GPS LAP TIMERS (Distorted Results)

[Rotary]

Ionospheric effects are one source of error, but it's usually averaged out by the receiver. It was more likely a combination of some or all of GDOP (geometric dilution of position), multipath effects and interference. It doesn't take much to throw it out by 10 or so metres - which is still very accurate for a relatively cheap receiver.

At Wakefield Park, I would say Multipath effects can be eliminated as a source of distortion, as it’s a mostly flat terrain which rises slightly at the back end, with only one small building near the main straight and no trees or other large object to reflect off causing multipath distortion.

In contrast Eastern Creek is full of objects for signals to reflect of, with large grandstands, long pits with viewing rooms above, many trees and walls close to the track massive dirt mounds close to track, and at one place the track runs down, then alongside a large hill. (worked well at EC)

GDOP or Geometric Dilution of precision, this can be worked out using mathematical equations, that as Caff said, tells us how good or bad a sat fix is (only 4 sats are required)
if this value is high, then the error reading of those 4 sats will be high, if GDOP is low then the error will be low.
But if there are more sats available, we can calculate the GDOP for the other sats and then for each and every combination of sats, then choose the best combination of satelites that would give the most accurate positioning.
Seems to me that all GPS devices would be designed this way in order to be as accurate as possible, and they seem to be.
So an error from this would only come if the sat fix is low(4 maybe 5) and the sats are too close in orbit, if there are more sats to choose from then this error will be mostly eliminated or reduced within the claimed accuracies
(anything I may be missing?)

I have tested the accuracy of the Qstarz unit together with a Gtech Pro unit(10hz Data Sampling), and they correspond within 1% of each other in both Longitudinal and Lateral G's, I was surprised that the difference would be so small from 2 completely different devices and with the QStarz only operating at 5Hz.
Gives me confidence that the Qstarz GPS unit is as accurate as claimed, similar in claimed accuracy as PBox(positioning wise) but PBox does sample at 10Hz.

When closely examining lines taken on a track, I would say its accurate to about 2m most of the time

So contrary to this thread I do highly recommend it,

[JBT]

No, you're not really missing anything except if all these small errors occur at the same time you can have discrepancies. It's also a case of "which 8 satellites?". Take the extreme situation that they are all at a small angular separation and low on the horizon, the position fix accuracy will be pretty ordinary. Even if the angular separation is small and they are higher the accuracy will be below average. It will only happen for relatively short periods of time, but they could be just at the time you are reading positions.

[Rotary]

cheers, I think I’m starting to understand the system a bit better now(still much to read up on), I never took the time to look into it until now, so in a way its good the problem came up, got me thinking so appreciate all the responses.

I think I may do a few long running tests, to see if comes up, how often etc. also read more into the hardware and softwares role to better understand where a weakness may lie.