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Test Accuracy of GPS


KayakGZ

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My experience about the lack of bias matches yours. It would be tough for the receiver to introduce it.

 

Any receiver clock error that could introduce a position error in a given moment should be largely self-corrected when, looking at signals from all available satellites, the disagreement in fix solutions that demonstrate a difference in frequency of the GPSr clock and satellite clocks can be used to make a correction to the timing of the receiver clock, making the quality of the fix better. So there should be no positional bias being introduced by that one key element of the receiver, and the math really supports a wider EPE for that element anyway, not an 'offset' type bias. As far as I've ever been able to figure it, you'd really need a really, really strange set of multipath circumstances or some bad code to introduce 'bias' of the sort we're discussing, so that's why I'm comfortable ignoring it as a variable. There's no reason why, over time, the distribution you describe shouldn't occur as a matter of course around the 'reference point'. It's for that reason that I find the previous discussion of precision vs. accuracy a bit different than taken by some others in this particular application.

 

What fascinates me are the rapidly changing short term bias issues that seem to crop up in the field. They're what can make placing and finding a cache based upon the coordinates a challenge at times, and are why it's always good to get some kind of statistical coordinate average when placing them. You can spend 10 minutes being pointed 15' to one side of a point on the ground, and in a matter of moments, be pointed 15' in the opposite direction to the other side. Very often, it's not the kind of gradual drift you might expect if it was just the movement of the constellation; it's a pretty marked jump. Ideas?

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What fascinates me are the rapidly changing short term bias issues that seem to crop up in the field. They're what can make placing and finding a cache based upon the coordinates a challenge at times, and are why it's always good to get some kind of statistical coordinate average when placing them. You can spend 10 minutes being pointed 15' to one side of a point on the ground, and in a matter of moments, be pointed 15' in the opposite direction to the other side. Very often, it's not the kind of gradual drift you might expect if it was just the movement of the constellation; it's a pretty marked jump. Ideas?

Does the word, pointed, imply that the device compass needle is being used as the primary reference? Isn't the compass needle direction derived from manipulation of data? If so, might a continuous track log be used for further contemplation?

 

My attention span is very short and is limited to perhaps one or two open field ground zero tests per firmware version release on the Delorme PN-40/60 gps devices over the years. My conclusions regarding the Garmin handheld gps device compass needle pointing within 60 feet of ground zero is derived only from reading posts on these forum threads. It does, however, seem that there may be differences in programming logic used by those two brands, and may also vary by firmware version.

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What fascinates me are the rapidly changing short term bias issues that seem to crop up in the field. They're what can make placing and finding a cache based upon the coordinates a challenge at times, and are why it's always good to get some kind of statistical coordinate average when placing them. You can spend 10 minutes being pointed 15' to one side of a point on the ground, and in a matter of moments, be pointed 15' in the opposite direction to the other side. Very often, it's not the kind of gradual drift you might expect if it was just the movement of the constellation; it's a pretty marked jump. Ideas?

Does the word, pointed, imply that the device compass needle is being used as the primary reference? Isn't the compass needle direction derived from manipulation of data? If so, might a continuous track log be used for further contemplation?

My bad choice of words .. Although there may be a 'needle' that corresponds to what I'm describing, what I mean is that the physical position that provides the lowest estimated distance to the target (a minimum of 0 feet, though rarely that low) can move as I described. Sometimes, it can make such a move in rather short order, and I've always wondered what sometimes makes that time so brief. MOST of the time, the wander is a bit slower, but not always. This can be seen by setting a unit up in 1 second track mode in a stationary position (clear sky, no obstructions, including no obstruction by the observer!) and observing a plot of the motion of the track vs. time. Most of the time the drift is relatively slow and easy, and sometimes it can get pretty jumpy. The latter can cause a pretty good 'bee dance' for a novice.
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