True wind from GPS?

[Welti]

I got aquestion in a quiz, basically asking if one had proper calibration and sufficient sea trials, if one could expect to get a reasonably accurate true wind from a GPS? I thought it would be "no" since the GPS only knows SOG, but I got it wrong. (?) What am I missing?

[Jim Cate]

Umm... the GPS has zero knowledge of wind of any sort, so on its own (that is, without an input of apparent wind) it surely can not output true wind.

Jim

[Welti]

Apparently the folks over at Nauticed know something we don't (?) Or maybe it is just a badly framed question. They may be assuming we have additional sensors on the boat, to get speed in water?

[El Pinguino]

I think the question is 'what is the quiz master missing' - which is a wind instrument and a speed log.
Anybody wanting to open a book on how many posts this question generates. I reckon on the high side of 100.

[fxykty]

Someone will define all the different ‘true’ winds and why one is better than the others. Yawn.

If you’re mostly in areas with minimal current flows on average and on a boat that doesn’t make much leeway then a ground true wind generated by GPS (using COG and SOG) will be close enough. That goes double for a boat that doesn’t have a well calibrated boat speed sensor. And triple for a boat that moves really fast.

If you do have a well calibrated boat speed sensor then you can use your boat’s heading and speed to calculate a boat true wind. This is particularly useful for calculating exact tacking and gybing angles and for sailing to polar targets.

But otherwise, from a comfort and safety point of view, we want (some, any) true wind to inform us off the actual strength of wind so we know more accurately what to expect when we harden up or bear away at our next waypoint. Or whether we should reef first before turning more upwind. Whether it’s with respect to ground or water doesn’t really matter.

[Pizzazz]

The question most likely was targeting the difference between true wind (adjusts measured wind by boat speed through water) and ground wind (...by boat speed relative to ground). If we need to answer the actual question though:

1. GPS gives you course heading and speed
2. The boat sailing characteristics relative to the wind direction are known

As long as you can measure the angle of the boom to the heading of the boat, with sufficient trials, yes, I think you should be able to get reasonable wind speed and direction.

[Welti]

OK, so we need two things to get true wind: apparent wind and boats motion relative to water (which wind is in reference to). If I know the apparent wind, through another sensor, I can maybe calculate leeway, using boat direction and velocity from GPS, and apparent wind plus an equation, and/or maybe plus some type of polar data for the boat. BUT the velocity that GPS gives is course over ground, not relative to water, which leeway is relative to. That is what is throwing me off.

[El Pinguino]

Originally Posted by Pizzazz

The question most likely was targeting the difference between true wind (adjusts measured wind by boat speed through water) and ground wind (...by boat speed relative to ground). If we need to answer the actual question though:

1. GPS gives you course heading and speed
2. The boat sailing characteristics relative to the wind direction are known

As long as you can measure the angle of the boom to the heading of the boat, with sufficient trials, yes, I think you should be able to get reasonable wind speed and direction.

GPS does not give you heading.

[noelex 77]

For a cruising boat switching to ground wind (together with apparent wind) using the SOG input from the GPS rather than the STW input from the paddle wheel is worth trying.

The main advantage lies in removing the often inaccurate and poorly calibrated paddlewheel information and replacing this with the more accurate and always in calibration GPS input.

Ground wind and true wind are slightly different when significant current is present. Ground wind is the actual real windspeed and direction that would be felt when stationary. It is the wind you feel would at anchor or when when tied to the dock. In an ideal world it would match the forecast wind.

[cr180]

We are over thinking this problem. The instrument vendors are simplistically taking incremental GPS data over time to calculate both heading and boat speed (yes it is speed over ground, leeway is ignored for simplicity). Then the heading and boat speed are used to calculate true wind from apparent wind measured by the sensor.

[wingssail]

Originally Posted by Welti

OK, so we need two things to get true wind: apparent wind and boats motion relative to water (which wind is in reference to). If I know the apparent wind, through another sensor, I can maybe calculate leeway, using boat direction and velocity from GPS, and apparent wind plus an equation, and/or maybe plus some type of polar data for the boat. BUT the velocity that GPS gives is course over ground, not relative to water, which leeway is relative to. That is what is throwing me off.

To calculate true wind you need apparent wind (speed and angle, so a wind sensor), and boat speed through the water. You can get boat speed from GPS if you discount current. In low current places it may suffice to give you a general reading.

A good water speed sensor connected to the wind speed sensor (through a instrument system) will give more accurate true wind readings. Instruments which can calculate leeway will additionally improve it.

NOW, That's true wind speed and direction relative to the boat. If you want true wind relative to the compass, sometimes called magnetic wind direction, You'll need a compass sensor in the system. GPS systems rarely give a boat's heading, only it's course over ground.

[s/v Jedi]

Apparent wind + speed through water = true wind

Apparent wind + speed over ground (GPS) = surface wind

[donradcliffe]

A highly calibrated system will not give you "true" wind speed, it will give you true wind speed over the water you are in. That is actually more useful than "true" wind speed when you are making decisions on sail selection or reefing.

Using GPS SOG instead of speed through the water will give you less accuracy, but in my experience it is still way better than a WAG. This is true when racing, as the wind may have picked up a bit on the downwind leg, and its time to call for the #1 or #3 jib at the leeward mark. It is more true for cats, when you are looking at going through the "cone of death".

Bottom line, if I was programming a MFD I would give the user the option of selecting SOG for the true wind functions.

[s/v Jedi]

Originally Posted by donradcliffe

A highly calibrated system will not give you "true" wind speed, it will give you true wind speed over the water you are in. That is actually more useful than "true" wind speed when you are making decisions on sail selection or reefing.

Using GPS SOG instead of speed through the water will give you less accuracy, but in my experience it is still way better than a WAG. This is true when racing, as the wind may have picked up a bit on the downwind leg, and its time to call for the #1 or #3 jib at the leeward mark. It is more true for cats, when you are looking at going through the "cone of death".

Bottom line, if I was programming a MFD I would give the user the option of selecting SOG for the true wind functions.

When you round the mark, you will be exposed to wind over water, not wind over ground. When you sail downwind and wind seems calm, it is the apparent wind you feel, the true wind display will show the wind you get when your stw equals zero.

[Dockhead]

Originally Posted by noelex 77

. . . Ground wind and true wind are slightly different when significant current is present. . .

I would disagree with the word "slightly". Ground wind and true may be nearly 180 degrees different from each other in a strong current and at low boat speed.

Originally Posted by noelex 77

. . Ground wind is the actual real windspeed and direction that would be felt when stationary. It is the wind you feel would at anchor or when when tied to the dock. In an ideal world it would match the forecast wind.

. .. .


When stationary with regard to the ground, not with regard to the water, and that's the problem. Sailing at 6 knots against a 6 knot current you are stationary with regard to the ground, but you are making 6 knots through the water. Ground wind is completely useless for sailing in this situation. Or even 6 knots against 2 knots of current, or with the current.


It is true that valid STW data is really really hard to get, and without valid STW data, your true wind calculations are useless. That's why many sailors give up and use SOG. But with any significant currents, the true wind you get using SOG is pretty much useless too.