Gps satellite navigation
gps satellite navigation system - Русский перевод – Словарь Linguee
With its vast land areas and territorial waters,[...] low population density and sub-Arctic to Arctic location, Norway benefits immensely from the GPS satellite navigation system.
Для Норвегии с ее протяженными[...]
участками суши и территориальными водами, низкой[...] плотностью населения, субарктическим и арктическим климатом применение спутниковой навигационной системы GPS имеет колоссальное [...]
при возникновении других помех.
Some themes deal with the fundamentals of satellite[...]
navigation, the principles[...] of the construction and operation of the GPS and GLONASS global satellite systems, regional satellite navigation systems and with the issues of enhancing the efficiency [...]
of their use,[...]
which will be interesting both for the experts working in this area and for the wide audience of the visitors taking an interest in navigation subjects.
Отдельные темы посвящены[...]
основам спутниковой[...] навигации, принципам построения и функционирования глобальных спутниковых систем GPS и ГЛОНАСС, региональных спутниковых навигационных систем и вопросам повышения эффективности [...]
которые будут интересны как специалистам, работающим в этой области, так и широкому кругу слушателей, интересующихся навигационной тематикой.
Практическое[...] использование сигналов, получаемых от существующих ГНСС, наиболее известными из которых являются Глобальная система позиционирования (GPS) Соединенных Штатов Америки и Глобальная навигационная спутниковая система (ГЛОНАСС) Российской Федерации, стало универсальным средством содействия расширению применения систем определения местоположения и навигационных систем.
To ensure high integrability the device was[...]
equipped with a range of[...] RS-232 interfaces that can be used to connect satellite navigation system (GPS), gyrocompass and other external navigation [...]
devices via NMEA 0183[...]
protocol. Also, the device has a possibility to be automatically connected to reserve power supply and a possibility to connect analog inputs and outputs.
В целях обеспечения высокой интегрируемости устройство было оснащено[...]
рядом RS-232 интерфейсов,[...] посредством которых обеспечивается подключение спутниковой навигационной системы (GPS), гирокомпаса и других внешних навигационных [...]
протоколу NMEA 0183, реализована функция автоматического подключения резервного канала питания, а также обеспечена возможность подключения аналоговых входов и выходов.
The new HDD series is an ideal proposition to telematics[...]
manufacturers improving the in-vehicle user[...] experience by integrating automotive infotainment systems, self-diagnostic systems, satellite GPS, navigation and communication tools into premium automobiles.
Жесткие диски новой серии идеально подходят для производителей средств обработки и передачи информации,[...]
позволяя расширить возможности пользователей[...] и интегрировать в автомобили премиум-класса информационно-развлекател ь ные и диагностические системы, а также инструменты спутниковой GPS-навигации и общения.
uses include sharing[...]
context-specific resources and delivering information relevant to a journey or a particular place.
благодаря ей можно[...]
обмениваться контекстно-специфическими ресурсами, а также информацией, имеющей отношение к определенному маршруту или месту.
According to Padva & Epstein, the Ruling sets out[...]
new transport safety[...] requirements, e.g. vehicles used to carry passengers, special or hazardous cargo should be equipped with satellite navigation systems GLONASS or GLONASS/GPS.
Как сообщает юридическое бюро "Падва и Эпштейн", постановление устанавливает новые требования к безопасности[...]
автомобилей, например[...] транспортные средства для перевозки пассажиров, специальных и опасных грузов, должны быть оснащены аппаратурой спутниковой навигации ГЛОНАСС или ГЛОНАСС/GPS.
For increase of level of safety of navigation, efficiency of use and competitiveness of an internal sailing charter, and also ecological safety at the[...]
expense of use of new[...] elements of uniform technology of satellite navigation and according to technical and operational parameters of the equipment of GNSS GLONASS/GPS by working out of control-correcting [...]
stations (CCS), their[...]
input in pre-production operation and survey should correspond to technical-operational requirements of Ministry of Transport of Russia.
Для повышения уровня безопасности судоходства, эффективности использования и конкурентоспособности внутреннего водного[...]
транспорта, а также[...] экологической безопасности за счет использования новых элементов единой технологии спутниковой навигации и в соответствии с техническими и эксплуатационными параметрами оборудования ГНСС ГЛОНАСС/GPS [...]
контрольно-корректирующих станций (ККС), вводе их в опытную эксплуатацию и освидетельствовании должны соответствовать технико-эксплуатационным требованиям Минтранса России.
ESP, ACC system with[...] two radars monitoring the distance, Lane Guard camera, air-conditioning, working with the turned off engine, telematics system, combines GSM mobile communication and GPS navigation.
Система динамической стабилизации ESP, система ACC с двумя[...]
радарами, следящими за[...] дистанцией, телекамера слежения за разметкой Lane Guard, кондиционер, работающий при выключенном двигателе, система телематики, объединяющая мобильную связь GSM и спутниковую навигацию GPS.
In particular, "the availability at the licensee at the right of ownership or at other legal grounds vehicles appropriate by the purpose, design, external and internal hardware specifications for the[...]
transportation of passengers and[...] equipped in accordance with established procedure by satellite navigation equipment GLONASS or GLONASS / GPS and allowed in the prescribed manner to participate [...]
В частности, обязательным становится «наличие у лицензиата на праве собственности или на ином законном основании транспортных средств, соответствующих по назначению, конструкции, внешнему и внутреннему[...]
оборудованию техническим[...] требованиям в отношении перевозок пассажиров, оснащенных в установленном порядке аппаратурой спутниковой навигации ГЛОНАСС или ГЛОНАСС/GPS и допущенных [...]
в установленном порядке[...]
к участию в дорожном движении».
The Committee took note of the full deployment[...] by the Russian Federation of the Global Navigation Satellite System (GLONASS), which currently comprised 31 [...]
Комитет принял к[...]
сведению завершение[...] развертывания Российской Федерацией Глобальной навигационной спутниковой системы (ГЛОНАСС), которая в настоящее [...]
время состоит из 31 космического аппарата.
A functioning space plane[...] might be useful for the Russians to service the new Glonass satellite network, Russia`s answer to the American GPS system, which is still on track to launch this year despite a [...]
disastrous crash that[...]
cost the program three satellites last December.
космический самолет может[...] оказаться полезным для русских, чтобы обслуживать их спутниковую сеть ГЛОНАСС, российский ответ на американскую систему GPS, которую по-прежнему планируется запустить в этом году, [...]
несмотря на катастрофу[...]
