Global Positioning System
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.
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) Block I. (b) Block II-IIA. (c) Block IIR. (d) Block IIF.
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.
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.
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).
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.
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.
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.)
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.
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 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.
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
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