The nervous tension was terrific. The three inarticulate men stood there while the seconds sped, Wagner staring around with desperation on his fat face, Silvy Ward clenching and unclenching his hands, Portiz leaning his drink-pliant body against the bank of keyers.
Suddenly Ward broke the silence.
"Wagner, get that trajectory keyer going. First check the interlock beam relay; the circuits seem to be okay, so it must be the relay. Portiz, get the portable glide beam transmitter unit and drive it out to the very base of the Hump on the eastern end of the field, and keep your receiver open on thirty-four megacycles; I'll give you directions from here. Come on, get going!"
Wherewith Ward spun around to the Pali-Vanyi radio range transmitter. There was a peculiar smile on his face as he released the controls on the goniometer unit which governs the direction of the signals by reducing or increasing the radio frequency in the four range radiators. They'll be on the beam, he thought; these Martian boys won't take any chances of missing on the first stab for it would take them so long to maneuver around for a second attempt that the element of surprise would be lacking and their prey would have gotten away. They'll ride the beam in from the west. When they get directly over the range station they'll get the vertical radio signal from the station location marker and know that the field lies ten miles to the east and Pali-Vanyi ten miles south of the field. Switching their course ninety degrees they'll drop in right over the city and let go with everything they've got.
They're probably on the beam now and four hundred miles to the west. They're due to hit the strato-winds which any astronaut knows will buck them around. The thunderheads will make their compass blotto so the only direction they can be sure of is due east on the beam. If we shift the beam slowly by rotating the goniometer counter-clockwise, the quadrants of the beam will be reversed. They'll swerve their course to follow, and gradually instead of getting the A signal to the south they'll be getting it from the east, and instead of an N from the north they'll have an N from the west. They'll come into Pali from the South—
The radio range at Pali-Vanyi resembled to a great extent the radio ranges used for centuries before by the Federal airways of the United States of America, Earth. The increasing use of ultra-high frequency waves had made obsolete the four towers of the intermediate frequency range. Small, compact, the new range system had through the long decades of scientific advancement after the war years of the 20th century reached a stage of efficiency a hundredfold greater than its predecessor.
A small antenna array atop the broadcast station consisting of four vertical radiators mounted at the terminals of a horizontal X replaced the towers of yesteryear. The four bars of the X pointed northeast, northwest, southwest and southeast.
The NW and SE radiators sent out a steady N (dash-dot) in Morse code, the SW and NE a steady A (dot-dash). At thirty second intervals the identification letters of Pali-Vanyi [dot-dash-dash-dot dot-dot-dot-dash] were transmitted from all radiators. A ship coming in from the west, directly on course, heard both N and A simultaneously and with equal strength so that they interlocked and formed a steady dash. A swerve to the north of the course meant that the N signal would be predominant in the ship's receiver; to the south, that the A would be predominant. Rotating the goniometer counter-clockwise would so change the radio frequency in the four signal radiators as to cause all on-course signals to swing similarly, and ships on the beam would follow it blindly around, especially when their compasses were put awry by natural causes. A 90 degrees swing would completely reverse the so-called N and A quadrants; hence the beam would completely lie. WHAT would the destroyer's speed be? Probably twelve or fourteen hundred m.p.h. Twenty minutes or less to swing the beam. With one hand Silvy Ward began to rotate the goniometer, casting an eye at a nearby chronometer. Ninety degrees, say, in eighteen minutes. Five degrees per minute. Easy now! With a free hand Ward reached out and snapped on the shortwave transceiver which was used in communications when testing experimental equipment. He picked up the microphone and called:
"Portiz, are you in position?"