Re: Centre of three positions on earth
Re formulae
Any decent surveying book will have the formulae
eg gps satellite surveying alfred leick isbn 0471059307 and admiralty manual of navigation by hmso
in my own spreadsheets I make use of the gauss method in the book by leick which is a reworked version of the andoyer-lambert technique.
compsys21 is a bit clumsy and modestly accurate i.e. ten metres or thereabouts. [when I say modest it is in comparison with road surveyors who set out curved lines on motorways to millimetric accuracy].
however when it comes to equal time points and circumcentres for aviators it is spot on.