Global Positioning System
"GPS"
redirects here. For other similar systems, see Global Navigation Satellite System.
For other uses of GPS, see GPS (disambiguation).
The Global Positioning System (GPS),
is currently the only fully functional Global Navigation Satellite System
(GNSS). More than two dozen GPS satellites are in medium Earth orbit, transmitting signals
allowing GPS receivers to determine the receiver's location, speed and direction.
Since the first experimental satellite was launched
in 1978, GPS has become an indispensable aid to navigation
around the world, and an important tool for map-making
and land
surveying. GPS also provides a precise time
reference used in many applications including scientific study of earthquakes,
and synchronization of telecommunications networks.
Developed by the United States Department of Defense,
it is officially named NAVSTAR GPS (NAVigation Satellite Timing
And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th
Space Wing. The cost of maintaining the system is approximately US$750 million per year,[1] including the
replacement of aging satellites, and research and development. Despite this
fact, GPS is free for civilian use as a public good.
Simplified method of operation
A GPS receiver calculates its position by measuring
the distance between itself and three or more GPS satellites.
Measuring the time delay between transmission and reception of each GPS radio
signal gives the distance to each satellite, since the signal travels at a
known speed. The signals also carry information about the satellites' location.
By determining the position of, and distance to, at least three satellites, the
receiver can compute its position using trilateration.[2]
Receivers typically do not have perfectly accurate clocks and therefore track
one or more additional satellites to correct the receiver's clock error.
Technical description
System
segmentation
The current GPS consists of three major segments.
These are the space segment (SS), a control segment (CS), and a user segment
(US).[3]
Space segment
SPACE SEGMENT The space segment comprises a network of satellites
. The complete GPS space system includes 24 satellites, 11,000 nautical
miles above the earth, take 12 hours each to go around
the earth once or one orbit. They are orbit in six different planes and 55
degrees inclination. These positions of satellites, we can receive signals from
six of them nearly of the time at any point on earth. Satellites are equipped
with very precise clocks that keep accurate time to within three nanoseconds ( 0.000000003 of a second or 3e-9)
This precision timing is important because the receiver must determine exactly
how long it takes for signals to travel from each GPS satellite to receiver.
Each satellite contains a supply of fuel and small servo engines so that it can
be moved in orbit to correct for positioning errors.
Each satellite contains four atomic clocks. These clocks are accurate to a nanosecond .
Each satellite emits two seperate signals , one for
military purposes and one for civilian use.
SOME SPECIFICATION OF SATELLITE
Weight 930 kg.(in
orbit)
Size 5.1 m.
Travel Velocity 4
km/sec
Transmit Signals
1575.42 MHz and 1227.60 MHz
Receive at 1783.74
MHz
Clocks 2 Cesium
and 2 Rubidium
Design life 7.5
year (later model BlockIIR 10 years)
The space segment (SS) is composed of the orbiting
GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design calls
for 24 SVs to be distributed equally among six circular orbital planes.[4] The
orbital planes are centered on the Earth, not rotating with respect to the
distant stars.[5] The six planes have approximately
55° inclination
(tilt relative to 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).[1]
Orbiting at an altitude of approximately 20,200 kilometers
(12,600 miles or
10,900 nautical miles; orbital radius of 26,600 km (16,500
mi or 14,400 NM)), each SV makes two complete orbits each sidereal
day, so it passes over the same location on Earth once each day. The orbits
are arranged so that at least six satellites are always within line of
sight from almost anywhere on Earth.[6]
As of February 2007, there are 30 actively
broadcasting satellites in the GPS constellation. The additional satellites
improve the precision of GPS receiver calculations by providing redundant
measurements. With the increased number of satellites, the constellation was
changed to a nonuniform arrangement. Such an arrangement was shown to improve
reliability and availability of the system, relative to a uniform system, when
multiple satellites fail.[7]
CONTROL SEGMENT The control
Segment of GPS consist of:
Master Control Station ( one station ):
The master control station is responsible for overall managment of the remote
monitoring and transmission sites. As the center for support operations
, It calculates any position or clock errors for each individual
satellite from monitor stations and then order the appropriate corrective
information back to that satellite.
Monitor Stations ( four stations ): Each of
monitor stations checks the exact altitude , position , speed , and overall of
the orbiting of satellites. A station can track up to 11 satellites at a time.
This check-up is performed twice a day by each station as the satellites go
around the earth.
