The new global telephone systems, with their multitude of orbiting satellites, worry radioastronomers. The Iridium system now entering service uses 66 satellites; at least one, and usually several, are above the horizon at any time anywhere on Earth. The radio glare of their transmitters blots out the feeble radiation from cosmic sources. Daedalus now points out that radioastronomy itself has pioneered a solution to the problem.

The satellites are all in communication with one another; but only one (the nearest) communicates with any given client station on the ground. But suppose all the satellites in sight of that station joined in the communication. They would collectively form a ‘very long baseline interferometer’ aerial in the sky, spread over millions of square kilometres of orbital space. With the right phasings between its elements, such an aerial is wonderfully directional. It could transmit to, and receive from, a mere few square metres around a specific ground station. Receivers elsewhere, such as radio telescopes, would detect nothing.

Not only would this system eliminate interference, it would drastically reduce the power needed for its transmitters. For instead of broadcasting wastefully to whole regions, the satellite array could target each ground station exactly. Its sensitivity as a receiving array would be equally enhanced. Properly phased, the collective satellite aerial would have a very high ‘gain’ for signals from that station. So the satellites could be put into higher orbits, their horizons would broaden, and fewer of them would be needed to cover the globe. The only snag is that their positions are constantly changing. And to compute the correct phasings needed to target each ground station, and keep them properly updated, those positions would have to be accurately known all the time.

No problem, says Daedalus. The most accurately located satellites of them all are, of course, those of the Global Positioning System. Several of those must also be in the sky over any point at any time. So combine the two systems in one set of satellites. Fewer satellites would crowd the heavens, transmitter power and radio spectrum space would be saved, and radioastronomers everywhere could relax. They would merely have to avoid using the satellite telephone while at the telescope. Otherwise their cosmic gaze would be disrupted by a sudden accurately targeted blast of chatter from the sky.