GPS Accuracy Explained – VDOP TDOP GDOP HDOP PDOP

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 as me then you might be surprised to know that a well-designed GPS receiver can 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.

PDOP – Position Dilution of Precision

The term ‘dilution of Precision’ (DOP) is used to describe factors that worsen the accuracy you receive.
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

 

Links to source at: nptel.ac.in

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 refraction (remember physics? age 15ish).

Different frequencies of signal from any one satellite can be used to minimise this effect. But not all satellites can do this.

More ion 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 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.

Garmin Fenix 5X 5 5S Forerunner 935 Review

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.

  • 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
  • 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. 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.
  • User – wearing the watch on the underside of your arm WILL restrict the signal and increase inaccuracy.
  • 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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27 thoughts on “GPS Accuracy Explained – VDOP TDOP GDOP HDOP PDOP

  1. I use a basic Garmin watch (Forerunner 25) that only has GPS (no GALILEO, etc). But between my friends I’m usually the one that gets the distance closer to the oficial race distance, even when there are tunnels and buildings involved. The guys with the Fenix and Spartan always have a bigger difference…

  2. in the webinar of ublox “Super-E: low power and good performance” , that you can download from here https://www.u-blox.com/en/white-papers
    they found similar results as you: with a sport watch while running in their test they found that gps 2D-accuracy (95% confidence) in urban environment is 25 meter, in suburban environment 15 meter with a standard accuracy gps receiver while running.
    Their test consisted in a comparison between a runner with a gps sport watch and a highly accurate gps receiver with centimeter accuracy that followed the runner.
    They commented that the biggest problems in a gps watch are:
    1- small dimension of antenna
    2- battery problem : in a sport watch the gps must use very low power and this compromise performance
    3- the swing of the arm while running
    Usually to measure accuracy they don’t use always the 95% accuracy, but others methods, for example the RMS (root mean square) accuracy (that corresponds to a probability of 63-68%) or CEP (probability 50%). See http://gpsworld.com/gps-accuracy-lies-damn-lies-and-statistics

    • awesome stuff. I love science telling me I’m right.
      Always best to ignore it the rest of the time 😉
      seriosuly though, thank you for that link. interesting that low power is so much of a factor, if yo uare running for an hour LOTS of people would be prepared to go for a ‘super gps mode’ where the battery is used heavily. but is it not, instead, more that chips are designed for lower powerconsumption and then the source of dilution is the chip.?

      • I agree that in smartphone and watches the main source of dilution is the chip and the firmware itself. The problem is that people want always longer battery life, and brands give more importance to battery than GPS performance. My training usually is just one hour, I wish that for example Garmin gives also an option “maximum power GPS”, where battery life is just two hours and the GPS chip eats all the juice of the battery. They could put a dual frequency chip with a reception of 10 times per second. Unfortunately I think that there is not a big market for this watch.

        • On the other hand it’s very easy for the average user to see how many hours the battery lasts. It’s more difficult to see how accurate is the gps track, because you should download the data and see the track in a map, but very few people do this.

          The brands want to sell products with a long battery life (that is what people want).
          It seems that gps accuracy is no more a priority.
          It’s seems to me that the gps is always less important in a sport watch.
          1- Suunto for example produces now a sport watch without gps (the Suunto 3).
          2- battery life of the sport watches is always longer and brands give a lot of options to extend battery life (Garmin ultratrack, Suunto fuse track etc), but in the same time I read your article where you write that it seems that gps accuracy of recent watches seems to be less than gps accuracy of old watches (for example, you write that the most accurate gps sport watch remains the 4 years old Polar V800, and you are still not fully convinced of the gps tracking of the new expensive Fenix Plus).
          3- I think that in recent watches the chipset are giving always more importance to accelerometer data to correct gps data. In this way they want that the gps chipset uses less battery power. But if the algorithm is not so clever, this could give worse gps tracking, and I think this is why old watches gave cleaner gps tracking. Their gps chipset used more power and the algorithm used less the data of accelerometer, ore used no data of accelerometer because a lot of old watches didn’t have an accelerometer inside.

