>Another way to look at it. > >So what you have is a circle on the Earth in which you can receive >transmissions -- outside that circle, you can't. And, the circle >moves as the satellite moves -- correct? > >Now, knowing where the satellite is, you want to know where the >reception circle is, correct? > >In your current analogy, you are using a cone/sphere intersection, >but you can also think about it as a sphere/sphere intersection. One >sphere is, of course, the Earth, but the other is the transmission >from the satellite -- which radiates outward like a sphere (even if >focused). > >So, you could look at this like the intersection of two spheres. The >circle of reception would be the locus of all points common to the >surfaces of both spheres. > >This may make for a simpler problem. Sort of. When you bring this circle out to the 2D latitude,longitude used for the map (one of the rectangular ones) then it is only a true circle if the satellite is directly over the equator, as it moves further away from the equator the circle deforms such that when it reaches its heighest latitude above about 60û it actually looks a little like a sinewave. I have actually done this part (to generate the footprint) as this is how I can tell if the function is producing the correct result. I now have to work the other way to determine if a location is within this footprint. A cone is used because the dishes used on many satellites and probes are parabolic, especially the ones in higher orbits. Yes some of the lower orbit satellites use normal antennas which would produce the sphere but the function used is generic for sensors and comms which facilitates the use of a cone rather than a sphere. I am open to other suggestions if code can be provided or guided to. Thanks, Regards, Ashley ~)~ ============================================================= Ashley Butterworth Email: macbse@... ============================================================= _________________________________________________________ Do You Yahoo!? Get your free @yahoo.com address at http://mail.yahoo.com