields. These EMFs are detected by a chip worn by the user which allows them to calculate time elapsed between both transmitters. Most of their transmitters have a detection range of 1-3 meters.

I'm wondering if you could build a similar system that uses a phone (iphone/android) instead of a timing chip. Could you make a device that a phone could detect when it passes within a few meters of it? at 80km/h?

I've researched this for a few hours and haven't found anything so I appreciate any thoughts.