In my last aside, I noted it was interesting the Hubble telescope was using a low earth orbit (LEO) to conduct its mission, considering some of the challenges inherent in such a low orbit. So let’s start with the first challenge.
A low earth orbiting (LEO) satellite is going around the Earth really quickly. There are some very good reasons for this speed having to do with the conservation of angular momentum, but I won’t go into those here as there are so many excellent explanations regarding Kepler’s Laws, specifically the second one, out there already.
SO, LEO satellites tend to really hustle in their orbits and the big problem is this: once the camera takes the picture, how does picture get to the ground? Especially when the satellite isn’t in view of one particular area of the Earth’s surface for more than 15 minutes (probably less)? The answer to this is: build a comprehensive ground system.
The mess below comes from Globalimaging.com, and is typical of how a lot of organizations view the satellite ground system.
One of the most useful pieces of ANY satellite constellation is the GROUND SYSTEM. The ground system, very simply described, is a communications network based on the Earth, which receives data from, and sends data to, satellites. The system also pushes the data across cables or other methods to a central operations center. Digitalglobe and RapidEye each have their own ground systems to do this.
This ground system is important, because it allows the operators, the people in the operations center “flying” the satellite, to send commands to the satellites and payloads on them. The operators also can request the satellite to give them information on the health of the satellite, its sub-systems, and the payload.
But the operations center of the ground system is just one node, happening to be located on one particular part, of the Earth’s surface. If the operations center is sitting at both of the Earth’s poles, this wouldn’t be a terrible problem, because about every 45 minutes, a satellite would be in view.
Using the logic of RapidEye’s brochure, this means the operations center would only be in view of one satellite in their 5-satellite constellation only once a day, too. And this can be a problem if you have customers in a rush for updated imagery (who isn’t?). Since RapidEye likely wants to deliver the payload data to their customers fairly quickly, they would need more than one antenna on the Earth to talk with their satellites.
It just so happens RapidEye has thought of this, and therefore also contracted more ground stations, probably dispersed as much as possible around the world, for what they call “direct downlink.” Unfortunately, with RapidEye, the number of other ground stations other than the command center, has been kept sort of nebulous (probably on purpose).
However, Digitalglobe can help here. For their 5-satellite constellation, at least according to this article, they now have 11 Remote Ground Terminals (RGTs) (one of these is in Wilkes-Barre, Virginia, another in Troll, Antarctica!!) around the globe shunting payload images and commands back and forth to the operations center in Longmont, Colorado.
As you can imagine, this is costly (and there may be political issues, too). But this also allows Digitalglobe and RapidEye to guarantee their customers a certain timeliness for their imagery products. Google and Bing get their satellite map view updates of completed highway construction. And both Digitalglobe and RapidEye specifically mention government customers needing “quick-turnaround” or “near real-time” products of certain locations on the Earth’s surface. I am sure neither are giving discounts on these products.
So, while hard and expensive, this part of the LEO challenge has been overcome, basically, by adding more ground stations to the ground system. But there are other challenges…