The flight paths of the satellites are tracked by
US Air Force monitoring stations in Hawaii, Kwajalein, Ascension
Island, Diego Garcia, and Colorado Springs, Colorado, along with
monitor stations operated by the National
Geospatial-Intelligence Agency (NGA).[8] The
tracking information is sent to the Air Force Space Command's master control
station at Schriever Air Force Base, Colorado Springs, Colorado, which is
operated by the 2d Space Operations Squadron (2 SOPS)
of the United States Air Force (USAF). 2 SOPS
contacts each GPS satellite regularly with a navigational update (using the
ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado
Springs). These updates synchronize the atomic clocks on board the satellites
to within one microsecond
and adjust the ephemeris of each satellite's internal orbital model. The
updates are created by a Kalman Filter which uses inputs from the ground
monitoring stations, space weather information, and other various inputs.[9]
USER SEGMENT As the pilot fly , the GPS receiver continuously caculates
the current position and display the correct position / heading.The GPS unit
listen to the satellite's signal and measure the time between the satellites
transmission and receipt of the signal. By the process of triangulation among
the several satellites being received, the unit computes the location of the
GPS receiver. GPS receiver has to see at least four satellites to compute a
three dimensional position (it can compute position with only three satellites
if know altitude). Not only latitude and Longitude ,
but altitude as well. There are numerous forms of display among the various manufacturer. No frequency tuning is required
, as the frequency of the satellite transmissions are already known by
the receiver.
The user's GPS receiver is the user segment (US) of
the GPS system. In general, 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 2006,
receivers typically have between twelve and twenty channels.
GPS receivers may include an input for differential
corrections, using the RTCM
SC-104 format. This is typically in the form of a RS-232 port at
4,800 bps speed. Data is actually sent at a much lower rate, which limits the
accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers
can outperform those using external RTCM data. As of 2006, even low-cost units
commonly include WAAS receivers.
Many GPS receivers can relay position data to a PC
or other device using the NMEA 0183 protocol. NMEA 2000[10]
is a newer and less widely adopted protocol. Both are proprietary
and controlled by the US-based National Marine Electronics Association.
References to the NMEA protocols have been compiled from public records,
allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary
protocols exist as well, such as the SiRF protocol. Receivers can interface with other devices using
methods including a serial connection, USB or Bluetooth.
Navigation
signals
GPS satellites broadcast three different types of
data in the primary navigation signal. The first is the almanac which
sends coarse time information along with status information about the
satellites. The second is the ephemeris,
which contains orbital information that allows the receiver to calculate the
position of the satellite. This data is included in the 37,500 bit Navigation
Message, which takes 12.5 minutes to send at 50 bps.
The satellites also broadcast two forms of clock
information, the Coarse / Acquisition code, or C/A which is
freely available to the public, and the restricted Precise code, or P-code,
usually reserved for military applications. The C/A code is a 1,023 bit long pseudo-random code broadcast at 1.023
MHz, repeating every millisecond. Each satellite sends a distinct C/A code,
which allows it to be uniquely identified. The P-code is a similar code
broadcast at 10.23 MHz, but it repeats only once a week. In normal operation,
the so-called "anti-spoofing mode", the P code is first encrypted
into the Y-code, or P(Y), which can only be decrypted by units
with a valid decryption key. Frequencies used by GPS include:
- L1
(1575.42 MHz)
- Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted
precision P(Y) code.
- L2
(1227.60 MHz) - P(Y) code, plus the new L2C code on the
Block IIR-M and newer satellites.
- L3
(1381.05 MHz) - Used by the Defense Support Program to signal
detection of missile launches, nuclear detonations, and other high-energy
infrared events.
- 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
(see 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 would provide this signal is set to be launched
in 2008.
Calculating
positions
The coordinates are calculated according to the World Geodetic System WGS84 coordinate system. To calculate its
position, a receiver needs to know the precise time. The satellites are
equipped with extremely accurate atomic
clocks, and the receiver uses an internal crystal oscillator-based clock that is
continually updated using the signals from the satellites.
The receiver identifies each satellite's signal by
its distinct C/A code pattern, then measures the time delay for each satellite.
To do this, the receiver produces an identical C/A sequence using the same seed number
as the satellite. By lining up the two sequences, the receiver can measure the
delay and calculate the distance to the satellite, called the pseudorange.
