Success Has Many Fathers - GPS Declassified: From Smart Bombs to Smartphones - page 37

as an aid for measuring longitude, with one satellite over the Greenwich merid-
ian and another one passing over New Orleans, both at altitudes of about four
thousand miles.
The second and third systems used Doppler measurements. A year before
the Soviets launched Sputnik, Lovell Lawrence Jr., an early rocket scientist who
directed the U.S. Army's Redstone project, published “Navigation by Satel-
lites” in the initial issue of Missiles and Rockets . He discussed placing satellites
in geosynchronous orbits, but that was beyond existing technological capa-
bilities. Geosynchronous orbits require altitudes high enough that satellites
orbit the earth once each day, remaining roughly over one point on the earth.
Such orbits require much effort to attain and maintain. A low orbit, six hun-
dred miles in altitude, was more feasible at the time.
Sputnik's launch on October 4, 1957, led many people to track it. This was
important for predicting Sputnik's orbit and studying how it decayed over time.
Three individuals or groups who tracked Sputnik later proposed satellite-based
navigational systems. This was no coincidence and warrants more attention
than it often receives. Each recognized that the methods used for tracking
could be transformed or inverted for navigation. Space tracking uses one or
more ground stations to measure a satellite's orbit. Satellite navigation systems
use one or more satellites to estimate a receiver's position.
As noted in chapter 1, three days after the Sputnik launch, William Guier
and George Weiffenbach at the Applied Physics Laboratory at Johns Hopkins
University listened to the satellite's twenty-megahertz signal and noted its
pitch change over time due to the Doppler shift. They developed a space-based
navigation system the following March using this shift. The first satellite launch,
Transit 1a, in 1959, was unsuccessful. Transit 1b was successfully launched in
1960. The system used low-altitude satellites, at altitudes of about six hundred
miles, and it entered service in 1964. It provided receivers two-dimensional
position fixes periodically throughout the day. It was a useful system, espe-
cially for Polaris missile submarines, but over time it became clear that aircraft
needed a three-dimensional, continuously available system, which would
require a new approach.
Roy Anderson, a consulting engineer at General Electric's office in Sche-
nectady, New York, followed Sputnik's track using radio direction-inder equip-
ment set up in a camping tent. norad (North American Aerospace Defense
Command) subsequently asked him to track each new satellite for forty-eight
hours after launch. 2 He recalled the story of tracking Pioneer 4: