The Global Positioning
System

System (GPS), originally Navstar GPS,[1] is a
satellite-based radionavigation system owned by the United States
government and operated by the
United States

United States Air Force.[2] It is a
global navigation satellite system that provides geolocation and time
information to a
GPS receiver

GPS receiver anywhere on or near the
Earth
.jpg/600px-Planets_everywhere_(artist’s_impression).jpg)
Earth where
there is an unobstructed line of sight to four or more GPS
satellites.[3] Obstacles such as mountains and buildings block the
relatively weak
GPS

GPS signals.
The
GPS

GPS does not require the user to transmit any data, and it
operates independently of any telephonic or internet reception, though
these technologies can enhance the usefulness of the
GPS

GPS positioning
information. The
GPS

GPS provides critical positioning capabilities to
military, civil, and commercial users around the world. The United
States government created the system, maintains it, and makes it
freely accessible to anyone with a
GPS

GPS receiver.
The
GPS

GPS project was launched by the
U.S. Department of Defense

U.S. Department of Defense in 1973
for use by the
United States

United States military and became fully operational in
1995. It was allowed for civilian use in the 1980s. Advances in
technology and new demands on the existing system have now led to
efforts to modernize the
GPS

GPS and implement the next generation of GPS
Block IIIA satellites and Next Generation Operational Control System
(OCX).[4] Announcements from Vice President
Al Gore

Al Gore and the White
House in 1998 initiated these changes. In 2000, the U.S. Congress
authorized the modernization effort,
GPS

GPS III. During the 1990s, GPS
quality was degraded by the
United States

United States government in a program
called "Selective Availability", however, this is no longer the case,
and was discontinued in May 2000 by law signed by former President
Bill Clinton.[5] New
GPS receiver

GPS receiver devices using the L5 frequency to
begin release in 2018 are expected to have a much higher accuracy and
pinpoint a device to within 30 centimeters or just under one
foot.[6][7]
The
GPS

GPS system is provided by the
United States

United States government, which can
selectively deny access to the system, as happened to the Indian
military in 1999 during the Kargil War, or degrade the service at any
time.[8] As a result, a number of countries have developed or are in
the process of setting up other global or regional navigation systems.
The Russian Global
Navigation

Navigation
Satellite

Satellite
System

System (GLONASS) was developed
contemporaneously with GPS, but suffered from incomplete coverage of
the globe until the mid-2000s.[9]
GLONASS

GLONASS can be added to
GPS

GPS devices,
making more satellites available and enabling positions to be fixed
more quickly and accurately, to within two meters.[10] China's BeiDou
Navigation

Navigation
Satellite

Satellite
System

System is due achieve global reach in 2020. There
are also the
European Union

European Union Galileo positioning system, and India's
NAVIC. Japan's
Quasi-Zenith Satellite System

Quasi-Zenith Satellite System (scheduled to commence in
November 2018) will be a
GPS

GPS satellite-based augmentation system to
enhance GPS's accuracy.
Contents
1 History
1.1 Predecessors
1.2 Development
1.3 Timeline and modernization
1.4 Awards
2 Basic concept of GPS
2.1 Fundamentals
2.2 More detailed description
2.3 User-satellite geometry
2.4 Receiver in continuous operation
2.5 Non-navigation applications
3 Structure
3.1 Space segment
3.2 Control segment
3.3 User segment
4 Applications
4.1 Civilian
4.1.1 Restrictions on civilian use
4.2 Military
5 Communication
5.1 Message format
5.2
Satellite

Satellite frequencies
5.3 Demodulation and decoding
6
Navigation

Navigation equations
6.1 Problem description
6.2 Geometric interpretation
6.2.1 Spheres
6.2.2 Hyperboloids
6.2.3 Inscribed sphere
6.2.4 Spherical cones
6.3 Solution methods
6.3.1 Least squares
6.3.2 Iterative
6.3.3 Closed-form
7 Error sources and analysis
8 Accuracy enhancement and surveying
8.1 Augmentation
8.2 Precise monitoring
8.3 Timekeeping
8.3.1 Leap seconds
8.3.2 Accuracy
8.3.3 Format
8.4 Carrier phase tracking (surveying)
9 Regulatory spectrum issues concerning
GPS

GPS receivers
10 Other systems
11 See also
12 Notes
13 References
14 Further reading
15 External links
History[edit]
The
GPS

GPS project was launched in the
United States

United States in 1973 to overcome
the limitations of previous navigation systems,[11] integrating ideas
from several predecessors, including a number of classified
engineering design studies from the 1960s. The U.S. Department of
Defense developed the system, which originally used 24 satellites. It
was initially developed for use by the
United States

United States military and
became fully operational in 1995. Civilian use was allowed from the
1980s.
Roger L. Easton

Roger L. Easton of the Naval Research Laboratory, Ivan A.
Getting of The Aerospace Corporation, and
Bradford Parkinson

Bradford Parkinson of the
Applied Physics Laboratory are credited with inventing it.[12]
The design of
GPS

GPS is based partly on similar ground-based
radio-navigation systems, such as
LORAN

LORAN and the Decca Navigator,
developed in the early 1940s.
Predecessors[edit]
When the
Soviet Union
.jpg/460px-Soviet_Union-1964-stamp-Chapayev_(film).jpg)
Soviet Union launched the first artificial satellite (Sputnik
1) in 1957, two American physicists, William Guier and George
Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory
(APL) decided to monitor its radio transmissions.[13] Within hours
they realized that, because of the Doppler effect, they could pinpoint
where the satellite was along its orbit. The Director of the APL gave
them access to their UNIVAC to do the heavy calculations required.
The next spring, Frank McClure, the deputy director of the APL, asked
Guier and Weiffenbach to investigate the inverse problem —
pinpointing the user's location, given that of the satellite. (At the
time, the Navy was developing the submarine-launched Polaris missile,
which required them to know the submarine's location.) This led them
and APL to develop the TRANSIT system.[14] In 1959, ARPA (renamed
DARPA
.png)
DARPA in 1972) also played a role in TRANSIT.[15][16][17]
Emblem of the 50th Space Wing
TRANSIT was first successfully tested in 1960.[18] It used a
constellation of five satellites and could provide a navigational fix
approximately once per hour.
In 1967, the U.S. Navy developed the
Timation satellite, which proved
the feasibility of placing accurate clocks in space, a technology
required for GPS.
In the 1970s, the ground-based OMEGA navigation system, based on phase
comparison of signal transmission from pairs of stations,[19] became
the first worldwide radio navigation system. Limitations of these
systems drove the need for a more universal navigation solution with
greater accuracy.
While there were wide needs for accurate navigation in military and
civilian sectors, almost none of those was seen as justification for
the billions of dollars it would cost in research, development,
deployment, and operation for a constellation of navigation
satellites. During the
Cold War

Cold War arms race, the nuclear threat to the
existence of the
United States

United States was the one need that did justify this
cost in the view of the
United States

United States Congress. This deterrent effect
is why
GPS

GPS was funded. It is also the reason for the ultra secrecy at
that time. The nuclear triad consisted of the
United States

United States Navy's
submarine-launched ballistic missiles (SLBMs) along with United States
Air Force (USAF) strategic bombers and intercontinental ballistic
missiles (ICBMs). Considered vital to the nuclear deterrence posture,
accurate determination of the SLBM launch position was a force
multiplier.
Precise navigation would enable
United States

United States ballistic missile
submarines to get an accurate fix of their positions before they
launched their SLBMs.[20] The USAF, with two thirds of the nuclear
triad, also had requirements for a more accurate and reliable
navigation system. The Navy and Air Force were developing their own
technologies in parallel to solve what was essentially the same
problem.
To increase the survivability of ICBMs, there was a proposal to use
mobile launch platforms (comparable to the Russian SS-24 and SS-25)
and so the need to fix the launch position had similarity to the SLBM
situation.
In 1960, the Air Force proposed a radio-navigation system called
MOSAIC (MObile
System

System for Accurate
ICBM

ICBM Control) that was essentially
a 3-D LORAN. A follow-on study, Project 57, was worked in
1963 and it was "in this study that the
GPS

GPS concept was born." That
same year, the concept was pursued as Project 621B, which had
"many of the attributes that you now see in GPS"[21] and promised
increased accuracy for Air Force bombers as well as ICBMs.
Updates from the Navy TRANSIT system were too slow for the high speeds
of Air Force operation. The
Naval Research Laboratory

Naval Research Laboratory continued
advancements with their
Timation (Time Navigation) satellites, first
launched in 1967, and with the third one in 1974 carrying the first
atomic clock into orbit.[22]
Another important predecessor to
GPS

GPS came from a different branch of
the
United States

United States military. In 1964, the
United States

United States Army orbited
its first Sequential Collation of Range (SECOR) satellite used for
geodetic surveying.[23] The
SECOR system included three ground-based
transmitters from known locations that would send signals to the
satellite transponder in orbit. A fourth ground-based station, at an
undetermined position, could then use those signals to fix its
location precisely. The last
SECOR satellite was launched in 1969.[24]
Development[edit]
With these parallel developments in the 1960s, it was realized that a
superior system could be developed by synthesizing the best
technologies from 621B, Transit, Timation, and
SECOR in a
multi-service program.
During Labor Day weekend in 1973, a meeting of about twelve military
officers at the Pentagon discussed the creation of a Defense
Navigation

Navigation
Satellite

Satellite
System

System (DNSS). It was at this meeting that the
real synthesis that became
GPS

GPS was created. Later that year, the DNSS
program was named Navstar, or
Navigation

Navigation
System

System Using Timing and
Ranging.[25] With the individual satellites being associated with the
name Navstar (as with the predecessors Transit and Timation), a more
fully encompassing name was used to identify the constellation of
Navstar satellites, Navstar-GPS.[26] Ten "Block I" prototype
satellites were launched between 1978 and 1985 (an additional unit was
destroyed in a launch failure).[27]
The effects of the ionosphere on radio transmission through the
ionosphere was investigated within a geophysics laboratory of Air
Force Cambridge Research Laboratory. Located at Hanscom Air Force
Base, outside Boston, the lab was renamed the Air Force Geophysical
Research Lab (AFGRL) in 1974. AFGRL developed the Klobuchar Model for
computing ionospheric corrections to
GPS

