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How accurate is your GPS?
I went for a run last night and, at times, the track I looked at later was over 20m off. When I drive in London the car Satnav often has no idea where I am.
If you have experienced the same inaccuracies as me then you might be surprised to know that a well-designed GPS receiver can, in fact, achieve an accuracy of better than 5m especially if multiple constellations and multiple bands are used to improve position accuracy.
You might also know that GPS can be used to measure your elevation/altitude but the accuracy of that, in ideal conditions, is not quite so good.
But the error is not simply from the device you are using or the building you are near. There are also continually varying factors linked to the satellites’ positions above you. That’s why you can do the same run on different days and see differently plotted tracks of your route.
The term ‘Dilution of Precision’ (DOP) is used to describe factors that worsen the accuracy you receive.
PDOP – Position Dilution of Precision
Complex maths derives a simple, single number for each kind of DOP. If that number is less than 1.0 then that’s “ideal” but even less than 2 is still “excellent”. There are 4 kinds of DOP:
- Geometric or Position (3D) dilution of precision (GDOP or PDOP – 3D);
- Horizontal dilution of precision (HDOP);
- Vertical dilution of precision (VDOP); and
- Time dilution of precision (TDOP).
The maths behind a better spread of satellites means that you get less dilution and hence a better signal. Something like this diagram shows
I’m most interested in PDOP and I almost always get 1.0<PDOP<2.0 for my runs. But NOT always.
Other factors are at play too
Refraction – in the atmosphere
The troposphere and ionosphere can make the effective distance from the satellite to your watch longer due to atmospheric refraction (remember physics? age 15ish).
Using two signal frequencies from one or more satellite constellations can be used to minimise this effect. But not all satellite constellations can do this and may only broadcast over one PUBLIC frquency.
More on Dual Frequency (JBarbeau Via @Mirko) on medium.com
Reflection – near the ground (Multipath Effects)
If you run close to buildings you will sometimes see your post-run track veer away from the building and into the middle of the road. Your watch is picking up a reflected signal from the building which must have travelled further. Hence the maths puts you somewhere other than where you really are. I reckon this can give at least 1-3m of error in my experience.
Look here at the blue line in the bottom left as I run close to a building on Old Bridge St.
Ephemeris – Satellite Location and Timing
Trilateration of distance (not triangulation of angles) is derived from each satellite’s time signals. However the atomic clock in the satellite can be slightly inaccurately synchronised. This can cause another 1-3m of error
However when you sync your sports watch you also sync the “ephemeris data” showing the satellites exact locations for the next week or so.
I *think* that sometimes when you see a post-workout GPS track that runs parallel to your real track only in one direction such as when you run South to North then this could be due to incorrect ephemeris data, most likely from it being sync’d incorrectly at some stage rather than an error in the source itself.
Something like the blue line being too far North here but ‘about right’ from East-to-West
There are other sources of SIGNAL error but those are the main ones for us.
Improving GPS Accuracy
GPS Chip manufacturers and those who integrate them into a sports watch can improve our accuracy. But we can help ourselves too.
- Constellation level – GPS can be augmented with additional satellites from GLONASS and GALILEO. Simply there are more AVAILABLE satellites to choose from. The following chart ‘proves’ that GLONASS+GALILEO is better yet my experience tells me otherwise, a lot of you tell me the same. So maybe that is a theoretical improvement in ideal scenarios that don’t translate well to running watches?
- I’ve recently seen as few as 2 GALILEO satellites available on some runs with the Fenix 5 Plus and 935. Maybe that’s why the accuracy wasn’t so great that day?
- Satellite level – some satellites transmit dual frequency (this does NOT MEAN GPS and GLONASS). Dual frequencies will help both atmospheric refraction and multipath errors. For example GALILEO CAN allow a dual frequency signal to be used HOWEVER as of SEP 2019 absolutely ZERO sports watches use this DUAL-BAND service from GALILEO (Source: I know stuff 😉 )
- Device level – a good antennae design (this paper came out top of my Google search: link to semanticscholar.org). This is often bandied about as the be-all and end-all. Sure it must be important but…
- Algorithm – from which satellites in which orbit to choose for the calculation through to how a swinging arm is accounted for. There’s a LOT of modelling that takes place.
- User – Most of us know to wait until the watch beeps to tell us it has a signal. With Assisted GPS (A-GPS) if your current ephemeris data is up to date then you get a quick lock. If your data is not up-to-date it is ‘downloaded’ from the satellites. Which takes longer, sometimes several minutes longer than when you are told you have a satellite lock. However, I never knew until recently that the 3D position take even longer to acquire. So if you are using GPS for elevation and are bothered about its accuracy then, maybe, wait a bit longer. (Ideally sync your device online before you go out and wait 15 minutes…yep 15
- User – wearing the watch on the underside of your arm WILL restrict the signal and increase inaccuracy. THE ANTENNAE IS POINTING THE WRONG WAY!
- User – even wearing on left/right wrists can make a difference if one side is more frequently closer to a building.
- User – wearing devices close to each other may cause interference and knock may affect signal reception?? Not sure about that but it can’t help.
- User – For some reason I find that a good stationary ‘soak’ recording a 15 minute blank track in my back garden before heading off helps. That’s anecdotal..no science behind that one. Give it a go.
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