Professor Graham Bailey


SATELLITE NAVIGATION

Contents

  • GPS (Global Positioning System)
  • Differential GPS
  • WAAS (Wide Area Augmentation System)
  • GLONASS (Global Navigation Satellite System)
  • EGNOS (European Geostationary Navigation Overlay Service)
  • MSAS (MTSAT Satellite-Based Augmentation System)
  • Galileo
  • Plasmaspheric Effects
  • Satellite Navigation Systems

    gps_01 gps_02
    An artist's impression of a GPS satellite
    The constellation of GPS satellites

    GPS (Global Positioning System)

    GPS is a satellite-based navigation system made up of a network of 24 satellites. Each satellite is built to last about 10 years. At times more than 24 satellites are operational as new ones are launched to replace older ones. The system was originally intended for military applications. It is funded and controlled by the US Department of Defense. In the 1980s the system was made available for civilian use.

    The satellites in the GPS system are in six equally-spaced orbital planes, inclined at 55° to the equatorial plane, at an altitude of about 20,200 km. Each satellite circles the earth once every 12 hours. The orbits are such that the satellites repeat the same track and configuration over any point each 24 hours (4 minutes earlier each day). The satellites are arranged so that a GPS receiver on Earth can always receive signals from at least four satellites at any given time.

    GPS satellites transmit information on two radio frequencies. The L1 frequency of 1575.42 MHz carries both civilian and military grade information while the L2 frequency of 1227.60 MHz carries only military grade information. Use of this frequency provides a correction for the effects of the ionosphere on the signal propagation and, hence, a greater accuracy for the receiver position. However, the L2 frequency is encrypted which denies direct access of the signal to the civilian user. The encrypted signal can only be used by users with specially equipped receivers and authorised by the US Department of Defense. Dual frequency receivers for civilian use have been developed to receive the un-encrypted signal and to reduce as many errors as possible in the signal. Dual frequency receivers are much more expensive than single frequency receivers as the algorithms that are used to obtain useful information from the L2 signal are highly proprietary and costly to develop.

    Each GPS satellite contains several high-precision atomic clocks and constantly transmits radio signals using a unique identifying code. Four unmanned monitor stations located around the world receive these signals and continuously track each satellite. One station is located in Hawaii and the other three are located as close as possible to the equator: Ascension Island in the Mid Atlantic, Kwajalein in the Pacific, and Diego Garcia in the Indian Ocean. This tracking data is forwarded to the Master Control Station located at the Schriever Air Force Base, a few miles east of Colorado Springs in Colorado. At the Master Control Station changes in the satellite's position and timing are determined. These changes are then transmitted back to the satellite on a daily basis to ensure that the satellite is transmitting accurate information about its orbital path.

    GPS receivers on Earth contain a computer which uses a triangulation technique to calculate the receiver's location from the signals transmitted by the satellites. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. From the time difference the distance of the satellite from the GPS receiver can be determined. A GPS receiver must be locked on to the signal of at least three satellites to determine a 2-dimensional position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3-dimensional position (latitude, longitude and altitude).

    Today's GPS receivers are extremely accurate due to their parallel multi-channel design. Parallel-channel receivers typically have between 5 and 12 receiver circuits, each devoted to one particular satellite signal, so strong locks can be maintained on all the satellites in view at all times. Parallel-channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks even in dense foliage or urban settings with tall buildings.

    Factors that can degrade the GPS signal and thus affect the accuracy include the following:

    Differential GPS (DGPS)

    Accuracy can be improved by combining the GPS receiver with a Differential GPS receiver. Differential GPS works by placing a GPS receiver (called a reference station) at a known location. Since the reference station knows its exact location, it can determine the errors in the satellite's signals. It does this by measuring the ranges to each satellite using the signals received and comparing these measured ranges with the actual ranges calculated from its known position. The difference between the measured and calculated range for each satellite in view becomes a differential correction. The differential corrections for each tracked satellite are formatted into a correction message and transmitted to DGPS receivers. These differential corrections are then applied to the DGPS receiver's calculations, removing some of the common errors and improving accuracy. The level of accuracy obtained is dependent upon the DGPS receiver and the similarity of its environment to that of the reference station, especially its proximity to the station. The reference station receiver determines the error components and provides corrections to the DGPS receiver in real time. Typical DGPS receiver accuracy is 1-5 metres.

    WAAS (Wide Area Augmentation System)

    WAAS is a system of satellites and ground stations that provide GPS signal corrections. A WAAS-capable receiver can give position accuracy better than 3 metres for 95% of the time.

    WAAS is being developed by the Federal Aviation Administration (FAA) and the Department of Transportation (DOT) for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

    WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites (satellites with a fixed position over the equator). The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

    Currently, WAAS satellite coverage is only available in North America. For some users the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. An advantage of WAAS is that it does not require additional receiving equipment, while DGPS does.