          • does it really matter what the post-run track looks like? for most devices it’s ok enough. BUT I take adherence to the actual route as being a (part) proxy for inaccurate instant pace
            battery life is definately seen as more importnat by more people. even the fenix 5s plus i am using now has run out of juice a couple of times. mainly because i’m so used to NOT charging watches these days!
            suunto 3…not sure how well that will do. there IS a market (maybe a big one) but it’s competitive at those price points and suunto have no pedigree there.
            2 recent/old watches – yes i dont think gps tech has evolved to increase accuracy that much. save battery..sure it improved there. it comes again to the desire fo the watch company to get the max accuracy from the chip
            3. yes there are moves to integrate multiple functions in one chip. this is another strategy for saving power and space.

  3. “it seems that gps accuracy of recent watches seems to be less than gps accuracy of old watches”

    Yep. My (very) old FR610 generates more accurate tracks than my new(er) F3 or FR935 (better than the F3). But… I don’t see any real benefit from “super accurate” (read: precise & accurate) GPS tracks in casual sporting watches – so, I don’t care too much about it.

    Better battery life means less of this annoying charging procedure. 😂

    Even the bad GPS of my F3(HR) was good enough to see afterwards where I was running/walking/riding. 🤷‍♂️

    “does it really matter what the post-run track looks like? for most devices it’s ok enough.”

    Yep.

    • ‘super accurate’ always sounds more professional than ‘precise and accurate’ 😉

      annoying charging procedure…yup. that’s one benefit of my process of always anaully uploading stuff. i tend to faff around and when i’ve finisehd the watch is invariable fully charged as well as the data transferred.

      • My experience with gps sport watches:
        1- walking and easy runs and long slow runs: ok
        2- tempo run when I run near my race pace : quite ok, but I usually prefer to use the manual lap function near km marks on the roads.
        3 – intervals of 1km or 500 meter: gps is useless, I use the manual lap and I have signs on the track or on the road with the exact distance
        4- race : quite ok, but when the race goes around town near building it often has bad moments. The problem is that also the km signs are usually wrong.

        I would say that in training GPS is really useful for me half of the times. The other half I must run on roads where I have km signs, but I wish I could run only with GPS without the constraint to run always on the same road

        My ideal sensor: an armband sensor like the Scosche Rhythm that is both heart rate monitor and GPS receiver with high accuracy :
        A) frequency of reception of 10Hz – ten times in a second, versus 1 time in a second of the normal watches
        B) multiple frequency
        C) and why not, Boris, also PPP or RTK 🙂

        I don’t think that all of the above could be installed now in a watch.

        • Another thing, Boris, I see from the web site you linked that the service swipos of Agnes for differential GPS in Switzerland (Suisse) costs circa 2000 € in one year. With the commercial service of Galileo (E6 frequency) , even if right now it’s not operational, we could have a similar service for free in 2020.
          First problem: we need the right PPP receiver, and for sport usage Garmin doesn’t give us this type of receiver 🙁
          Second problem: my expectations about gps accuracy are very high, but my budget is very low 🙁
          Third problem: with the free commercial Galileo PPP service (E6 frequency) we are probably near a revolution in gps accuracy (high precision gps will be delivered not only to the military, but also to mass market), but it will probably impact other devices, perhaps even smartphone, but it’s difficult that it will impact smartwatch, they are too limited (low power, small dimension for antenna and battery, it’s impossible to be always in open sky – one of the prerequisites of PPP- and the signal is too disturbed by the movement of the arm)

          So maybe it would be enough to have just the second frequency of Galileo E5b, that it’s more clever than the the E1 frequency and could avoid multipath errors. We will see with the Fenix6, I hope, if this second frequency it’s enough for us runners.

