Eric Greenwell wrote:
John Sinclair wrote:
I asked Dick Johnson, why couldn't I fly in calm conditions
(morning) and hold a given airspeed (say 60 knots)
and a given heading (say west) for 10 minutes, then
reverse heading to east (to cancel out any wind) and
then analyze the GPS trace to determine my ships L/D
at 60 knots. We have an accurate distance covered and
fairly accurate altitude lost, so why can't we crunch
the numbers?
Dick said the GPS info wasn't accurate enough. I thought
it was a good idea, but I defer to the master.

How long ago did you ask him? GPS is much more accurate in the last
few
years, especially if using the WAAS ablities. But, let's say you know

the distance to only +/- 100 feet (it's typically more like +/- 30
feet), then flying only a mile (5000 feet) would be a 2% error, or
one
L/D point for a 50:1 glider. Good enough for us, I think.

I don't think this is quite right. Consider a 50:1 glider descending
100 meters:
It will go 100 x 50 = 5000 meters
I think vertical navigation errors are typically 1.5 times the
horizontal.
If horizontal accuracy is 10 meters, then vertical will be about 15
So when we've measured this exactly 50:1 glider sinking 100 meters, the
two worst cases we would read for a measurement a
(5000-Herror)/(100+Verror) = 4990/115 = 43.4
(5000+Herror)/(100-Verror) = 5010/85 = 58.9

With errors of 3 meters and 5 meters, it gets closer.
4997/105 = 47.6
5003/95 = 52.6

With errors of 1 and 2 meters
4999/102 = 49.0
5001/98 = 51.0

If you use the GPS for the altitude instead of the pressure altitude,

you might have to fly off a 1000 feet or so of altitude, I suppose.
Maybe Dick was referring to GPS altitude?

Probably. If I understand the technique he uses correctly, he flies a
test glider at a particular airspeed, timing the descent, to get a sink
rate for that airspeed. Then he plots the sinkrates to get the polar.
I can't see why he'd be interested in horizontal position at all.
I don't know what kind of accuracy is possible from the barometric
altimeters. It could be that a skilled pilot/data recorder can get
accuracy below the typical 20 foot tick mark on the altimeter face.

OTOH, you are correct, if I'm reading the FAA docs right, WAAS should
be able to give vertical accuracy better than 2 meters 95 percent of
the time. I don't know if the COTS handhelds can actually deliver that.
On Sam Wormsley's GPS site, there's a link that suggests that a Garmin
GPS 76 maintained about 6 meter vertical accuracy 95 percent of the
time. That's very close to the 20 foot resolution on a barometric
altimeter, and that's absolute position, rather than relative position.
That is, we don't care where we started and where we ended, but rather
how far we descended. If the error offset is pretty much the same at
the beginning and end of a run, then the relative accuracy may be much
better than the absolute position.

It might be interesting to look at the GPS speed during a descent. If
the airspeed and heading is being held constant, and the airmass is
uniform, then the GPS speed should be constant. Excursions might
indicate shears that would affect the quality of the data. You might
be able to improve accuracy by throwing out segments that indicated
non-steady-state behavior.

Tim Ward


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Eric Greenwell
Washington State
USA