The orbital position data from the Navigation
Message is then used to calculate the satellite's precise position. Knowing the
position and the distance of a satellite indicates that the receiver is located
somewhere on the surface of an imaginary sphere centered on that satellite and
whose radius is the distance to it. When four satellites are measured
simultaneously, the intersection of the four imaginary spheres reveals the
location of the receiver. Earth-based users can substitute the sphere of the
planet for one satellite by using their altitude. Often, these spheres will
overlap slightly instead of meeting at one point, so the receiver will yield a
mathematically most-probable position (and often indicate the uncertainty).
Calculating a position with the P(Y) signal is
generally similar in concept, assuming one can decrypt it. The encryption is
essentially a safety mechanism; if a signal can be successfully decrypted, it
is reasonable to assume it is a real signal being sent by a GPS satellite. In
comparison, civil receivers are highly vulnerable to spoofing since correctly
formatted C/A signals can be generated using readily available signal
generators. RAIM
features will not help, since RAIM only checks the signals from a navigational
perspective.
Accuracy and
error sources
The position calculated by a GPS receiver requires
the current time, the position of the satellite and the measured delay of the
received signal. The position accuracy is primarily dependent on the satellite
position and signal delay.
To measure the delay, the receiver compares the bit
sequence received from the satellite with an internally generated version. By
comparing the rising and trailing edges of the bit transitions, modern
electronics can measure signal offset to within about 1% of a bit time, or
approximately 10 nanoseconds for the C/A code. Since GPS signals propagate
nearly at the speed of light, this represents an error of about 3
meters. This is the minimum error possible using only the GPS C/A signal.
Position accuracy can be improved by using the
higher-speed P(Y) signal. Assuming the same 1% accuracy, the faster P(Y) signal
results in an accuracy of about 30 centimeters.
Electronics errors are one of several
accuracy-degrading effects outlined in the table below. When taken together,
autonomous civilian GPS horizontal position fixes are typically accurate to
about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's
accuracy.
Sources of errors |
|
Source |
Effect |
Ionospheric
effects |
± 5
meter |
Ephemeris
errors |
± 2.5
meter |
Satellite
clock errors |
± 2
meter |
Multipath
distortion |
± 1
meter |
Tropospheric
effects |
± 0.5
meter |
Numerical
errors |
± 1
meter or less |
Atmospheric
effects
Changing atmospheric conditions change the speed of
the GPS signals as they pass through the Earth's atmosphere and ionosphere.
Correcting these errors is a significant challenge to improving GPS position
accuracy. These effects are minimized when the satellite is directly overhead,
and become greater for satellites nearer the horizon, since
the signal is affected for a longer time. Once the receiver's approximate
location is known, a mathematical model can be used to estimate and compensate
for these errors.
Because ionospheric delay affects the speed of
radio waves differently based on frequency, a characteristic known as dispersion, both frequency bands can be used to
help reduce this error. Some military and expensive survey-grade civilian
receivers compare the different delays in the L1 and L2 frequencies to measure
atmospheric dispersion, and apply a more precise correction. This can be done
in civilian receivers without decrypting the P(Y) signal carried on L2, by
tracking the carrier wave instead of the modulated
code. To facilitate this on lower cost receivers, a new civilian code signal on
L2, called L2C, was added to the Block IIR-M satellites, first launched in
2005. It allows a direct comparison of the L1 and L2 signals using the coded
signal instead of the carrier wave.
The effects of the ionosphere are generally
slow-moving, and can be averaged over time. The effects for any particular
geographical area can be easily calculated by comparing the GPS-measured
position to a known surveyed location. This correction is also valid for other
receivers in the same general location. Several systems send this information
over radio or other links to allow L1 only receivers to make ionospheric
corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation
Systems such as WAAS,
which transmits it on the GPS frequency using a special pseudo-random number
(PRN), so only one antenna and receiver are required.
Humidity also causes a variable delay, resulting in errors
similar to ionospheric delay, but occurring in the troposphere.
This effect is much more localized, and changes more quickly than the
ionospheric effects, making precise compensation for humidity more difficult.
Altitude also causes a variable delay, as the signal passes through less
atmosphere at higher elevations. Since the GPS receiver measures altitude
directly, this is a much simpler correction to apply.
