Satellite Navigation (GPS)


Of the various applications that satellites have been used for, one of the most promising is that of global positioning. Made possible by Global Navigation Satellite Systems, global positioning enables any user to know his or her exact position on Earth. Nowadays, the only fully functioning system is the American Global Positioning System (GPS). However, the European system, known as Galileo, is expected to be operative in 2012.

Since ancient times, mankind has tried to find its bearings by using milestones and stars. A new era has begun, however, thanks to satellite communication. New devices will be necessary to take advantage of both GPS and Galileo systems.

Navigation is defined as the process of planning, reading, and controlling the movement of a craft or vehicle from one place to another. The word navigate is derived from the Latin root navis, meaning "ship," and agere meaning "to move" or "to direct." All navigational techniques involve locating the navigator’s position by comparing it to known locations or patterns.

Since ancient times, human beings have been developing ingenious ways to navigate. Polynesians and modern navies developed the use of angular measurements of the stars. Everyone engages in some form of navigation in everyday life. When we use our eyes, common sense, and landmarks to find our way when driving to work or walking to a store, we are essentially navigating. Nevertheless, with the development of radios, the need for another class of navigation aids came along. This new phase in navigation called for more accurate information of position, intended course, and/or transit time to a desired destination. Examples of these navigational aids include a simple clock to determine velocity over a known distance, an odometer to keep track of the distance travelled, and more complex navigation aids that transmit electronic signals such as radio beacons, VHF omnidirectional radio ranges (VORs), long-range radio navigation (LORAN), and OMEGA. With artificial satellites, more precise line-of-sight radio-navigation signals became possible.

The position of anyone with a proper radio-navigation receiver can be computed by means of the signals from one or more radio-navigation aids. In addition to computing the user’s position, some radio-navigation aids provide velocity determination and time dissemination. The user’s receiver processes these signals, computes its position, and performs the required computational calculations (e.g., range, bearing, estimated time of arrival) so that the user can reach a desired location.

Radio-navigation aids can be classified as either ground-based or space-based. For the most part, the accuracy of ground-based radio-navigation aids is proportional to their operating frequency. Highly accurate systems generally transmit at relatively short wavelengths and the user must remain within the line of sight, whereas systems broadcasting at lower frequencies (longer wavelengths) are not limited to line of sight but are less accurate[Kaplan96], [Parkinson96].

GPS Predecessors

In the early 1960s, several U.S. governmental organizations—including the Department of Defense (DOD), the National Aeronautics and Space Administration (NASA), and the Department of Transportation (DOT)—were interested in developing satellite systems for position determination. The optimum system was viewed as having the following attributes: global coverage, continuous/all weather operation, the ability to serve high-dynamic platforms, and high accuracy.

The system Transit became operational in 1964 and its operation was based on the measurement of the Doppler shift of a tone at 400MHz sent by polar orbiting satellites at altitudes of about 600 nautical miles (ionospheric group delay was corrected by transmitting two frequencies). Transit satellites travelled along well-known paths and broadcasted their signals on a well-known frequency.

The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. If the frequency shift is measured over a short time interval, the receiver can determine its location on one side or the other of the satellite. Many measurements such as these, combined with precise knowledge of the satellite’s orbit, can enable a receiver to compute a particular position. This first system had its limitations, as it offered an intermittent service with limited coverage with periods of 35min. to 100min. of unavailability. However, because of its low velocity, its two-dimensional nature was suitable for shipboard navigation rather than for high dynamic uses, as aircrafts. The technology developed for Transit, which included both satellite prediction algorithms and more than 15 years of space system reliability, exceeding expectations more than two or three times, has proved to be extremely useful for GPS. Limitations of early developed spaced-based systems (the U.S. Transit and the Russian Tsikada system) led to the development of both the U.S. Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS).

Overcoming these early systems’ shortcomings required either an enhancement of Transit or the development of another satellite navigation system with the desired capabilities previously mentioned. By 1972, breakthroughs were made by installing high-precision clocks in satellites. These satellites, known as Timation, were used principally to provide highly precise time and time transfer between various points on Earth. They additionally provided navigational information. Several variants of the original Transit system were proposed, among them the inclusion of highly stable space-based atomic clocks in order to achieve precise time transfer. Modifications were made to Timation satellites to provide a ranging capability for two-dimensional position determination, employing side-tone modulation for satellite-to-user ranging.

