Editing Global Positioning System (section)

=== Message format ===
:{|class="wikitable" style="float:right; margin:0 0 0.5em 1em;" border="1"
|+ {{nowrap|GPS message format}}
! Subframes !! Description
|-
| 1 || Satellite clock,<br />GPS time relationship
|-
| 2–3 || Ephemeris<br />(precise satellite orbit)
|-
| 4–5 || Almanac component<br />(satellite network synopsis,<br />error correction)
|}

Each GPS satellite continuously broadcasts a ''navigation message'' on L1 (C/A and P/Y) and L2 (P/Y) frequencies at a rate of 50 bits per second (see [[bitrate]]). Each complete message takes 750 seconds ({{frac|12|1|2}} minutes) to complete. The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long. Subframes 4 and 5 are [[commutation (telemetry)|subcommutated]] 25 times each, so that a complete data message requires the transmission of 25 full frames. Each subframe consists of ten words, each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes in a frame times 25 frames in a message, each message is 37,500 bits long. At a transmission rate of 50-bit/s, this gives 750 seconds to transmit an entire [[GPS Almanac|almanac message (GPS)]]. Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.<ref>{{cite web |url=http://gpsinformation.net/gpssignal.htm |title=Satellite message format |publisher=Gpsinformation.net |access-date=October 15, 2010 |archive-url=https://web.archive.org/web/20101101021138/http://gpsinformation.net/gpssignal.htm |archive-date=November 1, 2010 |url-status=live }}</ref>

The first subframe of each frame encodes the week number and the time within the week,<ref>{{cite web |author=Dana |first=Peter H. |title=GPS Week Number Rollover Issues |url=http://www.colorado.edu/geography/gcraft/notes/gps/gpseow.htm |archive-url=https://web.archive.org/web/20130225182002/http://www.colorado.edu/geography/gcraft/notes/gps/gpseow.htm |archive-date=February 25, 2013 |access-date=August 12, 2013}}</ref> as well as the data about the health of the satellite. The second and the third subframes contain the ''[[ephemeris]]'' – the precise orbit for the satellite. The fourth and fifth subframes contain the ''almanac'', which contains coarse<!-- "Coarse" is correct, as in "not precision"--> orbit and status information for up to 32 satellites in the constellation as well as data related to error correction. Thus, to obtain an accurate satellite location from this transmitted message, the receiver must demodulate the message from each satellite it includes in its solution for 18 to 30 seconds. To collect all transmitted almanacs, the receiver must demodulate the message for 732 to 750 seconds or {{frac|12|1|2}} minutes.<ref>{{cite web|url=https://www.losangeles.spaceforce.mil/ |title=Interface Specification IS-GPS-200, Revision D: Navstar GPS Space Segment/Navigation User Interfaces |publisher=Navstar GPS Joint Program Office |page=103 |url-status=live |archive-url=https://web.archive.org/web/20120908003700/http://www.losangeles.af.mil/shared/media/document/AFD-070803-059.pdf |archive-date=September 8, 2012 }}</ref>

All satellites broadcast at the same frequencies, encoding signals using unique [[code-division multiple access]] (CDMA) so receivers can distinguish individual satellites from each other. The system uses two distinct CDMA encoding types: the coarse<!-- "Coarse" is correct, as in "not precision"-->/acquisition (C/A) code, which is accessible by the general public, and the precise (P(Y)) code, which is encrypted so that only the U.S. military and other NATO nations who have been given access to the encryption code can access it.<ref>{{cite book
|title=Satellite Systems for Personal Applications: Concepts and Technology
|first1=Madhavendra
|last1=Richharia
|first2=Leslie David
|last2=Westbrook
|publisher=John Wiley & Sons
|year=2011
|isbn=978-1-119-95610-5
|page=443
|url=https://books.google.com/books?id=MqPQ5CbgQ48C&pg=PT443
|access-date=February 28, 2017
|archive-url=https://web.archive.org/web/20140704134423/http://books.google.com/books?id=MqPQ5CbgQ48C&pg=PT443
|archive-date=July 4, 2014
|url-status=live
}}</ref>

The ephemeris is updated every 2 hours and is sufficiently stable for 4 hours, with provisions for updates every 6&nbsp;hours or longer in non-nominal conditions. The almanac is updated typically every 24&nbsp;hours. Additionally, data for a few weeks following is uploaded in case of transmission updates that delay data upload.{{citation needed|date=April 2021}}