Those Ghost Satellites

Radio listeners, both professional and ham, sometimes hear signals that sound as if they came from a satellite. When they check, they find that no satellite was near them. Such signals need not originate in an unannounced Russian satellite or spaceship departing for Mars. According to Owen Garriott of Stanford University, they may come from a well-known satellite that is passing over an area on the other side of the earth, exactly opposite the listener's antenna.

The way it works, says Garriott, is that the satellite broadcasts its signal in all directions. Some of the waves pass around the earth, just as water flows around a stone. Meeting on the opposite side, they come to a sort of focus at the point on the earth that is farthest from the satellite. There they reinforce each other enough to be picked up by listeners below.

Since Sputnik III was launched seven months ago, Stanford has picked up its orbiting ghost during three periods of several weeks each. The ghost could be heard for three to five minutes (v. five to 15 minutes for the Sputnik itself).

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