GPS location.[28] Of note is work
done by Australian Space Scientist Elizabeth Essex-Cohen at AFGRL in
1974. She was concerned with the curving of the path of radio waves
traversing the ionosphere from NavSTAR satellites.[29]
After Korean Air Lines Flight 007, a
Boeing 747

Boeing 747 carrying 269 people,
was shot down in 1983 after straying into the USSR's prohibited
airspace,[30] in the vicinity of
Sakhalin

Sakhalin and Moneron Islands,
President
Ronald Reagan

Ronald Reagan issued a directive making
GPS

GPS freely available
for civilian use, once it was sufficiently developed, as a common
good.[31] The first Block II satellite was launched on February 14,
1989,[32] and the 24th satellite was launched in 1994. The GPS
program cost at this point, not including the cost of the user
equipment, but including the costs of the satellite launches, has been
estimated at about USD 5 billion (then-year dollars).[33]
Initially, the highest quality signal was reserved for military use,
and the signal available for civilian use was intentionally degraded
(Selective Availability). This changed with President Bill Clinton
signing a policy directive to turn off
Selective Availability

Selective Availability May 1,
2000 to provide the same accuracy to civilians that was afforded to
the military. The directive was proposed by the U.S. Secretary of
Defense, William Perry, because of the widespread growth of
differential
GPS

GPS services to improve civilian accuracy and eliminate
the U.S. military advantage. Moreover, the U.S. military was actively
developing technologies to deny
GPS

GPS service to potential adversaries
on a regional basis.[34]
Since its deployment, the U.S. has implemented several improvements to
the
GPS

GPS service including new signals for civil use and increased
accuracy and integrity for all users, all the while maintaining
compatibility with existing
GPS

GPS equipment. Modernization of the
satellite system has been an ongoing initiative by the U.S. Department
of Defense through a series of satellite acquisitions to meet the
growing needs of the military, civilians, and the commercial market.
As of early 2015, high-quality, FAA grade, Standard Positioning
Service (SPS)
GPS

GPS receivers provide horizontal accuracy of better than
3.5 meters,[35] although many factors such as receiver quality and
atmospheric issues can affect this accuracy.
GPS

GPS is owned and operated by the
United States

United States government as a
national resource. The Department of Defense is the steward of GPS.
The
Interagency GPS Executive Board (IGEB) oversaw
GPS

GPS policy matters
from 1996 to 2004. After that the National Space-Based Positioning,
Navigation

Navigation and Timing Executive Committee was established by
presidential directive in 2004 to advise and coordinate federal
departments and agencies on matters concerning the
GPS

GPS and related
systems.[36] The executive committee is chaired jointly by the Deputy
Secretaries of Defense and Transportation. Its membership includes
equivalent-level officials from the Departments of State, Commerce,
and Homeland Security, the
Joint Chiefs of Staff

Joint Chiefs of Staff and NASA. Components
of the executive office of the president participate as observers to
the executive committee, and the FCC chairman participates as a
liaison.
The
U.S. Department of Defense

U.S. Department of Defense is required by law to "maintain a
Standard Positioning Service (as defined in the federal radio
navigation plan and the standard positioning service signal
specification) that will be available on a continuous, worldwide
basis," and "develop measures to prevent hostile use of
GPS

GPS and its
augmentations without unduly disrupting or degrading civilian uses."
Timeline and modernization[edit]
Main article: List of
GPS

GPS satellites
Summary of satellites[37][38][39]
Block
Launch
period
Satellite

Satellite launches
Currently in orbit
and healthy
Suc-
cess
Fail-
ure
In prep-
aration
Plan-
ned
I
1978–1985
10
1
0
0
0
II
1989–1990
9
0
0
0
0
IIA
1990–1997
19
0
0
0
0
IIR
1997–2004
12
1
0
0
12
IIR-M
2005–2009
8
0
0
0
7
IIF
2010–2016
12
0
0
0
12
IIIA
From 2017
0
0
0
12
0
IIIF
—
0
0
0
22
0
Total
70
2
0
34
31
(Last update: March 9, 2016)
8 satellites from Block IIA are placed in reserve
USA-203

USA-203 from Block IIR-M is unhealthy
[40] For a more complete list, see list of
GPS

GPS satellite launches
In 1972, the USAF Central Inertial Guidance Test Facility (Holloman
AFB) conducted developmental flight tests of four prototype GPS
receivers in a Y configuration over White Sands Missile Range, using
ground-based pseudo-satellites.[41]
In 1978, the first experimental Block-I
GPS

GPS satellite was
launched.[27]
In 1983, after Soviet interceptor aircraft shot down the civilian
airliner KAL 007 that strayed into prohibited airspace because of
navigational errors, killing all 269 people on board, U.S.
President
Ronald Reagan

Ronald Reagan announced that
GPS

GPS would be made available for
civilian uses once it was completed,[42][43] although it had been
previously published [in
Navigation

Navigation magazine] that the CA code
(Coarse/Acquisition code) would be available to civilian users.
By 1985, ten more experimental Block-I satellites had been launched to
validate the concept.
Beginning in 1988, Command & Control of these satellites was moved
from Onizuka AFS, California to the 2nd
Satellite

Satellite Control Squadron
(2SCS) located at Falcon Air Force Station in Colorado Springs,
Colorado.[44][45]
On February 14, 1989, the first modern Block-II satellite was
launched.
The
Gulf War

Gulf War from 1990 to 1991 was the first conflict in which the
military widely used GPS.[46]
In 1991, a project to create a miniature
GPS receiver

GPS receiver successfully
ended, replacing the previous 23 kg military receivers with a 1.25 kg
handheld receiver.[16]
In 1992, the 2nd Space Wing, which originally managed the system, was
inactivated and replaced by the 50th Space Wing.
By December 1993,
GPS

GPS achieved initial operational capability (IOC),
indicating a full constellation (24 satellites) was available and
providing the Standard Positioning Service (SPS).[47]
Full Operational Capability (FOC) was declared by Air Force Space
Command (AFSPC) in April 1995, signifying full availability of the
military's secure Precise Positioning Service (PPS).[47]
In 1996, recognizing the importance of
GPS

GPS to civilian users as well
as military users, U.S. President
Bill Clinton

Bill Clinton issued a policy
directive[48] declaring
GPS

GPS a dual-use system and establishing an
Interagency GPS Executive Board to manage it as a national asset.
In 1998,
United States

United States Vice President
Al Gore

Al Gore announced plans to
upgrade
GPS

GPS with two new civilian signals for enhanced user accuracy
and reliability, particularly with respect to aviation safety and in
2000 the
United States

United States Congress authorized the effort, referring to it
as
GPS

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

United States Government signed an agreement with the
European Community establishing cooperation related to
GPS

GPS and
Europe's Galileo system.
In 2004,
United States

United States President
George W. Bush

George W. Bush updated the national
policy and replaced the executive board with the National Executive
Committee for Space-Based Positioning, Navigation, and Timing.[49]
November 2004,
Qualcomm

Qualcomm announced successful tests of assisted
GPS

GPS for
mobile phones.[50]
In 2005, the first modernized
GPS

GPS satellite was launched and began
transmitting a second civilian signal (L2C) for enhanced user
performance.[51]
On September 14, 2007, the aging mainframe-based Ground Segment
Control
System

System was transferred to the new Architecture Evolution
Plan.[52]
On May 19, 2009, the
United States

United States Government Accountability Office
issued a report warning that some
GPS

GPS satellites could fail as soon as
2010.[53]
On May 21, 2009, the
Air Force Space Command

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."[54]
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

Navigation Limited of Sunnyvale,
Calif.[55]
On February 25, 2010,[56] the
U.S. Air Force

U.S. Air Force awarded the contract to
develop the
GPS

GPS Next Generation Operational Control
System

System (OCX) to
improve accuracy and availability of
GPS

GPS navigation signals, and serve
as a critical part of
GPS

GPS modernization.
Awards[edit]
On February 10, 1993, the
National Aeronautic Association selected the
GPS

GPS Team as winners of the 1992 Robert J. Collier Trophy, the nation's
most prestigious aviation award. This team combines researchers from
the Naval Research Laboratory, the USAF, the Aerospace Corporation,
Rockwell International

Rockwell International Corporation, and
IBM

IBM Federal Systems Company.
The citation honors them "for the most significant development for
safe and efficient navigation and surveillance of air and spacecraft
since the introduction of radio navigation 50 years ago."
Two
GPS

GPS developers received the National Academy of Engineering
Charles Stark Draper Prize for 2003:
Ivan Getting, emeritus president of
The Aerospace Corporation

The Aerospace Corporation and an
engineer at the Massachusetts Institute of Technology, established the
basis for GPS, improving on the
World War II

World War II land-based radio system
called
LORAN

LORAN (Long-range
Radio

Radio Aid to Navigation).
Bradford Parkinson, professor of aeronautics and astronautics at
Stanford University, conceived the present satellite-based system in
the early 1960s and developed it in conjunction with the U.S. Air
Force. Parkinson served twenty-one years in the Air Force, from 1957
to 1978, and retired with the rank of colonel.
GPS

GPS developer
Roger L. Easton

Roger L. Easton received the National Medal of
Technology on February 13, 2006.[57]
Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force
Space and Missile Pioneers Hall of Fame at Lackland A.F.B., San
Antonio, Texas, March 2, 2010 for his role in space technology
development and the engineering design concept of
GPS

GPS conducted as
part of Project 621B.
In 1998,
GPS

GPS technology was inducted into the
Space Foundation Space
Technology Hall of Fame.[58]
On October 4, 2011, the
International Astronautical Federation (IAF)
awarded the Global Positioning
System

System (GPS) its 60th Anniversary
Award, nominated by IAF member, the American Institute for Aeronautics
and
Astronautics
.jpg/440px-Hubble_Space_Telescope_over_Earth_(during_the_STS-109_mission).jpg)
Astronautics (AIAA). The IAF Honors and Awards Committee
recognized the uniqueness of the
GPS

GPS program and the exemplary role it
has played in building international collaboration for the benefit of
humanity.
Basic concept of GPS[edit]
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Fundamentals[edit]
The
GPS

