Editing Global Positioning System (section)

=== Demodulation and decoding ===
<!-- Demodulation is done with carrier frequency; decoding is done with Gold Code. -->

[[File:gps ca gold.svg|thumb|right|upright=0.8|Demodulating and Decoding GPS Satellite Signals using the Coarse<!-- "Coarse" is correct, as in "not precision"-->/Acquisition [[Gold code]]]]

Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary [[sequence]] known as a [[Gold code]]. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver.<ref>{{cite web|url=http://www.navcen.uscg.gov/?pageName=gpsAlmanacs|title=GPS Almanacs, NANUS, and Ops Advisories (including archives)|publisher=United States Coast Guard|work=GPS Almanac Information|access-date=September 9, 2009|archive-url=https://web.archive.org/web/20100712223936/http://www.navcen.uscg.gov/?pageName=gpsAlmanacs|archive-date=July 12, 2010|url-status=live}}</ref><ref>"George, M., Hamid, M.; and Miller, A. {{PDFWayback|date=20071122063244|url=http://www.xilinx.com/support/documentation/application_notes/xapp217.pdf|archive-url=https://web.archive.org/web/20071122063244/http://www.xilinx.com/support/documentation/application_notes/xapp217.pdf|archive-date=2007-11-22|url-status=live|title=Gold Code Generators in Virtex Devices|126&nbsp;KB}}.</ref>

If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range&nbsp;1 through 32. If the almanac information is not in memory, the receiver enters a search mode until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A&nbsp;code pattern. There can be a delay of up to 30&nbsp;seconds before the first estimate of position because of the need to read the ephemeris data.

Processing of the navigation message enables the determination of the time of transmission and the satellite position at this time. For more information see [[GPS signals#Demodulation and decoding|Demodulation and Decoding, Advanced]].