GPS systems have an inherent degree of fudge factor*, which will become more apparent on a small course such as a BMX track. For such a situation I'd add a vote to the Cateye mentioned previously.
*The GPS system is operated by the US military, and the error factor does vary depending on how active they are. If they're in a particularly sensitive mood, the error can be several hundred metres. Interesting side point: I once slept with my Bryton GPS unit running, primarily so I could see what my heart rate did while I was asleep. When I plugged it in & downloaded the plot the following day, my bedside table, where the computer unit was resting, had apparently walked around my house and into the neighbouring properties several times, covering well in excess of a kilometre! Not bad going for an inanimate object! :lol: