the5krunner

GPS Accuracy Explained – VDOP TDOP GDOP HDOP PDOP

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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.

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:

The maths behind a better spread of satellites means that you get less dilution and hence a better signal. Something like this diagram shows

Source (now deleted): https://nptel.ac.in/courses/105104100/lectureB_11

 

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

Other Errors

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.

Source: gpsworld.com

What is GPS 3? GPS III vs GALILEO and GLONASS – which is best?

 

 

 

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