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Accuracy of the lap times : magnetic strip vs RaceChrono

The result for 17 laps, tested on Ales[FR] track :

magnetic_strip_vs_racechrono.png



Most importantly, the average precision is 0.03 seconds. Very impressive!

If I remember correctly, in his documentation aol say : better +-0.15sec with GPS 5hz.







How do I located the magnetic strip : a magnetic sensor plugged to GPS button during 40 laps.

magnetic_strip-plus-RC-Ales.jpg

From there, I have defined the M point, this position indicate a new start/finish line in RaceChrono.





Details of GPS points identified by the magnetic sensor

magnetic_strip-Ales.png



GPS time is accurate to 0.2s (5 Hz) and my average speed in this area is 144kph. This gives a possible distance error of 144 x 1000 x 0.2 / 3600 = 8 meters. It is approaching 8,9m. That is why I have located the point M at the beginning of the detection area.





You can download the spreadsheet that comes from these calculations. Also contains the 40 GPS points identified by the magnetic sensor and more things ...





Here you have two bin RaceChrono files of this session, also added nmea original file in zip archive file. And here, you can download "track.bin" which contains as start/finish line the GPS position of the magnetic strip.



I made this on a motorcycle, the GPS is attached to the rear (mounted on duck tail), as it is well clear of everything, including me the driver.

Track is dry, wind is very low, sky is blue and the temperatures are between 18 and 29 °C. Weather is perfect !

I completed 68 full laps during the day, I started the track at 9:26, I left at 17:40 (local time, for UTC time remove 2hours).

Comments

  • aolaol
    edited July 2011
    Very nice analysis! My estimate of +-0.15sec is on average environment. The error might be less or more depending of the environment. Here's few things that affect the accuracy:

    - GPS-receiver used. Some are better than others. Internal GPSes on the phones can always be considered inferior to external ones.
    - GPS installation. GPS always works better when there is unshielded visibility to the satellites from the antenna. Plastic and glass reflect less of the GPS signal than steel roof, but unshielded view is always best.
    - Geographical location: The satellites hover mostly over the places like USA, Australia, UK, Continental Europe. In places like Finland and Sweden satellites mostly lie in the horizon and the visibility is easily blocked by landscape or buildings.
    - Track landscape: View to satellites are easily blocked by landscape and forest in historic race tracks like Nurburgring Nordschleife.
    - Weather matters. Rare atmospheric activity can affect accuracy greatly especial ly when DGPS-satellite is not visible (or if GPS-receiver doesn't support DGPS). Also rain and clouds can affect the accuracy somewhat.
    - Magnetic/light trap location: If the GPS trap and the physical trap locations do not match exactly it will naturally affect on lap times. Misplacement is easily detected when cool down and warm-up lap have consistently much more error than fast laps.

    I'd say that in relatively bad environment +-0.5s is possible or even worse. But in perfect environment and well planned installation very good accuracy can be achieved as you have proved here! Thanks for doing the analysis!
  • edited July 2011
    Alastaro[FIN], Motorcycle, GPS located in windshield, transponder located in front fender.
    Official race timing Magnetic strip
    vs
    RaceChrono & Qstarz 818XT 10Hz

    Laptimes 1:33,xxx - 1:34,xxx

    Relative difference:
    -0,007
    -0,032
    +0,015
    +0,004
    +0,022
    -0,003
    -0,022
    +0,003

    Absolute:
    Min diff: 0:00,003
    Max diff: 0:00,032

    Average difference of 8 laps:
    0:00,0135

    Sum of the differences of 8 laps:
    - 0:00,020
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