This book looks very interesting. Basic (yuch - gimme that FORTRAN) source code disk included (choice of Mac or PC) for many many algorithms. Methods Of Orbit Determination For The Microcomputer http://www.willbell.com/math/mc10.HTM This is an excellent reference book written at a level most people with a reasonable math background can understand. Escobal, P.R., Methods of Orbit Determination, John Wiley & Sons, Inc., 1965. http://www.sprl.db.erau.edu/~minow/sat_refs.html A 113 Kbyte list of astro books http://www.rmplc.co.uk/eduweb/sites/bookman/abooks/auth.html Phil progers@mindspring.com ********