By Steven Shepard, Contributing Columnist
Science, science fiction…which is it?
In October 1945, science fiction writer Arthur C. Clarke published a paper in Wireless World entitled, “Extra-Terrestrial-Relays: Can Rocket Stations Give World-Wide Radio Coverage?” In his paper, Clarke proposed the concept of a platform orbiting above the Earth that would serve as a relay facility for radio signals sent to it that could then be retransmitted back to Earth with far greater coverage (‘footprint’) than was achievable through the terrestrial transmission techniques of the time. He describes his platform in the article:
Let us now suppose that such a station were built in this orbit. It could be provided with receiving and transmitting equipment (the problem of power will be discussed later) and could act as a repeater to relay transmissions between any two points on the hemisphere beneath, using any frequency which will penetrate the ionosphere. If directive arrays were used, the power requirements would be very small, as direct line of sight transmission would be used. There is the further important point that arrays on the Earth, once set up, could remain fixed indefinitely.
Moreover, a transmission received from any point on the hemisphere could be broadcast to the whole of the visible face of the globe, and thus the requirements of all possible services would be met.
Theory of Orbiting Satellites
According to Clarke, his platforms would orbit 42,000 kilometers (25,200 miles) above the equator, at a rotational speed identical to the rotation speed of the Earth. As a consequence, the satellite would appear to be stationary to Earth-bound users since they would move at the same orbital velocity as the Earth itself.
Figure 1: Arthur C. Clarke’s original diagram for his concept of a three-satellite orbiting system for relaying radio signals around the Earth.
Ringing the equator like a string of pearls, these devices provide a variety of services including voice, broadcast television, video-on-demand, interactive data, and point-of-sale applications. And while satellites are viewed as technological marvels, they really aren’t — basically, they’re just big repeaters that have the ability to take a received signal, clean it up, modulate it into a different frequency band, and then retransmit it back down to Earth.
But because satellites have a VAST line-of-sight — basically, the entire visible hemisphere over which they are orbiting — the retransmitted signal can be delivered to an enormous geography, making signal distribution very cost-effective.
It’s interesting to note that because these geosynchronous satellites orbit above the equator, the receivers have to be pointed at them. As a result, a satellite dish antenna that is in the extreme southern United States, or in Latin America, is pointed almost straight up, while dishes in the northern U.S. or in Canada are practically lying on the ground.
Satellites are wondrous devices, because they actually work. The real magic isn’t the radio signal management that they perform; that’s relatively trivial. What’s particularly fascinating is the degree to which they have to be hardened for the rigors of space.
Communications Satellite Environmental Challenges
Think about it. Not only do they have to endure exposure to the temperature extremes of a very unforgiving environment (from near-absolute zero [-273 degrees] to well over a hundred degrees), they also have to withstand bursts of gamma radiation from the sun, which would fry the electronics of most unprotected devices.
And because they orbit with one side facing the sun and one facing the Earth, they have sophisticated heat management systems, deployable louvers and heating systems to help them cope with rather hostile operating conditions.
One final note about these extraordinary things: They can be enormous. A fully configured communications satellite can be 100 feet long and 30 feet in diameter, with solar panels that are even larger – hundreds of square feet. Think about it. That’s equivalent to a ten-story structure.