Error on speed data

jujupointcom · Post by **jujupointcom** » Wed May 20, 2015 10:10 pm

Hi Harry,

First of all, after using your app for years, I still found it great ! Good Job.

I have an issue on a track log regarding speed : it is far "below" expected values. VMax is around 130 km/h when 160 was expected. It seems that all speed data are lowered.

I have run 2 different days on the same track, but with 2 different configurations :
- Day 1 : iPhone 5C, in landscape, using Ram Mount behind the windshield -> speed data sounds correct
- Day 2 : iPhone 5C, in landscape, using Ram Mount, behind the windshield, external GPS receiver (DUAL XGPS150A, patched for 5Hz) -> speed data are lower than expected.

The GPS receiver was in the arm rest, between the seats, but I am driving a roadster, so no roof above it.
I would expect more accurate results using such a receiver, but I was disappointed to notice that speed is so inaccurate. Mesured time sounds good by the way (I was 3s faster Day 2 than Day 1).

Any issue with the XGPS receiver ? Positioning of it or the way it is fixed to the car ?

Thanks,

Julien

Post by **Harry** » Thu May 21, 2015 6:02 am

Hi,

GPS Speeds you see in LapTimer are 1:1 what the sensor delivers. Maybe the mounting position... I'd suggest you contact Dual in this. Speed measurements have no influence on timing accuracy. But bad values are an indicator for bad GPS accuracy - which certainly has an influence on timing accuracy too.

Harry

jujupointcom · Post by **jujupointcom** » Thu May 21, 2015 6:41 am

Hi,

Ok, thanks. I will make some try fixing the receiver in a better way. Thanks for your answer, it closes some interrogation Inhad regarding my setup.

Yellowjacketboss302 · Thu Jun 04, 2015 11:34 pm

Hi Harry,
Same thing happened to me.
Are you saying that you get speed directly from the XGPS unit? I thought you got position data, then computed speed yourself.

gplracerx · Post by **gplracerx** » Fri Jun 05, 2015 2:46 am

On the GPS data page, how many satellites show up in the display in the upper left corner? Or, you can export lap data as a .csv file. The number of satellites used for the fix is listed in one of the columns. Less than six may be too few.

Post by **Harry** » Fri Jun 05, 2015 6:26 am

No, speed is delivered by the XGPS using the so called Doppler effect. Although one could derive speed from positions, this is an outdated approach not used by modern GPSes any more. It can be done, but is typically very noisy. Smoothing (Kalman) needed will add even more lag than what you see from Doppler measured speeds.

Harry

gplracerx · Post by **gplracerx** » Fri Jun 05, 2015 3:06 pm

Although one could derive speed from positions, this is an outdated approach not used by modern GPSes any more.

Which is why I don't understand why Circuit Tools throws out the speed data and calculates speed and acceleration from the position data. I guess you can't expect much from free software.

jujupointcom · Post by **jujupointcom** » Fri Jun 05, 2015 6:15 pm

I have contacted Dual, they do not explain the issue.
I have tried to better positioned the unit, and fix it well in the car and seems far better.
I have to test again to see if it is reproductive.

Yellowjacketboss302 · Post by **Yellowjacketboss302** » Sat Jun 06, 2015 2:14 am

Thanks for the speed clarification, Harry. Do you smooth the speed values?

Post by **Harry** » Sat Jun 06, 2015 6:48 am

No, LapTimer uses the incoming GPS speed values unchanged. Usually they are "over smoothed" already...