One is a lonely number. It takes a while for one low earth orbiting (LEO) satellite’s payload (the camera) to see the entire Earth’s surface. If you want to get pictures more quickly, say the same day something like a hurricane or a forest fire is happening, you want a CONSTELLATION of satellites orbiting the Earth.
Just like stars in the sky forming nice groupings, more than 1 satellite of the same system of satellites is called a constellation. There are several companies out there with constellations of imagery satellites. RapidEye and Digitalglobe are ones that might be known to you. We’re going to use RapidEye’s constellation for our example.
RapidEye has decided, for now, to use five satellites with 98 minute low earth orbits for imaging the Earth’s surface (actually, Digitalglobe is also currently using five satellites in their constellation). With a constellation of only five satellites, RapidEye claim in their brochure they can revisit a spot on the Earth’s surface daily and their imaging payloads collect 5 million square kilometers of images as well.
How does this work?
Do you remember the last orange you imagined holding in your hand, rotating it carefully while rolling a coin perpendicular to the rotation? Do you also remember imagining a line coming off the coin as it rolled over the orange’s surface? (There’s a picture below to help you remember).
Well, that line, if the coin was the satellite, represents what most space operators call a ground track. If you flatten that orange into a map of the Earth (hard to do in real-life, easy when you imagine it), the ground track of the coin/satellite on the Earth’s surface would look something like this (it’s the yellow line on the Earth’s surface):
Pretty lonely right? The ground track would be the line the satellite is making as it flies around the Earth and had a nice long marker attached to it drawing the line. And you can see the offset (where you see what looks like two ground tracks–it’s actually the same track), beginning near Australia and ending north of South America. That is why using only one satellite will be relatively slow in collecting pictures of the Earth’s surface.
When you add four more satellites and their ground tracks, it looks something like this (and is the reason why RapidEye says they can get any image on the Earth’s surface to their customer in a single day):
So, in this case, a constellation of satellites are better if you’re in a hurry. Every 98 minutes, all five of RapidEye’s satellites are somewhere over the Earth’s surface, their payloads taking lots of pictures. Digitalglobe’s satellites are doing the same thing.
There is at least one company trying to outdo RapidEye and Digitalglobe: Skybox. Skybox wants to have a constellation of 24 “cubesats” (linked to now, but to be explained later). This verges on a space concept called “near-real time” which we will also discuss later.
If you’re wondering what I’m using for the ground tracks and tracking satellites, the pictures on this page are from an online satellite tracking tool called “Real Time Satellite Tracking.” There’s also a link in one of the right columns of this blog. The tool is pretty neat!
So, with five satellites, and at least one revisit guarantee of particular points on the Earth’s surface, everything’s perfect, right?
Well, no, of course not!