Multipath
effects
GPS signals can also be affected by multipath
issues, where the radio signals reflect off surrounding terrain; buildings,
canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A
variety of techniques, most notably narrow correlator
spacing, have been developed to mitigate multipath errors. For long
delay multipath, the receiver itself can recognize the wayward signal and
discard it. To address shorter delay multipath from the signal reflecting off
the ground, specialized antennas may be used. Short delay reflections are
harder to filter out since they are only slightly delayed, causing effects
almost indistinguishable from routine fluctuations in atmospheric delay.
Multipath effects are much less severe in moving
vehicles. When the GPS antenna is moving, the false solutions using reflected
signals quickly fail to converge and only the direct signals result in stable
solutions.
Ephemeris and
clock errors
The navigation message from a satellite is sent out
only every 12.5 minutes. In reality, the data contained in these messages tend
to be "out of date" by an even larger amount. Consider the case when
a GPS satellite is boosted back into a proper orbit; for some time following
the maneuver, the receiver's calculation of the satellite's position will be
incorrect until it receives another ephemeris update. The onboard clocks are
extremely accurate, but they do suffer from some clock drift.
This problem tends to be very small, but may add up to 2 meters (6 ft) of
inaccuracy.
This class of error is more "stable" than
ionospheric problems and tends to change over days or weeks rather than
minutes. This makes correction fairly simple by sending out a more accurate
almanac on a separate channel.
Selective
availability
The GPS includes a feature called Selective
Availability (SA) that introduces intentional errors between 0
meters and up to a hundred meters (300 ft) into the publicly available
navigation signals, making it difficult to use for guiding long range missiles
to precise targets. Additional accuracy was available in the signal, but in an
encrypted form that was only available to the United States military, its
allies and a few others, mostly government users.
SA typically added signal errors of up to about 10
meters (30 ft) horizontally and 30 meters (100 ft) vertically. The inaccuracy
of the civilian signal was deliberately encoded so as not to change very
quickly, for instance the entire eastern U.S. area might read 30 m off, but 30
m off everywhere and in the same direction. In order to improve the usefulness
of GPS for civilian navigation, Differential
GPS was used by many civilian GPS receivers to greatly improve accuracy.
During the Gulf War, the
shortage of military GPS units and the wide availability of civilian ones among
personnel resulted in a decision to disable Selective Availability. This was
ironic, as SA had been introduced specifically for these situations, allowing
friendly troops to use the signal for accurate navigation, while at the same
time denying it to the enemy. But since SA was also denying the same accuracy
to thousands of friendly troops, turning it off or setting it to an error of 0
meters (effectively the same thing) presented a clear benefit.
In the 1990s, the FAA started pressuring the military
to turn off SA permanently. This would save the FAA millions of dollars every
year in maintenance of their own radio
navigation systems. The military resisted for most of the 1990s, but SA was
eventually "discontinued"; the amount of error added was "set to
zero"[11] in 2000 following an announcement by U.S.
President Bill Clinton, allowing users access to an undegraded L1 signal. Per
the directive, the induced error of SA was changed to add no error to the
public signals (C/A code). Selective Availability is still a system capability
of GPS, and error could, in theory, be reintroduced at any time. In practice,
in view of the hazards and costs this would induce for US and foreign shipping,
it is unlikely to be reintroduced, and various government agencies, including the
FAA,[12]
have stated that it is not intended to be reintroduced.
The US military has developed the ability to
locally deny GPS (and other navigation services) to hostile forces in a specific
area of crisis without affecting the rest of the world or its own military
systems.
GPS jamming
Jamming of any radio navigation system, including satellite
based navigation, is possible. The U.S. Air Force conducted GPS jamming exercises
in 2003 and they also have GPS anti-spoofing capabilities. In 2002, a detailed
description of how to build a short range GPS L1 C/A jammer was published in Phrack issue 60[13]
by an anonymous author. There has also been at least one well-documented case
of unintentional jamming, tracing back to a malfunctioning TV antenna
preamplifier.[14] If stronger signals were
generated intentionally, they could potentially interfere with aviation GPS
receivers within line of sight. According to John Ruley, of AVweb, "IFR
pilots should have a fallback plan in case of a GPS malfunction".[15]
Receiver Autonomous
Integrity Monitoring (RAIM), a feature of some aviation and marine
receivers, is designed to provide a warning to the user if jamming or another
problem is detected. GPS signals can also be interfered with by natural geomagnetic
storms, predominantly at high latitudes.[16]
The U.S. government believes
that such jammers were also used occasionally during the 2001 war in Afghanistan. Some officials
believe that jammers could be used to attract the precision-guided munitions
towards non-combatant infrastructure; other officials believe
that the jammers are completely ineffective. In either case, the jammers may be
attractive targets for anti-radiation missiles. During the Iraq War, the
U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb.[17]
Relativity
According to the theory of relativity, due to their constant
movement and height relative to the Earth-centered inertial reference frame, the clocks on the satellites
are affected by their speed (special relativity) as well as their
gravitational potential (general relativity). For the GPS satellites,
general relativity predicts that the atomic clocks at GPS orbital altitudes
will tick more rapidly, by about 45,900 nanoseconds
(ns) per day, because they are in a weaker gravitational field than atomic
clocks on Earth's surface. Special relativity predicts that atomic clocks
moving at GPS orbital speeds will tick more slowly, by about 7,200 ns per day,
than stationary ground clocks. When combined, the discrepancy is 38 microseconds
per day; a difference of 4.465 parts in 1010.[18].
