Skyman - if you mean a 'VHF Direction Finding Approach' (yes - I am old enough to have done them
) it relies on the sytem of using a CRT to display the bearing of a received radio signal (as used on 121.5 for 'triangulation').
The a/c is steered into the overhead by the controller who calls for transmissions by the a/c to establish a homing heading, and when the o/head is detected, steers the a/c onto the appropriate o/b track for the appropriate time before turning it i/b and steering it onto the correct i/b track - all done by calling for check transmissions which show the a/c's bearing from the station on his CRT.
It took a skilled controller and accurate flying and responses by the a/c. Typically DH's of around 6-800 feet could be achieved subject to terrain. It was used at airfields where no radar or ILS existed. Q code 'QGH'.
If you want 'calculations' for this, best wait until one of those 'skilled' ATC controllers comes along and gives away the secrets
EDIT: Whoops! There is that very man - we crossed