GPS concept is based on time and the known position of GPS
specialized satellites. The satellites carry very stable atomic clocks
that are synchronized with one another and with the ground clocks. Any
drift from true time maintained on the ground is corrected daily. In
the same manner, the satellite locations are known with great
precision.
GPS

GPS receivers have clocks as well, but they are less stable
and less precise.
GPS

GPS satellites continuously transmit data about their current time and
position. A
GPS receiver

GPS receiver monitors multiple satellites and solves
equations to determine the precise position of the receiver and its
deviation from true time. At a minimum, four satellites must be in
view of the receiver for it to compute four unknown quantities (three
position coordinates and clock deviation from satellite time).
More detailed description[edit]
Each
GPS

GPS satellite continually broadcasts a signal (carrier wave with
modulation) that includes:
A pseudorandom code (sequence of ones and zeros) that is known to the
receiver. By time-aligning a receiver-generated version and the
receiver-measured version of the code, the time of arrival (TOA) of a
defined point in the code sequence, called an epoch, can be found in
the receiver clock time scale
A message that includes the time of transmission (TOT) of the code
epoch (in
GPS

GPS time scale) and the satellite position at that time
Conceptually, the receiver measures the TOAs (according to its own
clock) of four satellite signals. From the TOAs and the TOTs, the
receiver forms four time of flight (TOF) values, which are (given the
speed of light) approximately equivalent to receiver-satellite ranges.
The receiver then computes its three-dimensional position and clock
deviation from the four TOFs.
In practice the receiver position (in three dimensional Cartesian
coordinates with origin at the Earth's center) and the offset of the
receiver clock relative to the
GPS

GPS time are computed simultaneously,
using the navigation equations to process the TOFs.
The receiver's Earth-centered solution location is usually converted
to latitude, longitude and height relative to an ellipsoidal Earth
model. The height may then be further converted to height relative to
the geoid (e.g., EGM96) (essentially, mean sea level). These
coordinates may be displayed, e.g., on a moving map display, and/or
recorded and/or used by some other system (e.g., a vehicle guidance
system).
User-satellite geometry[edit]
Although usually not formed explicitly in the receiver processing, the
conceptual time differences of arrival (TDOAs) define the measurement
geometry. Each TDOA corresponds to a hyperboloid of revolution (see
Multilateration). The line connecting the two satellites involved (and
its extensions) forms the axis of the hyperboloid. The receiver is
located at the point where three hyperboloids intersect.[59][60]
It is sometimes incorrectly said that the user location is at the
intersection of three spheres. While simpler to visualize, this is
only the case if the receiver has a clock synchronized with the
satellite clocks (i.e., the receiver measures true ranges to the
satellites rather than range differences). There are significant
performance benefits to the user carrying a clock synchronized with
the satellites. Foremost is that only three satellites are needed to
compute a position solution. If this were part of the
GPS

GPS concept so
that all users needed to carry a synchronized clock, then a smaller
number of satellites could be deployed. However, the cost and
complexity of the user equipment would increase significantly.
Receiver in continuous operation[edit]
The description above is representative of a receiver start-up
situation. Most receivers have a track algorithm, sometimes called a
tracker, that combines sets of satellite measurements collected at
different times—in effect, taking advantage of the fact that
successive receiver positions are usually close to each other. After a
set of measurements are processed, the tracker predicts the receiver
location corresponding to the next set of satellite measurements. When
the new measurements are collected, the receiver uses a weighting
scheme to combine the new measurements with the tracker prediction. In
general, a tracker can (a) improve receiver position and time
accuracy, (b) reject bad measurements, and (c) estimate receiver speed
and direction.
The disadvantage of a tracker is that changes in speed or direction
can only be computed with a delay, and that derived direction becomes
inaccurate when the distance traveled between two position
measurements drops below or near the random error of position
measurement.
GPS

GPS units can use measurements of the
Doppler shift

Doppler shift of
the signals received to compute velocity accurately.[61] More advanced
navigation systems use additional sensors like a compass or an
inertial navigation system to complement GPS.
Non-navigation applications[edit]
For a list of applications, see § Applications.
In typical
GPS

GPS operation as a navigator, four or more satellites must
be visible to obtain an accurate result. The solution of the
navigation equations gives the position of the receiver along with the
difference between the time kept by the receiver's on-board clock and
the true time-of-day, thereby eliminating the need for a more precise
and possibly impractical receiver based clock. Applications for GPS
such as time transfer, traffic signal timing, and synchronization of
cell phone base stations, make use of this cheap and highly accurate
timing. Some
GPS

GPS applications use this time for display, or, other
than for the basic position calculations, do not use it at all.
Although four satellites are required for normal operation, fewer
apply in special cases. If one variable is already known, a receiver
can determine its position using only three satellites. For example, a
ship or aircraft may have known elevation. Some
GPS

GPS receivers may use
additional clues or assumptions such as reusing the last known
altitude, dead reckoning, inertial navigation, or including
information from the vehicle computer, to give a (possibly degraded)
position when fewer than four satellites are visible.[62][63][64]
Structure[edit]
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The current
GPS

GPS consists of three major segments. These are the space
segment, a control segment, and a user segment.[65] The U.S. Air Force
develops, maintains, and operates the space and control segments. GPS
satellites broadcast signals from space, and each
GPS receiver

GPS receiver uses
these signals to calculate its three-dimensional location (latitude,
longitude, and altitude) and the current time.[66]
Space segment[edit]
See also:
GPS (satellite)

GPS (satellite) and List of
GPS

GPS satellites
Unlaunched
GPS

GPS block II-A satellite on display at the San Diego Air
& Space Museum
A visual example of a 24 satellite
GPS

GPS constellation in motion with
the earth rotating. Notice how the number of satellites in view from a
given point on the earth's surface, in this example in Golden,
Colorado, USA(39.7469° N, 105.2108° W), changes with time.
The space segment (SS) is composed of 24 to 32 satellites in medium
Earth
.jpg/600px-Planets_everywhere_(artist’s_impression).jpg)
Earth orbit and also includes the payload adapters to the boosters
required to launch them into orbit.
The space segment (SS) is composed of the orbiting
GPS

GPS satellites, or
Space Vehicles (SV) in
GPS

GPS parlance. The
GPS

GPS design originally called
for 24 SVs, eight each in three approximately circular
orbits,[67] but this was modified to six orbital planes with four
satellites each.[68] 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).[69] The orbital
period is one-half a sidereal day, i.e., 11 hours and 58 minutes so
that the satellites pass over the same locations[70] or almost the
same locations[71] every day. The orbits are arranged so that at least
six satellites are always within line of sight from almost everywhere
on the Earth's surface.[72] 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°.[73]
Orbiting at an altitude of approximately 20,200 km
(12,600 mi); orbital radius of approximately 26,600 km
(16,500 mi),[74] each SV makes two complete orbits each sidereal
day, repeating the same ground track each day.[75] 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 2016[update],[76] there are 32 satellites in the
GPS

GPS constellation, 31 of which are in use. The additional satellites
improve the precision of
GPS receiver

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 reliability and availability of the
system, relative to a uniform system, when multiple satellites
fail.[77] About nine satellites are visible from any point on the
ground at any one time (see animation at right), ensuring considerable
redundancy over the minimum four satellites needed for a position.
Control segment[edit]
Ground monitor station used from 1984 to 2007, on display at the Air
Force Space & Missile Museum.
The control segment (CS) is composed of:
a master control station (MCS),
an alternate master control station,
four dedicated ground antennas, and
six dedicated monitor stations.
The MCS can also access
U.S. Air Force

U.S. Air Force
Satellite

Satellite Control Network
(AFSCN) ground antennas (for additional command and control
capability) and NGA (National Geospatial-Intelligence Agency) monitor
stations. The flight paths of the satellites are tracked by dedicated
U.S. Air Force

U.S. Air Force monitoring stations in Hawaii,
Kwajalein

Kwajalein Atoll,
Ascension Island, Diego Garcia,
Colorado Springs, Colorado

Colorado Springs, Colorado and Cape
Canaveral, along with shared NGA monitor stations operated in England,
Argentina, Ecuador, Bahrain,
Australia

Australia and Washington DC.[78] The
tracking information is sent to the
Air Force Space Command

Air Force Space Command MCS at
Schriever Air Force Base

Schriever Air Force Base 25 km (16 mi) ESE of Colorado
Springs, which is operated by the 2nd Space Operations Squadron
(2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts each
GPS

GPS satellite regularly with a navigational update using dedicated or
shared (AFSCN) ground antennas (
GPS

GPS dedicated ground antennas are
located at Kwajalein, Ascension Island, Diego Garcia, and Cape
Canaveral). These updates synchronize the atomic clocks on board the
satellites to within a few nanoseconds of each other, and adjust the
ephemeris of each satellite's internal orbital model. The updates are
created by a
Kalman filter

Kalman filter that uses inputs from the ground monitoring
stations, space weather information, and various other inputs.[79]
Satellite

Satellite maneuvers are not precise by
GPS

GPS standards—so to change a
satellite's orbit, the satellite must be marked unhealthy, so
receivers don't use it. After the satellite maneuver, engineers track
the new orbit from the ground, upload the new ephemeris, and mark the
satellite healthy again.
The operation control segment (OCS) currently serves as the control
segment of record. It provides the operational capability that
supports
GPS

GPS users and keeps the
GPS

GPS operational and performing within
specification.
OCS successfully replaced the legacy 1970s-era mainframe computer at
Schriever Air Force Base

Schriever Air Force Base in September 2007. After installation, the
system helped enable upgrades and provide a foundation for a new
security architecture that supported U.S. armed forces.
OCS will continue to be the ground control system of record until the
new segment, Next Generation
GPS

GPS Operation Control System[4] (OCX), is
fully developed and functional. The new capabilities provided by OCX
will be the cornerstone for revolutionizing GPS's mission
capabilities, enabling[80]
Air Force Space Command

Air Force Space Command to greatly enhance
GPS

GPS operational services to U.S. combat forces, civil partners and
myriad domestic and international users. The
GPS

GPS OCX program also will
reduce cost, schedule and technical risk. It is designed to provide
50%[81] sustainment cost savings through efficient software
architecture and Performance-Based Logistics. In addition,
GPS

