In this image from the Chicago Tribune you can see two of the bots. one is orange, the other yellow. they should have been in a straight line since they were being used to guide the swimmers down the course.
i was told they position via gps and their gps just didn't work downtown.
GPS can be fiddly when you're in a pit or surrounded by lots of tall things that block RF or make the signals bounce around. Clearly the event needed a local GPS augmentation signal :)