в декабре прошлого года, из-за которой были потеряны сразу три спутника этой программы.
Средняя околоземная орбита выделяется в[...] пределах от 1500 км до 36 000 км, и используется она в основном для навигационных группировок, таких как Глобальная система местоопределения США (ГСМ).
As a member State of ESA, as well as through[...]
cooperation agreements with the European[...] Union, Norway is now taking part in the development of Europe’s global navigation satellite system, Galileo.
Будучи членом ЕКА, а также на условиях заключенных[...]
с Евросоюзом соглашений о[...] сотрудничестве, Норвегия принимает участие в разработке европейской глобальной спутниковой навигационной системы "Галилео".
to the private sector[...]
and other CIS countries (Ukraine and Kazakhstan).
другим странам СНГ (Украина, Казахстан)[...]
предоставляются услуги спутниковой связи.
– GPRS, 3G, WiFi др.
Throughout its history, Toshiba SDD has[...]
revolutionized the design and development[...] of small form factor storage devices and its HDDs can be found inside the world's leading GPS navigation systems, portable media players and entertainment systems.
Это подразделение компании известно своими революционными[...]
разработками в производстве устройств[...] хранения малого формфактора, и поэтому его жесткие диски используются в навигационных и развлекательных системах, а также портативных медиапроигрывателях ведущих [...]
object monitored in the conditions of multiple urban buildings.
объекта в условиях плотной городской застройки.
С этим прибором[...] будущего поколения клиент будет отлично подготовлен: наряду с сигналами американской GPS, он принимает сигналы российской GLONASS, а также европейской системы GALILEO.
The Committee was informed[...] that the Russian Global Orbital Navigation Satellite System (GLONASS) was freely available to [...]
the public in the Asian[...]
and Pacific region and that those new technologies could serve the purpose of disaster preparedness and mitigation.
Комитет также[...] информировали, что российской Глобальной навигационной спутниковой системой (ГЛОНАСС) могут свободно пользоваться [...]
в Азиатско-Тихоокеанском регионе и что эти новые технологии могут служить целям обеспечения готовности к бедствиям и смягчения их последствий.
Development of the motor transport navigation system Over 8 thousand motor[...] transport vehicles were equipped and connected to the system of satellite navigation.
Развитие системы[...] навигации на автотранспорте Системой спутниковой навигации оснащено более 8 тыс. единиц автотранспорта.
The Subcommittee noted that Italy continued to be an active member of ICG,[...]
having been one of[...] the founders of the European EGNOS and Galileo satellite navigation system, and was developing national application projects [...]
aimed at fostering[...]
the use of satellite navigation and harmonizing them with European projects.
Подкомитет отметил, что Италия как один из основателей[...]
Европейской геостационарной[...] службы навигационного покрытия (EGNOS) и спутниковой навигационной системы "Галилео" остается одним из активных членов [...]
МКГ и разрабатывает[...]
национальные прикладные проекты, нацеленные на расширение использования спутниковой навигации, и согласует их с европейскими проектами.
The flight tests proved the proper functioning[...]
of the aircraft avionics,[...] most notably the inertial reference system and the satellite navigation systems GPS and Glonass, in flights in high north latitude [...]
area (up to 78 degree).
В полетах проверялось и было подтверждено[...]
надлежащее[...] функционирование систем авионики самолета, прежде всего инерциальной навигационной системы и систем спутниковой навигации GPS и ГЛОНАСС, в условиях [...]
высоких северных широт (до 78о).
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].
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
Other Satellite Navigation Systems (GPS)
GLONASS satellite system
GLONASS is an all-weather global navigation satellite system developed by Russia. The GLONASS satellite system has much in common with the GPS system. The nominal constellation of the GLONASS system consists of 21 operational satellites plus three spares at a nominal altitude of 19,100 km. Eight GLONASS satellites are arranged in each of three orbital planes (see Figure 11.1). GLONASS orbits are approximately circular, with an orbital period of 11 hours and 15 minutes and an inclination of 64.8° [1, 2].
Similar to GPS, each GLONASS satellite transmits a signal that has a number of components: two L-band carriers, C/A-code on L1, P-code on both L1 and L2, and a navigation message. However, unlike GPS, each GLONASS satellite transmits its own carrier frequencies in the bands 1,602-1,615.5 MHz for L1 and 1,246-1,256.5 MHz for L2, depending on the channel number. These two bands are on their way to being shifted to 1,598.0625-1,604.25 MHz and 1,242.9375-1,247.75 MHz, respectively, to avoid interference with radio astronomers and operators of low-Earth-orbiting satellites. With this shift, each pair of GLONASS satellites will be assigned the same L1 and L2 frequencies. The satellite pairs, however, will be placed on the opposite sides of the Earth (antipodal), which means that a user cannot see them simultaneously.
Figure 11.1 GLONASS system.
GLONASS codes are the same for all the satellites. As such, GLONASS receivers use the frequency channel rather than the code to distinguish the satellites. The chipping rates for the P-code and the C/A-code are 5.11 and 0.511 Mbps, respectively. The GLONASS navigation message is a 50-bps data stream, which provides, among other things, the satellite ephemeris and the channel allocation . The signal of GLONASS system is not affected by either SA or antispoofing. The GLONASS system completed 24 working satellites in January 1996. Unfortunately, however, the number of GLONASS satellites had dropped to only seven satellites by May 2001 . It is expected that a new genera-m tion GLONASS satellites, GLONASS-M, will be launched in the near future. GLONASS-M has a lifetime of 5 years, improved onboard atomic clocks, and the facility to transmit the C/A code on both L1 and L2 carrier frequencies .
GPS and GLONASS systems may be integrated to improve geometry and positioning accuracy, particularly under poor satellite visibility, such as in urban areas. There are, however, two problems with GPS/GLONASS integration. The first one is that both GPS and GLONASS systems use different coordinate frames to express the position of their satellites. GPS uses the WGS 84 system, while GLONASS uses the Earth Parameter System 1990 (PZ-90) system. The two systems differ by as much as 20m on the Earth’s surface. The transformation parameters between the two systems may be obtained by simultaneously observing reference points in both systems. Various research groups have developed various sets of transformation parameters [2, 5]. However, accurate determination of the transformation parameters is still unavailable. The second problem with the GPS/GLONASS integration is that both systems use different reference times. The offset between the two time systems changes slowly and reaches several tens of microseconds. One way of determining the time offset is by treating it as an additional variable in the receiver solution.
