New Moons Rising - GPS Declassified: From Smart Bombs to Smartphones - page 20

An astute reader may perceive here what scientists and engineers in those
early days first comprehended—that if you could pinpoint a single radio-
emitting satellite in orbit using multiple receivers on the ground, it should be
possible by inverting the process to pinpoint a single receiver's location using
multiple orbiting satellites—the essence of satellite navigation. However, the
first operational satellite navigation system emerged from a different tracking
method. A few days after Sputnik captured the world's attention, two physi-
cists at Johns Hopkins University's Applied Physics Laboratory, George Wei-
ffenbach and William Guier, calculated and eventually were able to predict
Sputnik's orbits by analyzing the Doppler shift of its radio signal as the satel-
lite circled the earth. 111 Doppler shift is the apparent change in pitch that occurs
as a sound source moves by a listener. Trains, emergency vehicle sirens, race
cars, and jets planes have made this effect so common that people today rarely
give it a second thought. Frank McClure, chairman of the Applied Physics
Laboratory (apl) Research Center, reviewed Guier and Weiffenbach's findings
and challenged them (coincidently, the same day Vanguard I was launched)
to “invert the solution,” that is, to see if they could calculate a receiving sta-
tion's position using a known satellite orbit. 112 They succeeded, and later in
1958 the apl's Richard Kershner led a federally funded project to build a sys-
tem of radio-emitting satellites and worldwide tracking stations for Doppler
measurements. 113 In 1964 the completed network became the Naval Naviga-
tion Satellite System, commonly called Transit. The military made it available
for commercial use in 1967 (a pattern repeated later with gps), and it operated
until 1996, helping ships and submarines plot their position to within about
five hundred feet anywhere in the world in any weather. 114
The advent of satellites in orbit and dreams of sending humans into space
led to the creation of nasa, the National Aeronautics and Space Administra-
tion, in 1958. Project Vanguard and most of the nrl people who worked on it
were absorbed into the new civilian agency. 115 As satellites increased in size,
number, and complexity, nasa reconfigured the network of Minitrack ground
stations to keep pace. New polar and geosynchronous orbits (satellites travel-
ing the same speed as the earth's rotation to remain at a fixed location above
the equator) prompted the building of new sites and the closing of others, and
Minitrack was renamed the Spacecraft Tracking and Data Acquisition Net-
work. 116 It was followed by the Manned Spaceflight Network, also land-based,
and both were replaced in the mid-1980s by the satellite-based Tracking and
Data Relay Satellite System. 117