To account for this, the frequency standard onboard each satellite is given a
rate offset prior to launch, making it run slightly more slowly than the
desired frequency on Earth; specifically, at 10.22999999543 MHz instead of
10.23 MHz.[19]
Another relativistic effect to be compensated for
in GPS observation processing is the Sagnac
effect. The GPS time scale is defined in an inertial system
but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system; a system in
which simultaneity
is not uniquely defined. The Lorentz transformation between the two
systems modifies the signal run time, a correction having opposite algebraic
signs for satellites in the Eastern and Western celestial hemispheres. Ignoring
this effect will produce an East-West error on the order of hundreds of
nanoseconds, or tens of meters in position.[20]
The atomic clocks on board the GPS satellites are
precisely tuned, making the system a practical engineering application of the
scientific theory of relativity in a real-world system.
Techniques to
improve accuracy
Augmentation
Augmentation methods of improving accuracy rely on
external information being integrated into the calculation process. There are
many such systems in place and they are generally named or described based on
how the GPS sensor receives the information. Some systems transmit additional
information about sources of error (such as clock drift, ephemeris, or
ionospheric delay), others provide direct measurements of how much the signal
was off in the past, while a third group provide additional navigational or
vehicle information to be integrated in the calculation process.
Examples of augmentation systems include the Wide Area Augmentation System, Differential
GPS, and Inertial Navigation Systems.
Precise
monitoring
The accuracy of a calculation can also be improved
through precise monitoring and measuring of the existing GPS signals in
additional or alternate ways.
The first is called Dual Frequency
monitoring, and refers to systems that can compare two or more signals, such as
the L1 frequency to the L2 frequency. Since these are two different
frequencies, they are affected in different, yet predictable ways by the
atmosphere and objects around the receiver. After monitoring these signals, it
is possible to calculate how much error is being introduced and then nullify
that error.
Receivers that have the correct decryption key can
relatively easily decode the P(Y)-code transmitted on both L1 and L2 to measure
the error. Receivers that do not possess the key can still use a process called
codeless to compare the encrypted information on L1 and L2 to gain much
of the same error information. However, this technique is currently limited to
specialized surveying equipment. In the future, additional civilian codes are
expected to be transmitted on the L2 and L5 frequencies. When these become
operational, non-encrypted users will be able to make the same comparison and
directly measure some errors.
A second form of precise monitoring is called Carrier-Phase
Enhancement (CPGPS). The error, which this corrects, arises because the
pulse transition of the PRN is not instantaneous, and thus
the correlation (satellite-receiver sequence matching)
operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which
has a period
1000 times smaller than that of the C/A bit period, to act as an additional clock
signal and resolve the uncertainty. The phase difference error in the
normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS
working to within 1% of perfect transition reduces this error to 3 centimeters
(1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS
normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute
accuracy.
Relative Kinematic Positioning (RKP)
is another approach for a precise GPS-based positioning system. In this
approach, determination of range signal can be resolved to an accuracy of less
than 10 centimeters
(4 in). This is done by resolving the number of cycles in which the signal is
transmitted and received by the receiver. This can be accomplished by using a
combination of differential GPS (DGPS) correction data, transmitting GPS signal
phase information and ambiguity resolution techniques via statistical tests —
possibly with processing in real-time (real-time kinematic positioning, RTK).
