Editing Global Positioning System (section)

== Communication ==
{{Main|GPS signals}}

The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military.<ref>{{cite web |title=GPS.gov: Performance Standards & Specifications |url=https://www.gps.gov/technical/ps/ |website=www.gps.gov |access-date=June 21, 2024}}</ref>

=== 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}}

=== Satellite frequencies ===
:{|class="wikitable" style="float:right; width:30em; margin:0 0 0.5em 1em;" border="1"
|+ {{nowrap|GPS frequency overview<ref name="handbook-pent">{{cite book|last1=Penttinen|first1=Jyrki T.J.|title=The Telecommunications Handbook: Engineering Guidelines for Fixed, Mobile and Satellite Systems|publisher=John Wiley & Sons|isbn=978-1-119-94488-1|url=https://books.google.com/books?id=HRQmBgAAQBAJ|language=en|date=2015}}</ref>{{rp|607}}}}
! Band !! Frequency !! Description
|-
| '''L1''' || 1575.42&nbsp;MHz || Coarse-acquisition<!-- "Coarse" is correct, as in "not precision"--> (C/A) and encrypted precision (P(Y)) codes, plus the L1 civilian ([[GPS signals#L1C|L1C]]) and military (M) codes on Block III and newer satellites.
|-
| '''L2''' || 1227.60&nbsp;MHz || P(Y) code, plus the [[L2C]] and military codes on the Block IIR-M and newer satellites.
|-
| '''L3''' || 1381.05&nbsp;MHz || Used for nuclear detonation (NUDET) detection.
|-
| '''L4''' || 1379.913&nbsp;MHz || Being studied for additional ionospheric correction.
|-
| '''L5''' || 1176.45&nbsp;MHz || Used as a civilian safety-of-life (SoL) signal on Block IIF and newer satellites.
|}

All satellites broadcast at the same two frequencies, 1.57542&nbsp;GHz (L1 signal) and 1.2276&nbsp;GHz (L2&nbsp;signal). The satellite network uses a CDMA spread-spectrum technique<ref name="handbook-pent" />{{rp|607}} where the low-bitrate message data is encoded with a high-rate [[pseudorandom number generator|pseudo-random]] (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A&nbsp;code, for civilian use, transmits data at 1.023&nbsp;million [[chip (CDMA)|chips]] per second, whereas the P&nbsp;code, for U.S. military use, transmits at 10.23&nbsp;million chips per second. The actual internal reference of the satellites is 10.22999999543&nbsp;MHz to compensate for [[Theory of relativity|relativistic effects]]<ref>{{cite book|title=Global Positioning System. Signals, Measurements and Performance|edition=2nd|first1=Pratap|last1=Misra|first2=Per|last2=Enge|publisher=Ganga-Jamuna Press|year=2006|isbn=978-0-9709544-1-1|page=115|url={{google books|plainurl=y|id=pv5MAQAAIAAJ}}|access-date=August 16, 2013}}</ref><ref>{{cite book
|title=A Software-Defined GPS and Galileo Receiver. A single-Frequency Approach|first1=Kai|last1=Borre|first2=Dennis|last2=M. Akos|first3=Nicolaj|last3=Bertelsen|first4=Peter|last4=Rinder|first5=Søren Holdt|last5=Jensen|publisher=Springer|year=2007|isbn=978-0-8176-4390-4|page=18|url={{google books|plainurl=y|id=x2g6XTEkb8oC}}}}</ref> that make observers on the Earth perceive a different time reference with respect to the transmitters in orbit. The L1&nbsp;carrier is modulated by both the C/A and P&nbsp;codes, while the L2&nbsp;carrier is only modulated by the P&nbsp;code.<ref name=avionicswest /> The P&nbsp;code can be encrypted as a so-called P(Y)&nbsp;code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y)&nbsp;codes impart the precise time-of-day to the user.

The L3 signal at a frequency of 1.38105&nbsp;GHz is used to transmit data from the satellites to ground stations. This data is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and near space.<ref>{{cite web |url=https://fas.org/spp/military/program/nssrm/initiatives/usnds.htm |title=United States Nuclear Detonation Detection System (USNDS) |website=Fas.org |access-date=November 6, 2011 |archive-url=https://web.archive.org/web/20111010123718/http://www.fas.org/spp/military/program/nssrm/initiatives/usnds.htm |archive-date=October 10, 2011 |url-status=dead }}</ref> One usage is the enforcement of nuclear test ban treaties.

