Editing Global Positioning System (section)

=== Satellite frequencies ===
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|+ {{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>