How does ski race timing work? (Part 1)

You've seen displays of ski race timing when watching World Cup or Olympic races on TV.  The running time in the corner of the screen that freezes at the "splits" and shows how the racer on course is doing against the best time posted so far.  The leaderboard that shows that the times of top 5 racers are jammed within a period of 12/100ths of a second.  You may have wondered, "How does this information get  from the race course to my TV screen?"  and "How can the timing be so precise that a winner can be reliably determined by a margin of as little as one hundredth of a second?"

Try starting and stopping a stopwatch as quickly as you can.  Chances are, the watch will show that this action takes you 2 to 3 tenths of a second.  Now consider a time period of 1/100th of a second.  If two racers were to cross the finish line at a speed of 90 km/h (a realistic speed into the finish of a downhill course) and one crossed the line 1/100th of a second before the other, the separation between them would be 25 cm, or a little less than one ski boot length.  That is why you see racers reaching out to break the finish line with their hand.  If done correctly, that can shave one or two 100ths of a second off their time.

So, obviously the timing needs to be very precise throughout a race.  All racers must be timed from exactly the same start point to exactly the same finish point throughout a race, using chronometers that have negligible "drift" over the time period taken for the whole field to complete the course.

Let's start at the top of the course.  The start gate consists of a block with two or more switches in it, and a wand.  The start gate is mounted on a solid post set into the hard snow, so that the wand is below the racers' knees.  When the racers' lower legs push the wand open enough to trip the switches, the switch closures are detected by the electronic chronometers that are connected to the gate via wires.  These wires run out of the start gate and all the way down the mountain to the timing building, where two synchronized timers with precision quartz oscillators receive the signal to indicate that the racer has started.  The chronometers send the time-stamped start data to the race timing software, where the start is attributed to a particular racer that was confirmed to be entering the course.  The race software then starts tracking elapsed time on course and generates the graphics for displaying this running time "downstream" on TV.

In Part 2: moving on down the course to the intermediate timing points, and the "speed trap".