Harry's GPS Suite Forum

Hello,

I use the GrandPrix version (just updated before the trouble) with an iPhone6 (iOS 9.3.1) and a Dual XPGS160. I never changed the expert settings.
I only drive on ATP Papenburg.
All worked fine last year and also on 09.April this year. But last time on 23. April I got wrong laptimes. The weather was not so good but the GPS signal was very good.
I compared the laps and the normal times have had much more than 750 positions GPS, ACCEL (up to 799), all faster laps have had only 700-750 positions (and usually slightly shorter track length). But on 23.04. I have had up to 6 seconds faster laptimes and only 600-700 positions.
The track length was often too short.

I can't remember that I changed the settings in the App and I wonder what I can do to get exact data, at the moment it is complete useless.
Wlan was off, internal GPS was off, I used a new mount for the iPhone at the same position but I still use the standard calibration. G-values are not so important for me at the moment.
Very important are exact laptimes for me and it seems that we are far away from exact.
I wonder what exactly I can do to get exact laptimes and if it make sense to buy the vbox sport or the e.g. the driftbox. At the moment I lost the trust in my actual system.

BTW: I would like to measure the 0-100km/h and 0-200km/h values and I can't find it in the German version. Could you give me an hint with the German menu?

Thanks
André

Please export *one* lap showing a wrong lap time to .hlptrl and I will have a look into it. Please keep in mind that GPS based system are always exposed to disturbances and guarantee accuracy claimed for a certain share of measurements only (typically 95%). So whatever GPS sensor you use or will use, there are days it will be off or will be disturbed. This is the reason reference systems will always be transponder or light barrier systems, not GPS. They are not helpful to analyze your drive however. For more information, please check viewtopic.php?f=39&t=3479

Any accuracy reported by a non pro GPS system will always be a statistic guess. A receiver may report 5 meters accuracy and be off by 100 meters at this point in time. There are system available compensating this but they will be beyond the budget of amateur racers.

From the export I can check if the lap time derived from data measured is calculated correctly. In case the sensor input has been wrong, it is hard to recognize this after the fact. I need to add that I have never seen a lap time derived wrong from data available in the last years... So in the end it will always be about the level of real accuracy delivered by the sensor.

Except malfunctions always possible for hardware, the VBOX Sport or Driftbox will not be different to the XGPS in principle. They may be even more sensible to disturbances because they use GPS only (no GLONASS).

In case you see wrong numbers or strange behavior of the receiver, power cycle it - maybe there is a malfunction. LapTimer has a function built in to monitor atmospheric disturbances too. The Kp-Index is displayed in GPS View. Sun storms e.g. will generate a warning in LapTimer Health Status. On the 24th of April, conditions where at 4 where a 7 is considered a storm. No good value, but not critical either.

To measure acceleration times, got Lap List ‣ lap ‣ Data Recordings ‣ Charts. Touch a starting point and an end point. Drag them so you get roughly the starting speed and finishing speed wanted. LapTimer will display all kinds of delta information between the two cursor lines. In case the diagram is to tight to select - zoom in.

- Harry

Thank You Harry
I sent you an email with the file.

What does "beyond the budget" means in detail? Do you have some search keys for me?
Better to search a mobile transponder system with battery which we can place next to the track?

Regards
André

Thanks and answered in detail.

To sum up for other readers:

• Dual XGPS had issues with accuracy, a reduced number of fixes per lap is an indicator for accuracy issues just like bigger accuracies shown are. Not sure what the reason has been, but it is the root cause.
• Lap time derived is in line with the GPS input.
• A short mathematical exercise shows that an error of 6 seconds per lap means the so called GPS drift has changed by 33 meters in one lap. This is beyond what is expected, these drifts usually develop very slow. This is not an individual fix's accuracy, this may vary by 33 meters when the sensor loses or nearly loses the lock. In the data provided, the fixes around start finish where o.k. - so a GPS drift needs to be the cause.
• Keep in mind: GPS is fine to record lap times at an accuracy of 0.05 seconds or even below, but only for a certain share of samples (typically 95%). In case you are focussed on lap times alone, or if these are relevant to decide on some kind of ranking, go for a transponder system. This however does not give you a lot help to improve - it does not provide any information what happened during your drive. This in turn is the reason pros will use both types of systems.

- Harry

Beyond the budget means that a system with centimeter accuracy would cost many thousands of US dollars. They don't even list prices for those items. You have to get a quote. You need something like a Racelogic VBOX3i with a DGPS Base Station. With the single antenna VBOX3i, that gives you 80cm positional accuracy. For many thousands of dollars more, you can buy a VBOX3i RTK and an RTK base station and get 2cm accuracy.