Going Mainstream - GPS Declassified: From Smart Bombs to Smartphones - page 126

nal when it chose in 1991 not to use gps to time its networks in five states. 29
Private investors also had to contemplate whether the government would reli-
ably maintain the system over the long term and whether service would remain
free of charge or be subject to taxes or user fees.
From the military's perspective, national security concerns trumped poten-
tial domestic or worldwide economic benefits. Following the successful debut
of gps in the Persian Gulf War, U.S. military leaders feared that enemies would
soon take advantage of the signals for hostile use. gps capability offered Third
World adversaries an inexpensive way to upgrade scud missiles or even to
launch what the Pentagon termed “the poor man's cruise missile” against the
United States or U.S. interests abroad. 30 The strategy to counter this threat was
to carefully limit access to the more accurate, encrypted, and jam-resistant
military signal (the Precise Positioning Service, or pps) and to “dither” the
civilian signal (the Standard Positioning Service, or sps), rendering it less accu-
rate. In late August 1991, a few months after the Air Force reactivated Selec-
tive Availability following the war, the U.S. government revised its international
trade regulations to reflect the dichotomy, classifying civilian receivers as unre-
stricted “general destination items” but maintaining tight export restrictions
on military-grade receivers, treating them as “munitions.” 31 By December
1993, the Department of Defense reported that about eleven thousand military-
grade gps devices had been sold or provided to allies, with nearly nine thou-
sand going to Europe and hundreds each to Japan, Israel, Canada, South Korea,
and Australia. 32
Even as the military degraded the civilian signal, Pentagon officials acknowl-
edged “an inherent quandary” posed by simultaneous multimillion-dollar
programs the Coast Guard and Federal Aviation Administration pursued to
boost sps accuracy for enhanced safety around seaports and airports. 33 These
local augmentation systems used the technique known as differential gps
(dgps), which employs gps receivers and ground-based radio beacons at fixed
locations with precisely surveyed latitude, longitude, and elevation coordi-
nates. The reference stations make it possible for computers to detect tiny
inaccuracies in the signals from space, calculate the differences, and transmit
corrections to users via the ground beacons. At that time Selective Availability
reduced the civilian signal's accuracy to within about 328 feet (100 meters),
and the precise military signal was accurate to within about 38 feet, but dgps
could provide accuracies within about 10 feet. 34 While the military could pre-
sumably shut down dgps systems serving U.S. territory in the event of an attack,