> >I think I have it - assuming the satellite illuminates the whole side of the >earth facing it - I.E., the satellite's beam is *not* focussed to a smaller >cone. > >If this is the case, then can't you just compare the distances AT and AM? >Where A is the satellite, M is the receiver on earth, and T is the >tangential contact of the cone with the earth? > >If AM < AT then M has to be visible > >If AM > AT them M is not visible > >Cheers, Peter > >PS I'm a little puzzled by a cone being perpendicular to tangents - >perhaps this is a way of saying the axis of the cone goes through the >confluence of the perpendiculars to all the tangents? It is sort of like this except that the beam is focused so comparing the lengths would not be enough. The perpendicular to the tangents can be explained by an example. Take a basketball and place a flat piece of paper on it. The radius between the center and the point where the paper touches (it only touches a point in reality) is perpendicular to the piece of paper it is tangental. Think of 2D shapes such as a parabola and the tangents is just the line that touches at a point and has the same slope as that point of the curve. Calculus. Ie imagine a circle centered about the origin radius of 5. The tangent located on the top half of the circle on the y axis would be the line y=5 or similarly the tangent located on the x axis would be x = 5 at one end and x = -5 at the other. The cone being perpenicular to the tangents means that the axis of the cone is like a continuation of the radius passing through the sphere, it is perpenicular to the surface of the sphere. Ie using our example above it would be perpendicular to the piece of paper. Or another example is take an empty ice-cream cone and sit it on a counter top, The tangents at the surface run along the counter top ie the counter top is the tangent and the cones axis is perpendicular to it. I hope this explains it, thanks, Regards, Ashley ~)~ ============================================================= Ashley Butterworth Email: macbse@... ============================================================= _________________________________________________________ Do You Yahoo!? Get your free @yahoo.com address at http://mail.yahoo.com