Success Has Many Fathers - GPS Declassified: From Smart Bombs to Smartphones - page 41

the head of the Viking project, looked as upset that day as any person he had
ever seen. The 1955 Project Vanguard proposal that Easton cowrote with Rosen
listed navigation as a beneficial use of satellites. It stated, “It would also be
possible to determine the absolute longitudes and latitudes by observation of
the satellite. Such observations would also yield the height of the observer
above the center of the earth.” 9 After the nrl's proposal prevailed over Wer-
nher von Braun and the Army's proposal as the official American satellite pro-
gram in the International Geophysical Year, Easton designed Vanguard 1 and
the Minitrack system to track satellites at the igy-designated frequency of 108
megahertz. However, Sputnik transmitted at 20 and 40 megahertz, so track-
ing it required modifying Minitrack.
From Tracking to Navigation
After Sputnik's launch, Soviet spy satellites became a concern. These satellites
would be silent, not emitting a signal most of the time; consequently Minitrack
could not use their signal for space tracking. The new system Easton proposed
in January 1958, Space Surveillance (formally the Naval Space Surveillance
System, or navspasur), was an important development not only for its role
in tracking satellites over coming decades. It created a technical problem—
keeping the system's clocks synchronized—and Easton's solution of putting
the synchronizing clock in a satellite led directly to his proposal for a naviga-
tion satellite system.
Space Surveillance worked as follows: “The system concept of navspasur
is that of a continuous wave (cw) multistatic radar. A high-powered transmit-
ter generates a large fan beam of energy, commonly called the 'fence,' which
reflects signals from an orbiting object back to separate receiving stations.
These receiving stations use large arrays of antennas as an interferometer to
determine the angle and angle rates of arrival from the reflected signals. By
observing the target satellite from several stations, the position can be deter-
mined; using multiple penetrations, the orbit can be inferred.” 10 Thus the trans-
mitter sends a beam into space; if it hits a satellite, the beam is reflected to
receiver stations east and west of the transmitter. Easton's colleague, Martin
Votaw, has called Space Surveillance “Minitrack with the transmitter on the
ground rather than in the satellite.” 11 Space Surveillance required receivers
about one hundred times the size of Minitrack's. 12 Smaller satellites or those
at higher orbits are more difficult to detect. arpa, the Advanced Research Proj-
ects Agency, was established in 1958 in response to Sputnik to prevent tech-