GPS OCX is
expected to cost millions less than the cost to upgrade OCS while
providing four times the capability.
The
GPS

GPS OCX program represents a critical part of
GPS

GPS modernization
and provides significant information assurance improvements over the
current
GPS

GPS OCS program.
OCX will have the ability to control and manage
GPS

GPS legacy satellites
as well as the next generation of
GPS III

GPS III satellites, while enabling
the full array of military signals.
Built on a flexible architecture that can rapidly adapt to the
changing needs of today's and future
GPS

GPS users allowing immediate
access to
GPS

GPS data and constellation status through secure, accurate
and reliable information.
Provides the warfighter with more secure, actionable and predictive
information to enhance situational awareness.
Enables new modernized signals (L1C, L2C, and L5) and has M-code
capability, which the legacy system is unable to do.
Provides significant information assurance improvements over the
current program including detecting and preventing cyber attacks,
while isolating, containing and operating during such attacks.
Supports higher volume near real-time command and control capabilities
and abilities.
On September 14, 2011,[82] the
U.S. Air Force

U.S. Air Force announced the completion
of
GPS

GPS OCX Preliminary Design Review and confirmed that the OCX
program is ready for the next phase of development.
The
GPS

GPS OCX program has missed major milestones and is pushing the GPS
IIIA launch beyond April 2016.[83]
User segment[edit]
Further information:
GPS

GPS navigation device
GPS

GPS receivers come in a variety of formats, from devices integrated
into cars, phones, and watches, to dedicated devices such as these.
The first portable
GPS

GPS unit, Leica WM 101 displayed at the Irish
National Science Museum at Maynooth.
The user segment (US) is composed of hundreds of thousands of U.S. and
allied military users of the secure
GPS

GPS Precise Positioning Service,
and tens of millions of civil, commercial and scientific users of the
Standard Positioning Service (see
GPS

GPS navigation devices). In general,
GPS

GPS receivers are composed of an antenna, tuned to the frequencies
transmitted by the satellites, receiver-processors, and a highly
stable clock (often a crystal oscillator). They may also include a
display for providing location and speed information to the user. A
receiver is often described by its number of channels: this signifies
how many satellites it can monitor simultaneously. Originally limited
to four or five, this has progressively increased over the years so
that, as of 2007[update], receivers typically have between 12 and
20 channels. Though there are many receiver manufacturers, they
almost all use one of the chipsets produced for this purpose.[citation
needed]
A typical OEM
GPS receiver

GPS receiver module measuring 15×17 mm.
GPS

GPS receivers may include an input for differential corrections, using
the
RTCM SC-104 format. This is typically in the form of an RS-232
port at 4,800 bit/s speed. Data is actually sent at a much lower
rate, which limits the accuracy of the signal sent using
RTCM.[citation needed] Receivers with internal D
GPS

GPS receivers can
outperform those using external
RTCM data.[citation needed] As of
2006[update], even low-cost units commonly include Wide Area
Augmentation
System

System (WAAS) receivers.
A typical
GPS receiver

GPS receiver with integrated antenna.
Many
GPS

GPS receivers can relay position data to a PC or other device
using the
NMEA 0183 protocol. Although this protocol is officially
defined by the National Marine Electronics Association (NMEA),[84]
references to this protocol have been compiled from public records,
allowing open source tools like gpsd to read the protocol without
violating intellectual property laws.[clarification needed] Other
proprietary protocols exist as well, such as the
SiRF and MTK
protocols. Receivers can interface with other devices using methods
including a serial connection, USB, or Bluetooth.
Applications[edit]
This section needs additional citations for verification. Please help
improve this article by adding citations to reliable sources.
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how and when to remove this template message)
Main article: GNSS applications
See also:
GPS

GPS navigation device
While originally a military project,
GPS

GPS is considered a dual-use
technology, meaning it has significant military and civilian
applications.
GPS

GPS has become a widely deployed and useful tool for commerce,
scientific uses, tracking, and surveillance. GPS's accurate time
facilitates everyday activities such as banking, mobile phone
operations, and even the control of power grids by allowing well
synchronized hand-off switching.[66]
Civilian[edit]
This antenna is mounted on the roof of a hut containing a scientific
experiment needing precise timing.
Many civilian applications use one or more of GPS's three basic
components: absolute location, relative movement, and time transfer.
Agriculture:
GPS

GPS has made a great evolution in different aspects of
modern agricultural sectors. Today, a growing number of crop producers
are using
GPS

GPS and other modern electronic and computer equipment to
practice Site Specific Management (SSM) and precision agriculture.
This technology has the potential in agricultural mechanization (farm
and machinery management) by providing farmers with a sophisticated
tool to measure yield on much smaller scales as well as precise
determination and automatic storing of variables such as field time,
working area, machine travel distance and speed, fuel consumption and
yield information.[85][86]
Astronomy: both positional and clock synchronization data is used in
astrometry and celestial mechanics.
GPS

GPS is also used in both amateur
astronomy with small telescopes as well as by professional
observatories for finding extrasolar planets.
Automated vehicle: applying location and routes for cars and trucks to
function without a human driver.
Cartography: both civilian and military cartographers use GPS
extensively.
Cellular telephony: clock synchronization enables time transfer, which
is critical for synchronizing its spreading codes with other base
stations to facilitate inter-cell handoff and support hybrid
GPS/cellular position detection for mobile emergency calls and other
applications. The first handsets with integrated
GPS

GPS launched in the
late 1990s. The U.S.
Federal Communications Commission

Federal Communications Commission (FCC) mandated
the feature in either the handset or in the towers (for use in
triangulation) in 2002 so emergency services could locate
911 callers. Third-party software developers later gained access
to
GPS

GPS APIs from
Nextel

Nextel upon launch, followed by Sprint in 2006, and
Verizon

Verizon soon thereafter.
Clock synchronization: the accuracy of
GPS

GPS time signals (±10 ns)[87]
is second only to the atomic clocks they are based on, and is used in
applications such as
GPS

GPS disciplined oscillators.
Disaster relief/emergency services: many emergency services depend
upon
GPS

GPS for location and timing capabilities.
GPS-equipped radiosondes and dropsondes: measure and calculate the
atmospheric pressure, wind speed and direction up to 27 km from
the Earth's surface.
Radio

Radio occultation for weather and atmospheric science
applications.[88]
Fleet tracking: used to identify, locate and maintain contact reports
with one or more fleet vehicles in real-time.
Geofencing: vehicle tracking systems, person tracking systems, and pet
tracking systems use
GPS

GPS to locate devices that are attached to or
carried by a person, vehicle, or pet. The application can provide
continuous tracking and send notifications if the target leaves a
designated (or "fenced-in") area.[89]
Geotagging: applies location coordinates to digital objects such as
photographs (in
Exif

Exif data) and other documents for purposes such as
creating map overlays with devices like Nikon GP-1
GPS

GPS aircraft tracking
GPS

GPS for mining: the use of RTK
GPS

GPS has significantly improved several
mining operations such as drilling, shoveling, vehicle tracking, and
surveying. RTK
GPS

GPS provides centimeter-level positioning accuracy.
GPS

GPS data mining: It is possible to aggregate
GPS

GPS data from multiple
users to understand movement patterns, common trajectories and
interesting locations.[90]
GPS

GPS tours: location determines what content to display; for instance,
information about an approaching point of interest.
Navigation: navigators value digitally precise velocity and
orientation measurements.
Phasor measurements:
GPS

GPS enables highly accurate timestamping of power
system measurements, making it possible to compute phasors.
Recreation: for example, Geocaching, Geodashing,
GPS

GPS drawing,
waymarking, and other kinds of location based mobile games.
Robotics: self-navigating, autonomous robots using a
GPS

GPS sensors,
which calculate latitude, longitude, time, speed, and heading.
Sport: used in football and rugby for the control and analysis of the
training load.[91]
Surveying: surveyors use absolute locations to make maps and determine
property boundaries.
Tectonics:
GPS

GPS enables direct fault motion measurement of earthquakes.
Between earthquakes
GPS

GPS can be used to measure crustal motion and
deformation[92] to estimate seismic strain buildup for creating
seismic hazard maps.
Telematics:
GPS

GPS technology integrated with computers and mobile
communications technology in automotive navigation systems.
Restrictions on civilian use[edit]
The U.S. government controls the export of some civilian receivers.
All
GPS

GPS receivers capable of functioning above 18 km (60,000
feet) altitude and 515 m/s (1,000 knots), or designed or modified
for use with unmanned air vehicles like, e.g., ballistic or cruise
missile systems, are classified as munitions (weapons)—which means
they require State Department export licenses.[93]
This rule applies even to otherwise purely civilian units that only
receive the L1 frequency and the C/A (Coarse/Acquisition) code.
Disabling operation above these limits exempts the receiver from
classification as a munition. Vendor interpretations differ. The rule
refers to operation at both the target altitude and speed, but some
receivers stop operating even when stationary. This has caused
problems with some amateur radio balloon launches that regularly reach
30 km (100,000 feet).
These limits only apply to units or components exported from the
United States. A growing trade in various components exists, including
GPS

GPS units from other countries. These are expressly sold as ITAR-free.
Military[edit]
Attaching a
GPS

GPS guidance kit to a dumb bomb, March 2003.
M982 Excalibur

M982 Excalibur GPS-guided artillery shell.
As of 2009, military
GPS

GPS applications include:
Navigation: Soldiers use
GPS

GPS to find objectives, even in the dark or
in unfamiliar territory, and to coordinate troop and supply movement.
In the
United States

United States armed forces, commanders use the Commander's
Digital Assistant and lower ranks use the Soldier Digital
Assistant.[94]
Target tracking: Various military weapons systems use
GPS

GPS to track
potential ground and air targets before flagging them as
hostile.[citation needed] These weapon systems pass target coordinates
to precision-guided munitions to allow them to engage targets
accurately. Military aircraft, particularly in air-to-ground roles,
use
GPS

