Contents: GPS errors

1. Error summary table
2. Multipath errors
3. Ionospheric errors
4. Space weather
5. Clock errors
6. Orbit or ephemeris errors
7. Tropospheric errors
8. Receiver errors

Error summary

Error	VALUE (Approx)
Ionosphere	4.0 meters (watch out for ‘solar storms’)
Clock	2.1 meters
Orbit	2.1 meters
Troposphere	0.7 meters
Receiver	0.5 meters
Multipath	1.0 meter (In use, this can be much worse)
TOTAL	10.4 meters

Don’t take those figures too literally, they all vary and different authoritative sources will give slightly different errors for the same things. However, they give an idea of what causes the worst errors. In use, the nature of the ground you’re crossing and how much sky is visible will likely be more significant.

Even very old GPS receivers like my first unit the Garmin GPS12XL (1998), that can only use the American GPS constellation are impressive. They should provide a fix that is within 15 metres of where the receiver actually is. Modern receivers typically do better than that.

The bottom line is that GPS accuracy is phenomenally good, if the GPS receiver has an uninterrupted view of most of the sky and not too close to any signal-reflecting objects, like rocks, trees and buildings. However, that’s not always the case.

Multipath errors (± 1m but often more)

Looking at the error summary table above you’d think that multipath errors weren’t worth worrying about. Errors of ± 1m. What’s the problem? Under an open sky that might be true, but multipath errors are often much bigger than the total of all the other errors in the summary table. They are likely to be responsible for the biggest errors you see on your adventures.

Hikes don’t always stay in wide open spaces. We might walk through woods, between buildings, by a cliff, through a gully or just be walking by large rocks or a dry stone wall. Reflected signals off any hard surfaces can all upset the GPS receiver. 30 metre errors can be generated by a nearby wood or cliff. If you get into a deep rocky canyon errors could be much larger. Impossible position ‘spikes’ might occur if the receiver gets really confused.

If the sky is getting hard to see, the GPS receiver won’t be able to see many satellites and could lose the plot completely. It might fail to get a fix. At the bottom of a deep slot canyon, with just a sliver of sky overhead, it would be tough for a GPS receiver to get a good fix. Dense foliage can also cause problems but in the typical British woodland I’ve walked through, my GPS units have continued to work, albeit with larger errors.

Having said all that, GPS units usually do very well indeed. If you spend your time hiking the British hills then you won’t often see errors bigger than 30m and on open ground accuracy will typically be just a few metres.

Not all of the radio signals arriving at a GPS receiver have come in a straight line, directly from the GPS satellites. The signals reflect off trees, rocks, cliffs and buildings etc. Those signals reflected off nearby objects may have enough strength to be problematic.

Reflected signals must travel further to the receiver which could make the receiver think it’s further away from the satellites than it really is. The aerials associated with GPS receivers are designed to try and reject multi-path reflections but they can’t eliminate this error.

The quad-helix aerials housed by the ‘thumbs’ on Garmin handhelds are generally thought to be better than the flat patch aerials found in the SatMap and eTrex models. Quad-helix aerials perform better held vertically and patch aerials held horizontally.

Typically, with plenty of sky visible, it won’t matter a jot which aerial type you have or how you hold your device. All will work fine. However if you do take the unit into tricky places for GPS reception then the aerial type might make a difference.

In my experience, receivers seem to perform surprisingly well, even in quite challenging spots.

When studying my tracklogs I don’t think I’ve seen tracking errors significantly over 30 metres that weren’t very brief events. Usually errors are just a few metres. A cliff, or trees on one-side-only tend to be the cause of consistent track offsets and errors are larger in woodland.

Here’s a typical tracklog recorded using the Locus Map 4 app on my Samsung S10e phone. Day 2 of a wild-camping trip, I was walking over Helvellyn (950m) in the Lake District. If you choose the Satellite map layer and zoom-in to the track where I’m obviously following a well trodden path, then you can see the errors for yourself. Even amongst the trees it doesn’t get more than about 20m out.

Just be aware that if you take your unit into a very challenging place, it might struggle to position you with it’s usual accuracy or even fail to get a good fix occasionally.

