Global Positioning System (GPS
Global Positioning System (GPS
The Global Positioning System is a
satellite-based system providing worldwide continuous position, velocity, time,
and related data to civil and military users. It has a growing number of
applications in the fields of marine, land, and aerospace navigation and precise
time and time transfer, as in surveying, geodesy, and mapping; precision
farming; air-traffic control; asset location and tracking; and timing of
communication systems and power grids. Since the 1960s, GPS has grown from a
navigation concept to an operational system of about 24 spacecraft (Fig. 1)
serving millions of users. Over a million GPS receivers a year were produced
during 1997–1999.
Fig. 1 Constellation of operational GPS
spacecraft.
GPS has performed extremely well and has
generally exceeded expectations. However, a number of deficiencies have been
identified, and some significant improvements are needed that could be
implemented with the new GPS replenishment spacecraft.
Satellites
The satellites' limited lifetime in orbit,
about 7.5 years for the current operational Block II and IIA spacecraft (Fig.
2), establishes the need and deployment schedule for their replacements.
Twenty-one third-generation (Block IIR) replenishment spacecraft have been
ordered by the U.S. Department of Defense to continue the GPS constellation to
2010 and possibly beyond. In July 1997, the first of these was launched to
replace the Block II and IIA spacecraft that will phase out by about 2005.
Fig. 2 Generations of GPS spacecraft. (a)
The Department of Defense has also
contracted for 6 of a planned 30 fourth-generation, follow-on (IIF) spacecraft.
These are to replace the IIR spacecraft and will carry the GPS constellation
well beyond 2010. The Delta 2 launch vehicle (Fig. 3) carries these spacecraft
to their medium-altitude orbits (20,180 km or 10,898 nautical miles above the
Earth).
Fig. 3 Delta 2 ready to launch a Block II
spacecraft.
Modernization
activities
A
number of committees have investigated the needs and deficiencies of the GPS in
order to determine what capabilities and features should be incorporated into a
future system to satisfy both military and civil users. The modernization will
include a new frequency, new signals, higher signal power levels, more extensive
ground tracking, and more frequent spacecraft position updates, all of which
will dramatically improve accuracy, integrity, and other aspects of performance.
The management of GPS has also changed and now involves coordinated civil and
military funding and oversight. These factors, combined with the increasing
worldwide importance of navigation systems and services, provide a basis for
integrating GPS into an international Global Navigation Satellite System (GNSS)
consisting of a number of independent but coordinated elements. The modernized
GPS will continue to play a central role in providing position, velocity,
attitude, and time services in an economical manner.
Selective
availability
Removal of selective availability (SA) is
scheduled between 2000 and 2006, in accordance with the Presidential Decision
Directive on GPS of March 29, 1996. This removal will provide undegraded
accuracy of the signals for civil users. This modification, together with the
additional civil signal frequencies (which include means for correcting
ionospheric delay errors), will improve civil GPS performance by an order of
magnitude or more, to a position determination accuracy of 5 m (15 ft) or
better, by 2010 (Fig. 4).
Civil
signals
New civil signals are planned, including
one centered at frequency L2 (1227.6 MHz), which has heretofore been used
exclusively by the military (Fig. 5). The long-standing civil signal centered at
L1 (1575.42 MHz) will be retained. On January 25, 1999, Vice President Gore
announced that a third civil frequency, L5, had been selected at 1176.45 MHz in
the Aeronautical Radionavigation Services band. This selection was intended
principally to satisfy aviation safety concerns but also to benefit applications
requiring real-time kinematic measurements. (Basically, these are precision
measurements of carrier phases for a number of GPS signals that are measured
between two differential receivers. They are done in real time, or near real
time, and provide almost survey-quality position information, currently about
5–20 cm or 2–8 in.) The new arrangement provides to civil users capabilities for
correction of errors caused by ionospheric delays, increased signal robustness,
and improved techniques for resolving the cycle ambiguities associated with
precision carrier-phase measurements.
|
Fig. 5 GPS current and modernized signals. (a)
Civil signal spectrum. (b) Military signal
spectrum.
Signal
structures
The new civil signal at L5 is planned at a
code rate ten times that of the Coarse/ Acquisition (C/A) code, which is now
available to civilian users, and with a 1-millisecond period. These
specifications will improve measurement accuracy, reduce noise, and provide
improved mitigation of multipath errors. New military signals named M codes will
provide improved measurement accuracy, a desirable power distribution in the
spectrum, and a capability for direct access. (In order to acquire the P/Y code,
currently employed in military applications, authorized users must normally
first access the civil C/A code, which contains information about the timing of
the military code. No such procedure is needed to access the M code.) The civil
signals have most of their power in the center of their bands, while the
military signals have most of their power in the outer regions of their bands
(Fig. 5). The existing civil and military signals will remain available
throughout the decade 2000–2010, while the new military (M-code) signals and the
new civil signals at L2 and L5 are to be introduced during the latter part of
the decade.