GPS to find targets.
Missile and projectile guidance:
GPS

GPS allows accurate targeting of
various military weapons including ICBMs, cruise missiles,
precision-guided munitions and artillery shells. Embedded GPS
receivers able to withstand accelerations of 12,000 g or about
118 km/s2 have been developed for use in 155-millimeter
(6.1 in) howitzer shells.[95]
Search and rescue.
Reconnaissance: Patrol movement can be managed more closely.
GPS

GPS satellites carry a set of nuclear detonation detectors consisting
of an optical sensor called a bhangmeter, an X-ray sensor, a
dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that
form a major portion of the
United States

United States Nuclear Detonation Detection
System.[96][97] General William Shelton has stated that future
satellites may drop this feature to save money.[98]
GPS

GPS type navigation was first used in war in the 1991 Persian Gulf
War, before
GPS

GPS was fully developed in 1995, to assist Coalition
Forces to navigate and perform maneuvers in the war. The war also
demonstrated the vulnerability of
GPS

GPS to being jammed, when Iraqi
forces installed jamming devices on likely targets that emitted radio
noise, disrupting reception of the weak
GPS

GPS signal.[99]
Communication[edit]
Main article:
GPS

GPS signals
The navigational signals transmitted by
GPS

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.
Message format[edit]
GPS

GPS message format
Subframes
Description
1
Satellite

Satellite clock,
GPS

GPS time relationship
2–3
Ephemeris
(precise satellite orbit)
4–5
Almanac component
(satellite network synopsis,
error correction)
Each
GPS

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 (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 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 almanac message (GPS). Each 30-second frame begins precisely on
the minute or half-minute as indicated by the atomic clock on each
satellite.[100]
The first subframe of each frame encodes the week number and the time
within the week,[101] 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 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 12 1/2
minutes.[102]
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/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.[103]
The ephemeris is updated every 2 hours and is generally valid for 4
hours, with provisions for updates every 6 hours or longer in
non-nominal conditions. The almanac is updated typically every
24 hours. Additionally, data for a few weeks following is
uploaded in case of transmission updates that delay data
upload.[citation needed]
Satellite

Satellite frequencies[edit]
GPS

GPS frequency overview[104]:607
Band
Frequency
Description
L1
1575.42 MHz
Coarse-acquisition (C/A) and encrypted precision (P(Y)) codes, plus
the L1 civilian (L1C) and military (M) codes on future Block III
satellites.
L2
1227.60 MHz
P(Y) code, plus the
L2C

L2C and military codes on the Block IIR-M and
newer satellites.
L3
1381.05 MHz
Used for nuclear detonation (NUDET) detection.
L4
1379.913 MHz
Being studied for additional ionospheric correction.
L5
1176.45 MHz
Proposed for use as a civilian safety-of-life (SoL) signal.
All satellites broadcast at the same two frequencies, 1.57542 GHz
(L1 signal) and 1.2276 GHz (L2 signal). The satellite
network uses a CDMA spread-spectrum technique[104]: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[105][106] that make observers
on the
Earth
.jpg/600px-Planets_everywhere_(artist’s_impression).jpg)
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.[73] 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

United States Nuclear Detonation (NUDET) Detection
System

System (USNDS) to
detect, locate, and report nuclear detonations (NUDETs) in the Earth's
atmosphere and near space.[107] One usage is the enforcement of
nuclear test ban treaties.
The L4 band at 1.379913 GHz is being studied for additional
ionospheric correction.[104]:607
The L5 frequency band at 1.17645 GHz was added in the process of
GPS

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 of 2010.[108] On
February 5th 2016, the 12th and final Block IIF satellite was
launched.[109] 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

GPS signal, will eventually support safety-of-life applications
for aviation and provide improved availability and accuracy."[110]
In 2011, a conditional waiver was granted to
LightSquared

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

LightSquared to form a study group with the
GPS

GPS community to test GPS
receivers and identify issue that might arise due to the larger signal
power from the
LightSquared

LightSquared terrestrial network. The
GPS

GPS community had
not objected to the
LightSquared

LightSquared (formerly MSV and SkyTerra)
applications until November 2010, when
LightSquared

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 impact of the lower 10 MHz of
spectrum is minimal to
GPS

GPS devices (less than 1% of the total GPS
devices are affected). The upper 10 MHz intended for use by
LightSquared

LightSquared may have some impact on
GPS

GPS devices. There is some
concern that this may seriously degrade the
GPS

GPS signal for many
consumer uses.[111][112]
Aviation Week magazine reports that the
latest testing (June 2011) confirms "significant jamming" of
GPS

GPS by
LightSquared's system.[113]
Demodulation and decoding[edit]
Demodulating and Decoding
GPS

GPS
Satellite

Satellite Signals using the
Coarse/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.[114][115]
If the almanac information has previously been acquired, the receiver
picks the satellites to listen for by their PRNs, unique numbers in
the range 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 code pattern. There can be a
delay of up to 30 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 Demodulation and Decoding, Advanced.
Navigation

Navigation equations[edit]
Further information: GNSS positioning calculation
See also: Pseudorange
Problem description[edit]
The receiver uses messages received from satellites to determine the
satellite positions and time sent. The x, y, and z components of
satellite position and the time sent are designated as [xi, yi, zi,
si] where the subscript i denotes the satellite and has the value 1,
2, ..., n, where n ≥ 4. When the time of message reception
indicated by the on-board receiver clock is t̃i, the true reception
time is ti = t̃i − b, where b is the receiver's clock bias from the
much more accurate
GPS

GPS clocks employed by the satellites. The receiver
clock bias is the same for all received satellite signals (assuming
the satellite clocks are all perfectly synchronized). The message's
transit time is t̃i − b − si, where si is the satellite time.
Assuming the message traveled at the speed of light, c, the distance
traveled is (t̃i − b − si) c.
For n satellites, the equations to satisfy are:
d
i
=
(
t
~
i
−
b
−
s
i
)
c
,
i
=
1
,
2
,
…
,
n
displaystyle d_ i =left( tilde t _ i -b-s_ i
right)c,;i=1,2,dots ,n
where di is the geometric distance or range between receiver and
satellite:
d
i
=
(
x
−
x
i
)
2
+
(
y
−
y
i
)
2
+
(
z
−
z
i
)
2
displaystyle d_ i = sqrt (x-x_ i )^ 2 +(y-y_ i )^ 2 +(z-z_ i )^
2
Defining pseudoranges as
p
i
=
(
t
~
i
−
s
i
)
c
displaystyle p_ i =left( tilde t _ i -s_ i right)c
, we see they are biased versions of the true range:
p
i
=
d
i
+
b
c
,
i
=
1
,
2
,
.
.
.
,
n
displaystyle p_ i =d_ i +bc,;i=1,2,...,n
.[116][117]
Since the equations have four unknowns [x, y, z, b]—the three
components of
GPS receiver

GPS receiver position and the clock bias—signals from
at least four satellites are necessary to attempt solving these
equations. They can be solved by algebraic or numerical methods.
Existence and uniqueness of
GPS

GPS solutions are discussed by Abell and
Chaffee.[59] When n is greater than 4 this system is overdetermined
and a fitting method must be used.
The amount of error in the results varies with the received
satellites' locations in the sky, since certain configurations (when
the received satellites are close together in the sky) cause large
errors. Receivers usually calculate a running estimate of the error in
the calculated position. This is done by multiplying the basic
resolution of the receiver by quantities called the geometric dilution
of position (GDOP) factors, calculated from the relative sky
directions of the satellites used.[118] The receiver location is
expressed in a specific coordinate system, such as latitude and
longitude using the
WGS 84

WGS 84 geodetic datum or a country-specific
system.[119]
Geometric interpretation[edit]
The
GPS

GPS equations can be solved by numerical and analytical methods.
Geometrical interpretations can enhance the understanding of these
solution methods.
Spheres[edit]
The measured ranges, called pseudoranges, contain clock errors. In a
simplified idealization in which the ranges are synchronized, these
true ranges represent the radii of spheres, each centered on one of
the transmitting satellites. The solution for the position of the
receiver is then at the intersection of the surfaces of three of these
spheres.[120] If more than the minimum number of ranges is available,
a near intersection of more than three sphere surfaces could be found
via, e.g. least squares.
Hyperboloids[edit]
If the pseudorange between the receiver and satellite i and the
pseudorange between the receiver and satellite j are subtracted, pi
− pj, the common receiver clock bias (b) cancels out, resulting in a
difference of distances dj − dj. The locus of points having a
constant difference in distance to two points (here, two satellites)
is a hyperbola on a plane and a hyperboloid of revolution in 3D space
(see Multilateration). Thus, from four pseudorange measurements, the
receiver can be placed at the intersection of the surfaces of three
hyperboloids each with foci at a pair of satellites. With additional
satellites, the multiple intersections are not necessarily unique, and
a best-fitting solution is sought instead.[59][60][121][122][123]
Inscribed sphere[edit]
The receiver position can be interpreted as the center of a inscribed
sphere (insphere) of radius bc, given by the receiver clock bias b
(scaled by the speed of light c). The insphere location is such that
it touches other spheres (see Problem of Apollonius#Applications). The
circumscribing spheres are centered at the
GPS

GPS satellites, whose radii
equal the measured pseudoranges pi. This configuration is distinct
from the one described in section #Spheres, in which the spheres'
radii were the unbiased or geometric ranges di.[123][124]
Spherical cones[edit]
The solution space [x, y, z, b] can be seen as a four-dimensional
geometric space. In that case each of the equations describes a
spherical cone,[125] with the cusp located at the satellite, and the
base a sphere around the satellite. The receiver is at the
intersection of four or more of such cones.
Solution methods[edit]
Least squares[edit]
When more than four satellites are available, the calculation can use
the four best, or more than four simultaneously (up to all visible
satellites), depending on the number of receiver channels, processing
capability, and geometric dilution of precision (GDOP).
Using more than four involves an over-determined system of equations
with no unique solution; such a system can be solved by a
least-squares or weighted least squares method.[116]
(
x
^
,
y
^
,
z
^
,
b
^
)
=
arg
min
(
x
,
y
,
z
,
b
)
∑
i
(
(
x
−
x
i
)
2
+
(
y
−
y
i
)
2
+
(
z
−
z
i
)
2
+
b
c
−
p
i
)
2
displaystyle left( hat x , hat y , hat z , hat b right)=
underset left(x,y,z,bright) arg min sum _ i left( sqrt (x-x_ i )^
2 +(y-y_ i )^ 2 +(z-z_ i )^ 2 +bc-p_ i right)^ 2
Iterative[edit]
Both the equations for four satellites, or the least squares equations
for more than four, are non-linear and need special solution methods.
A common approach is by iteration on a linearized form of the
equations, such as the Gauss–Newton algorithm.
The
GPS

