GPS Signal Structure. Sources: GPS Satellite Surveying, Leick Kristine Larson Lecture Notes http://www.colorado.edu/engineering/ASEN/asen4519/asen4519.html. GPS Signal Requirements. Method (code) to identify each satellite
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Modulo-2 arithmetic: 0 + 0 = 0; 0 + 1 = 1; 1 + 0 = 1; 1 + 1 = 0
Bit shifts aligned
MUST MOD-2 ADD RECEIVER-GENERATED CODE TO RECOVER
All spectral components of y1(t) are 90° out of phase
with those of y2(t). This allows this the two signals to
be separated in the receiver.
Note that C/A code is below noise
level; signal is multiplied in the
Receiver by the internally calculated
code to allow tracking.
C/A-code chip is 1.023 Mhz
P-code chip is 10.23 Mhz
Power = P(t) = y2(t)
The calculated power spectrum
derives from the Fourier
transform of a square wave
of width 2π and unit amplitude.
Common function in DSP
called the “sinc” function.
Correlation of receiver generated PRN code (A) with incoming data
stream consisting of multiple (e.g. four, A, B, C, and D) codes
Since C/A-code is 1023 chips long and repeats every 1/1000 s, it is inherently ambiguous by 1 msec or ~300 km. Must modulo-2 add the transmitted and received codes after correlation to increase SNR and narrow bandwidth.
Ashtech Z-12 & µZ
From Ashjaee & Lorenz, 1992