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Critical Issues in Global Navigation Satellite Systems - THE GLOBAL GROWTH OF GNSS, THE MECHANICS OF GNSS, Space Segment, Control Segment, User Segment

gps time accuracy satellites

Ina Freeman
University of Birmingham, UK

Jonathan M. Auld
NovAtel Inc., Canada


Since the dissolution of the USSR, the GLONASS system has become the responsibility of the Russian Federation, and on September 24, 1993, GLONASS was placed under the auspices of the Russian Military Space Forces. The Russian government authorized civilian utilization of GLONASS in March 1995. This system declined (Langley, 1997) and did not evolve, making the system questionable for civilian or commercial use (Misra & Enge, 2001). Recognizing this, the European Union announced its intent to develop a separate civilian system known as Galileo. In 2004, the Russian government made a commitment to bring back GLONASS to a world-class system and has increased the number of functional satellites to 10 with more anticipated to a level concurrent with the American GPS.

Today, other countries of the world have recognized the importance and commercial value of GNSS and are taking steps to both broaden the technology and utilize it for their populations. The European Space Agency (ESA) has entered the second development phase to make Galileo interoperable with the U.S. GPS by developing appropriate hardware and software. Its first satellite launch is scheduled for 2008. China, India, Israel, and South Africa all have expressed an interest in joining Europe in developing the 30-satellite Galileo GNSS under the auspices of the Galileo Joint Undertaking (GJU), a management committee of the European Space Agency and the European Commission. The Japanese government is exploring the possibility of a Quazi-Zenith system that will bring the number of GNSS globally to four in the next 10 years. Thus, the globalization of navigation and positioning standards is progressing, albeit under the watchful eye of the United States, who may fear a weakening of its military prowess, and of Europe, who wants sovereign control of essential navigation services.


GNSS requires access to three segments: specialized satellites in space (space segment); the operational, tracking, or control stations on the ground (control segment); and the appropriate use of localized receiver equipment (user segment). The following diagrammed system (NovAtel, Inc. Diagrams) uses a plane as the user, but it could be any user, as the same mechanics apply throughout all systems:

The following is a description of GPS; however, the same principles apply to all GNSSs.

Space Segment

GPS relies on 24 operational satellites (a minimum of four satellites in six orbital planes, although there are often more, depending upon maintenance schedules and projected life spans) that travel at a 54.8-degree inclination to the equator in 12-hour circular orbits, 20,200 kilometers above earth (http://www.spacetechnology.com/projects/gps/). They are positioned so there are usually six to eight observable at any moment and at any position on the face of the earth. Each carries atomic clocks that are accurate to within one 10-billionth of a second and broadcast signals on two frequencies (L1 and L2) (Anonymous, 1998).

The satellite emits a Pseudo Random Code (PRC) that is a series of on and off pulses in a complex pattern that reduces the likelihood of emulation or confusion of origin and that uses information theory to amplify the GPS signal, thus reducing the need for large satellite dishes. The PRC calculates the travel time of a signal from a satellite. Furthermore, each satellite broadcasts signals on two distinct frequencies that are utilized to correct for ionospheric distortions. The signals are received and identified by their unique patterns. Receivers on the earth’s surface then use these signals to mathematically determine the position at a specific time.

Because the GPS signals are transmitted through two layers of the atmosphere (troposphere and ionosphere), the integrity and availability of the signal will vary (Naim, 2002). A Satellite Based Augmentation System (SBAS) augments the GPS system. This system monitors the health of the GPS satellites, provides corrections to users of the system, and is dependent upon geosynchronous satellites to provide data to the user. The SBAS system relies on the statistical principle that the more measurements taken, the greater the probability of accuracy, given consistency of all other parameters. The U.S. has structured an SBAS that is referred to as the Wide Area Augmentation System (WAAS) for use by the commercial aviation community. Work is currently underway to further enhance the WAAS with both lateral (LNAV) and vertical navigation (VNAV) capabilities, specifically useful in aviation (Nordwall, 2003). The U.S. also is investigating the capability of a Ground Based Augmentation System (GBAS) called a Local Area Augmentation Systems (LAAS) that would further enhance aviation by allowing instrument landings and take-offs under all weather conditions. With further research and reduction of costs, it could be more widely spread. SBASs are being and have been structured in various countries (e.g., the Indian GPS and GEO [Geosynchronous] Augmented Navigation (GAGAN) (anticipated); the Japanese MTSAT [Multifunction Transport Satellite] Satellite Augmentation Service [MSAS]; the Chinese Satellite Navigation Augmentation Service [SNAS]; the Canadian WAAS [CWAAS] [anticipated]; and the European Geostationary Navigation Overlay Service [EGNOS]). The satellites cover all areas of the globe, diagrammed as follows (NovAtel Inc., 2004):

Control Segment

The accuracy and currency of the satellites’ functioning and orbits are monitored at a master control station operated by the 2 nd Satellite Control Squadron at Schriever Air Force Base (formerly Falcon), Colorado, which also operates five monitor stations dispersed equally by longitude at Ascension Island; Diego Garcia; Kwajalein, Hawaii; and Colorado Springs; and four ground antennas colocated at Ascension Island; Diego Garcia; Cape Canaveral; and Kwajalein. The incoming signals all pass through Schriever AFB where the satellite’s orbits and clocks are modeled and relayed back to the satellite for transmission to the user receivers (Misra & Enge, 2001).