GPS was initially developed assuming use of a numerical
least-squares solution method—i.e., before closed-form solutions
were found.
Closed-form[edit]
One closed-form solution to the above set of equations was developed
by S. Bancroft.[117][126] Its properties are well known;[59][60][127]
in particular, proponents claim it is superior in low-GDOP situations,
compared to iterative least squares methods.[126]
Bancroft's method is algebraic, as opposed to numerical, and can be
used for four or more satellites. When four satellites are used, the
key steps are inversion of a 4x4 matrix and solution of a
single-variable quadratic equation. Bancroft's method provides one or
two solutions for the unknown quantities. When there are two (usually
the case), only one is a near-
Earth
.jpg/600px-Planets_everywhere_(artist’s_impression).jpg)
Earth sensible solution.[117]
When a receiver uses more than four satellites for a solution,
Bancroft uses the generalized inverse (i.e., the pseudoinverse) to
find a solution. However, a case has been made that iterative methods
(e.g., Gauss–Newton algorithm) for solving over-determined
non-linear least squares (NLLS) problems generally provide more
accurate solutions.[128]
Leick et al. (2015) states that "Bancroft's (1985) solution is a very
early, if not the first, closed-form solution."[129] Other closed-form
solutions were published afterwards,[130][131] although their adoption
in practice is unclear.
Error sources and analysis[edit]
Main article: Error analysis for the Global Positioning System
GPS

GPS error analysis examines error sources in
GPS

GPS results and the
expected size of those errors.
GPS

GPS makes corrections for receiver
clock errors and other effects, but some residual errors remain
uncorrected. Error sources include signal arrival time measurements,
numerical calculations, atmospheric effects (ionospheric/tropospheric
delays), ephemeris and clock data, multipath signals, and natural and
artificial interference. Magnitude of residual errors from these
sources depends on geometric dilution of precision. Artificial errors
may result from jamming devices and threaten ships and aircraft[132]
or from intentional signal degradation through selective availability,
which limited accuracy to ≈6–12 m, but has been switched off since
May 1, 2000.[133][134]
Accuracy enhancement and surveying[edit]
Main article: GNSS enhancement
This article duplicates the scope of other articles, specifically,
GNSS enhancement. Please discuss this issue on the talk page and edit
it to conform with's Manual of Style. (November 2013)
Augmentation[edit]
Integrating external information into the calculation process can
materially improve accuracy. Such augmentation systems are generally
named or described based on how the information arrives. Some systems
transmit additional error information (such as clock drift, ephemera,
or ionospheric delay), others characterize prior errors, while a third
group provides additional navigational or vehicle information.
Examples of augmentation systems include the Wide Area Augmentation
System

System (WAAS), European Geostationary
Navigation

Navigation Overlay Service
(EGNOS),
Differential GPS

Differential GPS (DGPS), inertial navigation systems (INS)
and Assisted GPS. The standard accuracy of about 15 meters (49 feet)
can be augmented to 3–5 meters (9.8–16.4 ft) with DGPS, and
to about 3 meters (9.8 feet) with WAAS.[135]
Precise monitoring[edit]
Accuracy can be improved through precise monitoring and measurement of
existing
GPS signals

GPS signals in additional or alternate ways.
The largest remaining error is usually the unpredictable delay through
the ionosphere. The spacecraft broadcast ionospheric model parameters,
but some errors remain. This is one reason
GPS

GPS spacecraft transmit on
at least two frequencies, L1 and L2.
Ionospheric delay

Ionospheric delay is a
well-defined function of frequency and the total electron content
(TEC) along the path, so measuring the arrival time difference between
the frequencies determines TEC and thus the precise ionospheric delay
at each frequency.
Military receivers can decode the P(Y) code transmitted on both L1 and
L2. Without decryption keys, it is still possible to use a codeless
technique to compare the P(Y) codes on L1 and L2 to gain much of the
same error information. However, this technique is slow, so it is
currently available only on specialized surveying equipment. In the
future, additional civilian codes are expected to be transmitted on
the L2 and L5 frequencies (see
GPS

GPS modernization). All users will
then be able to perform dual-frequency measurements and directly
compute ionospheric delay errors.
A second form of precise monitoring is called Carrier-Phase
Enhancement (CPGPS). This corrects the error that arises because the
pulse transition of the PRN is not instantaneous, and thus the
correlation (satellite–receiver sequence matching) operation is
imperfect. CP
GPS

GPS uses the L1 carrier wave, which has a period of
1
s
1575.42
×
10
6
=
0.63475
n
s
≈
1
n
s
displaystyle frac 1,mathrm s 1575.42times 10^ 6
=0.63475,mathrm ns approx 1,mathrm ns
, which is about one-thousandth of the C/A
Gold code bit period
of
1
s
1023
×
10
3
=
977.5
n
s
≈
1000
n
s
displaystyle frac 1,mathrm s 1023times 10^ 3
=977.5,mathrm ns approx 1000,mathrm ns
, to act as an additional clock signal and resolve the uncertainty.
The phase difference error in the normal
GPS

GPS amounts to 2–3 meters
(7–10 ft) of ambiguity. CP
GPS

GPS working to within 1% of perfect
transition reduces this error to 3 centimeters (1.2 in) of
ambiguity. By eliminating this error source, CP
GPS

GPS coupled with DGPS
normally realizes between 20–30 centimeters (8–12 in) of
absolute accuracy.
Relative Kinematic Positioning

Relative Kinematic Positioning (RKP) is a third alternative for a
precise GPS-based positioning system. In this approach, determination
of range signal can be resolved to a precision of less than 10
centimeters (4 in). This is done by resolving the number of
cycles that the signal is transmitted and received by the receiver by
using a combination of differential
GPS

GPS (DGPS) correction data,
transmitting
GPS

GPS signal phase information and ambiguity resolution
techniques via statistical tests—possibly with processing in
real-time (real-time kinematic positioning, RTK).
Timekeeping [edit]
Leap seconds[edit]
While most clocks derive their time from Coordinated Universal Time
(UTC), the atomic clocks on the satellites are set to
GPS

GPS time (GPST;
see the page of
United States

United States Naval Observatory). The difference is
that
GPS

GPS time is not corrected to match the rotation of the Earth, so
it does not contain leap seconds or other corrections that are
periodically added to UTC.
GPS

GPS time was set to match UTC in 1980, but
has since diverged. The lack of corrections means that
GPS

GPS time
remains at a constant offset with
International Atomic Time (TAI) (TAI
−
GPS

GPS = 19 seconds). Periodic corrections are performed to the
on-board clocks to keep them synchronized with ground clocks.[136]
The
GPS

GPS navigation message includes the difference between
GPS

GPS time
and UTC. As of January 2017,
GPS

GPS time is 18 seconds ahead of UTC
because of the leap second added to UTC on December 31, 2016.[137]
Receivers subtract this offset from
GPS

GPS time to calculate UTC and
specific timezone values. New
GPS

GPS units may not show the correct UTC
time until after receiving the UTC offset message. The GPS-UTC offset
field can accommodate 255 leap seconds (eight bits).
Accuracy[edit]
GPS

GPS time is theoretically accurate to about 14 nanoseconds.per, or
relative to, what?[138] However, most receivers lose accuracy in the
interpretation of the signals and are only accurate to 100
nanoseconds.[139][140]
Format[edit]
As opposed to the year, month, and day format of the Gregorian
calendar, the
GPS

GPS date is expressed as a week number and a
seconds-into-week number. The week number is transmitted as a ten-bit
field in the C/A and P(Y) navigation messages, and so it becomes zero
again every 1,024 weeks (19.6 years).
GPS

GPS week zero started
at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the
week number became zero again for the first time at 23:59:47 UTC
on August 21, 1999 (00:00:19 TAI on August 22, 1999). To
determine the current Gregorian date, a
GPS receiver

GPS receiver must be provided
with the approximate date (to within 3,584 days) to correctly
translate the
GPS

GPS date signal. To address this concern the modernized
GPS

GPS navigation message uses a 13-bit field that only repeats every
8,192 weeks (157 years), thus lasting until the year 2137
(157 years after
GPS