Chinese regional satellite navigation system (Beidou system)
China has recently launched two domestically built navigation satellites, which form the first generation of a satellite-based navigation system [6-8]. It is an all-weather regional navigation system, which is known as the Beidou Navigation System. The satellites are placed in geostationary orbits at an altitude of approximately 36,000 km above the Earth’s surface. The primary use of the system is in land and marine transportation. China is also planning to build its second-generation satellite positioning and navigation system, which will have more satellites and more coverage area.
The current satellite-based global navigation systems, GPS and GLONASS, do not meet all of the civil aviation requirements. To overcome these limitations, regional augmentation systems are currently being developed. A regional augmentation system typically combines one or more satellite constellations such as GPS and GLONASS, geostationary satellites equipped with navigation transponders and a number of ground reference stations . Merging the various interoperable regional systems leads to a Global Navigation Satellite System (commonly known as GNSS-1) that meets the civil aviation requirements. In fact, the International Civil Aviation Organization (ICAO) has endorsed the GNSS as the core system for international aviation use .
Various regional augmentation systems are currently being developed as part of the worldwide GNSS. The United States is developing a GPS-based regional system called the Wide Area Augmentation System (WAAS), which covers North America with the possibility of extending to include South America. Europe is developing a similar regional system called European Geostationary Navigation Overlay System (EGNOS),which is based on both GPS and GLONASS. It covers Europe and North Africa with the possibility of extending to include all of Africa and the Middle East. A third regional GPS-based system, called Multi-function Transport Satellite (MTSAT), is being developed in Japan, and covers parts of Asia and the Pacific region. Australia is also in the process of developing its own regional system. The regional systems are expected to merge and be interoperable .
Future European global satellite navigation system (Galileo system)
Galileo is a satellite-based global-navigation system proposed by Europe. Galileo is a civil-controlled satellite system to be delivered through a public-private partnership . Three different constellation types were investigated to ensure the optimum selection of the Galileo architecture, namely low Earth orbits (LEO), medium Earth orbits (MEO), and inclined geosynchronous orbits (IGSO). Combinations of various constellation types were also studied. Following this study, the Galileo decision makers adopted a constellation of 30 MEO satellites. The satellites will be evenly distributed over three orbital planes at an altitude of about 23,000 km. This selection ensures that more uniform performance is obtained for all regions (i.e., independent of the region’s latitude). The signal characteristics of the Galileo system were to be determined sometime in 2001 .
Galileo will be compatible at the user level with the existing GPS and GLONASS systems. However, unlike GPS and GLONASS, Galileo will provide two levels of services: a basic, free-of-direct-charge service and a chargeable service that offers additional features. Some security measures, such as withholding of the service, have been studied to ensure that the system is properly used. A European political body, independent of Galileo management, will have the authority to take the proper measures in the event of a crisis.
The Galileo development plan will be divided into three different phases.
1. The definition phase was concluded at the end of 2000.
2. The development and validation phase began in 2001 and has been extended for a period of 4 years. This phase comprises a more detailed definition of the Galileo system (e.g., frequency allocation). As well, it includes the construction of the various segments of the system (space, ground, and receiver). Some prototype satellites will be launched in 2004, along with the establishment of a minimal ground infrastructure, to validate the system.
3. The constellation deployment phase is scheduled to begin in 2006 and extend until 2007. With the experience gained during the system validation phase, operational satellites will be gradually launched during this phase. In addition, ground infrastructure will be completed.The target date for the gradual introduction of Galileo operational service is 2008 or shortly thereafter. At that time, EGNOS service will be provided in parallel until it is phased out in 2015 .
GPS Satellite Navigation Explained
GPS receivers are able to obtain their location by acquiring a signal from some of the 27 satellites that orbit the globe. The GPS system, originally for use by the US military, is now available to all for location-finding and navigation.
With a GPS receiver, road warriors, pilots, ramblers, and nautical folk can find out where they are, plan routes, help get on the wrong track when lost, and generally make travelling a whole lot easier.
A GPS receiver makes use of signals received from the satellites, using three or more to pinpoint your position. Obviously, you need line-of-sight for this to work, so your receiver must be able to see the sky.
A very small number of mobile phones come with a built-in GPS receiver. This means that you can use the phone either in-car or on foot to work out your position and perform route-planning and door-to-door navigation.
Our personal favourite GPS-enabled mobile is the Microsoft Windows Mobile powered o2 XDA Orbit (also branded as the T-Mobile MDA Compact III). This is a high-spec mobile phone, GPS receiver and PDA in one package. It also has built-in Wi-fi and Bluetooth, and a 2 megapixel camera.
Available from O2's online store (o2 XDA Orbit) or from T-Mobile (MDA Compact IV), free on selected tariffs.
See our o2 XDA Orbit for more details
If you're a fan of Nokia phones, there's the powerful Nokia N95 or the Nokia 6220, both with GPS and a 5 meg camera.
This bundle includes a GPS receiver terminated with a Pocket PC connector (IPAQ, Pocket Loox plus others), an in-car mount, and the Pocket PC mapping software. Considerably cheaper than a dedicated in-car unit, and just as powerful, this is a superb solution. You can achieve the same effect with separate units (on this page), but this is a neat, single-box solution for those with an existing Pocket PC. Features:
- Fast and reliable route planning on highly detailed, up-to-date maps
- Automatic, one-second recalculation when you move off the route
- Automatic positioning, zooming, and guidance by GPS
- Guidance by means of clear and timely verbal instructions, symbolic arrows, and maps
- Extensive lists of useful or interesting locations
- Real-time traffic information and route adjustment (Navigator version 3)
- Destination selection by means of: Favourites, Point of Interest, Clicking on map, Entering address
Prices are around the £200 mark, but do vary - try Amazon , Maplin, and PC World to see who's running the best offers.
For details of TomTom Navigator, see our TomTom Navigator page.
If you're looking to use GPS with a Pocket PC, Palm or laptop that has built-in Bluetooth, then a cable-free option such as the Socket, is the neatest option available. The Socket, pictured here is rechargeable, and runs with any NMEA compatible GPS program. Low power consumption allowing for trips up to 6 hours on a single charge. A little flashing blue light indicates that it's working.
- Belkin Bluetooth GPS: Around £190 from Amazon
- Navman GPS 4100: Around £150 from Amazon
- Socket Bluetooth GPS: Around £240 from Amazon
Pictured on the left is the Garmin GPS 12, an affordable 12 parallel channel GPS receiver that features fast satellite acquisition, great performance, housed in a rugged, waterproof case. Navigation features include a moving map plotter that displays waypoint names, symbols or comments, proximity waypoint alarms, average and maximum speed data and trip timers. Requires 4 x AA type batteries to give up to 24 hours performance. Includes Lat/Long, UTM, Ordnance Survey, Irish, Swiss, Swedish, German and Maidenhead Grids.