User Segment

The civilian use of the GPS system was made possible through the evolution of miniaturization of circuitry and by continually decreasing costs, resulting in more than 1,000,000 civilian receivers in 1997 (Misra & Enge, 2001). This technology has revolutionized the ability of the man-on-the-street to do everything from telling time and location with a wristwatch to locating a package en route.

The receiver plays a key role in the Differential GPS (DGPS) used to enhance the accuracy calculated with GPS. DGPS uses a reference station receiver at a stationary surveyed location to measure the errors in the broadcast signal and to transmit corrections to other mobile receivers. These corrections are necessitated by a number of factors, including ionosphere, troposphere, orbital errors, and clock errors. The positioning accuracy achievable can range from a few meters to a few centimeters, depending on the proximity of the receiver.

There are a number of different types of GNSS systems, all operating on the same concept but each having different levels of accuracy, as indicated in the following table.



On June 10, 1993, the U.S. Federal Aviation Administration, in the first step taken toward using GPS instead of land-based navigation aids, approved the use of GPS for all phases of flight. Since that time, GPS has been used for flight navigation, enhancing conservation of energy. In-flight navigation was integrated with take-offs and landings in September 1998, when the Continental Airlines Airbus MD80 used GPS for all phases of flight (Bogler, 1998). This technology also allows for more aircraft to fly the skies, because separation of flight paths is possible with the specific delineation capable only with GPS. Further, GPS is integral to sea navigation for giant ocean-going vessels in otherwise inaccessible straights and through passages to locate prime fishing areas and for small fishing boats.

Navigation is not restricted to commercial enterprises. Individuals such as hikers, bikers, mountaineers, drivers, and any other person who may traverse distance that is not clearly signed can use GPS to find their way.


GNSS can be used to determine the position of any vehicle or position. Accuracy can be as high as 1-2 centimeters with access to a reference receiver and transmitter. GPS is currently an integral part of search and rescue missions and surveying (in particular, it is being used by the Italian government to create the first national location survey linked to the WGS-84 coordinated reference frame). Receivers are used by geologists, geodesists, and geophysicists to determine the risk of earthquakes, sheet ice movement, plate motion, volcanic activity, and variations in the earth’s rotation; by archeologists to locate and identify difficult to locate dig sites; by earth movers to determine where to work; and by farmers to determine their land boundaries. In the future, self-guided cars may become a reality.

Asset Tracking

The world of commerce uses GPS to track its vehicle fleets, deliveries, and transportation scheduling. This also includes tracking of oil spills and weather-related disasters such as flooding or hurricanes, and tracking and locating containers in storage areas at ports.


This system began with the primary purpose of locating people who contacted emergency services via cell phones. It now also tracks emergency vehicles carrying E-OTD (Enhanced Observed Time Difference) equipment to facilitate the determination of the fastest route to a specified destination.


GPS can be used as easily to explore locations as it can be used to locate people and vehicles. This ability enhances the accuracy of maps and models of everything from road maps to ecological surveys of at-risk animal populations to tracking mountain streams and evaluating water resources both on earth and in the troposphere.


GPS can be coordinated with communication infrastructures in many applications in transportation, public safety, agriculture, and any primary applications needing accurate timing and location abilities. GPS facilitates computerized and human two-way communication systems such as emergency roadside assistance and service, enhancing the speed of any transaction.


GPS is used in agriculture for accurate and efficient application of fertilizer or herbicides and for the mechanized harvesting of crops.


With the use of DGPS, construction may be completed using CAD drawings without manual measurements, reducing time and costs. Currently, GPS assists in applications such as setting the angle of the blade when digging or building a road. It also assists with monitoring structural weaknesses and failures in engineering structures such as bridges, buildings, towers, and large buildings.


With the use of two cesium and two rubidium atomic clocks in each satellite and with the automatic corrections that are part of the system, GPS is an ideal source of accurate time. Time is a vital element for many in commerce, including banks, telecommunication networks, power system control, and laboratories, among others. It is also vital within the sciences, including astronomy and research.


The uses of GPS are varied and include individually designed applications such as the tracking of convicts in the community via the use of an ankle band, the location of high-value items such as the bicycles used in the Tour de France, and surveillance.


The future of GNSS is emerging at a phenomenal pace. Already in prototype is a new GPS navigation signal at L5. When used with both WAAS and LAAS, this will reduce ionospheric errors and increase the integrity of the received data. The introduction of the interoperable Galileo system will enhance further the GPS system and further refine the precision of the measurements. Commerce continually speaks of globalization and the positive effects of this phenomenon upon humankind. With the increasing usage of GNSS systems, this globalization becomes a seamless system with governments and private enterprises interacting across national borders for the benefit of all. As commercial enterprises around the world become increasingly dependent on GNSS, these invisible waves may bring together what governments cannot.

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