GPS time and
date
While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic
clocks on the satellites are set to GPS time. The difference is that
GPS time is not corrected to match the rotation of the Earth, so it does not
contain leap
seconds or other corrections which are periodically added to UTC. GPS time
was set to match Coordinated Universal Time (UTC) in
1980, but has since diverged. The lack of corrections means that GPS time
remains synchronized with the International Atomic Time (TAI).
The GPS navigation message includes the difference
between GPS time and UTC, which as of 2006
is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC
and 'local' time. New 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) which, at the current rate of change of the Earth's
rotation, is sufficient to last until the year 2330.
As opposed to the year, month, and day format of
the Julian calendar, the GPS date is expressed as a
week number and a day-of-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 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). In order to
determine the current Gregorian date, a GPS receiver must be provided
with the approximate date (to within 3,584 days) in order to correctly
translate the GPS date signal. To address this concern the modernized GPS
navigation messages use a 13-bit field, which only repeats every 8,192 weeks
(157 years), and will not return to zero until near the year 2137.
GPS
modernization
Having reached Fully Operational Capability on July
17, 1995,[21] the GPS completed its original design goals.
However, additional advances in technology and new demands on the existing
system led to the effort to "modernize" the GPS system. Announcements
from the Vice Presidential and the White House in 1998 heralded the beginning
of these changes and in 2000, the U.S. Congress reaffirmed the effort; referred
to it as GPS III.
The project aims to improve the accuracy and
availability for all users and involves new ground stations, new satellites,
and four additional navigation signals. New civilian signals are called L2C,
L5 and L1C; the new military code is called M-Code. A goal
of 2013 has been established with incentives offered to the contractors if they
can complete it by 2011.
Applications
Military
GPS allows accurate targeting of various military
weapons including cruise missiles and precision-guided munitions. To help
prevent GPS guidance from being used in enemy or improvised weaponry, the US
Government controls the export of civilian receivers. A US-based manufacturer
cannot generally export a receiver unless the receiver contains limits
restricting it from functioning when it is simultaneously (1) at an altitude
above 18 kilometers (60,000ft) and (2) traveling at over 515 m/s (1,000 knots).[22]
The GPS satellites also carry nuclear detonation
detectors, which form a major portion of the United States Nuclear
Detonation Detection System.[23]
Navigation
GPS Navigation System using TomTom software
- Automobiles can
be equipped with GPS receivers at the factory or as after-market
equipment. Units often display moving maps and information about location,
speed, direction, and nearby streets and landmarks.
- Aircraft
navigation systems usually display a "moving map" and are often
connected to the autopilot for en-route navigation. Cockpit-mounted GPS
receivers and glass cockpits are appearing in general aviation aircraft of all sizes, using
technologies such as WAAS or LAAS to increase accuracy. Many of these systems may be
certified for instrument flight rules navigation,
and some can also be used for final approach and landing operations. Glider pilots
use GNSS Flight Recorders to log GPS data
verifying their arrival at turn points in gliding competitions. Flight computers
installed in many gliders also use GPS to compute wind speed aloft, and
glide paths to waypoints such as alternate airports or mountain
passes, to aid en route decision making for cross-country soaring.
- Boats and ships
can use GPS to navigate all of the world's lakes, seas and oceans.
Maritime GPS units include functions useful on water, such as “man
overboard” (MOB) functions that allow instantly marking the location where
a person has fallen overboard, which simplifies rescue efforts. GPS may be
connected to the ships self-steering gear and Chartplotters
using the NMEA 0183
interface. GPS can also improve the security of shipping traffic by
enabling AIS.
- Heavy
Equipment can use GPS in construction, mining and precision agriculture. The blades and
buckets of construction equipment are controlled automatically in
GPS-based machine guidance systems. Agricultural equipment may use GPS to
steer automatically, or as a visual aid displayed on a screen for the
driver. This is very useful for controlled traffic and row crop operations
and when spraying. Harvesters with yield monitors can also use GPS to
create a yield map of the paddock being harvested.
- Bicycles
often use GPS in racing and touring. GPS navigation allows cyclists to
plot their course in advance and follow this course, which may include
quieter, narrower streets, without having to stop frequently to refer to
separate maps. Some GPS receivers are specifically adapted for cycling
with special mounts and housings.
- Hikers, climbers, and even ordinary pedestrians in
urban or rural environments can use GPS to determine their position, with
or without reference to separate maps. In isolated areas, the ability of
GPS to provide a precise position can greatly enhance the chances of
rescue when climbers or hikers are disabled or lost (if they have a means
of communication with rescue workers).
