# How accurate is a single GPS at detecting distance traveled?

I read that consumer grade GPS is accurate 5-10 meters at resolving global positioning coordinates.

However, my question is how accurate is consumer grade GPS at detecting distance traveled.

For example:

Assume I record a position with GPS and consider it my origin of (0,0,0). One second later I recheck my position & calculate the difference which result is (10mm, 20mm, 30mm).

For this sort of example how much error should I expect?

Thanks!!

• depending on the application, you might have to assume worst case: That each of the two points is off by 10 meters. So your result could be +/- 20 meters. – Mark Jun 26 '16 at 20:41
• Have you ever looked at the data that your GPS puts out just sitting still? There's quite a bit of "noise" in the LSBs (several meters worth). Detecting a change of tens of mm in all of that noise requires very long integration times. – Dave Tweed Jun 26 '16 at 22:37
• If you've got a big GPS antenna, a receiver capable of tracking a dozen or more satellites and a good timebase you can get much better than 5-10m accuracy. Back at Uni there was a guy building a search n rescue drone, his gps was the size of a deck of cards and could get down to 10cm accuracy while flying at 50kph, and it was an off-the-shelf part so consumer GPS can be as good as you want just about, provided you have the money (I think this guy's unit was around the \$500 mark for just the GPS receiver) – Sam Jun 27 '16 at 0:15
• @Tom: GPS accuracy is bounded, with standard GPS the resolution is limited to ~2m. It sounds like your friend had a GPS/IMU combo, and probably some augmentation (WAAS and friends). Adding the IMU (and some clever filtering) allows you to filter out the jitter and massively improves position accuracy. To get ~cm accuracy with GPS alone, DGPS and a stable/long integration platform is necessary. – uint128_t Jun 27 '16 at 16:11

Relative positions are more accurate than absolute positions for a L1 C/A user ("consumer grade GPS"). Differential GPS and RTK positioning take advantage of this effect.

Measurement of the carrier phase would yield centimeter level accuracy, as the L1-signal has 20cm wavelength. Users without access to the military code can however not relate this carrier phase to the timing information exactly, there is always an ambiguity. You can therefore only use code phase, which gives the 10m accuracy at best.

The situation is better if you have access to information from two receivers or from one receiver, that had continuous lock on (some) satellites while moved. You do then know the difference of carrier cycles. For two receivers, you are able to resolve the ambiguity after some observation time, for one with continuous lock, the ambiguity will cancel off when subtracting.

For a commercial receiver, where you do not have access to the raw observables but only to the position output, this still means, that the traveled distance is more accurate than the absolute position. This has to do with that fact, that receivers use "carrier phase aiding" and the last known position fix to some extent to improve their solution. The fix would be much more noisy without such tricks.

I would expect around 20cm distance accuracy in optimal conditions (no multipath, short time (some seconds) between observations, no loss/acquisition of signal between observations).

It can be quite accurate, but depending on the conditions of the environment (open field vs city canyon) and the position of the GPS (in your pocket vs sitting open with a good sky view) it can have a fair amount of random walk.

The answer is "it depends" and you need to test in your application to find out if it will be suitable.

• Thanks! I plan to use this in an open field using an Arduino checking the delta as often as possible. When you say "quite" accurate would you guess "mm","cm" or "m" level accuracy for a 1sec delta in my setup? – Jamey McElveen Jun 26 '16 at 21:13
• I am always in the assumption that millimeter GPS accuracy is only available in the military grade GPS units, not common consumers'. Correct me if I'm wrong. – soosai steven Jun 26 '16 at 22:26
• @soosaisteven: No, techniques such as RTK can deliver centimeter-level accuracy even with the civilian signal. But this level of performance is not generally available in a cheap consumer-grade module, most of which do not make or provide carrier phase measurements. – Dave Tweed Jun 26 '16 at 22:45
• Look, I mean, if you sit still and count all the drifting around as 'movement' it's going to be garbage. If you're riding your bike down the street, it's probably going to be pretty good. Having the thing hit a target within 1-2m isn't unreasonable, but if you're going to try to take the data literally with some stationary or slow-moving object, you'll need some kind of filtering. – Daniel Jun 26 '16 at 23:19

You can expect several meters of inaccuracy. Unless you are moving several 10s of meters between "samples" then you should expect no useful delta position information.