If you're looking for a handheld GPS unit, try Maplin, or for a real bargain, try a second-hand unit on eBay.co.uk
The tiny i-gotU GPS tracker can be used to capture your current position, or track your progress on a journey, then let you use that data on your PC or online.
You can overlay your position and waypoints onto Google Maps and Google Earth, and also use it to geo-tag photos and on photo services such as Flickr and Picaso.
This device is cheap, waterproof, rechargeable, and a snip at around £40. If you're a biker or a hiker, or own a digital camera, find out more about the basic i-gotU GPS tracker on FrequencyCast's i-gotU review.
We've also reviewed the higher-spec version with motion detection - thei-gotU GT600 with motion detection.
A snazzy plug-and-play solution for Pocket PCs. The Pretec CompactGPS comes bundled with TomTom Citymaps which makes it ideal for use with Pocket PCs. Pretec's CompactGPS-LP is in CompactFlash form-factor which is a popular for both the new and old generation of Windows powered Pocket or Handheld PCs. It is designed for easy integration with a wide range of navigation software applications. Featuring all view tracking capability, CompactGPS provides robust performance in applications that require high vehicle dynamics and high signal blockage operations. With 8-Channel architecture technology, Pretec's CompactGPS card can provide rapid Time-First-Fix (TIFF) under all start-up conditions for fast data acquisition and reacquisition.
Available for under £120 from Amazon
Alternatives: Bid for a bargain on Ebay.co.uk
See our GPS CF card page for details of Pocket PC CF card solutions
A product designed for the Psion Series 5, 5mx, Series 7 and netBook, this GPS unit has a magnetic base for the roof of your car, and uses SiRF GPS technology. It's powered by your car's 12 volt cigarette lighter, and integrates well with TomToms route planning software. The connection to the Psion is via the RS-232 (PsiWin) port.It's hard to get hold of these devices new these days, but they're frequently to be found at very low prices on eBay.co.uk
Audio Review: Check out our Audio feature on GPS Watches, apps and trackers - FrequencyCast Show 43
Global navigation satellite system
Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. A GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line of sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.
As of 2007, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of being restored to full operation. The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2013. China has indicated it may expand its regional Beidou navigation system into a global system. India's IRNSS, a regional system is intended to be completed and operational by 2012.
GNSS that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows: [cite web |url=http://www.ifatca.org/docs/gnss.pdf |publisher=IFATCA |title=A Beginner’s Guide to GNSS in Europe |format=PDF]
* GNSS-1 is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), with Satellite Based Augmentation Systems (SBAS) or Ground Based Augmentation Systems (GBAS). In the United States, the satellite based component is the Wide Area Augmentation System (WAAS), in Europe it is the European Geostationary Navigation Overlay Service (EGNOS), and in Japan it is the Multi-Functional Satellite Augmentation System (MSAS). Ground based augmentation is provided by systems like the Local Area Augmentation System (LAAS).
* GNSS-2 is the second generation of systems that independently provides a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation. This system consists of L1 and L2 frequencies for civil use and L5 for system integrity. Development is also in progress to provide GPS with civil use L2 and L5 frequencies, making it a GNSS-2 system.¹Fact|date=June 2008
* Core Satellite navigation systems, currently GPS, Galileo and GLONASS.* Global Satellite Based Augmentation Systems (SBAS) such as Omnistar and StarFire. * Regional SBAS including WAAS(US), EGNOS (EU), MSAS (Japan) and GAGAN (India). * Regional Satellite Navigation Systems such a QZSS (Japan), IRNSS (India) and Beidou (China). * Continental scale Ground Based Augmentation Systems (GBAS) for example the Australian GRAS and the US Department of Transportation National Differential GPS (DGPS) service. * Regional scale GBAS such as CORS networks. * Local GBAS typified by a single GPS reference station operating Real Time Kinematic (RTK) corrections.
History and theory
Early predecessors were the ground based DECCA, LORAN and Omega systems, which used terrestrial longwaveradio transmitters instead of satellites. These positioning systems broadcast a radio pulse from a known "master" location, followed by repeated pulses from a number of "slave" stations. The delay between the reception and sending of the signal at the slaves was carefully controlled, allowing the receivers to compare the delay between reception and the delay between sending. From this the distance to each of the slaves could be determined, providing a fix.
The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: the satellites traveled on well-known paths and broadcast 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. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position.
Part of an orbiting satellite's broadcast included its precise orbital data. In order to ensure accuracy, the US Naval Observatory (USNO) continuously observed precisely the orbits of these satellites. As a satellite's orbit deviated, the USNO would send the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain the most recent accurate information about its orbit.
Modern systems are more direct. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time.
Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
Civil and military uses
The original motivation for satellite navigation was for military applications. Satellite navigation allows for hitherto impossible precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from mis-directed weapons. (See smart bomb). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war.
In these ways, satellite navigation can be regarded as a force multiplier. In particular, the ability to reduce unintended casualties has particular advantages for wars where public relations is an important aspect of warfare. For these reasons, a satellite navigation system is an essential asset for any aspiring military power. Fact|date=July 2008
GNSS systems have a wide variety of uses:
* Navigation, ranging from personal hand-held devices for trekking, to devices fitted to cars, trucks, ships and aircraft* Time transfer and synchronization* Location-based services such as enhanced 911* Surveying* Entering data into a geographic information system* Search and rescue* Geophysical Sciences* Tracking devices used in wildlife management* Asset Tracking, as in trucking fleet management* Road Pricing* Location-based media
Note that the ability to supply satellite navigation signals is also the ability to deny their availability. The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires.
Current global navigation systems
The United States' Global Positioning System (GPS), which as of 2007 is the only fully functional, fully available global navigation satellite system. It consists of up to 32 medium Earth orbit satellites in six different orbital planes, with the exact number of satellites varying as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is currently the world's most utilized satellite navigation system.
