Gps going at different speeds!!

[janda]

I was comparing the readouts of my Gps.es, a Raytheon 198, Garmin 120xl, and Magellan Sportrak while on passage last year. I was surprised that the 198 was 6 seconds adrift in the time and the Magellan insisted it was going a knot faster then the other two! Any one else noticed anything like this?

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[steve28]

I did see a big diffrence in speed on my new raynav 300 gps, only at low speeds. After telling Raymarine about this they agreed this was a fault in the gps firmware and they corrected it with an update.
Maybe this is the case with your senario ?

steve

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[mldpt]

is it possible the odd one out is set to MPH
Mike

<hr width=100% size=1>www.sailingforfun.co.uk

 

[omega2]

Make sure they are all set to the same measurement ie, mph, kilometers, or knots, but even then if you get them all to read the same let me know cos my LMS 350A, reads different to the Koden radar even though the radar is being fed by the 350A, and Raytheon 620 with a 120 lpt standing alone is out with those two, and the VDO log is out with all of them. The only way is a stop watch and a fixed measured distance and of course the ability to steer straight. Why worry if you get to the place you are aiming at.

<hr width=100% size=1>J HAMER

 

[Talbot]

I would be very surprised if your VDO log was reading the same as the GPS unless you were at 0 tidal current, as the log reads speed through the water, and the GPS - speed over the ground!!!!!!!

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[Boathook]

Need to make sure that they are taking the speed over the same time period such as 4 seconds or even 60 seconds. This can make a hell of a difference.

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[Omatako]

I think you're talking about refresh rate. Most GPS units have a refresh rate setting which provides the display with the average over the refresh period. So if you refresh every minute, the display will show the average speed over the minute and so on. If one is set to refresh every minute and the other every 10 minutes, the average speeds will be different and thus the results on your display. I suspect this may be your problem.

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[bob_tyler]

Re: Gps giving at different positions!!

My chart plotter is "installed" in my house for the winter.

Whereas the Navman Chart plotter shows my position as:

50 09 29 N
5 05 16 W

My handheld Magellan shows, at the same time alongside it:

50 09 16 N
5 05 08 W

Both are set to WGS84.

Can anyone explain the constant dirrerence please? (In simple terms if possible)

The Chart Plotter show the position of my boat very precisely on the Charts when installed aboard.

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[AndCur]

Re: Gps giving at different positions!!

I had a samilar problem last season with my GPS and repeater it was caused by the GPS reading Magnetic and the repeater reading true. I was able to set the GPS to read true which solved the problem.

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[duncan]

Re: Gps giving at different positions!!

the plotter is displaying decimals and the handheld minutes possibly?

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[Solitaire]

Re: Gps giving at different positions!!

It's fairly simple! No two GPS's will recieve the signal at the same time and therefore a certain degree of error is inbuilt.If you were to sit looking at even just your one GPS in a stationary position the figures would be changing - not by much but they will change. If you then plotted thoses changes you would get a series of random fixes all close together. There are names for these errors and the bigest influencing one is Receiver Clock Error. Others are atmospeheric delays in the signal, multipath errors and elctronic noise. Mathmatically it is known as the Dilution of Precision.

GPS is accurate to within about 10 metres, sometimes even smaller than that but never spot on. The figures you quoted are perfectly within the "level of confidence" for the system and if you work out exactly the difference in metres - it is so small as to make no real difference. Perhaps a reason however for not putting waypoints in that are too close to a hazard or buoy! Also look at the this link -
Boating is <A target="_blank" HREF=http://www.powerboat-training.co.uk>Serious Fun

 

[mhouse]

Re: Gps giving at different positions!!

Are they both set up for the same altitude?

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