Narration: Ryan Fitzgibbons
300 trillion. That's about how many photons in each laser pulse fired by the ATLAS instrument. It sounds like a lot, but as the photons sail through the atmosphere and bounce off the Earth, they reflect back, just like any light in all directions. ATLAS receives only the photons that reflect straight back to the instrument, just about a dozen photons. That window of photons received must be very small in order to keep the timing error of those photons very small, which allows scientists to calculate elevation very precisely.