The formerly Soviet, and now Russian, "Global'naya Navigatsionnaya Sputnikovaya Sistema", or GLONASS, was a fully functional navigation constellation but since the collapse of the Soviet Union has fallen into disrepair, leading to gaps in coverage and only partial availability. The Russian Federation has pledged to restore it to full global availability by 2010 with the help of India, who is participating in the restoration project. [ [http://www.rin.org.uk/pooled/articles/BF_NEWSART/view.asp?Q=BF_NEWSART_156825 India signs GLONASS agreement] ] [ [http://www.spacedaily.com/news/gps-05zzzzzg.html India, Russia Agree On Joint Development Of Future Glonas Navigation System] ]
Proposed Global Navigation Systems
The Indian Regional Navigational Satellite System (IRNSS) is an autonomous regional satellite navigation system being developed by Indian Space Research Organisation which would be under the total control of Indian government. The government approved the project in May 2006, with the intention of the system to be completed and implemented by 2012. It will consist of a constellation of 7 navigational satellites by 2012. All the 7 satellites will placed in the Geostationary orbit (GEO) to have a larger signal footprint and lower number of satellites to map the region. It is intended to provide an absolute position accuracy of better than 20 meters throughout India and within a region extending approximately 2,000 km around it. A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India.
China has indicated they intend to expand their regional navigation system, called "Beidou" or "Big Dipper", into a global navigation system; a program that has been called "Compass" in China's official news agency Xinhua. The Compass system is proposed to utilize 30 medium Earth orbit satellites and five geostationary satellites. Having announced they are willing to cooperate with other countries in Compass's creation, it is unclear how this proposed program impacts China's commitment to the international "Galileo" position system.
Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a French precision navigation system. [ [http://www.jason.oceanobs.com/html/doris/welcome_uk.html DORIS information page] ]
The European Union and European Space Agency agreed on March 2002 to introduce their own alternative to GPS, called the Galileo positioning system. At a cost of about GBP £2.4 billion, [cite news |publisher=BBC News |url=http://news.bbc.co.uk/1/hi/sci/tech/5286200.stm |title=Boost to Galileo sat-nav system |date=25 August2006 |accessdate=2008-06-10] the system is scheduled to be working from 2012. The first experimental satellite was launched on 28 December2005. 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 greatly increase the accuracy.
The Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional time transfer system and enhancement for GPS covering Japan. The first satellite is scheduled to be launched in 2008. [cite news |url=http://www.space.com/spacenews/archive04/budgetarch_090704.html |title=Japan Seeking 13 Percent Budget Hike for Space Activities |publisher=SPACE.com |date=7 September2004 |accessdate=2008-06-10]
GNSS Augmentation involves using external information, often integrated into the calculation process, to improve the accuracy, availability, or reliability of the satellite navigation signal. There are many such systems in place and they are generally named or described based on how the GNSS 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, the European Geostationary Navigation Overlay Service, the Multi-functional Satellite Augmentation System, Differential GPS, and Inertial Navigation Systems.
Low Earth orbit satellite phone networks
The two current operational low Earth orbit satellite phone networks are able to track transceiver units with accuracy of a few kilometers using doppler shift calculations from the satellite. The coordinates are sent back to the transceiver unit where they can be read using AT commands or a graphical user interface [ [http://common.globalstar.com/doc/common/en/products/gsp1700_usermanual.pdf Globalstar GSP-1700 manual] ] [http://www.skyhelp.net/acrobat/jan_05/Iridium%20SBD-FAQ%201-05.pdf] . This can also be used by the gateway to enforce restrictions on geographically bound calling plans.
Topics to be covered
* Differential satellite navigation* GNSS reflectometry* Phase-counting differential satellite navigation* Trends within GNSS
*Differential GPS*Global Positioning System*Galileo (satellite navigation)*GLONASS*Wide Area Augmentation System*European Geostationary Navigation Overlay Service*GNSS reflectometry*GPS and Geo Augmented Navigation*SIGI*RAIM
Information on specific GNSS systems
* [http://www.esa.int/esaNA/GGG63950NDC_egnos_0.html ESA information on EGNOS] * [http://www.astronautix.com/craft/beidou.htm Information on the Beidou system] * [http://www.wsv.de/fvt/funknavi/funknavi.html German Federal Waterways Administration Traffic Technologies Centre] Information on GPS / DGPS
Organizations related to GNSS
* [http://www.unoosa.org/oosa/en/SAP/gnss/icg.html United Nations International Committee on Global Navigation Satellite Systems (ICG)] * [http://gnss.or.kr/e-html/introduction/intro_01.html Korean GNSS Technology Council (GTC)] * [http://www.ion.org/meetings/#gnss Institute of Navigation (ION) GNSS Meetings] * [http://igscb.jpl.nasa.gov/ The International GNSS Service (IGS), formerly the International GPS Service] * [http://www.ignss.org/ International Global Navigation Satellite Systems Society Inc (IGNSS)] * [http://www.iers.org/MainDisp.csl?pid=84-63 International Earth Rotation and Reference Systems Service (IERS) International GNSS Service (IGS)] * [http://facility.unavco.org/science_tech/gnss_modernization.html UNAVCO GNSS Modernization] * [http://www.apecgit.org/ Asia-Pacific Economic Cooperation (APEC) GNSS Implementation Team] * [http://www.fai.org/gliding/gnss/ Fédération Aéronautique Internationale (FAI) GNSS Flight Recorder Approval Committee (GFAC)]
at Nav Manufacturers
* [http://www.garmin.co.uk Garmin]
upportative or illustrative sites
* [http://rhp.detmich.com/gps.html GPS and GLONASS Simulation] (Java applet) Simulation and graphical depiction of the motion of space vehicles, including DOP computation.
Wikimedia Foundation. 2010.
Satellite Navigation Using GPSSatellite Navigation Using GPS
Satellite Navigation Using GPS
T.J. Martín Mur & J.M. Dow
Orbit Attitude Division, European Space Operations Centre (ESOC), Darmstadt, Germany
The Global Positioning System (GPS) is currently being used for a wide variety of applications. A GPS receiver aboard a spacecraft can provide the means for autonomous navigation and also allows a very accurate reconstitution of the trajectory of the spacecraft when onboard recorded measurements are combined with ground-based measurements.
This article outlines some of the basic concepts involved and presents the activities that the European Space Operations Centre is carrying out in the field of satellite navigation using GPS.
The launch of the first Sputnik triggered the initial challenge in satellite navigation: the determination of the characteristics of the orbit of the satellite, using the variations in the signal that was being radiated by the satellite. Within a short time the idea of using the inverse process was developed: if, by knowing your position you could determine the orbit of a satellite, then it should also be possible to use the signal transmitted by a satellite in a known orbit, in order to determine your own position. This concept was implemented in a series of satellites sponsored and operated by the US Armed Forces. Firstly, the Transit satellites were deployed, then the Timation and finally, the NAVSTAR GPS system.
