Gary Chatters wrote: > I have no idea of how the math works for computing heading. Checkout FORWARD and INVERSE (and their 3d equivalents) at http://www.ngs.noaa.gov/TOOLS/Inv_Fwd/Inv_Fwd.html While these are hosted on a NOAA site, I'm sure I've seen it on a NASA site as well. IIRC, there is also a C port somewhere. Iain