One System, Two Narratives - GPS Declassified: From Smart Bombs to Smartphones - page 56

assertion sometimes made that Timation was just a study plan is incorrect. 11
The March 1971 Timation Development Plan states, “The satellites provide the
necessary data for the navigator to determine his latitude, longitude, altitude,
and time. Not all navigators need to determine all these parameters. The user
with the most stringent requirements will use four satellites in view and the
equipment to receive signals from all four satellites; reduced requirements will
result in one or more of the four signals being ignored by user's equipment.
This system was configured within the jcs [Joint Chiefs of Staff ] navigation
accuracy requirements.” 12
Figure 4.2, from the same page of the plan, shows an airplane receiving sig-
nals from four satellites. This illustrates the most stringent requirement of
three-dimensional position and time transfer. This is one of many primary
source documents that refute the myth that Timation was a two-dimensional
system that required an atomic clock in the receiver. Some people continue to
assert the myth today. 13
nrl's James Buisson, a physicist, and Thomas McCaskill, a mathematician,
studied the optimal Timation configurations for the number of orbital planes,
number of satellites, and orbital altitude. An important consideration was the
placement of ground stations. For security purposes, the plan was to locate
them only in the United Sates or secure U.S territories. Easton commented,
“nrl found that the critical item in a satellite navigational system for unbe-
lievable accuracies is the ground station location. The ground station location
surprisingly determines the next item, the satellite constellation.” 14 As men-
tioned earlier, a problem with Project 621b was its European constellation
ground station. The U.S.-based restriction on Timation ground stations created
a gap in the Indian Ocean where no station was available to update the satel-
lites. A problem the people developing Timation faced was uncertainty about
when atomic clocks would be developed that were sufficient to replace crystal
oscillators (quartz clocks) in navigation satellites. 15 Timation I and II had crys-
tal oscillators, and Timation III, scheduled to be launched in 1974, carried them
in addition to two experimental rubidium atomic clocks made by the German
company Efratom. Midaltitude satellite orbits, as is true for all orbits up to geo-
synchronous altitude, have the trade-of that higher altitudes give greater cov-
erage whereas satellites at lower altitudes orbit more rapidly, permitting more
frequent ground station updates. The June 1972 constellation study by Buisson
and McCaskill proposed a three-by-nine constellation (three orbital planes
with nine satellites each) at eight-hour orbits with evenly spaced planes.