The focus of these programs was to provide the military forces of the US and its allies with precise positioning capabilities. In response the Soviet Union also developed and deployed similar Global Navigation Satellite Systems (GNSS): Tsikada and GLONASS.
From the very beginning it was realised that these systems could also be used for a wide range of scientific and other civil applications. New tracking methods that were not foreseen by the original developers of the systems, like carrier tracking, were proposed and, as soon as it was possible, successfully tested and used.
One of the applications that was soon envisioned was the use of GPS for navigation of spacecraft. The first onboard receiver was installed and flown in a Landsat satellite even before the complete GPS constellation was deployed. Since that time, more receivers have been flown on satellites, at first as a demonstration of increasingly precise uses and now as the main operational means of navigation.
The NAVSTAR Global Positioning System
The NAVSTAR Global Positioning System, usually called GPS, consists of three components: a space segment of GPS satellites, a control segment that monitors and operates those satellites and a user segment that employs GPS receivers to observe and record transmissions from the satellites and perform position, velocity, attitude and time calculations.
The GPS space segment The space segment is based on three-axis stabilised satellites orbiting in near-circular orbits with a period of half a sidereal day and an inclination of 55 degrees. There are six orbital planes, each with four satellites. This constellation provides global coverage with more than four satellites in view at all times.
The significance of the visibility of at least four satellites is that the GPS system is intended to allow instantaneous real time determination of the user position (3 variables) and the time of the fix (one more variable). Previous positioning systems, like the methods used in the Transit and Tsikada systems, were based on the processing of several passes of data (requiring hours to days) and did not provide the instantaneous solutions that GPS (or GLONASS) offers.
The GPS satellites carry very stable atomic clocks that are used to derive the ranging signals. The basic signal for civil use, L1, has a frequency of 1575.42 MHz and it is modulated with a Clear Acquisition (C/A) Pseudo Random Noise (PRN) code at 1.023 MHz that is different for every satellite. The signal is also modulated with a 10.23 MHz Precise (P) code that is usually encrypted and only available to authorised users. Additionally, there is a 50-bit-per-second modulation which is used to transmit the satellite ephemerides (predicted orbit and clock) and other information. Authorised users also have access to the Precise code on a second frequency, L2, which allows users to correct for ionospheric propagation delays. Some receivers are able to measure the delay between the signal in the L1 frequency and the L2 frequency without access to the P code. There are plans to add, in future satellites, another frequency for civil users so they can easily correct for ionospheric delays.
The GPS control segment The GPS control segment tracks and monitors the signal from the GPS space segment and estimates the orbits and clock behaviour of the satellites. This information is uploaded to the satellites so it can be transmitted to users.
The GPS user segment The GPS user segment can perform two basic measurements of the GPS signals. The first method compares the C/A or P code that it is receiving with a locally generated copy in order to compute the transmission delay between the satellite and the receiver. This measurement is called pseudo-range. Pseudo-ranges to four or more satellites can be used to determine the position of the user once the position of the GPS satellites has been obtained using the ephemerides of the navigation message.
The second and more precise method is to obtain the difference in phase between the received carrier signal and a receiver-generated signal at the same frequency. This measurement is known as the carrier phase observable and it can reach millimetre precision. However, it lacks the accuracy of the pseudo-range because once the tracking is started, the phase can only be identified with an ambiguity of an unknown number of times the carrier wavelength (about 19 cm for L1).
Figure 1. Common observability of GPS satellites by ground and onboard receivers allows a better determination of the satellite orbit.
Use of GPS for spacecraft navigation
The number of applications and users of the GPS system has exploded in the last years, well beyond any expectations. The latest receivers are inexpensive, small, offer very good performance and are easy to use.
One of the first scientific applications of GPS was to precisely determine the position of fixed ground antennas in order to study the dynamics of the Earth surface. It was soon realised that in order to obtain the best results it would be necessary to compute very precise orbits of the GPS satellites. A number of groups started doing this and, as a result, the first orbits that were precisely obtained using GPS were those of the GPS satellites themselves.
GPS has many advantages for the tracking of satellites orbiting the Earth. It provides unsurpassed observability because low-Earth satellites are able to track six or more GPS satellites, with tracking arcs amounting to about half of the user satellite orbit. This cannot be achieved by any ground-based tracking station. This ability also renders the method robust. There is a high level of redundancy because orbits can be determined with as few as two GPS satellites being tracked at any time. When four satellites are being tracked, GPS allows for real-time autonomous determination of the position of the satellite, with an accuracy equivalent to that obtained with non-precise ground tracking methods. If a precise dual-frequency receiver is used and data is processed together with ground based data, GPS possibly provides the best accuracy that can be achieved in precise orbit determination.
The first user satellite to fly a precise GPS receiver was the TOPEX/Poseidon altimeter satellite. Since then, other satellites have been flown with different types of receivers. Table 1 details the different applications for a GPS receiver aboard a spacecraft.
Table 1. Applications of a GPS receiver for space navigation
- To determine the position and velocity of a satellite.
- To accurately determine the time of observations from other tracking or scientific instruments.
- To determine the attitude of a satellite. This can be accomplished by comparing the measurements obtained from different antennas.
- To collect GPS measurements that will allow a precise reconstitution of the orbit of the satellite.
- To collect GPS measurements that can be used to reconstitute the characteristics of the medium travelled through by the signal: ionosphere and troposphere.
- The relative navigation of two spacecraft (currently being validated).
- The tracking of the launch and early-orbit phases of rockets.
- The tracking of re-entering spacecraft, even to the point of autonomous landing.
ESA has been involved in GPS activities since the late eighties with the development of space qualified GPS and GPS/GLONASS receivers, the ESOC activities which support spacecraft navigation and, recently, within the ARTES program, the European Geostationary Navigation Overlay Service (EGNOS). EGNOS will complement the GPS system in order to provide European users with increased availability, integrity and accuracy for real-time applications such as aircraft navigation.
It is foreseen that ESA will participate in the development of future Global Navigation Satellite Systems (GNSS) that may replace GPS, GLONASS and their augmentations for such purposes in the future.
GPS has been proposed as the tracking or scientific instrument for several ESA spacecraft. It is the main positioning instrument envisioned for the Automated Transfer Vehicle (AT), both for absolute navigation and for navigation relative to the International Space Station. The ATV Rendezvous Pre- development (ARP) program is being carried out to validate methods for relative navigation that will be used for ATV, including relative GPS navigation. Other spacecraft for which a GPS receiver has been proposed include several of the Earth Explorer candidates and other future observation and scientific satellites (Metop, STEP).
