The despun section
is a component of dual-spin geostationary satellites
. All geostationary satellites
, and indeed all geostationary spacecraft
, use either dual-spin stablilization
stabilization to maintain altitude
We focus on the case of dual-spin stabilization. Picture the satellite as a cylinder (such as a can of beans), rotating about its y-axis, (such that the consumer may see the entire label). Now, we take this cylinder, and we add another one to the top of it, joined by a rotary coupler and a despin bearing assembly. Got that? The first section, we spin at, say 60RPM, and the other section, we spin at just one rotation per day. It is this latter section, spinning much more slowly, that we call the despun section.
The despun section of a satellite will hold instruments and equipment that must point steadily at one area. This includes the communications payload, all antennae and cameras, the spectrometer, the photopolarimeter, and the radiometer, all oriented toward the earth. The despun section gets its information from the spinning section, where one finds the equipment shelf and solar arrays. The despun section is controlled by these arrays, which take signals from the Sun and the Earth, on the spinning section; the spinning section mechanically keeps the despun section’s rotation at a much lower rate, thereby preventing the satellite from going into a flat spin.
Another function of a despun section would be for the ease of repair. Imagine that you are an astronaut (or cosmonaut) needing to fix, say, a camera. We look to the previous paragraph and discover that this is on the despun section, so we send you out, fix the camera, and return; easy. Now, we need to fix a solar array, which we know is on the spinning section. We exit the spinning section from within, grabbing hold real tight, as now the entire Universe spins around us once per second (or much, much more). This is to say that once or twice every second, you see the planet Earth, our collective home, speed past you from "up" to "down", or vice versa. Very difficult to work in these conditions, and our very presence on so fast a spinning object introduces a wobble to the system; no good. To repair the spinning section, we would have to have it despun.
Hooray for the despun section!