GPS week zero).
Carrier phase tracking (surveying)[edit]
Another method that is used in surveying applications is carrier phase
tracking. The period of the carrier frequency multiplied by the speed
of light gives the wavelength, which is about 0.19 meters for the
L1 carrier. Accuracy within 1% of wavelength in detecting the
leading edge reduces this component of pseudorange error to as little
as 2 millimeters. This compares to 3 meters for the C/A code
and 0.3 meters for the P code.
However, 2 millimeter accuracy requires measuring the total
phase—the number of waves multiplied by the wavelength plus the
fractional wavelength, which requires specially equipped receivers.
This method has many surveying applications. It is accurate enough for
real-time tracking of the very slow motions of tectonic plates,
typically 0–100 mm (0–4 inches) per year.
Triple differencing followed by numerical root finding, and a
mathematical technique called least squares can estimate the position
of one receiver given the position of another. First, compute the
difference between satellites, then between receivers, and finally
between epochs. Other orders of taking differences are equally valid.
Detailed discussion of the errors is omitted.
The satellite carrier total phase can be measured with ambiguity as to
the number of cycles. Let
ϕ
(
r
i
,
s
j
,
t
k
)
displaystyle phi (r_ i ,s_ j ,t_ k )
denote the phase of the carrier of satellite j measured by receiver i
at time
t
k
displaystyle t_ k
. This notation shows the meaning of the subscripts i, j, and k. The
receiver (r), satellite (s), and time (t) come in alphabetical order
as arguments of
ϕ
displaystyle phi
and to balance readability and conciseness, let
ϕ
i
,
j
,
k
=
ϕ
(
r
i
,
s
j
,
t
k
)
displaystyle phi _ i,j,k =phi (r_ i ,s_ j ,t_ k )
be a concise abbreviation. Also we define three functions, :
Δ
r
,
Δ
s
,
Δ
t
displaystyle Delta ^ r ,Delta ^ s ,Delta ^ t
, which return differences between receivers, satellites, and time
points, respectively. Each function has variables with three
subscripts as its arguments. These three functions are defined below.
If
α
i
,
j
,
k
displaystyle alpha _ i,j,k
is a function of the three integer arguments, i, j, and k then it is
a valid argument for the functions, :
Δ
r
,
Δ
s
,
Δ
t
displaystyle Delta ^ r ,Delta ^ s ,Delta ^ t
, with the values defined as
Δ
r
(
α
i
,
j
,
k
)
=
α
i
+
1
,
j
,
k
−
α
i
,
j
,
k
displaystyle Delta ^ r (alpha _ i,j,k )=alpha _ i+1,j,k -alpha _
i,j,k
,
Δ
s
(
α
i
,
j
,
k
)
=
α
i
,
j
+
1
,
k
−
α
i
,
j
,
k
displaystyle Delta ^ s (alpha _ i,j,k )=alpha _ i,j+1,k -alpha _
i,j,k
, and
Δ
t
(
α
i
,
j
,
k
)
=
α
i
,
j
,
k
+
1
−
α
i
,
j
,
k
displaystyle Delta ^ t (alpha _ i,j,k )=alpha _ i,j,k+1 -alpha _
i,j,k
.
Also if
α
i
,
j
,
k
a
n
d
β
l
,
m
,
n
displaystyle alpha _ i,j,k and beta _ l,m,n
are valid arguments for the three functions and a and b are constants
then
(
a
α
i
,
j
,
k
+
b
β
l
,
m
,
n
)
displaystyle (a alpha _ i,j,k +b beta _ l,m,n )
is a valid argument with values defined as
Δ
r
(
a
α
i
,
j
,
k
+
b
β
l
,
m
,
n
)
=
a
Δ
r
(
α
i
,
j
,
k
)
+
b
Δ
r
(
β
l
,
m
,
n
)
displaystyle Delta ^ r (a alpha _ i,j,k +b beta _ l,m,n )=a
Delta ^ r (alpha _ i,j,k )+b Delta ^ r (beta _ l,m,n )
,
Δ
s
(
a
α
i
,
j
,
k
+
b
β
l
,
m
,
n
)
=
a
Δ
s
(
α
i
,
j
,
k
)
+
b
Δ
s
(
β
l
,
m
,
n
)
displaystyle Delta ^ s (a alpha _ i,j,k +b beta _ l,m,n )=a
Delta ^ s (alpha _ i,j,k )+b Delta ^ s (beta _ l,m,n )
, and
Δ
t
(
a
α
i
,
j
,
k
+
b
β
l
,
m
,
n
)
=
a
Δ
t
(
α
i
,
j
,
k
)
+
b
Δ
t
(
β
l
,
m
,
n
)
displaystyle Delta ^ t (a alpha _ i,j,k +b beta _ l,m,n )=a
Delta ^ t (alpha _ i,j,k )+b Delta ^ t (beta _ l,m,n )
.
Receiver clock errors can be approximately eliminated by differencing
the phases measured from satellite 1 with that from
satellite 2 at the same epoch.[141] This difference is designated
as
Δ
s
(
ϕ
1
,
1
,
1
)
=
ϕ
1
,
2
,
1
−
ϕ
1
,
1
,
1
displaystyle Delta ^ s (phi _ 1,1,1 )=phi _ 1,2,1 -phi _ 1,1,1
Double differencing[142] computes the difference of receiver 1's
satellite difference from that of receiver 2. This approximately
eliminates satellite clock errors. This double difference is:
Δ
r
(
Δ
s
(
ϕ
1
,
1
,
1
)
)
=
Δ
r
(
ϕ
1
,
2
,
1
−
ϕ
1
,
1
,
1
)
=
Δ
r
(
ϕ
1
,
2
,
1
)
−
Δ
r
(
ϕ
1
,
1
,
1
)
=
(
ϕ
2
,
2
,
1
−
ϕ
1
,
2
,
1
)
−
(
ϕ
2
,
1
,
1
−
ϕ
1
,
1
,
1
)
displaystyle begin aligned Delta ^ r (Delta ^ s (phi _ 1,1,1
)),&=,Delta ^ r (phi _ 1,2,1 -phi _ 1,1,1 )&=,Delta ^ r (phi _
1,2,1 )-Delta ^ r (phi _ 1,1,1 )&=,(phi _ 2,2,1 -phi _ 1,2,1
)-(phi _ 2,1,1 -phi _ 1,1,1 )end aligned
Triple differencing[143] subtracts the receiver difference from
time 1 from that of time 2. This eliminates the ambiguity
associated with the integral number of wavelengths in carrier phase
provided this ambiguity does not change with time. Thus the triple
difference result eliminates practically all clock bias errors and the
integer ambiguity. Atmospheric delay and satellite ephemeris errors
have been significantly reduced. This triple difference is:
Δ
t
(
Δ
r
(
Δ
s
(
ϕ
1
,
1
,
1
)
)
)
displaystyle Delta ^ t (Delta ^ r (Delta ^ s (phi _ 1,1,1 )))
Triple difference results can be used to estimate unknown variables.
For example, if the position of receiver 1 is known but the
position of receiver 2 unknown, it may be possible to estimate
the position of receiver 2 using numerical root finding and least
squares. Triple difference results for three independent time pairs
may be sufficient to solve for receiver 2's three position
components. This may require a numerical procedure.[144][145] An
approximation of receiver 2's position is required to use such a
numerical method. This initial value can probably be provided from the
navigation message and the intersection of sphere surfaces. Such a
reasonable estimate can be key to successful multidimensional root
finding. Iterating from three time pairs and a fairly good initial
value produces one observed triple difference result for
receiver 2's position. Processing additional time pairs can
improve accuracy, overdetermining the answer with multiple solutions.
Least squares

Least squares can estimate an overdetermined system. Least squares
determines the position of receiver 2 that best fits the observed
triple difference results for receiver 2 positions under the
criterion of minimizing the sum of the squares.
Regulatory spectrum issues concerning
GPS

GPS receivers[edit]
In the United States,
GPS

GPS receivers are regulated under the Federal
Communications Commission's (FCC) Part 15 rules. As indicated in the
manuals of GPS-enabled devices sold in the United States, as a Part 15
device, it "must accept any interference received, including
interference that may cause undesired operation."[146] With respect to
GPS

GPS devices in particular, the FCC states that
GPS

GPS receiver
manufacturers, "must use receivers that reasonably discriminate
against reception of signals outside their allocated spectrum."[147]
For the last 30 years,
GPS

GPS receivers have operated next to the Mobile
Satellite

Satellite Service band, and have discriminated against reception of
mobile satellite services, such as Inmarsat, without any issue.
The spectrum allocated for
GPS

GPS L1 use by the FCC is 1559 to
1610 MHz, while the spectrum allocated for satellite-to-ground
use owned by Lightsquared is the Mobile
Satellite

Satellite Service band.[148]
Since 1996, the FCC has authorized licensed use of the spectrum
neighboring the
GPS

GPS band of 1525 to 1559 MHz to the Virginia
company LightSquared. On March 1, 2001, the FCC received an
application from LightSquared's predecessor,
Motient Services, to use
their allocated frequencies for an integrated satellite-terrestrial
service.[149] In 2002, the U.S.
GPS

GPS Industry Council came to an
out-of-band-emissions (OOBE) agreement with
LightSquared

LightSquared to prevent
transmissions from LightSquared's ground-based stations from emitting
transmissions into the neighboring
GPS

GPS band of 1559 to
1610 MHz.[150] In 2004, the FCC adopted the OOBE agreement in its
authorization for
LightSquared

LightSquared to deploy a ground-based network
ancillary to their satellite system – known as the Ancillary Tower
Components (ATCs) – "We will authorize MSS ATC subject to conditions
that ensure that the added terrestrial component remains ancillary to
the principal MSS offering. We do not intend, nor will we permit, the
terrestrial component to become a stand-alone service."[151] This
authorization was reviewed and approved by the U.S. Interdepartment
Radio

Radio Advisory Committee, which includes the U.S. Department of
Agriculture, U.S. Air Force, U.S. Army, U.S. Coast Guard, Federal
Aviation Administration, National
Aeronautics

Aeronautics and Space
Administration, Interior, and U.S. Department of Transportation.[152]
In January 2011, the FCC conditionally authorized LightSquared's
wholesale customers—such as Best Buy, Sharp, and C Spire—to only
purchase an integrated satellite-ground-based service from
LightSquared

LightSquared and re-sell that integrated service on devices that are
equipped to only use the ground-based signal using LightSquared's
allocated frequencies of 1525 to 1559 MHz.[153] In December 2010,
GPS receiver

GPS receiver manufacturers expressed concerns to the FCC that
LightSquared's signal would interfere with
GPS receiver

GPS receiver devices[154]
although the FCC's policy considerations leading up to the January
2011 order did not pertain to any proposed changes to the maximum
number of ground-based
LightSquared

LightSquared stations or the maximum power at
which these stations could operate. The January 2011 order makes final
authorization contingent upon studies of
GPS

GPS interference issues
carried out by a
LightSquared

LightSquared led working group along with GPS
industry and Federal agency participation. On February 14, 2012, the
FCC initiated proceedings to vacate LightSquared's Conditional Waiver
Order based on the NTIA's conclusion that there was currently no
practical way to mitigate potential
GPS

GPS interference.
GPS receiver

GPS receiver manufacturers design
GPS

GPS receivers to use spectrum beyond
the GPS-allocated band. In some cases,
GPS

GPS receivers are designed to
use up to 400 MHz of spectrum in either direction of the L1
frequency of 1575.42 MHz, because mobile satellite services in
those regions are broadcasting from space to ground, and at power
levels commensurate with mobile satellite services.[155] However, as
regulated under the FCC's Part 15 rules,
GPS

GPS receivers are not
warranted protection from signals outside GPS-allocated spectrum.[147]
This is why
GPS

