Global Positioning System Reference
In-Depth Information
onstrating a major advance in navigation. Third, it was close to both nrl and
the Pentagon. The successful demonstration yielded further funding. An arti-
cle published to mark the fortieth anniversary of the test stated,
The plan was to take one measurement at Time of Closest Approach [tca]
and one on either side between tca and the horizon. As the data streamed
in, [Don] Lynch called out which set of side tone readings should be resolved.
Alick Frank then read the chart recorder deflections to James [Buisson, an
nrl physicist], who resolved them and passed the result back to Lynch to
be plotted on the intercept chart on the makeshift table. “The three points
that Don chose were beautiful. They intersected very closely,” Buisson rem-
inisced later. After thirteen action-packed minutes, the satellite sank below
the horizon and the pens fell silent. 25
Easton wrote in the May 1967 slide show referenced earlier that constella-
tion studies were deferred due to budget limitations but that both geosynchro-
nous and midaltitude (mainly eight- and twelve-hour) circular polar orbits
were being considered. Other tests were done with cars and boats, sometimes
with unexpected occurrences. James Buisson and other nrl scientists were in
a government van in Virginia, stopped on the side of the road waiting for the
Timation I satellite to rise so they could track it. Buisson recalled, “A police-
man stopped and asked us what we were doing. After we gave him a very
detailed and long explanation about our atomic clock and the satellite, etc., he
gave up trying to understand and said something like, 'Good luck on your exper-
iment,' and he drove off.”26 26
Different Needs, Different Systems
Rival navigation systems were proposed in the 1960s. The Joint Chiefs of Staff 's
Navigation Study Panel specified that navigation systems provide three-
dimensional instantaneous position fixes worldwide within a specified accu-
racy. Accuracy in a satellite navigation system depends on many factors,
including the accuracy of individual clocks, the synchronization between them,
precise knowledge of each satellite's orbit, and corrections for changes in the
signals caused by ionospheric distortion. Comparing two signals with differ-
ent frequencies permits calculating and correcting for ionospheric distortion,
so Timation II, launched in 1969, broadcast on two frequencies. nrl scientists
extensively studied possible constellation configurations for Timation and
determined that twenty-seven satellites, nine each in three evenly spaced orbital
 
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