http://www.geocentricperspective.com/geosat.pdf Neville this paper is very interesting as it does the math for me. However may I ask why you use the PDF format? this format is a stumbling block for me on many web sites and especially science subjects. for example if I want to select a section of text to paste here for a comment the selection is not easily usable and selectable and is rather blurry like a photo image. .. do you have a word doc format that you could attach to an email for our consumption. It would make it a lot easier for me to give you feedback.. Philip.