- GPS equipment for the visually
impaired is available. For more detailed information
see the article GPS for the visually impaired
Surveying and
mapping
- Surveying —
Survey-Grade GPS receivers can be used to position survey
markers, buildings, and road construction. These units use the
signal from both the L1 and L2 GPS frequencies. Even though the L2 code
data are encrypted,
the signal's carrier wave enables correction of some ionospheric
errors. These dual-frequency GPS receivers typically cost US$10,000 or more,
but can have positioning errors on the order of one centimeter or less
when used in carrier phase differential GPS mode.
- Mapping
and geographic information systems
(GIS) — Most mapping grade GPS receivers use the carrier
wave data from only the L1 frequency, but have a precise crystal oscillator which reduces errors
related to receiver clock jitter. This allows positioning errors on the order of
one meter or less in real-time, with a differential GPS signal received
using a separate radio receiver. By storing the carrier phase measurements
and differentially post-processing the data, positioning errors on
the order of 10 centimeters are possible with these receivers.
- Geophysics
and geology — High precision measurements of crustal strain can be made with differential
GPS by finding the relative displacement between GPS sensors. Multiple
stations situated around an actively deforming area (such as a volcano or fault
zone) can be used to find strain and ground movement. These
measurements can then be used to interpret the cause of the deformation,
such as a dike or sill beneath the surface of an active volcano.
Other uses
- Precise
time reference — Many systems that must be accurately synchronized
use GPS as a source of accurate time. GPS can be used as a reference
clock for time code generators or NTP clocks. Sensors (for seismology
or other monitoring application), can use GPS as a precise time source, so
events may be timed accurately. TDMA communications networks
often rely on this precise timing to synchronize RF generating equipment,
network equipment, and multiplexers.
- Mobile
Satellite Communications — Satellite communications
systems use a directional antenna (usually a "dish") pointed at
a satellite. The antenna on a moving ship or train, for example, must be
pointed based on its current location. Modern antenna controllers usually
incorporate a GPS receiver to provide this information.
- Emergency and Location-based services —
GPS functionality can be used by emergency services to locate cell phones.
The ability to locate a mobile phone is required in the United States by E911 emergency
services legislation. However, as of September 2006 such a system is not
in place in all parts of the country. GPS is less dependent on the
telecommunications network topology than radiolocation
for compatible phones. Assisted GPS reduces the power requirements of
the mobile phone and increases the accuracy of the location. A phone's
geographic location may also be used to provide location-based services
including advertising, or other location-specific information.
- Location-based games —
The availability of hand-held GPS receivers has led to games such as Geocaching,
which involves using a hand-held GPS unit to travel to a specific
longitude and latitude to search for objects hidden by other geocachers.
This popular activity often includes walking or hiking to natural locations.
Geodashing
is an outdoor sport using waypoints.
- Aircraft
passengers — Most airlines
allow passenger use of GPS units on their flights, except during landing
and take-off when other electronic devices are also restricted. Even
though consumer GPS receivers have a minimal risk of interference, a few
airlines disallow use of hand-held receivers during flight. Other airlines
integrate aircraft tracking into the seat-back television entertainment
system, available to all passengers even during takeoff and landing.[24]
- Heading
information — The GPS system can be used to determine heading
information, even though it was not designed for this purpose. A "GPS
compass" uses a pair of antennas separated by about 50 cm to detect
the phase difference in the carrier signal from a particular GPS satellite.[25]
Given the positions of the satellite, the position of the antenna, and the
phase difference, the orientation of the two antennas can be computed.
More expensive GPS compass systems use three antennas in a triangle to get
three separate readings with respect to each satellite. A GPS compass is
not subject to magnetic declination as a magnetic
compass is, and doesn't need to be reset periodically like a gyrocompass.
It is, however, subject to multipath effects.
- GPS
tracking systems use GPS to determine the location of
a vehicle, person, or pet and to record the position at regular intervals
in order to create a log of movements. The data can be stored inside the
unit, or sent to a remote computer by radio or cellular modem. Some
systems allow the location to be viewed in real-time
on the Internet with a web-browser.
- Weather
Prediction Improvements — Measurement of atmospheric
bending of GPS satellite signals by specialized GPS receivers in orbital
satellites can be used to determine atmospheric conditions such as air
density, temperature, moisture and electron density. Such information from
a set of six micro-satellites, launched in April 2006, called the
Constellation of Observing System for Meteorology, Ionosphere and Climate COSMIC has
been proven to improve the accuracy of weather prediction models.
