How does this work?

[MrOmnos]

In this video they talk about SLED (Signal Location by Exterior Drone) They talk about detecting active cell phones and probably even locating which I think is a great idea. But how are they doing this? Post your theory here.

[XOIIO]

Well I without watching the video I figure they have a small station that generates the same sort of frequency as various cell phones would use, but made to be a bit more directional so that it would

A) penetrate the ground better and
B) let them detect when a cell phone connects to it, as cell phones look for the strongest signal

Then they can get an idea of what sort of area someone is under to target their excavation.

[trophosphere]

I have never looked into cell phone tracking but right off the bat I would think of just interfacing with the wireless transceiver(s) of the victim's cell phones and using the raw signal to do things like calculate signal strength in order to get an idea if you are getting closer/farther to the source. If you had multiple sensors then you could also triangulate position. In addition, there are likely to be command queries where the search system can ask the cell phone for information such as cell ID, round trip time/ping, and so on that may be useful in helping improve accuracy.

As far as Bluetooth, I'm reminded of Apple's iBeacon implementation. I think there were a few crowd-source projects in the past which implemented this to find stuff with phones utilizing tags.

[MrOmnos]

I figure they have a small station that generates the same sort of frequency as various cell phones would use, but made to be a bit more directional so that it would

Probably a SDR with base GSM/3G/LTE stack?