signifiying

signifying full availability of the military’s secure Precise Positioning Service (PPS).[59]
In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[60] declaring GPS a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
In 1998, United States Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety, and in 2000 the United States Congress authorized the effort, referring to it as GPS III.
On May 2, 2000 “Selective Availability” was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally.
In 2004, the United States government signed an agreement with the European Community establishing cooperation related to GPS and Europe’s Galileo system.
In 2004, United States President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.[61]
In November 2004, Qualcomm announced successful tests of assisted GPS for mobile phones.[62]
In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.[63]
On September 14, 2007, the aging mainframe-based Ground segment Control System was transferred to the new Architecture Evolution Plan.[64]
On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.[65]
On May 21, 2009, the Air Force Space Command allayed fears of GPS failure, saying: “There’s only a small risk we will not continue to exceed our performance standard.”[66]
On January 11, 2010, an update of ground control systems caused a software incompatibility with 8,000 to 10,000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, California.[clarification needed][67]
On February 25, 2010,[68] the U.S. Air Force awarded the contract to Raytheon Company to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.
July 24, 2020, operation of the GPS constellation is transferred to the newly established U.S. Space Force as part of its establishment.[69]

Emblem of the 2nd Space Operations Squadron – the unit responsible for operating the constellation
On October 13, 2023, the Space Force activated PNT Delta (Provisional) to manage US navigation warfare assets. 2SOPS and GPS operations were realigned under this new Delta.[69]
Awards
boosters required to launch them into orbit. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits,[88] but this was modified to six orbital planes with four satellites each.[89] The six orbit planes have approximately 55° inclination (tilt relative to the Earth’s equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit’s intersection).[90] The orbital period is one-half of a sidereal day, about 11 hours and 58 minutes, so that the satellites pass over the same locations[91] or almost the same locations[92] every day. The orbits are arranged so that at least six satellites are always within line of sight from everywhere on the Earth’s surface (see animation at right).[93] The result of this objective is that the four satellites are not evenly spaced (90°) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30°, 105°, 120°, and 105° apart, which sum to 360°.[94]

Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi),[95] each SV makes two complete orbits each sidereal day, repeating the same ground track each day.[96] This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

As of February 2019,[97] there are 31 satellites in the GPS constellation, 27 of which are in use at a given time with the rest allocated as stand-bys. A 32nd was launched in 2018, but as of July 2019 is still in evaluation. More decommissioned satellites are in orbit and available as spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve accuracy but also improves reliability and availability of the system, relative to a uniform system, when multiple satellites fail.[98] With the expanded constellation, nine satellites are usually visible at any time from any point on the Earth with a clear horizon, ensuring considerable redundancy over the minimum four satellites needed for a position.

Control segment
1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique[168]: 607  where the low-bitrate message data is encoded with a high-rate 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 code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The actual internal reference of the satellites is 10.22999999543 MHz to compensate for relativistic effects[169][170] that make observers on the Earth perceive a different time reference with respect to the transmitters in orbit. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[94] The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

The L3 signal at a frequency of 1.38105 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.[171] One usage is the enforcement of nuclear test ban treaties.

The L4 band at 1.379913 GHz is being studied for additional ionospheric correction.[168]: 607 

The L5 frequency band at 1.17645 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.[172] On February 5, 2016, the 12th and final Block IIF satellite was launched.[173] 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.”[174]

Learn more
This section needs to be updated. (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 MHz of spectrum are minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10 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.[175][176] Aviation Week magazine reports that the latest testing (June 2011) confirms “significant jamming” of GPS by LightSquared’s system.[177]

Demodulation and decoding

Clickable image, highlighting medium altitude orbits around Earth,[b] from Low Earth to the lowest High Earth orbit (geostationary orbit and its graveyard orbit, at one ninth of the Moon’s orbital distance),[c] with the Van Allen radiation belts and the Earth to scale
Following the United States’s deployment of GPS, other countries have also developed their own satellite navigation systems. These systems include:

The Russian Global Navigation Satellite System (GLONASS) was developed at the same time as GPS, but suffered from incomplete coverage of the globe until the mid-2000s.[218] GLONASS reception in addition to GPS can be combined in a receiver thereby allowing for additional satellites available to enable faster position fixes and improved accuracy, to within two meters (6.6 ft).[219][220] In October 2011, the full orbital constellation of 24 satellites enabled full global coverage. The GLONASS satellites’ designs have undergone several upgrades, with the latest version, GLONASS-K2, launched in 2023.[221]
China’s BeiDou Navigation Satellite System began global services in 2018 and finished its full deployment in 2020. It consists of satellites in three different orbits, including 24 satellites in medium-circle orbits (covering the world), 3 satellites in inclined geosynchronous orbits (covering the Asia-Pacific region), and 3 satellites in geostationary orbits (covering China).[222]
The Galileo navigation satellite system, a global system being developed by the European Union and other partner countries, began operation in 2016,[223] and has been fully deployed by 2020. In November 2018, the FCC approved use of Galileo in the US.[224] As of September 2024, there are 25 launched satellites that operate in the constellation.[225][226][227] It is expected that the next generation of satellites will begin to become operational after 2026 to replace the first generation, which can then be used for backup capabilities.
Japan’s Quasi-Zenith Satellite System (QZSS) is a GPS satellite-based augmentation system to enhance GPS’s accuracy in Asia-Oceania, with satellite navigation independent of GPS scheduled for 2023.[228]
The Indian Regional Navigation Satellite System (Operational name ‘NavIC’, Navigation with Indian Constellation), deployed by India.
Backup system
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In the event of adverse space weather or the deployment of an anti-satellite weapon against GPS, the United States has no terrestrial backup system. The potential cost of such an event to the U.S. economy is estimated at $1 billion per day. The LORAN-C system was turned off in North America in 2010 and Europe in 2015. eLoran is proposed as an American terrestrial backup system, but as of 2024 has not received approval or funding.[229]

China continues to operate LORAN-C transmitters,[230] and Russia has a similar system called CHAYKA (“Seagull”).