    Other governments are developing satellite-based differential systems similar to WAAS. In Europe the Euro Geostationary Navigation Overlay Service (EGNOS) is being developed while in Asia it's the Japanese Multi-Functional Satellite Augmentation System (MSAS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

    GLONASS (Global Navigation Satellite System)

    GLONASS is the Russian counterpart to the United States' GPS system. It is operated for the Russian government by the Russian Space Forces.

    The operational space segment of GLONASS consists of 21 satellites in 3 orbital planes, with 3 on-orbit spares. The three orbital planes are separated by 120° and the satellites within the same orbit plane by 45°. Each satellite operates in circular 19,100 km orbits (slightly lower than that of the GPS satellites) at an inclination of 64.8° and each satellite completes an orbit in approximately 11 hours 15 minutes. The spacing of the satellites is arranged so that a minimum of 5 satellites are in view at any given time.

    GLONASS provides two separate levels of precision: deliberately degraded (for security purposes) signals for civilian users offer accuracy to within 100 meters, while its signals for military users offer accuracy of 10-20 metres.

    The first three test satellites were placed in orbit in October 1982 with the first operational satellites entering service in December 1983. The system was intended to be operational in 1991, but the constellation was not completed until December 1995.

    A characteristic of the GLONASS constellation is that the satellite orbits repeat after 8 days. As each orbit plane contains 8 satellites, there is a non-identical repeat, i.e., another satellite will occupy the same place in the sky, after one sidereal day. This differs from the GPS identical repeat period of one sidereal day.

    Due to the economic situation in Russia there were only eight satellites in operation in April 2002 rendering it almost useless as a navigation aid. However, due to improvements in the Russian economy there were 11 satellites in operation by March 2004. In addition, an advanced GLONASS satellite, the GLONASS-M, with an operational lifetime of 7 years, has been developed. A 3-satellite block of this new version was launched on 26 December 2004. Following a joint venture deal with the Indian Government the upgrade programme will provide a global navigation force of 18 satellites by 2007. This will be increased to 24 for completion by 2010-11.

    International partners and users are now showing greater interest in GLONASS and there are ongoing negotiations to expand cooperation, not only with India, but with the EU and other nations.

    EGNOS (European Geostationary Navigation Overlay Service)

    EGNOS is a satellite-based differential system similar to WAAS. It is being developed by the European Space Agency (ESA) under a tripartite agreement with the European Commission (EC) and the European Organisation for the Safety of Air Navigation (Eurocontrol). Several air traffic service providers are supporting the development programme with their own investments.

    EGNOS will provide improved performances over Europe to the existing GPS and GLONASS systems. It will disseminate, on the GPS L1 frequency, integrity signals giving real-time information on the health of the constellation. Correction data will improve the accuracy of the current services from about 20 metres to better than 5 metres. The EGNOS coverage area includes all European states and could be readily extended to include other regions, such as South America, Africa, and parts of Asia and Australia, within the coverage of three geostationary satellites being used.

    EGNOS offers all users of satellite radio navigation a high-performance navigation and positioning service, superior to that currently available in Europe. The system is composed of three transponders installed in geostationary satellites and a ground network of 34 positioning stations and four control centres, all interconnected. EGNOS is the first step of the Galileo programme.

    On 28 July 2005 the operations of EGNOS was transferred from ESA to the operating company European Satellite Services Provider (ESSP). EGNOS is now (July 2007) fully deployed and in its pre-operational phase. The system will undergo certification for safety-of-life applications before becoming fully operational.

    For further details and the latest information on EGNOS consult the webpages provided by ESA and the report provided by the BBC.

    MSAS (MTSAT Satellite-Based Augmentation System)

    MSAS is the wide-area augmentation system being developed by the Japan Civil Aviation Bureau (JCAB) for civil aviation. This space-based augmentation system will provide en-route through precision approach navigation services for all aircraft within Japan airspace. MSAS employs a ranging function to generate GPS-like signals and enable aircraft to use the Multi-functional Transport Satellites (MTSAT) as additional GPS satellites. MSAS is similar in function to WAAS and EGNOS and generates correction data for GPS and GLONASS.

    The system consists of Ground Monitor Stations (GMS), Monitor and Ranging Stations (MRS), Master Control Stations (MCS) and Navigation Earth Stations (NES). There are also monitoring stations in Hawaii and Australia to increase the service area beyond the Japanese borders. The GMS collect range measurements and send them to the MCS which monitor and control the system, calculates MTSAT orbit, ionospheric delay, and correction data, determine system integrity, collect range data and send the information to NES for uplink. The MRS receive the GPS/GLONASS/MTSAT signals and collect range data.

