Knt,s and GPS

[pcv]

How accurate are hand held GPS units for recoarding boats speed.
First spin round start bay on Saturday after winter refit, plus play with new toy ie GPS unit and was just wounderring how accurate would you say thy are.
Thanks

Pete

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[tcm]

they are very very accurate - to within 5metres. Or, they don't work at all. Much more accurate than car speedo which has to over-read by law.

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[AndrewB]

As TCM says, they are quite accurate - with regard to your speed over the Earth's surface. However, they don't measure your speed through the water, which can be different if there is any current. For all practical purposes, all types of GPS are equally accurate at measuring speed.

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[Talbot]

Think I wish to disagree with your statement about them all being as acurate at measuring speed. It is a known fact that the accuracy of the GPS is greatly dependant on the aerial. A fixed system will normally perform better than the the hand held. This is due to a combination of greater mathmatical capability in the system, and the better aerial. Not really a problem concerning the position, but more likely to be errors over predicted path and speed. The more satellites they can process simultaneously, the better, as that tends to reduce the errors.
Of course if you wait until the tidal current is stationary, then the read out from the GPS will give a good indication of speed through the water.

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[AndrewB]

You may well be right that there is a difference, but even a basic modern handheld GPS like a Garmin Etrex has a manufacturers claimed accuracy of 0.05m/s at yacht speeds, just the same as a typical small fixed unit. This is within 0.1 knots, negligible for most practical purposes.

 

[john_morris_uk]

The accuracy of the speed calculation depends on several factors.

Whether the boat is on a steady course (including sea state, yawing induced by wave action etc and this also why manufacturers ask you not to mount the GPS antenna on the masthead.)

Whether the filter setting of the GPS - ie the time the speed is calucated over - is short or long, and how long you have been on a particular course - if the time used is long, you obviously need to be on a steady course for that length of time for the speed to be calculated correctly.

Finally, the HDOP or accuracy of the fix will influence speed calculations, with a WAAS/EGNOS enabled system being more accurate than a non WAAS etc, but not quite as accurate as a full differential set. One of the main reasons some racing boats have WAAS/EGNOS enabled GPS's is that they get more accurate SOG and COG calculated more quickly.

The fact that resolution to 0.1 knots is available is no guarentee of speed being calculated to that accuracy. If you read the small print most manufacturers allow for the points I raise above. If they don't they are being economical with the truth.


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