True wind from GPS?

[wingssail]

Originally Posted by JimJohnston

In answer to the initial question "if one could expect to get a reasonably accurate true wind from a GPS?", the answer is 'yes' if one is using a Raymarine Axiom multi function display with a connection to the internet and a subscription to Windy.com, both of which are supported by the Axiom.

Alegedly the Axiom MFD will allow a user to provide an overlay screen from a connected website, like Windy.com which provides real time wind conditions with a fairly reasonable resolution. So the 'GPS' display will provide 'accurate true wind'. I haven't tried it yet but it's in the plan.

Interesting. I was not aware that Windy provides current wind direction and speed. Mostly Windy is a forecasting site, right?

Then there is the issue of resolution and locality.

I am skeptical that one could display with any accuracy True or Ground Wind information using Windy as the sensor.

[OneBoatman]

Happ When discussing the True speed of anything your reference is always measured as over the ground. Otherwise we call it relative by convention.
Wingsail Although the OPer may be a bit confused the question is whether GPS can help you determine True wind. The simple answer is that on some models you can use the GPS's display as the readout if your vessel also has a relative wind indicator. The paddlewheels used to measure the output of a relative indicator on a sailboat are driven by the wind not by the water. All modern GPSs can give you speed over the ground without the need for a speed log but most marine GPSs have algorythms allowing the user to select and display both speed over the ground ie True speed and may be equipped with a separate transducer to provide speed through the water for a captain seeking the effects of current. The waterborne paddles do tend to foul quite easily as you suggest.

[wingssail]

Originally Posted by OneBoatman

Happ When discussing the True speed of anything your reference is always measured as over the ground. Otherwise we call it relative by convention.
Wingsail Although the OPer may be a bit confused the question is whether GPS can help you determine True wind. The simple answer is that on some models you can use the GPS's display as the readout if your vessel also has a relative wind indicator. The paddlewheels used to measure the output of a relative indicator on a sailboat are driven by the wind not by the water. All modern GPSs can give you speed over the ground without the need for a speed log but most marine GPSs have algorythms allowing the user to select and display both speed over the ground ie True speed and may be equipped with a separate transducer to provide speed through the water for a captain seeking the effects of current. The waterborne paddles do tend to foul quite easily as you suggest.

Oneboatman, there are some different points of view about what is "True" wind speed and direction. The convention I adhere to are "True Wind" and "Ground Wind". Others, often flyers, might say those two terms are the same thing.

What I care about is the wind speed and direction felt by the boat and this of course is affected by current. I call this "True Wind" and this is the terminology used by my instrument systems.

GPS can help you determine current and this can be used to determine "Ground Wind" but I am usually only mildly interested in this.

[noelex 77]

I think the only correct answer to the question is indeed “no”.

GPS input can only be used for “ground wind”.

The only exception would be if the question was phrased along the lines of “is it possible to obtain accurate true wind in some circumstances using the SOG input from the GPS”. The answer is “yes” (when there is zero current).

Many sailors are confused by “true wind”. It does not help that the land based and meteorological definition of “true wind” is different to the nautical definition of “true wind”.

This anomaly came about because before GPS sailors had no means of measuring the land based definition of “true wind” at least when offshore and no one imagined that the capability would ever be possible.

They invented their own definition of “true wind” that did not eliminate the effect of all motion. With the advent of satellite position fixing it is now easy to calculate the more sensible land based definition of “true wind”, but as sailors already had their own alternative definition of “true wind” a new term was needed. Hence we have the rather confusing “true wind” and “ground wind”.

“Ground wind” is actually how most sailors would (incorrectly) define true wind. This is likely why the official quiz answer is wrong.

[IslandHopper]

Originally Posted by Sailor_Jeff

Yes, uper end expensive GPS units can. Mine does. The GPS gives you speed and course over "ground" (or water) "IF" your gps has NMEA inputs for wind speed and direction sensors it's computer can calculate true wind speed by subtracting wind sensor data from speed data and displaying it. My ship does that. I have ultrasonic sensors for wind speed and direction. It's all math at that point.

but it's an expensive GPS and sensors plus integration costs are high, day or "trailer-sailors" can't usually afford it.

Jeff

And the make and model # is?

[OneBoatman]

So the real question is what's the question? GPSs, the basic send and receive satellite system which replaced Loran, the prior ground based system gives us a true position and the change of position with time along with some memory and tracking features. Because it is a position system it alone won't provide information about the wind. However, what most of us buy and pay more for is a more complex instrument which marries the GPS position data to other data we provide by adding separate sensors such as a speed log, wind vane, or wind log to have much more than just position. The displays and algorythms built into the more expensive GPS/Chartplotters are indeed amazing. True wind is generally of little concern as sailors really only care about relative wind. We all navigate using True position and we measure our speed as over the ground in order to predict our arrival.

[Lodesman]

Originally Posted by ChrisOwens

Not to mention that, as commonly used in the marine world, "True wind" refers to wind speed relative to the water, not relative to the ground, and the GPS has no idea what the water is doing.

Is that commonly used in the "marine world" or just the "sailing world"? I can't see how the difference between ground and true wind as described, matters to motorboaters?

[Djarraluda]

Originally Posted by El Pinguino

GPS does not give you heading.

Oh dear, it had to happen so.....
NOr does GPS give a course. It only gives the Track.

[s/v Jedi]

Originally Posted by Djarraluda

Oh dear, it had to happen so.....
NOr does GPS give a course. It only gives the Track.