Later models of the Timation satellites employed the first atomic frequency standards (rubidium and cesium), which typically had a frequency stability of several parts per 1012 (per day) or better. This frequency stability greatly improves the prediction of satellite orbits (ephemerides) and also lengthens the required update time between control segment and satellites. This revolutionary work in space-qualified time standards was also important for the development of GPS.

At the same time as the Navy was considering the Transit enhancements and undertaking the Timation efforts, the Air Force conceptualized a satellite positioning system denoted as System 621B. By 1972, this programme had already demonstrated the operation of a new type of satellite-ranging signal based on pseudorandom noise (PRN). The signal modulation was essentially a repeated signal sequence of fairly random bits (ones or zeros) that possessed certain useful properties. The start ("phase") of the repeated sequence could be detected and used to determine the range of a satellite. The signals could be detected even when their power density was less than 1/100th that of ambient noise and all satellites could broadcast on the same nominal frequency because properly selected PRN codes were nearly orthogonal. The ability to reject noise also implied a powerful ability to resist most forms of jamming or deliberate interference.

The use of pseudorandom noise (PRN) modulation for ranging with digital signals provided three-dimensional coverage and continuous worldwide service. The use of PRN modulation with ranging (i.e., pseu-doranging), which could be considered the third foundation of the GPS system, was developed through Army research.

In 1969, the Office of the Secretary of Defense (OSD) established the Defense Navigation Satellite System (DNSS) programme to consolidate the independent development efforts of each military branch into a single joint-use system. The OSD also established the Navigation Satellite Executive Steering Group, which was put in charge of determining the viability of a DNSS and planning its development. This endeavour led to the forming of the GPS Joint Programme Office (JPO) in 1973, which set the development of Navstar GPS in motion. This was not exclusively the concept of any prior system but rather was a synthesis of them all. The JPO’s multibranch approach avoided any basis for further bickering because all contending parties were part of the conception process. From that point on, the JPO acted as a multiservice enterprise, with officers from all branches attending meetings that were previously exclusive. The system is generally referred to as simply GPS.

In 1973, the first phase of the programme was approved. It included four satellites (one was a refurbished test model), launch vehicles, three varieties of user equipment, a satellite control facility, and an extensive test programme. The first satellite prototype was launched in 1978. By this time, the initial control segment was deployed and working and five types of user equipment were undergoing preliminary testing.

More than four satellites were now required. The minimum number of satellites required to determine three-dimensional position is four. Any launch or operational failure would have gravely impacted the first phase of GPS testing. The problem of the need for spare satellites was solved by joining the Transit programme, which was followed by the development of two additional satellites. Apart from extending GPS, this joint endeavour avoided the possibility of having two systems competing against each other.

Even though today’s GPS system concept is the same as the one proposed in 1973, its satellites have expanded their functionality to support additional capabilities. Although the orbits are slightly modified, the original equipment designed to work with the very first four satellites would still work today[Kaplan96], [Parkinson96].


The European Union (EU) and European Space Agency (ESA) agreed on March 2002 to introduce an alternative to GPS, called the Galileo positioning system. The system is scheduled to be working in 2012.

The first experimental satellite was launched on December 28, 2005. Galileo is expected to be compatible with the modernized GPS system. The receivers will be able to combine the signals from both Galileo and GPS satellites to increase accuracy significantly.

In 1999, the European Commission presented its plans for a European satellite navigation system defined by a joint team of engineers from Germany, France, Italy, and the United Kingdom. Contrary to its American and Russian counterparts, Galileo is designed specifically for civilian and commercial purposes. The United States reserves the right to limit the signal strength or accuracy of the GPS systems or to shut down public GPS access completely (although it has never done the latter) so that only the U.S. military and its allies would be able to use it in time of conflict. Until 2000, the precision of the signal available to non-U.S. -military users was limited, due to a timing pulse distortion process known as selective availability. The European system will be subject to shutdown only for military purposes under extreme circumstances (although it may still be jammed by anyone with the right equipment). Both civil and military users will have complete and equal access to this system.

The European Commission faced certain challenges in finding funding for the project’s subsequent stage, because of national budget constraints across Europe. The United States government opposed the project, arguing that it would jeopardize the ability of the United States to shut down GPS in times of military operations in the wake of the September 11, 2001, attacks. In 2002, as a result of U.S. pressure and economic difficulties, the Galileo project was almost put on hold. However, a few months later, the situation changed dramatically. Partially in reaction to the pressure of the U.S. government, European Union member states decided it was important to have their own independent satellite-based positioning and timing infrastructure.

