Recalibrate from drift

[JohnRT]

Hi Harry,

I use the laptimer for autocross. I set trigger points in the morning when walking the course. Unfortunately by the time I run the course the GPS drift can be so high that all of the trigger points no longer work. In addition, for long waits between runs the drift can be significant. Would it be possible to add a recalibration mode, where one could go to a known marked location and re-home to account for drift?

John

[Harry]

One enters the next level of complexity when adding something like a "shift track/nudge feature" as mentioned in this article. An implementation would be to allow a "recalibration of a POI XY set already" by walking the position recorded earlier, and press a "recalibrate GPS to POI XY" button... Before adding something like this, I'd like to ask you to first check if the described phenomena is due to the drift, or due to low accuracy during initial POI definition. GPS receivers require quite some time to get to their optimal accuracy, so starting LapTimer and setting POIs immediately might be the reason for the effect mentioned. Usually LapTimer adds enough width to the left and right when detecting triggers to work around a certain drift (otherwise no track set would work). I'm aware AXes are typically tighter thus requiring additional accuracy... You may add driving directions to your triggers by the way as this widens the detection corridor without adding a risk of mis detection on counter straights etc.

I keep that shift track/nudge feature on my list.

- Harry

[gplracerx]

JohnRT,

Do you test the accuracy of your POI's by walking the course again to see if the timer starts, stops and records splits at the right place? I had to reset the starting line location at least twice this weekend to get it to work when walking the course. That also gives you a course map. I also started bringing a magnetic compass to help set directions for the POI's. There's a switch in Expert Settings to allow low speeds when detecting POI's that's useful for course walk testing.

I also had horrible problems this weekend with Bluetooth connection stability after 'upgrading' to iOS 7.0.2. The GPS update rate would frequently go to zero even though the XGPS160 was still connected (blue led on). Fortunately it never happened when I didn't have time to reboot.

[JohnRT]

I'd like to clarify what you mean when you say it takes time to achieve accuracy, does this mean turning the GPS on for a period of time to "warm up" before setting POI (which I do) or to stay at each POI for a period of time before setting them?

One of the challenges I believe I am facing, specific to autocross, is that there is often only one opportunity to set the POI during the course walk in the morning (and sometimes once in the afternoon). Depending on order it may be hours between setting the POI and actually driving the course. Usually there is no opportunity to reset the POI on course, but it would be possible to recalibrate based on a point off course.

I have used the course walk to create a map. I have never encountered problems during initial course walk. I may need to do more testing to determine what the exact problems are.

[gplracerx]

What GPS are you using? External units tend to have better precision than the internal GPS in the phone. The XGPS160 has excellent precision because it sees more satellites at any given time. I suspect that means it's less subject to drift from changes in atmospheric conditions. I did have a finish position drift far enough to trigger when I passed close to it part way through a run in the afternoon when it was ok in the morning. But that was before you could specify a direction for a trigger.

You could do it manually, I think. Measure a known reference point at the same time as you set the track POI's. Or measure the reference point, move away and then come back and measure it again. Keep doing that until you don't see any change. Then you know your GPS has stabilized. Then set your POI's and measure the reference point again. If it hasn't changed significantly you're set.

Then measure it again before you run. You could then change the latitude and longitude of your track POI's by the same change in the latitude and longitude of the reference point. You would have to be very careful that the GPS was placed exactly the same way on the reference point and nothing else in the vicinity has changed. Bluetooth external GPS units like the XGPS150, XGPS160 and the VBOX Sport should be placed on the ground. For a hand held unit, you should probably be facing the same direction and holding the unit at the same angle.

[Harry]

I'd like to clarify what you mean when you say it takes time to achieve accuracy, does this mean turning the GPS on for a period of time to "warm up" before setting POI (which I do) or to stay at each POI for a period of time before setting them?

The first - give several minutes after power up to stabilize the locked satellites.

Harry