Usually, consumer GPS receivers update positional data only once per second (1Hz). The one I have is capable of 5Hz updates, meaning it hands out position data every 0.2 seconds. The GPS Lap Timing software had not been tested with 5Hz receivers, as it’s developer Albert told me before I tried it.

Here’s what usually happens if all works well: You start up the software on your mobile phone, it hooks up with the GPS via bluetooth and you can “record” a track. On this you have a defined start/finish line (the point where you started the recording) and you can specify as many intemediate timing points as you like and one speed trap. Once the track is recorded, you can start a timing session. Depending on your settings, it either starts timing when you hit the start button or on first passing of the start/finish line. Now when you pass start/finish again (or an intermediate point), it’s highly unlikely that this happens at the exact time the GPS receiver hands out data. So, the software takes the time and position that was acquired before the finish line and the time and position that was acquired after the finish line. Depending on how far both points are from the finish line and on the speed you’re traveling at, the software interpolates the time at the finish line. Quite easy, really.

Here’s what obviously happened with my receiver (just the way I understood it from the explanation Albert gave me): Instead of receiving positional data once per second, the software received positional data five times per second. Obviously, this should result in better accuracy. However, the format at which my receiver put out the data was not fully understood (now there’s a real scientific technical term) by the software, so it ignored the millisecond data it received. This resulted in the software trying to interpolate two consecutive times rounded to a second. With 5Hz updates, it is very likely that those times rounded to a second are identical. And, of course, interpolation between two identical times is quite useless. But Albert altered the software and made it more flexible in reading data provided by a GPS receiver. He was really quick at doing that too and everything seems to work fine now.

I hope I’ll get a chance one day to compare the accuracy of GPS Lap Timing to other timing methods, but it’s not likely, to be honest. Extensive track testing pending…