The European Union and the European Space Agency agreed in 2002 to fund the project. The first stage of the Galileo programme was agreed upon officially in 2003 by the EU and the ESA. The plan was for private companies and investors to invest at least two-thirds of start-up costs, with the EU and ESA dividing the remaining cost. An encrypted higher-bandwidth Commercial Service with improved accuracy would be available at extra cost, with the base Open Service freely available to anyone with a Galileo-compatible receiver.

In 2007, it was agreed to reallocate funds from the EU’s agriculture and administration budgets and to soften the tendering process in order to woo more EU companies to join the project. In 2008, EU transport ministers approved the Galileo Implementation Regulation, which freed up funding from the EU’s agriculture and administration budgets.

This allowed the issuing of contracts to start construction of the ground station and satellites.

From its conception, a fundamental part of the Galileo programme was to be a worldwide system that would maximise its benefits by means of international cooperation. Such cooperation is foreseen to help to reinforce industrial know-how and to minimise the technological and political risks involved. This includes, quite naturally, cooperation with the two countries now operating satellite navigation systems. Europe is already examining a number of technical issues with the United States related to interoperability and compatibility with the GPS system. The objective is to ensure that everyone will be able to use both GPS and Galileo signals with a single receiver. Negotiations with the Russian Federation, which has valuable experience in the development and operation of its GLONASS system, are also ongoing.

In addition to the technical harmonisation required among Galileo and existing satellite navigation systems, international cooperation is necessary in the development of ground-based equipment and ultimately to promote widespread use of this technology. Such cooperation also falls in line with the objectives of the European Union with respect to foreign policy, co-operation with developing countries, employment, and the environment. Several non-European countries have already contributed to the Galileo programme in terms of system definition, research, and industrial cooperation. Since the European Council’s decision to launch the Galileo programme, even more countries have expressed the wish to be associated with the programme in one form or another. Indeed, the European Commission sees Galileo as highly relevant to all the countries of the world and remains committed to further collaboration with countries that share its vision of a high-performance, reliable, and secure global civil satellite navigation system. In 2003, China joined the Galileo project and invested heavily in the project over the following few years. In 2004, Israel signed an agreement with the EU to become a partner in the Galileo project. In 2005, the Ukraine, India, Morocco, and Saudi Arabia signed an agreement to take part in the project. At the time of publication, the most recently added member to the project was South Korea, which joined the programme in 2006.

In 2007, the 27 member states of the European Union collectively agreed to move forward with the project, with plans for bases in Germany and Italy[EU-Galileo].

Two Galileo System Test Bed satellites, dedicated to take the first step of the In-Orbit Validation phase towards full deployment of Galileo, can be found under the name of GIOVE, which stands for Galileo In-Orbit Validation Element. At the time of publication, the following milestones had been accomplished:

■ In 2005, GIOVE-A, the first GIOVE test satellite, was launched.

■ In 2008, GIOVE-B, with a more advanced payload than GIOVE-A, was successfully launched.

■ In 2008, the GIOVE-A2 satellite was ready to be launched. 1.1.3 Satellite Based Augmentation System (SBAS)

A Satellite Based Augmentation System (SBAS) is a system that supports wide-area or regional augmentation by using additional information sent by these satellites. In addition to the satellites, such systems are also composed of well-known multiple ground stations that take measurements of one or more of the Global Navigation Satellite System (GNSS) satellites, their signals, or other environmental factors that may influence the signal received by users. SBAS information messages are created from these measurements and sent to one or more satellites to be transmitted to users.

Therefore, Satellite Based Augmentation Systems use external information within the user’s receiver to improve the accuracy, reliability, and availability of the satellite navigation signal of a GNSS. There are many such systems in place that are generally named depending on the way that the external information reaches the receiver. Such information includes additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), direct measurements of how much the signal was off in the past, or additional vehicle information to be integrated in the calculation process.

Examples of augmentation systems of various SBAS are as follows. Note that the last two are commercial systems.

■ The Wide Area Augmentation System (WAAS), operated by the United States Federal Aviation Administration (FAA)

■ The European Geostationary Navigation Overlay Service (EGNOS), operated by the European Space Agency

■ The Wide Area GPS Enhancement (WAGE), operated by the United States Department of Defense for use by military and authorized receivers

■ The Multifunctional Satellite Augmentation System (MSAS) system, operated by Japan’s Ministry of Land, Infrastructure and Transport (JCAB)

■ The Quasi-Zenith Satellite System (QZSS), proposed by Japan

■ The GAGAN system, proposed by India

■ The StarFire navigation system, operated by John Deere

■ The Starfix DGPS System, operated by Fugro

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