Estimating Wind Speed with a GPS

[Tom Seim]

I had a nice flight last Saturday in somewhat windy conditions. I have a new
Garmin Pilot III that displays a fairly high resolution flight track. I
noticed that I could estimate wind speed (and, of course, direction) by
watching how much my thermal circles overlapped. Once set up in a thermal I
fly a reasonably consistent bank angle and speed. This results in a circle
diameter of 600-700 feet and a time period of 22 sec. This works out to a
wind drift of 20 mph (17 kt) if the circles touch (a drift of about 700 feet
per circle). The winds are 10 mph (9 kt) if the circles have a 50% overlap
(ever other circle touches). This gives me a quick (and rough) estimate of
the winds aloft.

Obviously this will not work if your are continuously adjusting your circle
position.

You can calculate your circle diameter by:

Diameter (feet) = 0.54 * Speed (kts) * Time Period (sec)

The wind speed per circle is:

Wind speed = 0.68 * Time Period (sec) / Diameter (feet)

--
Tom Seim, 2G DG-400
Richland, WA

[Al]

Alternativly you could look at your glide computer read it ;-)

Al
www.silentflight.com

Tom Seim <tom_s...@email.msn.com> wrote in message
news:Omm31P6r$GA.303@cpmsnbbsa04...

[Tom Seim]

Well, Al, if I had a glide computer then I probably would look at it.

Tom

Al <acro...@www.silentflight.com> wrote in message
news:sgebk56...@news.supernews.com...

[Al]

That aside the Lowrance airmap has a wind calculator in it too :-)

Al

Tom Seim <tom_s...@email.msn.com> wrote in message

news:eci1ov7r$GA.225@cpmsnbbsa03...

[JohnPegase]

Or when in level flight you could try looking at the gps's indicated ground
speed and subtract it from the asi's reading for the effective head/tail wind.
You just might find that easier than all the maths you were suggesting.

John Wright, 742

[Ray L.]

I like John's response. Like Al, I too have an Airmap 100, but
I don't use the windspeed calculator, (I didn't even know it had
one!) I just compare airspeed indication with ground speed
presented by the GPS.

Ray Lovinggood,
Carrboro, North Carolina, USA
LS-1d "W8"

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[Mike Borgelt]

On Wed, 26 Apr 2000 14:57:54 -0700, Ray L.
<rdlovinggo...@transystems.com.invalid> wrote:

>I like John's response. Like Al, I too have an Airmap 100, but
>I don't use the windspeed calculator, (I didn't even know it had
>one!) I just compare airspeed indication with ground speed
>presented by the GPS.

That might work OK at low altitudes but at 10,000 feet AMSL the True
Air Speed is about 17% higher than indicated so comparing IAS with GPS
groundspeed doesn't work. The Position Error of the Pitot/Static
system should also be taken into account. There should be a graph or
table of this in the Flight Manual.

Mike Borgelt

[Al]

Dont forget to add that it also gives wind through altitudes flown
In the home screen press left arrow once the scroll up and down to see wind
crossection.

Winpilot implements this very nicely too..

Regards

Al

Bruce Hoult <bruce...@pobox.com> wrote in message
news:brucehoult-27...@bruce.bgh...
> In article <Omm31P6r$GA.303@cpmsnbbsa04>, "Tom Seim"

> <tom_s...@email.msn.com> wrote:
>
> > I had a nice flight last Saturday in somewhat windy conditions. I have a
new
> > Garmin Pilot III that displays a fairly high resolution flight track. I
> > noticed that I could estimate wind speed (and, of course, direction) by
> > watching how much my thermal circles overlapped.
>

> The Cambridge GPS's have been doing this circle-drift calculation
> automatically for years.
>
> -- Bruce

[Ramy Yanetz]

Mike Borgelt wrote:
>
>
> That might work OK at low altitudes but at 10,000 feet AMSL the True
> Air Speed is about 17% higher than indicated so comparing IAS with GPS
> groundspeed doesn't work. The Position Error of the Pitot/Static
> system should also be taken into account. There should be a graph or
> table of this in the Flight Manual.
>
> Mike Borgelt

Or while thermaling in constant speed, look at the GPS ground speed,
subtract min from max and divide by 2. Simple math and a good exercise
to check if you are hypoxi :-)

Ramy Yanetz

[Bruce Hoult]

[Bert Willing]

Yep.

