TAS and windspeed from three (GPS)
groundspeeds.
Determine your groundspeed
on three headings that differ by 120 degrees (eg 40, 160 and 280 degrees), call
these v1, v2 and v3
Let
vms = (v1^2 + v2^2 + v3^2)/3
a1= v1^2/vms -1
a2= v2^2/vms -1
a3= v3^2/vms -1
mu= (a1^2 + a2^2 + a3^2)/6
Let bp and bm be the roots
of the quadratic b^2 -b + mu =0 ie:
bp= 1/2 +sqrt(1/4-mu)
bm= mu/bp
The TAS and windspeed are
then given by sqrt(vms*bp) and sqrt(vms*bm) provided that the TAS exceeds the
windspeed. If this is not the case, the roots are exchanged. This is a handy way
to check your TAS (and the calibration of your airspeed indicator) using your
GPS groundspeed, even though the wind is unknown.
Scott Krueger
AKA Grayhawk
N92EX IO320 Aurora, IL
(KARR)
.