Ionospheric errors (± 4m)

This layer of the earth’s atmosphere lies between 30 to 600 miles above the earth’s surface. It’s here that the ethereal Northern Lights or Aurora Borealis occurs. Electrically charged particles (ions) ejected from the sun collide with atoms of nitrogen and oxygen from the earth’s atmosphere to produce the amazing light show. There is a constant flow of ions (electrons and protons) from the sun which is commonly known as the solar wind.

The earth’s magnetic field effectively shields us from these charged particles by causing them to flow around the earth’s invisible magnetic field towards the poles.That is why the glowing aurora are usually best seen near the poles.

The ions in the solar wind transfer electric charge to the particles in this upper layer of our atmosphere, hence the name ionosphere. This electrically charged layer affects radio waves.

Radio waves and visible light are both different types of electromagnetic radiation. When light passes through glass it is refracted and slowed. The radio waves from GPS satellites are refracted and slowed in the same way by the ionosphere and to a much lesser extent by the troposphere too.

GPS distance calculations assume the signals always travel at the speed of light. The ionosphere’s slowing effect on GPS signals is not as dramatic as the effect glass has on light but GPS would be more accurate without this error.

The solar wind varies and affects different parts of the planet differently. No model of the atmosphere can remove all of this error.

Space weather

Space weather refers to the effects the solar wind has on the earth’s ionosphere. Mostly the ionosphere just spoils GPS accuracy a little, however when a big solar flare occurs, a huge amount of extra material is ejected into space in one go. The solar wind increases significantly. The ions fly towards earth at speeds of up to a million miles per hour. A particularly powerful flare may cause a ‘solar storm’ and these can seriously disrupt radio communications and GPS signals.

So many charged particles can arrive on earth at once that they cause electric current to flow through metal and through the ground. Satellite electronics can be damaged through the build up and discharge of static-electric charges and a big ‘storm’ can even cause electrical blackouts on earth.

These particularly big solar flares are called ‘coronal mass ejections’.

In 1989 a solar storm was powerful enough to cause the Great Quebec Blackout. Just 90 seconds after the mass of charged particles reached earth, the power grid in Quebec failed. Millions of people were left without light and heat.

Flares that are sufficiently powerful to do that happen infrequently so you’d be very unlucky to have your GPS let you down just when you really needed it. But maybe it’s worth knowing that ‘space weather’ can affect GPS navigation?

“The charged plasma of the ionosphere bends the path of the GPS radio signal similar to the way a lens bends the path of light. In the absence of space weather, GPS systems compensate for the “average” or “quiet”  ionosphere, using a model to calculate its effect on the accuracy of the positioning information. But when the ionosphere is disturbed by a space weather event, the models are no longer accurate and the receivers are unable to calculate an accurate position based on the satellites overhead.”

Space Weather Prediction Centre

More here…
Space weather and GPS systems.
Met office – space weather forecasts
NOAA – Space weather alerts, watches and warnings

Clock errors (± 2.1m)

When a timing error of just a millionth of a second will cause a location error of 300 metres, it’s no wonder that some GPS error is down to timekeeping.

Atomic clocks on the satellites keep exceptionally good time but they are still updated twice a day by even more stable ground-based clocks. Timing errors cannot be eliminated completely.

Orbit or ephemeris errors (± 2.1m)

This error is the difference between the expected position of the satellite and it’s actual orbit. The expected position is broadcast to a GPS receiver as ephemeris data. Ephemeris data is not updated continuously, so what your receiver has stored might be a couple of hours old.

Tropospheric errors (± 0.7m)

The troposphere is the layer of the earth’s atmosphere next to the ground where all our regular weather happens. The layer’s average depth is about 8 miles. Passenger jets like the 747 cruise around near the top of the troposphere at roughly 36,000 ft.

GPS satellites transmit using a frequency that was deliberately chosen to to be unaffected by weather. If there’s a lot of rain or snow in the air it is possible for the GPS signals to be weakened slightly but this is unlikely to affect GPS reception or accuracy. Clear blue skies or raging storm, it doesn’t matter.

GPS works well in the worst of weather.

Receiver errors (± 0.5m)

There is always a little ‘noise’ in the electronics doing the calculations. This causes small random errors.