--
---------------------
Bert Willing
Calif A21S
Come fly at La Motte du Caire in Southern France:
http://www.decollage.org/la_motte
Mike Borgelt <mbor...@ozemail.com.au> a écrit dans le message :
39076c03...@news.ozemail.com.au...

[Eric Greenwell]

An alternative method I've used takes one circle. During one circle,

-note the maximum GPS ground speed
-note the minimum GPS ground speed
-wind speed = (max - min)/2

If you can't do the arithmetic while flying, maybe you should drink
more water (or land).

Since I got my Cambridge GPS, I've used it's wind readout for thermal
flying. The above method is useful during wave flight when I'm not
circling enough times to trigger the Cambridge wind algorithm.

The other method I use in wave flying for a wind estimate is to:

-fly directly into the wind
-increase the IAS reading by (2 percent)/1000 feet to estimate TAS
-wind speed = (TAS - GPS ground speed)

Some day I'll have flight computer that does this for me.

In article <Omm31P6r$GA.303@cpmsnbbsa04>, tom_s...@email.msn.com
says...
= I had a nice flight last Saturday in somewhat windy conditions. I have a new
= Garmin Pilot III that displays a fairly high resolution flight track. I
= noticed that I could estimate wind speed (and, of course, direction) by
= watching how much my thermal circles overlapped. Once set up in a thermal I
= fly a reasonably consistent bank angle and speed. This results in a circle
= diameter of 600-700 feet and a time period of 22 sec. This works out to a
= wind drift of 20 mph (17 kt) if the circles touch (a drift of about 700 feet
= per circle). The winds are 10 mph (9 kt) if the circles have a 50% overlap
= (ever other circle touches). This gives me a quick (and rough) estimate of
= the winds aloft.
=
= Obviously this will not work if your are continuously adjusting your circle
= position.

--
>>Delete the "REMOVE" from my e-mail address to reply by e-mail<<

Eric Greenwell

[Artur Rutkowski]

I've been flying with different GPS devices, and all of them are not
accurate enough to estimate wind speed.

Why do I think so?

Could anybody tell me why the difference between ASI and GPS is bigger
when you have tail wind than when you have head wind in the same
flight e.g. after passing TP and turning 180 degs - the component should
be the same, but it really isn't.

Awaiting any explanation.

###########################################################################
# This is the most INCREDIBLE user @ figaro.ae.katowice.pl #
# #
# Art "Rudi" Rutkowski #
###########################################################################

[mike]

I have found just using min/max groundspeed to determine wind
direction has helped me enormously....winds aloft speed aside.
i use it to find blue streets, as well as pushing back into lift
when i loose it.

First thing off tow i do a constant speed circle, and find wind
direction.

[Chris Glen-Smith]

Artur Rutkowski wrote:
>
> Could anybody tell me why the difference between ASI and GPS is bigger
> when you have tail wind than when you have head wind in the same
> flight e.g. after passing TP and turning 180 degs - the component should
> be the same, but it really isn't.
>

How about this :- depending on altitude True Air Speed (TAS) is higher than the
Indicated Air Speed (ASI) ( see Mike Borgelt's post ) so when flying into wind
the wind speed is subtracted from the TAS and will bring the Speed Over Ground
(SOG) closer to the ASI speed ( if the wind speed is similar to the ASI - TAS
error) when flying with the wind the wind speed will be added to the TAS so
making the SOG further from the ASI speed.

Does that make sense or am I missing the point entirely ? - it wouldn't the
first
time :-)

Cheers
Chris

[ina...@my-deja.com]

Yup, that would be it.

If you are lucky enough to use CEGlide, there is a graphic on the glide
page that displays whether you are above or below glide slope given
your current altitude, speed, and distance to home. Next to the dot
that shows this status, there is an output of the estimated airspeed as
seen on your ASI, (i.e.: it compensates for altitude and wind). If you
are circling, this value is typically very close to the Indicated
Airspeed.

If you are not circling to get your headwind, you can increase or
decrease the displayed headwind (which overrides the one calculated
most recently while circling) until the estimated ASI matches the one
on your panel. Then the numbers will match the glide conditions as
long as you don't change your direction, since overriding the headwind
assumes the headwind is opposite the direction of flight.