- Photograph
annotation — Combining GPS position data with photographs
taken with a (typically digital) camera, allows
one to lookup the locations where the photographs were taken in a gazeteer,
and automatically annotate the photographs with the name of the location
they depict. The GPS device can be integrated into the camera, or the
timestamp of a picture's metadata can be combined with a GPS track log.[26][27]
- Skydiving —
Most commercial drop zones use a GPS to aid the pilot to "spot"
the plane to the correct position relative to the dropzone that will allow
all skydivers on the load to be able to fly their canopies back to the
landing area. The "spot" takes into account the number of groups
exiting the plane and the upper winds. In areas where skydiving through
cloud is permitted the GPS can be the sole visual indicator when spotting
in overcast conditions, this is referred to as a "GPS Spot".
History
The design of GPS is based partly on the similar
ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early
1940s, and used during World War II. Additional inspiration for the GPS
system came when the Soviet Union launched the first Sputnik
in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were
monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler
effect, the frequency of the signal being transmitted by Sputnik was higher
as the satellite approached, and lower as it continued away from them. They realized
that since they knew their exact location on the globe, they could pinpoint
where the satellite was along its orbit by measuring the Doppler distortion.
The first satellite navigation system, Transit, used by the United States Navy, was first successfully
tested in 1960. Using a constellation of five satellites, it could provide a
navigational fix approximately once per hour. In 1967, the U.S. Navy developed
the Timation
satellite which proved the ability to place accurate clocks in space, a
technology the GPS system relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal
phase comparison, became the first world-wide radio navigation system.
The first experimental Block-I GPS satellite was
launched in February 1978.[28] The GPS satellites were
initially manufactured by Rockwell International and are now manufactured by Lockheed
Martin.
Timeline
- In
1983, after Soviet interceptor aircraft shot down the
civilian airliner KAL 007 in restricted Soviet airspace,
killing all 269 people on board, U.S. President Ronald
Reagan announced that the GPS system would be made available for
civilian uses once it was completed.
- By
1985, ten more experimental Block-I satellites had been launched to
validate the concept.
- On
February
14, 1989,
the first modern Block-II satellite was launched.
- In
1992, the 2nd Space Wing, which originally managed the system, was
de-activated and replaced by the 50th
Space Wing.
- By
December 1993 the GPS system achieved initial operational capability[29]
- By
January
17, 1994 a
complete constellation of 24 satellites was in orbit.
- In
1996, recognizing the importance of GPS to civilian users as well as
military users, U.S. President Bill
Clinton issued a policy directive[30]
declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board
to manage it as a national asset.
- In
1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two new
civilian signals for enhanced user accuracy and reliability, particularly
with respect to aviation safety.
- On
May 2, 2000
"Selective Availability" was discontinued, allowing users
outside the US military to receive a full quality signal.
- In
2004, U.S. President George W. Bush updated the national policy,
replacing the executive board with the National Space-Based Positioning,
Navigation, and Timing Executive Committee.
- The
most recent launch was on 17
November 2006.
The oldest GPS satellite still in operation was launched in August 1991.
Awards
Two GPS developers have received the National Academy of
Engineering Charles Stark Draper prize year 2003:
- Ivan
Getting, emeritus president of The Aerospace Corporation and engineer
at the Massachusetts Institute of
Technology, established the basis for GPS, improving on the World
War II land-based radio system called LORAN (Long-range
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.
One GPS developer, Roger
L. Easton, received the National Medal of Technology on February 13,
2006 at the White House.[31]
On February 10,
1993, the National Aeronautic Association
selected the Global Positioning System Team as winners of the 1992 Robert
J. Collier Trophy, the most prestigious aviation award in the United
States. This team consists of researchers from the Naval Research Laboratory, the U.S.
Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems
Company. The citation accompanying the presentation of the trophy honors the
GPS Team "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."
Other systems
- GLONASS (GLObal
NAvigation Satellite System) is operated by Russia,
although with only twelve active satellites as of
2004. In Russia, Northern Europe and Canada, at least four GLONASS
satellites are visible 45% of time. There are plans to restore GLONASS to
full operation by 2008 with assistance from India.
- Galileo is being
developed by the European Union, joined by China, Israel, India, Morocco, Saudi
Arabia and South Korea, Ukraine
planned to be operational by 2010.