Where the boats are, and how they are rowing.

Each boat carries a small tracker: a GPS receiver, a radio and a battery in a sealed box. It reports where the boat is, how fast it is going, and which way it is pointing, to a receiver on the bank, which passes it here.

At a regatta that happens every second, for every boat on the course at once. The trackers take turns on the radio — each has its own tenth of a second — so forty boats never talk over one another. On the river it happens about once a minute, which is enough to follow a crew over ten kilometres and more, and lets the battery last the day.

The tracker also carries a motion sensor. The boat surges forward on every drive and checks on every recovery; counting that rhythm gives the stroke rate, which is sent along with the position.