  4. The DOP stuff seems to be very very important, and maybe the biggest factor when we see big errors in gps data.
    In this article
    http://eprints.nottingham.ac.uk/51823/1/B6-Basile%20v2.pdf
    the university of Nottingham writes about the fact that the poor gps signal visibility and continuity associated with urban environments together with the slow convergence/re-convergence time of Precise Point Positioning (PPP), usually makes PPP unsuitable for land navigation in cities.
    With PPP they usually reach an accuracy under 1 meter, but in the article we see that in the test they made in one moment the HDOP value jumped to more than 90, and so the error increased to about 24 meter. So even with receivers that use high precision accuracy (in this example PPP), with accuracy sometimes of just few centimeter, we can have big errors when there are few satellites in view (in the case of this article in urban environment).

    “Fig. 8 and 9 show the HDOP and the horizontal
    components of the position error for the kinematic test when
    GPS L1 and L5 and Galileo E1 and E5a, linearly combined
    into the IF combination, are processed in kinematic PPP mode
    with the POINT software. It is possible to see how the
    horizontal error is at centimeter level when, at the beginning of
    the kinematic test, the HDOP is well below 5. 33 minutes after
    the beginning of the simulation, a few satellites go behind the
    buildings, they are out of sight and the HDOP rises to more
    than 30. At this point the errors are as large as 1.63 m. After the
    receiver has lost track of these satellites, the horizontal solution
    does not converge to centimeter level. However the errors go
    below 50 cm after four minutes. Towards the end of the
    simulation, the HDOP jumps to more than 90, with the east
    error increasing to about 24 m”

    Sport watches now don’t use PPP technology, but in the same way they can have big errors due to the DOP stuff (few satellites in view for buildings or mountains). I think that we often don’t realise there are these big errors because the algorithm of the sport watches smooth these errors with the aid of the accelerometer inside.

  5. Looking at available data adding GLONASS to GPS will usually at least halve my HDOP/VDOP values so that it’s between 1 and 2, in reality turning it on seems to trigger some sort of offset and Suunto recommend only using it in urban environments (some tweaks in the algorithm maybe), so how the sources are integrated is a big issue even if they are available

  6. > Satellite level – some satellites transmit dual frequency…Dual frequencies will help both atmospheric refraction and multipath errors.

    The first smartphone in the world to leverage a dual band GNSS/GPS chipset was released in June 2018. Its use in smartphones is still (as of this writing) pretty new. I suspect it’s still not done yet in sport watch form factor devices. That’s not to say that it shouldn’t be, and it probably WILL be if demand is created for that feature.

    The good news: Dual band GNSS t can be used immediately. GPS, GLONASS, and Beidou all have (the secondary) lower frequency bands in their current operational satellite specifications.

    The bad: Your device must explicitly support it, which means it must be sold with a dual band GNSS receiver. The list of devices that does is small – but growing – the closest I’ve found to a comprehensive list at this point, for smartphones, is https://www.gsmarena.com/results.php3?sFreeText=Yes,%20with%20dual-band

  7. “…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…”

    Actually, there *is* science behind this. It takes 12.5 minutes to collect a full GPS Almanac.
    With a complete Almanac, Ephemeris lock after disruption is on the order of 6 to 30 seconds.
    It is not unusual to lose Ephemeris on one or more satellites under challenging conditions, and even the loss of just one satellite’s Ephemeris Signal will cause DOP degradation.

    • hi
      thank you. I sort of knew that in eneral (thank you for the precision of 12.5 minutes) but I also was told that after a PC sync the a-gps stuff is all up to date and so such a soak was not needed.
      anyway…”it can’t hurt!”

  8. Hi,

    I am doing some measurements of DOP values with garmin device and I am getting output of DOP values in VisualGPSView program. HDOP and VDOP are mostly between 1 and 2 and they are always excact same, for example if HDOP is 1,7 VDOP is also. I found that VDOP should be grater than HDOP so don’t why are the same. Also PDOP value is always 0.

    So I don’t know should be that way or shouldn’t, I am kind of confused .
    Does anyone know something about that?

      • thank you for reply

        I’ve tried with different GPS program and got the same sort of results (if that’s what you mean), I’ve also tried with different locations but thoose two values are allways excact same and PDOP is 0, don’t get it why ?
        Device (garmin) is kind of old and it’s working on it’s latest possible software version.

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