ESOC involvement in GPS activities
Within the European Space Agency, ESOC is responsible for the operation of ESA spacecraft. This includes the flight dynamics activities needed to achieve and maintain their desired orbit and attitude. In order to fulfil this task, ESOC began to prepare itself to support ESA missions that might use the GPS system as soon as GPS was proposed for spacecraft navigation.
ESOC had an excellent opportunity to do so by contributing to the success of the International GPS Service for Geodynamics (IGS). Scientists were proposing to install a permanent network of precise ground-based GPS receivers that would allow the monitoring of the movement of the Earth's surface in order to better understand plate tectonics and local deformations that are the cause of earthquakes. The data from these receivers could be processed in order to obtain precise orbits for the GPS satellites that would be used by geodesists in regional deformation studies. Additionally within the IGS Terms of Reference was a provision of support for other applications, including scientific satellite orbit determination. The assets which ESOC could contribute to the IGS were its network of ground stations in which receivers could be installed and its expertise, supported by in-house developed software, in precise orbit and geodetic parameter estimation.
The first receiver was installed in Maspalomas (Spain) in June 1992 (Fig. 2). Receivers have also been installed in Kourou (French Guyana) in July 1992, Kiruna (Sweden) in July 1993, Perth (Australia) in August 1993, Villafranca (Spain) in November 1994 and Malindi (Kenya) in November 1995. Precise estimation software was extended to include GPS measurement types for both ground-based and spacecraft-borne receivers. We have been providing data and increasingly precise GPS products for the last five years. Currently we provide:
- raw measurement data from our six ground stations,
- precise orbits of the GPS spacecraft,
- Earth orientation parameters (polar motion, length of day),
- station coordinate solutions for those stations included in our analysis,
- GPS satellite clock information.
Figure 2. Pillar-mounted GPS antenna and pillar in Maspalomas, Spain.
ESOC is currently an active IGS Analysis and Operational Data Centre and is especially involved in discussions to extend the IGS to use space-borne receivers.
The ESA GPS TDAF
The ESA GPS Tracking and Data Analysis Facility (GPS TDAF) has been developed in order to support the GPS activities carried out by ESOC. It includes a network of ground GPS receivers, the necessary communication interfaces to allow the remote operation and data downloading from ESOC, and the processing and analysis software needed to format the data and to obtain the precise products (Fig. 3). The system is highly automated, and includes an easy to operate interface for the retrieval and the processing of the data (Fig. 4). The GPS-TDAF is currently being extended to process GLONASS data and to include real-time monitoring capabilities that may be needed to support critical operational phases like rendez- vous.
Figure 3 : Current configuration of the GPS-TDAF.
Figure 4. Remote Station Control panel of the GPS-TDAF. This panel is used to monitor the daily retrieval tasks.
Other recent developments are:
- The calculation of global and local ionospheric models that can be used to correct one-frequency ranging and altimeter data.
- The implementation of a sequential filter to estimate spacecraft trajectories using the precise products obtained by the IGS analysis activities.
Role of the ESA operations ground segment in GPS navigation
The driving reason for implementing a GPS-TDAF in ESOC was to provide ESA with the capability to support the navigation of satellites equipped with GPS receivers. This support involves the following activities:
Support of critical real-time GPS applications GPS has been proposed as the absolute and relative positioning system for spacecraft going to the manned International Space Station. For this application it is clear that the ground segment cannot be in-the-loop for the calculation of real-time trajectories of the spacecraft involved because of the unavoidable delays that this will create. Still, the ground segment has a role in monitoring the integrity of the signals that are to be used for critical operations.
This can be accomplished using a ground network of GPS receivers that is able to track all the satellites that the orbiting spacecraft may use. The navigation data and observations of these precisely located ground stations can be processed in order to check their integrity and to estimate the error in the signal for each GPS satellite. Poorly performing satellites can be identified and the number of healthy GPS satellites observable by the user spacecraft during the critical operations can be predicted. This can also be done in real-time in order to detect satellite failures that can affect the navigation solution of the user spacecraft, so that the information can be delivered to Mission Control and the poor performing GPS satellites can be excluded from the onboard computed navigation solution.
Another role of the ground segment will be to support the validation of the receivers and their correct functioning before critical operations are started. It can also assist in the fast re-start of receivers by providing up-to-date almanacs and other initialisation data.
ESOC has installed GPS receivers at six ground stations and it is developing a real-time communication system that will allow for the continuous monitoring of the GPS spacecraft visible from these ground receivers.
Precise orbit determination using GPS GPS is one of the best tracking types for Precise Orbit Determination of Low Earth Orbit satellites because it combines high accuracy with unsurpassed observability. The high accuracy is obtained by using the GPS carrier phase observable, free of ionospheric errors, when dual frequency data is used. The observability is provided by the high number of GPS satellites that can be simultaneously tracked by an orbiting receiver.
ESOC has incorporated models for the most widely used GPS measurements in its Precise Orbit Determination software. This has been done both for the determination of the orbit of the GPS spacecraft and for the determination of the orbit of user spacecraft (spacecraft carrying a GPS receiver). The implementation has been validated using TOPEX/Poseidon data and the software is currently being used to support the ARP Flight Demonstrations. More information on these activities is given below.
Determination of the orbits and clocks of the GPS satellites For some applications it is not necessary to simultaneously solve for the orbits of the GPS spacecraft and the user spacecraft. The orbits and clock biases of the GPS spacecraft can be precisely computed and then held fixed for the computation of the orbit of the user spacecraft.
ESOC has been participating in the International GPS Service for Geodynamics (IGS) since its establishment, and producing precise orbit and clocks solutions for the GPS satellites. These ephemerides are estimated to be accurate to about 10 cm.
Our GPS orbit determination software is also being used for feasibility and validation experiments for the ARTES-9 EGNOS project.
Operational orbit determination using GPS The facilities implemented for Precise Orbit Determination can also be used for Operational Orbit Determination to produce a very accurate orbit prediction and to calibrate manoeuvres. This on-ground determined orbit can also be used during the spacecraft check-out to assess GPS-based onboard orbit determination.
For some applications it is not necessary to be able to produce a precise orbit prediction. In these cases the GPS- based onboard generated positions can be used on the ground as observable in order to determine the orbit that will be used for orbit control, mission planning and station visibility predictions. This process can also assess the quality of the onboard generated positions.
In this context, our GPS orbit determination software will be used operationally to determine the orbit of the Danish ørsted geomagnetic research microsatellite.
