Satelite precision (altitude, distance, 3d?)

I've been looking at some performance tests and the GPS and OBD2 data is not identical, and can differ quite a bit.

The GPS seems to be "faster" by almost a second at high speed tests, while the OBD2 is faster on everything from standing start (understandable) but also 80-120km/h.

So to get my head around this I started looking at altitudes, exact speed correlation between OBD2 and GPS and more.

My question is (finally!), what are these "DOP" units used for satelite precision? Is a lower figure better, and what can be considered "good" for accurate testing?

Also what's "3d precision"? I can understand coordinate and altitude, is 3d some kind of combination of these?

Is the number of satelites important at all or are these DOP values all that matters for precision?


  • The smaller the DOP number, the better.

    Your OBD2 speed readings can be affected by factors such as incorrect tire size.
  • Ok, so lower is better. But what's "good" and what's not? I have calibrated OBD2 speed with GPS speed and adjusted for that already, the inaccuracies seem to stem mostly from GPS being plain unreliable.
  • edited August 2010
    Actually I can't answer what is good DOP and what is bad... For me anything less than 0.50 has been all right. I guess you could calculate the precision in meters from the values, but I don't know how. Does anyone know the the theory of DOPs?
  • Hmm, my DOPs have been around 0.8-0.9, so maybe that's why the GPS is way off the OBD2... this is with 8-9 satelites btw if I remember correctly.

    A well calibrated OBD2 speed (compared with constants speeds and GPS) should still be reliable though, and it's the GPS logs I should be critical of (in my case, not use due to inaccuracy), right?

    That means GPS measurements are very apt for inaccuracies, and that you really need to check the conditions of your GPS signals for any performance test using GPS to be of any interest.
  • The DOP value seems to vary from GPS to GPS, so I'm not sure if it means anything that you're seeing little bit higher values.
  • OBD2 and GPS times can vary between 0,5 and 1 second though, and the variance with OBD2 should be minimal so I reckon it's the GPS that is off. I have seen irregularities in the GPS data as well, eg. a sudden jump in speed somewhere that takes nearly a second off a performance test.
  • gives an indication about DOP values and their meaning. The German article also has a good schematic to visualize the idea behind DOP. Due to the definition of DOP, <1 isn't possible. If you say everything below 0.5 seems good, maybe your numbers are of by a power? My gps receiver, which is very old, has gotten me numbers from 1.3 up to over 20 within only a few seconds.
  • You just proved I don't know anything about the subject :) I need to look my DOP numbers again to verify, but could be my GPS is giving some bogus numbers...
  • edited October 2010
    Looking at an old datalog I can see that the DOP values for coordinates and altitude are at 0.780 and 1.460 when the number of satelites is 10. When the number drops to 8, my DOP values 0.890 and 1,640.

    They go up when the number of satelites go down, which seems right, but I constantly have below 1 on coordinate precision.

    They could well be off by a power like obstacle says, 7,8 and 8,9 for coordinates seem to be a fair accuracy, considering it's no high grade GPS (Globaltop G66 5Hz). The values are stable as long as the number of satelites is constant. Maybe I should try sticking the GPS *outside* the car on the roof with Velcro or something and see what the precision is like :-)

    I wonder if 8-9 in DOP is good enough for performance tests, where you want 0,1s accuracy. It seems doubtful considering my results.

    EDIT: Oh, I never got an answer regarding "3d" precision. Is that just coordinate and altitude tacked into a single value?
  • Hm, do you mean PDOP? It's calculated from VDOP and HDOP.
  • No it's simply called "3D precision" in RaceChrono.

    I've got "Coordinate precision", "Altitude precision" and "3D precision" too choose from when selecting graphs to view in RaceChrono. I thought these were RaceChrono labels, are they from my GPS?
  • edited October 2010
    Yep, so "coordinate precision" is HDOP, "altitude precision is VDOP and the 3D precision is PDOP. These are RaceChrono labels, I did not remember I renamed them :)
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