How do I know my GPS is accurate?

Is there any way to check? I got 26.8kts at the end of a session when I was tired doing 1 run on a borrowed GPS last year.. Can't get over 26.2 in ideal conditions this year.. Hoping mine is underreading..

The only way to make sure it's accurate is to have two or even three GPSes. However the one is probably accurate. It takes a change in sailing style to get above 26-27 knots. As John says this is about the speed you get for back and forth sailing. Everyone thinks they are going 30+ when planing before getting a GPS. However this is often not the case.

Just be thankful you can get out when it's windy. Meanwhile I was stuck at bloody work all day and today is windless. At least it's not raining.

For speed readings, if it has good satellite coverage it has to be accurate. It's not measuring your speed like a speedo. It doesn't need calibration. It's doing calculations based on the timing signals from the satellites and the higher end ones do tricky stuff counting the wave length of the signal. If the base data is correct then the calculations will be correct.

If you have cloud cover and trees and buildings and solar flares and stuff then the signals will be dodgy and you can get bad calculations. Most GPS identify the bad calculations and leave them out. In the old days our speed would jump from 30kph to 300kph and back again. Most of that would not be an issue when sailing on the sea on a nice windy day.

Higher end GPS measure the quality of the signals received and store it with each reading. I doubt if yours would do that.

Sue, was the one you borrowed a "GT"?
It's possible the one you borrowed was reading a bit high if it was only a "cheapy"
I'm not sure if all displays use doppler, trackpoint calculation can easily be 1 to 2 knots high, especially if the satellite signal is a bit iffy.

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Gorgo said..

For speed readings, if it has good satellite coverage it has to be accurate. It's not measuring your speed like a speedo. It doesn't need calibration. It's doing calculations based on the timing signals from the satellites and the higher end ones do tricky stuff counting the wave length of the signal. If the base data is correct then the calculations will be correct.

If you have cloud cover and trees and buildings and solar flares and stuff then the signals will be dodgy and you can get bad calculations. Most GPS identify the bad calculations and leave them out. In the old days our speed would jump from 30kph to 300kph and back again. Most of that would not be an issue when sailing on the sea on a nice windy day.

Higher end GPS measure the quality of the signals received and store it with each reading. I doubt if yours would do that.

Most of the info here is incorrect for our use case (assuming GT-11/31 and to a lesser extent Garmin's):
- while it isn't like a speedo, it does need calibration of the various algorithm coefficients (as you said, they are calculations - different manufacturers blah blah).
- some of these manufacturer design choices, very much affect the usefulness for windsurfing (aka calculations wont be correct), which is why the GT-31 is recommended.
- Assuming we are talking about "doppler", no timing signals are used for calculating speed... it does indeed calculate wavelength of the signal (sort of... you could call a digitial-PLL as doing that)
- while cloud cover does affect the signal, it doesn't do so significantly compared to other sources of errors (eg: the mass of the earth)
- the GT-11/31 (and some Garmin's) do store the quality of each reading, this is known as dilution of precision (DOP), eg: horizontal/vertical dilution

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sboardcrazy said..
I got 26.8kts at the end of a session when I was tired doing 1 run on a borrowed GPS last year..

Are you certain that the borrowed GPS was displaying your speed in knots? Maybe it was set to show kilometres per hour.

Or just accept Sue that this is your top speed SO FAR - and work on what you will do to go faster - and that's not about posting a million times on seabreeze - or buying new kit - it's about what goes on in your head and what you can translate onto the water

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decrepit said..

Ben 555 said..

Or just accept Sue that this is your top speed SO FAR - and work on what you will do to go faster - and that's not about posting a million times on seabreeze - or buying new kit - it's about what goes on in your head and what you can translate onto the water

Or just go to Lake George next Jan-March
I've found that very flat water is the secret to speed, especially for lighter sailors.

Except that it might be very crowded there next year

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kato said..

decrepit said..

Ben 555 said..

Or just accept Sue that this is your top speed SO FAR - and work on what you will do to go faster - and that's not about posting a million times on seabreeze - or buying new kit - it's about what goes on in your head and what you can translate onto the water

Or just go to Lake George next Jan-March
I've found that very flat water is the secret to speed, especially for lighter sailors.

Except that it might be very crowded there next year

Hey guys Lake george is filling up very fast as have had big sea since the channel has been opened and has been raining heaps here so fresh water from inland is flowing in as well. will do an update sometime with a few pics.

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Ben 555 said..

Or just accept Sue that this is your top speed SO FAR - and work on what you will do to go faster - and that's not about posting a million times on seabreeze - or buying new kit - it's about what goes on in your head and what you can translate onto the water

GOLD!!!!

Hi all! How do I attach a gpx file and have it checked for the true 100mtr. dash on land not on a board? am trying to prove a friend wrong as he told me I can't run 18km/hr cuz so can't he. Let me know pls. anyone there's a buzz on our forum when I posted a 24max run and 23kph 100 mtr. dash ..just want to prove a point even when am turning 49yrs this 20th .. It would be nice if you people can analyze the track. I tried tracking the 'loose' points but don't seem to know how. Reply and I will send you the track immediately. Need to keep these 'critters' shut..

I know it's the wrong thread for running but there's no wind right now so just short distance running for now..Regards, JP