Doing Racing Line Analysis With GPS
One of the great benefits of using GPS data for your racing line analysis is that it knows where you are on track …. That is … until it doesn’t!
You are wanting to compare some logged data – it might be a previous session of yours, or some data from your friend or team-mate.
You load the data into your analysis software, pull up the GPS traces in a track map and … disaster …
… Your Data Doesn’t Line Up!
All you want is to be able to compare your differences so you can improve.
You want to spot the subtle stuff and use this tool to better explain your lap times.
Instead, you are now faced with two problems:
- First, whether your software will allow you to align the GPS traces.
- Second, your credibility.
The first one is often possible – although it’s never easy. If you are using an AiM data logger for example you can use something called “GPS Lap Insert” to try and at least align the startline (more on that here.)
Other systems typically have a way to correct the alignment by eye, but it is not always that good. Often you can find one part of the track seems to logically align but then another doesn’t.
In part because of this, and in part because you’d expect the data to line up on its own, the second issue – your credibility – can often be your bigger problem.
Dealing With “I Told You So …”
If you are not experienced, then you might now feel like it is hard to rely on your data to interpret track performance?
If you are with others – who might already be skeptical – then you could even feel undermined by your data system manufacturers promises.
The good news is this. There are several good engineering reasons why your data does not line up. The simple reason is that the satellites, which the GPS system relies upon, move.
This results in them thinking you are in a slight different place on the earth, each time you want to log data.
Roughly, here is why …
A Basic Introduction To How GPS Works
If you look at this animation above (thanks to wikipedia) then you can get an idea of the GPS system.
Each little black dot is a GPS satellite above the earth.
You can also see that there is a little point on the earth that is being tracked in the animation – Golden, Colorado, USA (39.7469°N 105.2108°W) if you’re interested. ?
Whenever a satellite can “see” that point, the animation above draws a straight line to it.
If you look closely, you will see the number of satellites that can see Golden varies. Below the globe you can also see a count of the “visible satellites.”
This is what affects your data systems positional accuracy. In simple terms, the more satellites that can see you the better.
The other issue is to do with angles and distance. The satellites work out your position using the distance you are away from them. When they are close to the horizon their ability to work out this distance reduces because of the small angles involved.
There is a lot more to it to be honest. In sum, the more accurately the satellite can calculate the distance you are from it, the more precisely it can give you your position.
Luckily, as long as you get a “GPS lock” on your data system, it does not really affect you within your 30 minute track session. The satellites don’t move so fast so your relative position is normally tracked pretty well.
So Is Racing Line Analysis With GPS Any Good?
I personally avoid using the GPS for racing line analysis because of these issues. You never know what satellites were in use during sessions. Different numbers of satellites and their relative position to one another will mean they place you in a slightly different place on the earth each time. Morning can be different too evening as well.
There are GPS systems that can be very accurate – for the INEOS 2hr marathon project I worked on we scanned the track to within 0.25 mm. This was possible however with industrial grade kit that cost a fortune and required a separate base station.
I do appreciate it can be unsettling when things that should line up and they don’t. Whilst it doesn’t look right, the good news is that your data is probably fine.
Use this explanation and the checklist below to give yourselves confidence in your data.
Checklist For Racing Line Analysis
Here is a checklist you can apply to ensure you are getting value from your data:
- To give yourself confidence your data is reliable, first look at your lap times. If they compare well with the circuit timing (or your stopwatch times) then your data is likely to be okay.
- If you have it, use onboard video instead for your racing line analysis. When you find something interesting in your data, find the same point in the video to see what was going on. It is more useful and more precise than any consumer level GPS system for line analysis.
- To find value in your logged GPS data, focus first on comparing your speed traces. The speed trace reveals a wealth of information on your driving and can often be enough to use on its own.
- Then look at longitudinal and then lateral acceleration. If you do this together with time slip (or delta-t) you will be able to compare data and work out precisely what is going on to improve.
If racing driver data analysis is new for you, then feel free to check out my FREE Data For Track Drivers – Mini Course that goes through the topics of racing line analysis, braking, turn-in, delta-t plus how to turn this insight into a good action plan back on track. It also includes a bonus at the end specifically looking in depth at how you can use the speed trace to highlight areas the driver is not confident in.
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