Receiver
system
Improved receivers such as narrow
correlator types, which provide considerably lower noise and other desirable
performance characteristics, will become generally available and commonly
employed. Also, the use of carrier-phase (real-time kinematic) measurements to
obtain precision position, velocity, attitude, and time determinations will
become commonplace. For example, position precision at the 2–10-cm (1–4 in.)
level will become available in moderate-cost receivers using phase and wide-lane
measurements of the three civil frequencies. (Wide-lane measurements are based
on the “lanes” formed by the wavelength corresponding to the difference in
frequency between signals. For example, the wide lane formed by the L1 and L2
frequencies, separated by 347.72 MHz, is about 86 cm.)
Spacecraft
The GPS spacecraft will offer increased
signal availability and power, and also have greater reliability and longer
lifetimes. Power in the new civil L2 (C/A-code) signal is to be consistent with
the L1 civil signal, which may be increased in power level by about 6 dB for
greater system robustness. Power in the military M-code signals is to be
substantially greater (by about 6–10 dB or more at times) than the current P/Y
military code signal power levels. While an increased number of spacecraft
(30–36) in the GPS constellation cannot be assured, there is strong interest in
this expansion.
Error
reduction
Systematic errors will be reduced not only
by the removal of selective availability and by ionospheric error correction but
also by substantial improvements in GPS receivers and in the control segment.
The GPS ground control system will be expanded by the addition of six or more
tracking stations, principally by incorporating those of the National Imagery
and Mapping Agency. More frequent uploads to the GPS spacecraft are also
planned. Control segment determinations of spacecraft position and prediction
errors will improve from about 2 m (80 in.) to 10–50 cm (4–20 in.).
Augmentations
Augmentations supporting improved
performance will become available worldwide before 2010. Augmentations include
the U.S. Coast Guard Differential Network (available now), the U.S. Federal
Aviation Administration's Wide Area Augmentation System (WAAS) and Local Area
Augmentation System (LAAS), the European Geostationary Navigation Overlay System
(EGNOS), and the Japanese Mobile Satellite Augmentation System (MSAS). Also, a
large number of other differential GPS systems are in use or will become
available that can provide highly precise position, velocity, attitude, and time
measurements.
International
implications
The GPS has become the de facto standard
for navigation satellite system operations, but there have been long-standing
concerns internationally because of the United States military origin and
control of the system. However, system and institutional changes have occurred
such that GPS now has a joint civil-military management structure and provides
independent civil and military capabilities, both of which are being
considerably improved. Additionally, GPS has an important role as a resource
worldwide. The management of GPS appears ready to take a significant role in an
international Global Navigation Satellite System. On February 10, 1999, the
European Commission requested the governments of the 15 states in the European
Union to support the development of an advanced system called the Galileo
project. Proposals have been made for the GPS frequencies and civil signal
structure to be integrated into the Galileo system and possibly into the MSAS as
well. The Europeans indicate a strong desire for Galileo to support their launch
vehicle, spacecraft, and ground control system industries. There is also the
potential for a coordinated global navigation satellite system capability
involving GPS as a principal element. The transition to an international system
is likely to occur before 2010.
Keith D. McDonald
Bibliography
K. D. McDonald, The GPS
modernization dilemma and some topics for resolution, Quart. Newsl. Inst.
Navig., vol. 8, no. 2, summer 1998
K. D. McDonald, The
modernization mantra, GPS World, Directions '99, 9(12):46, December 1998
K. D. McDonald, Technology, implementation and policy issues for the modernization of GPS and its role in a GNSS, J. Navig., vol. 51, no. 3, Royal Institute of Navigation, September 1998
Ali fazeli=egeology.blogfa.com
Additional
Galileo to Set Pace in
Satellite Navigation, Improve Safety, Generate Jobs, Commission Says, Public
Information Release, Pub. IP/99/102, European Commission,
K. D. McDonald et al.,
GPS: A shared national asset—A summary of the National Research Council report
on the future of GPS, Proceedings of the 8th International Technical Meeting of
the Satellite Division of the
New Global Positioning
System Modernization Initiative, Public Announcement, Office of the Vice
President, January 25, 1999
Presidential Decision
Directive on the Global Positioning System, announced by Vice President Albert
Gore and Secretary of Transportation Federico Pena, Doc. DOT 62–96, March 29,
1996
J. J. Spilker et al., A
family of split spectrum GPS civil signals, Proceedings of the 11th
International Technical Meeting of the Satellite Division of the Institute of
Navigation, Part 2, ION GPS-98, September 15–18, 1998
A. J. Van Dierendonck, L5
Signal Requirements, Revision 2, presentation to RTCA Working Group 1 on the
second and third civil frequency signals,
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