Viavi 87339, Operation Manual

Page: 121 / 198

Principals of Operation
6 -4
With the GPS satellites in constant motion, the number of satellites in view and their
relative location is dynamic. A 24 satellite configuration provides adequate satellite
coverage to perform three-dimensional position fixing. Failures of satellites and/or the
requirement for more than four satellites may result in inadequate satellite coverage.
Fig. 6-4 GPS SV Block Schematic
Fig. 6-5 GPS SV Signal Data Structure
Each GPS satellite transmits a unique signature assigned to it on the same carrier
frequency. This signature consists of a Pseudo Random Noise (PRN) Code of 1023 zeros
and ones, broadcast with a duration of 1 ms and continually repeated. The PRN code is
exclusively OR’d (modulo 2 added on), with 50 bit/s Navigation Data (Nav Data). The
combined code is then used to BPSK modulate a 1575.42 MHz carrier frequency. The
resultant signal is spread spectrum.