GPS and ground speed

In article ,
"BT" wrote:

"Mike Ash" wrote in message
...
In article ,
Stealth Pilot wrote:

if you were forced to use a gps unit for speed you could correct the
apparent value using the metrological forecast winds for the altitude.
it is a simple speed triangle calculation that could be done on a
circular sliderule like the jeppeson cr5.

The GPS unit could also derive the actual winds if you can fly a couple
of different headings at the same speed, or make a circle.
Glider-oriented GPS units typically do this automatically since it's so
useful to have and we tend to fly in circles all the time anyway.

I did not think that "glider oriented GPS units" computed the wind. It's the
software that correlates a reasonably constant diameter circle to compute a
center point of the circle and then correlates the movement (drift) of the
center point over the ground to compute a "best guess wind" from the
drifting. Poor pilot thermaling techniques will result in poor wind
estimates from the software.

I believe there are a couple of techniques depending on the data
available. A pure GPS unit is going to be stuck doing what you describe.
A unit which also has access to pitot/static data will be able to
compute airspeed as well as groundspeed and direction, and will be able
to derive wind from a couple of straight legs flown at different angles.
I believe both techniques can be found in practice, but I'm not familiar
enough with actual devices to say what does what.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon