[rlh930]

I got G-Tac Pro and tried using it at an autocross. Overall I'm pretty impressed, and it's certainly worth the $20, but there are a few things to mention:

1. Remember to set the iPhone not to power down (Settings/Auto-Lock/Never.

2. The GPS is slow to acquire unless it can see the sky. I bought a mount that places it in front of the ventilation ducts in the center of the dash (and has a power cable to keep the iPhone charged :-). This seems to keep the GPS happy.

3. You can't work the iPhone touch screen when you have gloves on, so it's hard to start/stop a run. You could of course cut a hole in the fingertip if you're not worried about fire protection. Otherwise you're stuck with a lot of dead time before and after the data you're really interested in.

4. The GPS position data doesn't look particularly accurate. The GPS track roughly corresponded to the AX course layout but by no means exactly. On the other hand the speed data looked reasonably accurate.

5. The accelerometer data was somewhat skewed. When I was stopped, the accelerometers showed me with around .1G of longitudinal and .01G lateral acceleration. This was consistent and if you export to a PC, you can easily remove it with Excel, but there doesn't appear to be a way to calibrate the unit except to fiddle with the iPhone's orientation in the car.

6. The time values for the GPS and accelerometer data were significantly out of sync (at least a second or more). The GPS speed and direction showed me at the end of a straight long after the accelerometer showed I had entered the turn at the end. Again, with a little experiment you could probably post-process this out with Excel.

Nevertheless, it's pretty incredible to get a completely self-contained data logger for $20.