Why space matters: Imaging satellite operations, part 9–cutting the cheese

Hopefully the beer was tasty and nutritious.

The question I posed in my previous post to you, dear (inebriated?) reader:  Why is it that moving the plane of a satellite’s orbit against the direction of the Earth’s rotation (backwards) is helpful to those operators of imagery ilk?

Let me clarify some terms here before we go on.  A satellite’s orbital plane is NOT a toy airplane flying around the satellite.  Instead, imagine a ball of mozzarella cheese representing the Earth, with that gosh-darn coin orbiting it representing the satellite.  If you attached a very long blade to the coin, insert the blade to the center of the cheese, then moved the coin in one orbit, you would likely cut the cheese in half.   So here’s the setup:

The nice flat surface of either half extended to the coin’s orbit around it now looks like a disk and represents the satellite’s orbital plane.  It’s a line extending through three different points: from the center of the coin at one part of the orbit, going through the center of the heavier object (the cheese/Earth) near the orbit’s center, and then extending to a point representing the center of the satellite in a different part of the orbit.  Like so:

See, I didn’t have to use terms like inclination and longitude of the ascending node to explain this part.  Read more here, if I’ve successfully convinced you to drink again.  Read it before you drink.

Why discuss those?  Well the Earth rotates on one plane and zips along another as it orbits the sun.  The satellites in our situation zip along a different plane, one nearly perpendicular to the Earth’s rotation.  Because these inclinations are slightly past perpendicular, the orbital plane of these imagery satellite orbits rotate opposite the Earth’s rotation at a rate around 1 degree of longitude per day.  In the space world, this is backwards rotation is called precession and it is the result of torque.  There’s more here that explains this better.

So that was a lot of jibber-jabber and maybe your brain is mushy, but hang in there.  The answer to the question at the beginning is simple, really.  The sun-synchronous orbit assures imagery operators the areas they are taking photos of have nearly the same kind of light every time they want to get a snapshot.  Does the interested organization want the satellite to always skirt the dusk/dawn lines of the Earth?  Or do they want to see certain surface areas in the morning or afternoon, when shadows might be helpful?  The sun-synchronous low earth orbit can do either one, and again, cuts down variables a notch.

So, you’re an imagery operator.  Up to this point, you know you might have 11 ground terminals (thanks to Digitalglobe) to upload and download data between your satellites and operations center.  And you have at least five satellites, all zipping around the Earth in a sun-synchronous low earth orbit.  Because you’re interested in changes to the Earth’s surface and recording them over time, you think that having those satellites crossing San Francisco’s latitude (37.78 degrees) at mid-morning would be useful.  You’re almost ready for your mission.

Or maybe not.  What to do, what to do…?  I think I’ll write more in another post, maybe about satellite sub-systems and imagery payloads.  Go have a beer, and I’ll see you then!