Geophysical parameter estimation Most of the activities listed previously are possible because networks of precise geodetic receivers are currently deployed to support these and other applications. For the most accurate applications, the position of the receivers in these networks has to be precisely determined, together with a number of other geophysical parameters. The accurate determination of the position of the ESA ground stations, the determination of Earth orientation parameters and the calculation of ionospheric calibrations can also support other projects that are not directly using GPS but need an accurate location of the position of tracking antennas and correction for ionospheric delays.
ESOC is contributing to the estimation of very precise station coordinate solutions that include the ESA ground stations, and through the IGS, also to the activities of the International Earth Rotation Service (IERS). We are currently testing the use of GPS derived ionospheric models in order to correct one-frequency ERS altimeter measurements and the S-band ranging and doppler measurements used for the routine control of most spacecraft.
TOPEX/Poseidon precise orbit determination
TOPEX/Poseidon (T/P) is a joint US/French altimetric spacecraft launched in August 1992. The main scientific goal of this mission is to produce sea level maps to study ocean circulation and variability. Accurate orbit determination is vital to the success of this or any other altimetric missions. In this case, the fundamental quantity measured is the geocentric height of the sea surface, and obtained as the difference between the radial orbit position and the altimeter measurement proper. The orbit determination requirements for T/P were set to a very demanding 13 cm error budget for the radial position. In order to satisfy this challenge an unprecedented effort was made to improve the gravity model of the Earth and, to further guarantee the best possible results in orbit determination, several tracking systems were placed onboard: retroreflectors for Satellite Laser Ranging (SLR), a DORIS receiver, and an experimental precise GPS receiver. Effectively, this has made the T/P spacecraft a veritable orbit determination laboratory that allows intercomparisons between different tracking techniques.
T/P has been of unique importance for the validation of techniques for GPS-based satellite navigation. It is equipped with a high precision dual-frequency GPS receiver producing long cycle-slip free carrier phase passes as well as pseudo- range measurements. It was launched when the GPS constellation was almost complete and when the IGS network of high precision GPS receivers had started to provide continuous globally distributed tracking data.
For our evaluation, a 10-day period was selected for the comparison of the orbit restitution capabilities of the three techniques: SLR, DORIS and GPS. For the GPS processing, T/P observations were used together with data from about 20 ground receivers from the IGS network. The orbit of the T/P spacecraft was then solved simultaneously with the orbits of the GPS spacecraft. The chosen data type was double-difference phase measurements involving two GPS satellites and two GPS receivers.
Comparisons were made to determine the ephemerides generated using only GPS and using a combination of SLR and DORIS, with external solutions obtained by the Delft University of Technology (DUT) and the Jet Propulsion Laboratory (JPL). The orbits show a remarkable agreement, with the difference in radial direction in the order of 2 cm, and along track and cross track differences in the order of 5 to 10 cm.
These results demonstrate the capability of the GPS-TDAF to produce very accurate results when precise data collected onboard can be combined with on-ground collected data from a network of high precision GPS receivers.
The ARP flight demonstrations
ESA is developing the unmanned Automated Transfer Vehicle (ATV) that will serve as a logistic / re-supply vehicle for the International Space Station (ISS). The ATV will perform a number of manoeuvres in order to rendezvous and dock with the ISS. GPS is baselined as the main positioning system for the ATV. It will be used for autonomous absolute position determination and autonomous relative position determination with respect to the ISS. For autonomous absolute position determination the ATV will be equipped with a one-frequency GPS receiver that will provide position, velocity and time solutions. For autonomous relative position determination the ISS will also be equipped with a GPS receiver that will provide GPS observables for transmission to the ATV. The ATV will process them together with its own GPS observables in order to determine its position and velocity relative to the ISS.
The ATV Rendezvous Pre-development (ARP) project covers the pre-development of rendezvous technologies critical to ATV. One of the aspects that are covered by this project is the validation of relative navigation using GPS observables. For this, three Flight Demonstrations (FD) are planned, in which the Space Shuttle will act as chaser and another spacecraft (Astrospas for FD1 and the MIR station for FD2 and FD3) will be the target. These spacecraft will carry one-frequency GPS receivers and will collect GPS data during the proximity operations. The data will be post-processed on the ground to validate the relative navigation algorithms.
The role of ESOC in these three ARP FDs is to compute reference trajectories (relative and absolute) for the spacecraft involved using all available measurements. These trajectories will be then used to compare with the results of the relative navigation filter. ESOC will obtain the trajectories using the following data:
- GPS observables (pseudo-range and phase) and onboard-derived positions from the two flying receivers.
- The ESOC precise orbit and clock solutions for the GPS satellites.
- Attitude data derived from the spacecraft Guidance, Navigation, and Control (GNC) system.
The data will be decoded and converted to an engineering format that will then be fed to a program which will produce the best estimate trajectories for the spacecraft. This program is called GPSBET (GPS Based Estimator of Trajectories) and it includes the following:
- Precise measurement models that use the GPS orbits and clocks computed by ESOC. The models include a centre of mass correction that is performed using the location of the particular antenna in the body-fixed axes and the attitude data.
- A multi-satellite orbit propagator that includes precise dynamic models and empirical accelerations.
- A Square Root Information Filter that processes all the information and produces filtered and smoothed estimates of the parameters.
We are currently processing data from the first ARP Flight Demonstration and we expect to achieve absolute positioning results with about 1 metre accuracy and even better relative-positioning accuracy.
The GPS system, originally deployed to allow very precise delivery of weapons, has demonstrated an incredible versatility of use for civil applications. GNSS systems are ideal to support many aspects of the navigation of spacecraft orbiting the Earth. They can support increased spacecraft autonomy and, when used together with ground collected measurements, they provide unsurpassed accuracy. The ESA GPS-TDAF is already supporting validation activities for navigation of spacecraft using GNSS systems and will also be able to support preparations for a European contribution to future Global Navigation Satellite Systems.
Additional information on these activities, as well as links to other related World Wide Web sites, can be found under http://nng.esoc.esa.de.
The development and operation of the GPS-TDAF has been possible thanks to the important contributions of C. García Martínez (GMV), J. Feltens (EDS) and M. A. Bayona (GMV), as well as those of S. Casotto and P. Duque and several trainees. Assistance of station and communications experts at ESOC and the ground stations is also gratefully acknowledged.About | Search | Feedback ESA Bulletin Nr. 90.Published May 1997.Developed by ESA-ESRIN ID/D.
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