Invisible Stars - GPS Declassified: From Smart Bombs to Smartphones - page 77

modulation) is generally clearer than am radio (based on amplitude modula-
tion). gps signals are quite weak after their twelve-thousand-mile journey, so
spread-spectrum techniques help receivers pick them out of the cacophony of
radio signals that fill the airwaves. 28 Military engineers utilize spread-spectrum
techniques to make radio communications less susceptible to enemy interfer-
ence (radio jamming is essentially noise) and to scramble signals so that enemy
eavesdroppers perceive transmissions as noise. 29
gps uses a type of spread-spectrum code called pseudorandom noise (prn)
to digitize information about each satellite and the navigation data that receiv-
ers use to determine location. Each satellite broadcasts three prn codes: the
coarse acquisition (C/A) code, the precision (P) code, and the (Y) code, which
replaces the P code whenever the military activates anti-spooing measures
designed to defeat intentionally misleading counterfeit signals. 30 When a user
turns on a gps device the receiver attempts to “acquire” or lock onto signals
from as many satellites or other sources as its circuitry is designed to handle.
Signals from a satellite directly overhead take about six-hundredths of a sec-
ond to reach the ground. Receivers first detect the C/A code, which modulates
l1 and repeats one thousand times every second. 31 This quick repetition helps
receivers acquire satellite signals faster. The longer P code repeats on a seven-
day cycle, making it harder to acquire. Because civilians use the C/A code and
the P and Y codes are reserved for military use, many references speak of the
two in combination as the P(Y) code. Most civilian receivers access only l1,
while military users access both l1 and l2; accessing both helps to correct for
atmospheric degradation. Otherwise, the accuracy of the civilian and military
signals as they travel through space is the same. 32
Each satellite transmits a unique C/A code. This identifier is different from
the satellite's space vehicle number, so it appears separately, as the satellite's
prn code, on the Coast Guard Navigation Center's Constellation Status web-
site. Another spread-spectrum technique, called code division multiple access
(cdma) arranges the digital sequences in a way that allows receivers to differ-
entiate between numerous satellites transmitting similar codes on the same
frequency.
Synchronicity
Aboard each satellite is an atomic clock, along with two or more backup clocks
in case of malfunction. Extremely precise timing is essential for gps accuracy,
and a number of technological advances in timekeeping had to occur before