Just curious, how is .1 sec accuracy achievable with a 1 hz refresh rate? I assume there is some intense interpolation happening but would like to know more.
Thanks!
.1 sec accuracy with 1 hz refresh rate?
Re: .1 sec accuracy with 1 hz refresh rate?
As GPS does not deliver positions continuously, but as distinct fixes with a certain update rate (e. g. 1 Hz), the fix will be never ,on‘ the POI position but slightly before or after. To make things a little more complicated, it will be slightly to the left or right too.
To get timing accuracy beyond update rates, LapTimer does sophisticated interpolation to calculate the actual point and time the start / finish line was passed. To accomplish this, LapTimer calculates the trigger‘s perpendicular projection to the virtual line between the GPS fix before and the GPS fix aftrer the trigger. Having calculated this point, the time this point was passed is calculated by linear interpolation from the speed at the fix before, the speed at the fix after, and the distance between these two points. As long as the speed development is near linear, this interpolation yields accuracy beyond 0.01 seconds. This is the reason, why receivers with 1 Hz update rate are fully sufficient for timing and equal accuracy of other technics.
- Harry
To get timing accuracy beyond update rates, LapTimer does sophisticated interpolation to calculate the actual point and time the start / finish line was passed. To accomplish this, LapTimer calculates the trigger‘s perpendicular projection to the virtual line between the GPS fix before and the GPS fix aftrer the trigger. Having calculated this point, the time this point was passed is calculated by linear interpolation from the speed at the fix before, the speed at the fix after, and the distance between these two points. As long as the speed development is near linear, this interpolation yields accuracy beyond 0.01 seconds. This is the reason, why receivers with 1 Hz update rate are fully sufficient for timing and equal accuracy of other technics.
- Harry
Re: .1 sec accuracy with 1 hz refresh rate?
Thanks, Harry, for that very quick and informative response! I am thoroughly convinced that update rate isn't a significant source of error by any means. That seems to leave two remaining sources of error: position and velocity. How accurate is the the iPhone 4 out of the box, and how much of an improvement is the TomTom mount?
Re: .1 sec accuracy with 1 hz refresh rate?
Hi,
While a TomTom was nearly a 'must' with the iPhone3G+3GS, it is an option today. However, there are still some advantages you get from an external 1Hz GPS device (TomTom, XGPS150):
- Harry
While a TomTom was nearly a 'must' with the iPhone3G+3GS, it is an option today. However, there are still some advantages you get from an external 1Hz GPS device (TomTom, XGPS150):
- Both can be accessed on NMEA level, which gives LT the opportunity to get unfiltered data.
- Some additional information available (Satellites information).
- Every GPS data going through the Apple location service gets filtered, check http://www.gps-laptimer.de/GPS_and_Accuracy.html for an example what happens and the full story. Recently there were several reports that speed specifically is filtered leading to problem with high / fast changing speeds.
- Without having it measured or compared 'fact based', they delivering still more accurate information.
- Harry