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GPS Errors


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Just Back from my 3km morning walk.

Always carry my Garmin Etrek with me to monitor pace & occupy my mind.

The first km is usually averaged out at 5.6km/h & by the end of the 3km the

average is usually 6.0km/h.


Added an extra block at the end this morning, so total distance I was expecting was

about 3.2 to 3.3km, I was pretty much on target for this figure as I approached my back gate

but when I looked again moments later my total distance had jumped to over 3.8km & average

was now 7.0km/h.


I've done this for years & know about GPS errors, but it instantly got me wondering if this at

all related to some of the Perfect Pass issues we see.


Have now only had 3 weekends on the water this season & PP performed spot on during the first

two outings.


On the third, we experienced 2 instances of the PP losing it during a run.

Giving I've just seen this 500m error It's got me thinking that a similar GPS

error whilst PP is in control could be responsible for some of the random problems

people see.






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I have gotten weird readings from my Gps putting me on a near by road when I am on a trail. That usually gives me extra distance because it jumps back and forth trying to figure out where I am at,


Measure it on google earth to get an accurate measurement.


I imagine that they do lose a satellite occasionally and that messes with the timing


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  • Baller

My aircraft gps specifies 15m accuracy. Nowhere near good enough for waterskiing precision. I was very sceptical about the ZO claims. Stan's MC had ZO and PP with magnet timing. Timing the boat with both systems on gave times that matched well with .01 discrepancies. ZO is accurate.

How can this be? Often ZO will beep early or late by a noticeable amount. But that error is consistent - entry gates are as off the same direction and magnitude as exit gates. Whatever dithers the signal does not change over the small distance or time interval of the course. Trust the ZO.

Drivers need to carefully monitor ZO. My aircraft gps lost signal - once. ZO will screw up one or two rides a tournament, but driver technique causes many of these. The gps error warning on ZO is not aggressive enough - a serious deficiency. Fortunately ZO is so reliable that it is not a real problem.

ZO rocks!



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No weather anomalies, so think I could rule that out,

wouldn't blame reflections or multi-pathing errors either as

I've walked this route 100's of time over recent years.

so could only be somthing atmospheric, solar or maybe a bit

of rogue data from a sat.



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  • Baller

Eric, this was expalined to me by some one much smarter then me. There are two GPS signals sent from the satellites. The most commonly used is the positioning one to find where you are on the earth and you are correct it's not very accurate although I was told it can be within three feet. The second is a time/distance based signal which we use to control the actual speed. It's accurate but dosen't know where it is, just how far it has gone from the setpoint.

The reason the beep is not always in the same spot is that the speed control uses the positioning signal to locate the course, and it is only accurate to at best the three feet mentioned above.

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Nothing like that.

Watched closer this morning, & saw average jump to 6.7km/h in

the first 50m.


After that I watched the moving speed a fair bit.

Set a steady pace that flicked between 5.7 & 5.8km/h & noticed

every few minutes that it would jump to numbers like 6.1 to 6.6km/h.


Unit has 1 second updates by the way, & when seeing these deviations, which typically lasted 2 or 3 seconds, I could not identify any significant environmental changes or significant structures that could expain the variations.


Might dig this out again & have a bit of a play, haven't used it in nearly 10 years, but did find it a bit interesting at the time.









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Interesting on Sunday. Two non engaging runs. Wasn't driving so not sure what was happening, but...

On the 25km drive back to town I could not get my TomTom SatNav to get a fix despite turning it on & off several times.


Interestingly a few hours later the TomTom grabbed Satellites straight away when I turned it on again.


Maybe there's some rogue data coming from one of the satellites ATM & causing issues when it's in view.


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My GPS had been spot on the last few mornings, total distance always about 3.12km,

until this morning when it jumped twice. About 500m each time, to give a total distance of

over 4 km.


East Tx, I guees you do realise that that's not just the antenna, but the complete GPS unit, so any thing could have been possible as far as faults are concerned.





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