    MSAS currently comprises two MTSAT satellites, MTSAT-1R and MTSAT-2. MTSAT-1R is the replacement satellite for the MTSAT-1 satellite, which was lost on 15 November 1999 when its launching rocket failed. The MTSAT-1R satellite was launched on 26 February 2005, the MTSAT-2 satellite was launched on 18 February 2006. In addition to their navigational payloads both satellites carry meteorological payloads. The meteorological payloads provide cloud imagery and continuous weather data from the Asia-Pacific region for processing by the Japanese weather forecasting community.

    Galileo

    galileo
    The Galileo satellite system

    Galileo will be Europe’s own state-of-the-art global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It is being developed by the European Space Agency (ESA) and the European Union. While providing autonomous navigation and positioning services, Galileo will at the same time be interoperable with GPS and GLONASS. A user will be able to take a position with the same receiver from any of the satellites in any combination. By offering dual frequencies as standard, Galileo will deliver real-time positioning accuracy down to the metre range, which is unprecedented for a publicly available system. It will guarantee availability of the service under all but the most extreme circumstances and will inform users within seconds of a failure of any satellite. This will make it suitable for applications where safety is crucial, such as running trains, guiding cars and landing aircraft. The combined use of Galileo and other Global Navigation Satellite Systems will offer much improved performances for all kinds of user communities all over the world.

    The fully deployed Galileo system will consist of 30 satellites (27 operational + 3 active spares), positioned in three circular Medium Earth Orbit (MEO) planes at an altitude of 24,000 km altitude and inclined at 56° to the equator. Two Galileo Control Centres (GCC) will be implemented on European ground to provide for the control of the satellites and to perform the navigation mission management. The data provided by a global network of twenty Galileo Sensor Stations (GSS) will be sent to the Galileo Control Centres through a redundant communications network. The GCC’s will use the data of the Sensor Stations to compute the integrity information and to synchronize the time signal of all satellites and of the ground station clocks. The exchange of the data between the Control Centres and the satellites will be performed through so-called up-link stations.

    The first test satellite, GIOVE-A, was launched in December 2005. The second test satellite, GIOVE-B, was launched in April 2008. Once the technology on board GIOVE-B is proven, the first four operational satellites will be launched.

    In early January 2010, the European Commission announced the award of three of the six contracts for the procurement of Galileo's initial operational capability. The contract for the system support services was awarded to Thales Alenia Space and covers the industrial services needed to support the European Space Agency for the integration and the validation of the Galileo system. The contract for a first order of 14 satellites was awarded to OHB System AG . The first satellite is expected to be delivered in July 2012 and the last one in March 2014. The remaining 12 to 14 satellites needed to reach the Full Operational Capability will be procured in subsequent work orders from either OHB or EADS Astrium GmbH under the framework contract signed with both manufacturers. The contract for the launch services was awarded to Ariane Space and covers the launch of five Soyuz launchers, each carrying two satellites. The first launch is scheduled for October 2012 from Kourou in French Guiana. The contracts were signed on 26 January between the European Space Agency, acting on behalf of the European Commission, and the companies involved.

    On 25 October 2010 ESA signed a contract with Spaceopal, the company chosen to provide ground-based services needed to operate the Galileo constellation. Spaceopal is a joint undertaking between the Italian company Telespazio and the German company Gesellschaft für Raumfahrtanwendungen (GfR). GfR was set up by the German Space Agency (DLR) to offer Galileo services.

    The remaining two procurement contracts, for the ground mission infrastructure and the ground control infrastructure, will be awarded in early 2011.

    Further details are provided in programme and procurement. For the latest information consult the webpages provided by ESA.

    Plasmaspheric Effects

    GPS is subject to inaccuracies due to the effect of the ionosphere on the propagation of radio signals. A correction for the ionosphere can be made when dual frequency receivers are used. However, many navigation uses of GPS rely on inexpensive single-frequency receivers with compensation for propagation effects being made through the use of an ionospheric model broadcast by the GPS satellites. Such models are usually based on measurements of total electron content or maximum plasma concentration. Measurements of this kind are dominated by the plasma in the ionosphere so that little account is taken in the GPS correction models of the contribution of the electrons on the long raypaths through the plasmasphere, the ionized region above the ionosphere. The effects of the plasmasphere on GPS have been largely neglected and it is only recently that significant effort has been devoted to this topic.


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    Every effort is made to keep the information on these pages accurate, but this is not a guarantee that it is correct. Pages last modified on 10 March 2011. These pages created and maintained by Professor Graham Bailey.

    Copyright © 2005 The Department of Applied Mathematics, University of Sheffield, UK.

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