By the Moon

When the Army Signal Corps bounced the first radio signal off the moon in 1946, there were high hopes that the moon would soon serve as a reflector to angle microwave radio messages around the curve of the earth. Nothing much happened for 14 years; the practical difficulties proved considerable. But last week the Navy proudly announced the establishment of man's first practical communications system by way of the moon, linking Washington and Hawaii. To celebrate the occasion, Navy officials displayed a radio photograph sent from Hawaii via the moon, of the carrier Hancock with its crew lined up on the flight deck to spell out "Moon Relay" (see cut).

The big advantage of the moon-relay system is that it makes possible the use of microwaves in the ultra-high frequency, 400-megacycle band that are normally unaffected by magnetic storms. Such microwaves cannot be used in normal long-range radio transmissions, since they do not bend around the curve of the earth, travel only along line-of-sight paths. When a message (Teletype, code or facsimile) is to be sent to Hawaii, an 84-ft. dish antenna at Annapolis, Md. is pointed at the moon. If the weather is overcast, the signal is aimed at the moon's calculated position--clouds do not affect it. The 100-kw. signal fades to a faint whisper during its 480,000-mile trip, but it slants down from space in an admirably dependable manner. In the heavy magnetic storm of November 1959, the moon relay got through to Hawaii when all other radio links failed.

Main disadvantage of the moon-relay system is that it cannot be used except when the moon is above the horizon at both the sending and the receiving stations. In the case of Annapolis and Hawaii--4,850 miles apart--this time, depending on the orbital position of the moon, is sometimes as little as five hours a day, and not more than ten.

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