The L4 band at 1.379913&nbsp;GHz is being studied for additional ionospheric correction.<ref name="handbook-pent" />{{rp|607}}

The L5 frequency band at 1.17645&nbsp;GHz was added in the process of [[GPS modernization]]. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that provides this signal was launched in May 2010.<ref name="dailytech1">{{cite news |url=http://www.dailytech.com/First+Block+2F+GPS+Satellite+Launched+Needed+to+Prevent+System+Failure/article18483.htm |title=First Block 2F GPS Satellite Launched, Needed to Prevent System Failure |work=DailyTech |access-date=May 30, 2010 |archive-url=https://web.archive.org/web/20100530023659/http://www.dailytech.com/First+Block+2F+GPS+Satellite+Launched+Needed+to+Prevent+System+Failure/article18483.htm |archive-date=May 30, 2010 }}</ref> On February 5, 2016, the 12th and final Block IIF satellite was launched.<ref>{{cite web|url=https://www.ulalaunch.com/about/news-detail/2016/02/05/united-launch-alliance-successfully-launches-gps-iif-12-satellite-for-u.s.-air-force|title=United Launch Alliance Successfully Launches GPS IIF-12 Satellite for U.S. Air Force|website=www.ulalaunch.com|access-date=February 27, 2018|archive-url=https://web.archive.org/web/20180228161519/https://www.ulalaunch.com/about/news-detail/2016/02/05/united-launch-alliance-successfully-launches-gps-iif-12-satellite-for-u.s.-air-force|archive-date=February 28, 2018|url-status=live}}</ref> The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. "L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy."<ref>{{cite web|title=Air Force Successfully Transmits an L5 Signal From GPS IIR-20(M) Satellite |url=https://www.losangeles.spaceforce.mil/?storyID=123144001 |publisher=LA AFB News Release |access-date=June 20, 2011 |url-status=live |archive-url=https://web.archive.org/web/20110521025953/http://www.losangeles.af.mil/news/story.asp?storyID=123144001 |archive-date=May 21, 2011 }}</ref>

{{update|section|date=May 2021}}

In 2011, a conditional waiver was granted to [[LightSquared]] to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify issues that might arise due to the larger signal power from the LightSquared terrestrial network. The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, essentially repurposing what was supposed to be a "quiet neighborhood" for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the effects from the lower 10&nbsp;MHz of spectrum are minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10&nbsp;MHz intended for use by LightSquared may have some effect on GPS devices. There is some concern that this may seriously degrade the GPS signal for many consumer uses.<ref name="gpsworld-20110301">{{cite web|url=https://www.gpsworld.com/the-system-test-data-predicts-disastrous-gps-jamming-by-fcc-authorized-broadcaster/ |title=The System: Test Data Predicts Disastrous GPS Jamming by FCC-Authorized Broadcaster |date=March 1, 2011 |publisher=GPS World |access-date=November 6, 2011 |archive-url=https://web.archive.org/web/20111011082258/http://www.gpsworld.com/gnss-system/news/data-shows-disastrous-gps-jamming-fcc-approved-broadcaster-11029 |archive-date=October 11, 2011 |url-status=live }}</ref><ref>{{cite web|url=http://www.saveourgps.org/studies-reports.aspx|title=Coalition to Save Our GPS|publisher=Saveourgps.org|access-date=November 6, 2011|archive-url=https://web.archive.org/web/20111030072958/http://saveourgps.org/studies-reports.aspx|archive-date=October 30, 2011}}</ref> ''[[Aviation Week]]'' magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system.<ref name="aviationweek1">{{cite magazine|title=LightSquared Tests Confirm GPS Jamming |url=http://www.aviationweek.com/aw/generic/story.jsp?id=news/awx/2011/06/09/awx_06_09_2011_p0-334122.xml&headline=LightSquared%20Tests%20Confirm%20GPS%20Jamming&channel=busav |magazine=Aviation Week |access-date=June 20, 2011 |archive-url=https://web.archive.org/web/20110812045607/http://www.aviationweek.com/aw/generic/story.jsp?id=news%2Fawx%2F2011%2F06%2F09%2Fawx_06_09_2011_p0-334122.xml&headline=LightSquared%20Tests%20Confirm%20GPS%20Jamming&channel=busav |archive-date=August 12, 2011 }}</ref>

=== 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]].