Harry's GPS Suite Forum

Hi guys,

It seems the only benefit of external gps aside from better timing accuracy is better resolution for vehicle speed. But a $10 odb2 bluetooth would be able to get wheel speed from the ecu directly. I am wondering what's the benefit of external gps when someone already has an odb2 connector.

Thanks.

Your lap times will be more accurate with the external GPS too. The internal GPS sensor in most phones is only 1 hz, meaning it gets one reading per second. (The external GPS is 5 hz, or 5 readings per second.)

In that situation, your laptimes could all be plus or minus 1 second. So if you think you ran a 2:02 lap, it really could have been a 2:01 or 2:03 due to the low refresh rate. (It may not update at the exact moment you cross the start/finish line.)

I'd say get both the external GPS and the OBD2 dongle as well. The OBD should give you throttle position, which is helpful in critiquing your laps if you're syncing to video.

vincenzo wrote:In that situation, your laptimes could all be plus or minus 1 second. So if you think you ran a 2:02 lap, it really could have been a 2:01 or 2:03 due to the low refresh rate. (It may not update at the exact moment you cross the start/finish line.)

No, that's a common misunderstanding how GPS based system work. Update rates have an impact on accuracy but it is *very* small. LapTimer uses sophisticated interpolation technics to derive accuracy of 0.1 or 0.05 seconds even for 1 Hz devices. Opposed to this, position accuracy has a big impact on timing accuracy - which is the reason a 1 Hz GPS with high accuracy will mostly produce higher timing accuracy than a 5 or 10 Hz GPS having problems to deliver good accuracy.

This does not mean the *internal* GPS should be preferred. It will typically show lower update rates and lower accuracy.

Harry

I'm not going to plug a $10 no-name Chinese manufactured OBDII dongle into my car. The OBDII port isn't read only. You can reprogram your ECU through the OBDII port.

I can confirm Harry's explanation directly from my own experience. I used Harry's Lap Timer on iPhone6 with its internal GPS senzor of 1Hz several times on track. I had also proffesional transponder from MyLaps beside it, that was provided by track operator. I was wondered that measured times were the same. Now I am testing external GPS sensor XGPS160 of 10Hz from Dual. I am expecting more fine data for afterward track and drive analyses. Will see.