Therefore, if you are in wave facing into the wind, then you can adjust
the Headwind value until they match, and the wind will be an accurate
setting (compensated to sea level, since that is what your ASI is
indicating anyway).

Ian Nadas
CEGlide
http://www.ccsi.com/~inadas

-------------------------------------------------------------------

How about this :- depending on altitude True Air Speed (TAS) is higher
than the Indicated Air Speed (ASI) ( see Mike Borgelt's post ) so when
flying into wind the wind speed is subtracted from the TAS and will
bring the Speed Over Ground (SOG) closer to the ASI speed ( if the wind
speed is similar to the ASI - TAS error) when flying with the wind the
wind speed will be added to the TAS so making the SOG further from the
ASI speed.

Does that make sense or am I missing the point entirely ? - it wouldn't
the first time :-)

Cheers
Chris
----------------------------------------------------------------------

Artur Rutkowski wrote:

Could anybody tell me why the difference between ASI and GPS is bigger
when you have tail wind than when you have head wind in the same flight
e.g. after passing TP and turning 180 degs - the component should be
the same, but it really isn't.

Sent via Deja.com http://www.deja.com/
Before you buy.

[Sierra SkyWare, Inc.]

Eric Greenwell <REMOVEeg...@prodigy.net> wrote in message
news:MPG.137221fec...@news.prodigy.net...
> [...]

>The other method I use in wave flying for a wind estimate is to:
>-fly directly into the wind
>-increase the IAS reading by (2 percent)/1000 feet to estimate TAS
>-wind speed = (TAS - GPS ground speed)

>Some day I'll have flight computer that does this for me.

Good news - perhaps this day is closer than you think :), as this feature is
ALREADY PRESENT in WinPilot version PRO. Because Borgelt B50 produces True
Air Speed reading every second, which when combined with Ground Speed from a
GPS allows computing (and showing on the screen in a visual form) the
current headwind/tailwind component (that is also updated every second).
It is also quite interesting to see the dynamics of headwind/tailwind
changes when you approach a thermal, or a shear line...

Best Regards,

Jerry Plaszowiecki
Ash-26e "MS"
Sierra SkyWare, Inc.
P.O.Box 2799
Minden, NV 89423
(775) 783-0097
http://www.winpilot.com

[jprre...@gmail.com]

On Wednesday, April 26, 2000 at 9:00:00 PM UTC-10, Eric Greenwell wrote:
> An alternative method I've used takes one circle. During one circle,
>
> -note the maximum GPS ground speed
> -note the minimum GPS ground speed
> -wind speed = (max - min)/2
> >

> Eric Greenwell

Eric:

I use your approach as well. During the circle run, I focus on GPS ground speeds and approximate direction where the max or min ground speed was observed. I then fly the direction where I thought I had the minimum ground speed. This is my guess at the head wind direction. I then vary my heading left or right to hone in on a better indication of the actual wind direction (It has the lowest ground speed).

When heading out to land, as I approach the LZ area, I will vary my course to find the minimum ground speed direction. That direction is the true wind direction at that moment. If it varies significantly from previous info, then it is concerning. I then am looking much harder at all the wind indicators on the ground that I can find. Question is there an active thermal in the LZ area, or did the wind just shift? Either way you know more by checking.

[weemacgregor]

On Wednesday, 26 April 2000 03:00:00 UTC-4, Tom Seim wrote:
> I had a nice flight last Saturday in somewhat windy conditions. I have a new

> Garmin Pilot III that displays a fairly high resolution flight track. I

> noticed that I could estimate wind speed (and, of course, direction) by

> watching how much my thermal circles overlapped. Once set up in a thermal I

> fly a reasonably consistent bank angle and speed. This results in a circle

> diameter of 600-700 feet and a time period of 22 sec. This works out to a

> wind drift of 20 mph (17 kt) if the circles touch (a drift of about 700 feet

> per circle). The winds are 10 mph (9 kt) if the circles have a 50% overlap

> (ever other circle touches). This gives me a quick (and rough) estimate of

> the winds aloft.

>
> Obviously this will not work if your are continuously adjusting your circle

> position.
>
> You can calculate your circle diameter by:
>
> Diameter (feet) = 0.54 * Speed (kts) * Time Period (sec)
>
> The wind speed per circle is:
>
> Wind speed = 0.68 * Time Period (sec) / Diameter (feet)
>
>
> --
> Tom Seim, 2G DG-400
> Richland, WA

Question:

Is the speed at which a thermal drifts the same as the winds aloft?

