I had forgotten that I had promised a response to that one. It has to do with the oblateness of the Earth. The Earth is not a sphere, but rather it is squashed a little bit in the Southern Hemisphere. The resulting non-uniformity in the gravity field can cause a phenomenon known as "precession" of a orbiting satellite. Not all orbits are affected equally, with the effect proportional to both inclination, and altitude.
Both I* and G* have multi-plane constellations at some inclination - all of I* planes are inclined at 86.4 degrees and G*'s are at 52 degrees. There is a "line" in space known as the ascending node, where the orbit intersects the ecliptic (the plane containing the Earth's orbit around the Sun). If you take a sheet of paper and draw a wheel with eight spokes, equally spaced around the wheel, then the spokes could represent the ascending node of each of G*'s eight planes, with the orbit coming up out of the paper at a 52 degree inclination.
So, for G* to take one satellite from one plane to the next all you have to do is let the phenomenon known as precession do its thing. You perform a in-plane delta-v (fire thrusters) reducing the altitude some amount. Since the precession rate is a function of altitude and inclination, the new orbit is going to precess at a different rate than the old orbit. The effect is that the ascending node (one of the spokes) of this one satellite's orbit is moving at a different rate than the ascending nodes of all of the remaining satellites. When the ascending node is lined up with the plane you want to move it to, you fire thrusters again, doing a in-plane maneuver, and raise the altitude back up to that of the remaining satellites. To quote Maurice, "easy-peasy". Note that inclination did not change. (Inclination maneuvers use a lot of fuel and neither I* nor G* carry enough fuel to do inclination maneuvers.)
But precession is a function of altitude *and* inclination. In fact, satellites in a equatorial orbit (zero degrees) and polar orbits (like I*) aren't affected by the oblateness of the Earth, and therefore, they do not precess. (Note that I* isn't perfectly polar, but for practical purposes of this discussion, it is close enough to make precession useless to move satellites from one plane to another.)
That means when I* launches a satellite to a certain plane, it is going to stay there. Contrary to many I* fans, G* management had nothing to do with this. It's all Kepler's fault! ;-)
Mr A |