Read up on satellite compasses. It’s simply two GPS receivers with one placed a couple inches in front of the other and the difference in position determines the heading. It’s accuracy is 0.1 degrees iirc.

[Cwens]

Originally Posted by Djarraluda

Oh dear, it had to happen so.....
NOr does GPS give a course. It only gives the Track.

You're right that track and course aren't the same thing. In theory. In practice, they pretty much are.



Strictly speaking, "Course" is an instantaneous velocity vector ("Where you're heading right now") while "Track" is a time series of locations ("where you've been"). GPS, as conventionally implemented, gives you track.


In reality, a boat can only change speed or direction of travel relatively slowly. A boat does not accelerate fast enough to snap your head clean off your shoulders, nor decelerate fast enough (even if it hits a rock) to throw you over the bow. It does not change direction fast enough to sling you over the sides. A boat under normal conditions is moving right now in almost the same direction and at almost the same speed as it was over the past half second or so. So, in practice, track over the past half second or so, is an extremely close approximation of course.

[Looking4Neptune]

In theory, if you have GPS/SOG, accurate polars, zero current, can judge the apparent wind angle, and perfectly trimmed sails, you should be able to:
1. Calculate the true wind angle from SOG and apparent wind angle.
2. Use the true wind angle and SOG to read the true wind speed from the polars.

The question did say you had adequate sea trials - I presume this was about developing your polars.

In theory. Probably won't be very accurate in real life.

Pat

[Bill_Giles]

The trick element of the question is ‘sufficient sea trials’. With these trials one can create polar diagrams. Or one might be luck and find one for this particular boat. These give boat speed for a given wind strength and boat heading. Normally these diagrams are used to give an ideal course when racing.

You merely have to point the boat into wind to get the true wind direction.

Then given the three variables one can read off one from knowing the other two. So gps will give you boat speed and direction the diagram will give you wind speed.

[Dockhead]

Originally Posted by Andrew Lippman

In order to calculate true wind, you need to know the apparent wind and angle and the boat speed and heading. . . .

You don't need heading unless you are displaying the True Wind as a compass direction, which is not the way it's normally displayed.

Normally Ground Wind is displayed as a compass direction; True Wind as an angle to the bow, reflecting the proper uses of these two different winds.

The exception might be for racing tactics, where you might want to know compass direction of True Wind in order to plot a layline on a chart, but that's a pretty narrow, perhaps archaic use case. I haven't plotted laylines by hand in years; it's much handier to let the plotter do it, with modern instruments.

[Dockhead]

Originally Posted by Looking4Neptune

In theory, if you have GPS/SOG, accurate polars, zero current, can judge the apparent wind angle, and perfectly trimmed sails, you should be able to:
1. Calculate the true wind angle from SOG and apparent wind angle.
2. Use the true wind angle and SOG to read the true wind speed from the polars.

The question did say you had adequate sea trials - I presume this was about developing your polars.

In theory. Probably won't be very accurate in real life.

Pat

Do you race? Do you always hit your target speed?

Naturally not. That's why good racing instruments can display % of target speed, a very useful readout to have.

In order to get wind speed out of your polar, you would need to know what % of target speed you are achieving. In order to get that -- you will need -- umm -- your STW. So we're back at the beginning.

And anyway, in the given problem, you are assuming zero current. If you really have zero current (which almost never exists in the ocean), then CSOG = STW and you can just use CSOG and forget the polars and all the rest.

At the end of the day, there is just no substitute for STW data. It's hard to get good STW data, but if you want to know True Wind, you need it. WingSail has shown us how racers generally do it. It's not impossible.

[Dockhead]

Originally Posted by s/v Jedi

Read up on satellite compasses. It’s simply two GPS receivers with one placed a couple inches in front of the other and the difference in position determines the heading. It’s accuracy is 0.1 degrees iirc.

Umm, it's not indeed the "difference in position". That's not the way satellite compasses work. They are much more clever and subtle than that -- in a word, they use the carrier phase to determine aspect between the antennae. Position is not considered in the calculation, indeed, none of the code transmitted from the satellites is used in the heading calculation, just the phase of the carrier wave, analysis of which gives very precise distance between the satellite and each antenna. I think we've had this conversation before.

Originally Posted by Djarraluda

Oh dear, it had to happen so.....
Nor does GPS give a course. It only gives the Track.

But I think what Djarraluda is saying is that COG (not HDG derived from a sat compass) is a TRACK because it only analyzes where you've already been.

Well, maybe, but COG, Course Over Ground, is how we think of it -- Course in relation to the ground as opposed to the water (which is HDG). COG is what our instruments call it.

This article: http://dictionary.sensagent.com/Trac...gation)/en-en/

does say that Track and COG can be used interchangeably:

"The track or course over ground, is the actual path followed by the vessel from A to B. In the above given scheme, the corrections are shown that must be implemented to obtain the track of the vessel. Some ambiguity exists in the fact that the path a navigator intends to follow, after evaluating and counteracting possible effects of wind and current, is also called a track." Ibid.

Originally Posted by ChrisOwens

. . . Strictly speaking, "Course" is an instantaneous velocity vector ("Where you're heading right now") while "Track" is a time series of locations ("where you've been"). GPS, as conventionally implemented, gives you track.. . .

I would have said the same thing until I read the resource quoted above. It seems we're wrong -- COG and Track are indeed the same thing, as those terms are used in navigation.