[moshe....@gmail.com]

On Thursday, April 27, 2000 at 3:00:00 AM UTC-4, Eric Greenwell wrote:
> An alternative method I've used takes one circle. During one circle,
>
> -note the maximum GPS ground speed
> -note the minimum GPS ground speed
> -wind speed = (max - min)/2
>

> If you can't do the arithmetic while flying, maybe you should drink
> more water (or land).
>
> Since I got my Cambridge GPS, I've used it's wind readout for thermal
> flying. The above method is useful during wave flight when I'm not
> circling enough times to trigger the Cambridge wind algorithm.
>

> The other method I use in wave flying for a wind estimate is to:
>
> -fly directly into the wind
> -increase the IAS reading by (2 percent)/1000 feet to estimate TAS
> -wind speed = (TAS - GPS ground speed)
>
> Some day I'll have flight computer that does this for me.
>
>
>

> --
> >>Delete the "REMOVE" from my e-mail address to reply by e-mail<<
>
> Eric Greenwell

Well Eric maybe now (20 years later) you have a glide computer that does that, but I still don't. I use Tophat (based on XCsoar) and it estimates the wind while you circle, presumably using the circling method you mentioned. But, with no pitot input, it's not very accurate. After all, the wind changes with altitude and over time. I'd love to have a simple sanity check, that would also help when I have not circled in a while.

What I have to do now is: see, e.g., 65 knots IAS, add 10 knots for the altitude (more or less, depending on the altitude), subtract the estimated headwind (say 15 knots), get a predicted ground speed of 60 knots, compare that with the GPS ground speed. If the GPS says the actual ground speed is 55 knots, then I know the headwind is 5 knots more than the estimate.

To avoid having to do all that math, especially the effect of altitude on TAS, I've asked for this to be added to Tophat, but there has been no development in a while:

Add an "info box" that one can choose to see on the screen, that displays the ESTIMATED indicated air speed. I.e., the computer would take the GPS ground speed, combine with the estimated wind speed to get an estimate of the true airspeed, and then adjust it for the altitude to show what the airspeed indicator should say. Compare that with the actual pitot-based speed indication.

E.g., if the IAS says 65 knots and the infobox predicts an indicated 60 knots then I'd know that there is actually 5 knots more headwind than the computer's estimate, since I need to fly faster than predicted to achieve the measured ground speed.

That's rough (ignores errors in wind direction, for one thing), but good enough to help make final glide calculations more reliable. (But you still won't know what wind you'll encounter later as you get lower.)

[Dave Nadler]

On Wednesday, June 3, 2020 at 7:33:52 PM UTC-4, weemacgregor wrote:
> Is the speed at which a thermal drifts the same as the winds aloft?

No.

[moshe....@gmail.com]

So then if a pilot, or glide computer, estimates the wind from the changing ground speed while circling in a thermal, is the wind speed underestimated?

[Sci Fi]

At 01:37 04 June 2020, moshe....@gmail.com wrote:
>On Wednesday, June 3, 2020 at 8:39:12 PM UTC-4, Dave Nadler wrote:
>> On Wednesday, June 3, 2020 at 7:33:52 PM UTC-4, weemacgregor wrote:
>> > Is the speed at which a thermal drifts the same as the winds aloft?

Yes
and
No... If the thermal is being produced by a power station chimney, then
20 feet higher than the chimney, the drift of the thermal is zero. It is
only when the thermal reaches a few thousand feet above the chimney that
the drift equals the wind aloft speed.

Also remember that for even higher altitudes your IAS is not your TAS, so
on a calm day, your GPS speed will differ from your IAS. ( about +2% per
1000ft.)

[Eric Greenwell]

Usually close enough, though, given all the other uncertainties in soaring
calculations. And, I think, they are the same if the vertical wind gradient is zero.

My flight computer (iGlide) displays two wind numbers:

- the drift while circling
- the "instant" wind from the Butterfly vario

My impression is the majority of the time, they agree within 2 knots and 10
degrees. The rest of the time they differ more than that, and sometimes
considerably more. Of course, the measurements are done very differently, and the
circling wind is not just once around, but averaged over several circles, which
are at different altitudes as the glider climbs in the thermal.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1