GPS operates next to the Mobile
Satellite

Satellite Service band,
and also why the Mobile
Satellite

Satellite Service band operates next to GPS.
The symbiotic relationship of spectrum allocation ensures that users
of both bands are able to operate cooperatively and freely.
The FCC adopted rules in February 2003 that allowed Mobile Satellite
Service (MSS) licensees such as
LightSquared

LightSquared to construct a small
number of ancillary ground-based towers in their licensed spectrum to
"promote more efficient use of terrestrial wireless spectrum."[156] In
those 2003 rules, the FCC stated "As a preliminary matter, terrestrial
[Commercial Mobile
Radio

Radio Service (“CMRS”)] and MSS ATC are
expected to have different prices, coverage, product acceptance and
distribution; therefore, the two services appear, at best, to be
imperfect substitutes for one another that would be operating in
predominately different market segments... MSS ATC is unlikely to
compete directly with terrestrial CMRS for the same customer base...".
In 2004, the FCC clarified that the ground-based towers would be
ancillary, noting that "We will authorize MSS ATC subject to
conditions that ensure that the added terrestrial component remains
ancillary to the principal MSS offering. We do not intend, nor will we
permit, the terrestrial component to become a stand-alone
service."[151] In July 2010, the FCC stated that it expected
LightSquared

LightSquared to use its authority to offer an integrated
satellite-terrestrial service to "provide mobile broadband services
similar to those provided by terrestrial mobile providers and enhance
competition in the mobile broadband sector."[157] However, GPS
receiver manufacturers have argued that LightSquared's licensed
spectrum of 1525 to 1559 MHz was never envisioned as being used
for high-speed wireless broadband based on the 2003 and 2004 FCC ATC
rulings making clear that the Ancillary Tower Component (ATC) would
be, in fact, ancillary to the primary satellite component.[158] To
build public support of efforts to continue the 2004 FCC authorization
of LightSquared's ancillary terrestrial component vs. a simple
ground-based LTE service in the Mobile
Satellite

Satellite Service band, GPS
receiver manufacturer Trimble
Navigation

Navigation Ltd. formed the "Coalition To
Save Our GPS."[159]
The FCC and
LightSquared

LightSquared have each made public commitments to solve
the
GPS

GPS interference issue before the network is allowed to
operate.[160][161] However, according to Chris Dancy of the Aircraft
Owners and Pilots Association, airline pilots with the type of systems
that would be affected "may go off course and not even realize
it."[162] The problems could also affect the Federal Aviation
Administration upgrade to the air traffic control system, United
States Defense Department guidance, and local emergency services
including 911.[162]
On February 14, 2012, the U.S.
Federal Communications Commission

Federal Communications Commission (FCC)
moved to bar LightSquared's planned national broadband network after
being informed by the National Telecommunications and Information
Administration (NTIA), the federal agency that coordinates spectrum
uses for the military and other federal government entities, that
"there is no practical way to mitigate potential interference at this
time".[163][164]
LightSquared

LightSquared is challenging the FCC's action.
Other systems[edit]
Main article:
Satellite

Satellite navigation
Comparison of geostationary, GPS, GLONASS, Galileo,
Compass

Compass (MEO),
International Space Station,
Hubble Space Telescope

Hubble Space Telescope and Iridium
constellation orbits, with the Van Allen radiation belts and the Earth
to scale.[a] The Moon's orbit is around 9 times larger than
geostationary orbit.[b] (In the SVG file, hover over an
orbit or its label to highlight it; click to load its article.)
Other satellite navigation systems in use or various states of
development include:
GLONASS

GLONASS – Russia's global navigation system. Fully operational
worldwide.
Galileo – a global system being developed by the
European Union

European Union and
other partner countries, which began operation in 2016,[165] and is
expected to be fully deployed by 2020.
Beidou – People's Republic of China's regional system, currently
limited to Asia and the West Pacific,[166] global coverage planned to
be operational by 2020[167][168]
IRNSS

IRNSS - A regional navigation system developed by the Indian Space
Research Organisation.
QZSS

QZSS - A regional navigation system in development that would be
receivable within Japan.
See also[edit]
Gladys West
GPS/INS
GPS

GPS navigation software
GPS

GPS navigation device
Indoor positioning system
Local Area Augmentation System
Local positioning system
Military invention
Mobile phone tracking
Navigation

Navigation paradox
Notice Advisory to Navstar Users
S-GPS
Trilateration
Wide Area Augmentation System
Notes[edit]
^ Orbital periods and speeds are calculated using the relations
4π²R³ = T²GM and V²R = GM, where R = radius
of orbit in metres, T = orbital period in seconds, V = orbital speed
in m/s, G = gravitational constant ≈ 6.673×10−11 Nm²/kg²,
M = mass of
Earth
.jpg/600px-Planets_everywhere_(artist’s_impression).jpg)
Earth ≈ 5.98×1024 kg.
^ Approximately 8.6 times (in radius and length) when the moon is
nearest (363 104 km ÷ 42 164 km) to 9.6 times when the moon is
farthest (405 696 km ÷ 42 164 km).
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^ "FCC Order, Granted
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LightSquared Subsidiary LLC, a Mobile Satellite
Service licensee in the L-Band, a conditional waiver of the Ancillary
Terrestrial Component "integrated service" rule" (PDF). Federal
Communications Commission. FCC.Gov. January 26, 2011. Retrieved
December 13, 2011.
^ "Data Shows Disastrous
GPS

GPS Jamming from FCC-Approved Broadcaster".
gpsworld.com. February 1, 2011. Archived from the original on February
6, 2011. Retrieved February 10, 2011.
^ "Javad Ashjaee
GPS

GPS World webinar". gpsworld.com. December 8, 2011.
Archived from the original on November 26, 2011. Retrieved December
13, 2011.
^ "FCC Order permitting mobile satellite services providers to provide
an ancillary terrestrial component (ATC) to their satellite systems"
(PDF). Federal Communications Commission. FCC.gov. February 10, 2003.
Retrieved December 13, 2011.
^ "
Federal Communications Commission

Federal Communications Commission Fixed and Mobile Services in the
Mobile
Satellite

Satellite Service". Federal Communications Commission. FCC.gov.
July 15, 2010. Retrieved December 13, 2011.
^ [6] Archived December 13, 2012, at the Wayback Machine.
^ "Coalition to Save Our GPS". Saveourgps.org. Archived from the
original on October 24, 2011. Retrieved November 6, 2011.
^ Jeff Carlisle (June 23, 2011). "Testimony of Jeff Carlisle,
LightSquared

LightSquared Executive Vice President of Regulatory Affairs and Public
Policy to U.S. House Subcommittee on Aviation and Subcommittee on
Coast Guard and Maritime Transportation" (PDF). Archived from the
original (PDF) on September 29, 2011. Retrieved December 13,
2011.
^ Julius Genachowski (May 31, 2011). "FCC Chairman Genachowski Letter
to Senator Charles Grassley" (PDF). Archived from the original (PDF)
on January 13, 2012. Retrieved December 13, 2011.
^ a b Tessler, Joelle (April 7, 2011). "Internet network may jam GPS
in cars, jets". The Sun News. Archived from the original on May 1,
2011. Retrieved April 7, 2011.
^ FCC press release "Spokesperson Statement on NTIA Letter –
LightSquared

LightSquared and GPS". February 14, 2012. Accessed 2013-03-03.
^ Paul Riegler, FBT. "FCC Bars
LightSquared

LightSquared Broadband Network Plan".
February 14, 2012. Retrieved February 14, 2012.
^ "Galileo navigation satellite system goes live". dw.com. Retrieved
December 17, 2016.
^ Beidou coverage
^ "Beidou satellite navigation system to cover whole world in 2020".
Eng.chinamil.com.cn. Retrieved October 15, 2010.
^ Levin, Dan (March 23, 2009). "Chinese Square Off With Europe in
Space". The New York Times. China. Retrieved November 6, 2011.
Further reading[edit]
"NAVSTAR
GPS

GPS User Equipment Introduction" (PDF).
United States

United States Coast
Guard. September 1996.
Parkinson; Spilker (1996). The global positioning system. American
Institute of
Aeronautics

Aeronautics and Astronautics.
ISBN 978-1-56347-106-3.
Jaizki Mendizabal; Roc Berenguer; Juan Melendez (2009).
GPS

GPS and
Galileo. McGraw Hill. ISBN 978-0-07-159869-9.
Nathaniel Bowditch (2002). The American Practical Navigator –
Chapter 11
Satellite

Satellite Navigation.
United States

United States government.
Global Positioning
System

System Open Courseware from MIT, 2012
External links[edit]
Wikimedia Commons has media related to Global Positioning System.
Global Positioning
System

System at Curlie (based on DMOZ)
FAA
GPS

GPS FAQ
GPS.gov—General public education website created by the U.S.
Government
U.S. Army

U.S. Army Corps of Engineers manual:"NAVSTAR HTML". Archived from the
original on August 22, 2008. Retrieved 2010-06-06. CS1 maint:
BOT: original-url status unknown (link) and"PDF (22.6 MB, 328 pages)"
(PDF). Archived from the original on June 25, 2008. Retrieved
2010-06-06. CS1 maint: BOT: original-url status unknown (link)
National Geodetic Survey Orbits for the Global Positioning System
satellites in the Global
Navigation

Navigation
Satellite

Satellite System
GPS

GPS and
GLONASS

GLONASS Simulation (Java applet) Simulation and graphical
depiction of space vehicle motion including computation of dilution of
precision (DOP)
Relativity Science Calculator – Explaining Global Positioning System
v
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NAVSTAR Global Positioning
System

System satellites
List of
GPS

GPS satellites
Block I
1
2
3
4
5
6
7†
8
9
10
11
Block II
1
2
3
4
5
6
7
8
9
Block IIA
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Block IIR
1†
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Block IIRM
1
2
3
4
5
6
7
8
Block IIF
1
2
3
4
5
6
7
8
9
10
11
12
Block IIIA
1
2
3
4
5
6
7
8
9
10
Block IIIF
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Italics indicate future missions. Signs † indicate launch failures.
v
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v
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1 Not assigned • 2 Unofficial designation • 3
Designation believed to be this type but unconfirmed
v
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