One System, Two Narratives - GPS Declassified: From Smart Bombs to Smartphones - page 62

Parkinson often focuses on the techniques used to determine the precise loca-
tions of the satellites in their orbits, rather than the configuration itself, as he
did in a 1999 interview:
For example, Transit gave us orbit determination. They really knew how to
do that, and we needed it, because gps satellites have to know where they
are. Roger Easton brought the atomic clock technology forward, and we
needed very stable time because in essence gps acts as a one-way radar.
That is enabled by knowing precisely when the gps signal was generated.
The third contributions are the digital signal structure and the concept of
operation. The Air Force was pretty close, and we took their signal structure
and refined it further, added some features to it that weren't there. All these
ideas contributed to the final gps of 1973 (which is still essentially
unchanged). 24
This may overstate Transit's role in gps. Two Timation satellites had already
been launched, and nrl tracked their positions precisely enough to estimate
the positions of receivers. Easton had designed two major space-tracking sys-
tems, Minitrack and Space Surveillance, so he knew how to track satellites.
And the problems posed by gravitational variations are far less for satellites at
higher, eight-thousand- or twelve-thousand-mile orbits than they were for the
Transit satellites at roughly seven-hundred-mile orbits. “The gps high altitude
satellites will avoid the drag which continuously changes the orbits of the tran-
sit birds,” Holmes wrote in an article published in the Naval Institute's Pro-
ceedings magazine. 25 Aviation Week & Space Technology , which closely covered
satellite navigation progress, reported, “The ability of usaf and Navy to resolve
their long-standing differences over the orbital configuration by basically adopt-
ing the Navy-proposed constellation arrangement has eliminated one of the
major obstacles to Pentagon approval for the program.” 26
Three alternatives are presented in the subsequent Development Concept
Paper Number 133, dated November 26, 1973. The first was to stop develop-
ment leading to gps. The second was to launch four synchronous repeater
Navigation Development Satellites (a 621b constellation) and three nts satel-
lites. The third, which the paper's authors strongly advocated, was to launch
two nts satellites and three subsynchronous Navigation Development Satel-
lites. This third option, with modifications, is what occurred. The 621b satel-
lites were canceled, and the Timation satellites were launched under different
names.