Expert: Jayendra Upadhye - 5/24/2005

Question
Dear Jayendra,
I was reading about the difference between GEO and GSO orbits. As i anderstand why a GSO satellite will always stay fixed by means of is earth footprint, i did not understand why is the ground track of a geosynchronous orbit (GEO) is not a straight line on a meridian with the equator as a center point, instead it is the shape of the figure eight?  if the satellite rotational period is equal to the earth's so the satellite should be fixed to a certain longitude.

sincerely,  Eitan Steiner.  

Answer
Hello Eitan,
For understanding certain physical concepts, we have to resort to "other means".
Such as ..
1 - Put it all into an equation and get the solution in terms of an wquation. (If i could do that, i would make einstein redundant!)..which is out of question for both of us.
2 - Imagine (this is really beautiful if you train yourself to do it) you are in space having a bir'd eye view of it all, and then mentally say "go"..and then observe the system from your standpoint.
This is easy (takes some practice), and we often get insights that others dont!

Now using the second method..picture a satellite in GEO. Mind you, a GEO neednot be circular, but could be elliptical, but with a period equalling a sidereal day of the earth. Such a satellite will be observed to speed up relative to the earth as it approaches perigee and slow down as it approaches apogee. naturally that adds horizontal relative motion components to its relative motion above the earth. (its footprint).
Additinally, A GEO may have sizable orbital inclination with respect to earth's equatorial plane. That when combined with the horizontal wanderings of the satellite will produce a figure 8 and other oddities.

In contrast, A GSO will be a circular orbit with period equalling that of the sidereal day, and having zero orbital inclination, thus qualifying to remain "fixed" above a spot on the earth's equator.

If a GSO is tilted, but yet circular, the footprint can become a line along a meridian of the earth.

Here is very good article with illustrations on this subject:
http://celestrak.com/columns/v04n07/

Hope that suffices.

Please do rate the answer if it has been helpful.
Jayen