# Period of revolution of Chinese Compass system

Given that the Russian GLONASS system has an orbital altitude of 19,140 Km and a period of revolution of 11 hours and 15 minutes, the US system has an orbital altitude of 20,180 Km and a period of revolution of 11 hours and 58 minutes and finally the Galileo system an orbital altitude of 23,222 Km and a period of revolution of 14 hours and 22 minutes, how does one calculate the period of revolution for the Chinese Compass system with an orbital altitude of 21,150 Km?

EDIT: I used Kepler's third law in my original calculation, but assumed that the orbital altitudes given are referenced to the centre of the earth. I made this assumption based on how I interpreted the math, and did not know that the orbital altitudes are referenced to mean sea level on Earth. Once I added the radius of the earth which is approximately 6,371 Km to the orbital altitudes I was able to calculate the period of revolution. So the actual orbital distance from the centre of the earth to the Chinese Compass system is 21,150 Km + 6,371 Km.

• I didn't downvote, but your assertion that "no question is a stupid question" is unfortunately not true on Stack Exchange at all. Questions that get answers that will be useful in general are great; questions that won't, aren't, and the site is not designed to welcome the latter. Aug 20, 2016 at 2:04
• @Kim - At the age of 55 I doubt if I am a kid doing homework assignments. LOL. I am an embedded engineer by training and I happily answer questions that people ask me. I last studied Kepler's laws some 34 years ago and am a bit rusty. We cannot be specialists in all fields. Now that I know how this site operates I shall word my questions a tad bit more carefully. Aug 20, 2016 at 3:05
• @kimholder and everyone - I left a comment about the benefits of including "what have you tried so far" etc in the question, and S'Ranger quickly updated the question accordingly (there was indeed work, just not mentioned). I deleted my comment as "cleanup", but the original "a pity" comment lingers - making it look like there is still discontent where (I assume) there isn't anymore. S'Ranger you may want to delete that comment unless you are deeply attached to it. After a certain delay, people are unable to reverse their down votes until you make an edit to the question (built-in to SE's UI).
– uhoh
Aug 20, 2016 at 3:53
• At least in this particular SE site, down votes are not at all personal. They are actually used to help improve questions by suggesting there's an issue. Sometimes they smart a bit, but (at least here) they are usually intended as constructive devices to flag where some fine tuning or attention is needed.
– uhoh
Aug 20, 2016 at 3:53