To do that, the system essentially bombards the space with an array of wireless signals and keepswatch on any changes in signal strength, building up a "picture" of the room in the process.
Start your journey in Suchitoto, a picturesque hill town located just 47km north of capital San Salvador, where cobbled streets zigzag back and forth, dipping down in the direction of majestic Lake Suchitoto, over which the town keepswatch.
Jennifer Lynch, a lawyer at the Electronic Frontier Foundation, a group based in San Francisco that keeps a watch on how digital technology encroaches on civil freedoms, also questions the quality of the data.