Lynn, The correlation works well for birds in a circular orbit, but elliptical orbits are another story. A high elliptical orbit could result in a long pass at a very low elevation, or a very short pass at 90 degrees if perigee is over the ground station. Since the current fleet of birds are in circular orbits it won't matter but if you are coding a program you might want to account for possible future HEO birds. I find both numbers to be useful (pass duration and max elevation). Best '73 - de Ed N8BBQ