Future European global satellite navigation 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 [10]. 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 mak- ers 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 char- acteristics of the Galileo system were to be determined sometime in 2001 [10].

Galileo will be compatible at the user level with the existing GPS and GLONASS systems. However, unlike GPS and GLONASS, Galileo will pro- vide 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 sys- tem 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 alloca- tion). As well, it includes the construction of the various segments of the system (space, ground, and receiver). Some prototype satel- lites 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 Gali- leo operational service is 2008 or shortly thereafter. At that time, EGNOS service will be provided in parallel until it is phased out in 2015 [10].

References

[1] Kleusberg, A., “Comparing GPS and GLONASS,”GPS World, Vol. 1, No. 6, November/December 1990, pp. 52–54.

[2] Langley, R. B., “GLONASS: Review and Update,”GPS World, Vol. 8, No. 7, July 1997, pp. 46–51.

[3] Navtech Seminars and GPS Supply, GPS/GNSS newsletter, May 17, 2001.

[4] Johnson, N. L., “GLONASS Spacecraft,”GPS World, Vol. 5, No. 11, November 1994, pp. 51–58.

[5] Bazlof, Y. A., et al., “GLONASS to GPS: A New Coordinate

Transformation,”GPS World, Vol. 10, No. 1, January 1999, pp. 54–58.

[6] CANSPACE, “China Puts Second Navigation Positioning Satellite into Orbit,”Canadian Space Geodesy Forum, December 21, 2000.

[7] CANSPACE, “Chinese Launch Navigation Satellite,”Canadian Space Geodesy Forum, October 31, 2000.

[8] CANSPACE, “Chinese Satellite Navigation System Update,”Canadian Space Geodesy Forum, January 9, 2001.

[9] Kayton, M., and W. R. Fried,Avionics Navigation Systems,2nd ed., New York: Wiley, 1997.

[10] Commission of the European Communities, “Commission Communication to the European Parliament and the Council on GALILEO,” Brussels, Belgium, November 22, 2000.

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Appendix A GPS Accuracy and Precision Measures

The termaccuracyis used to express the degree of closeness of a measure- ment, or the obtained solution, to the true value. The termprecision, how- ever, is used to describe the degree of closeness of repeated measurements of the same quantity to each other. In the absence of systematic errors, accuracy and precision would be equivalent [1]. For this reason, the two terms are used indiscriminately in many practical purposes. Accuracy can be measured by a statistical quantity called the standard deviation, assum- ing that the GPS measurements contain no systematic errors or blunders.

The lower the standard deviation, the higher the accuracy.

For the 1-D case, for example, measuring the length of a line between two points, the accuracy is expressed by the so-called root mean square (rms). The rms is associated with a probability level of 68.3%. For example, the accuracy of the static GPS surveying could be expressed as “5 mm + 1 ppm” (rms). This means that there is a 68.3% chance (or probability) that we get an error of less than or equal to “5 mm + 1 mm for every kilometer.”

In other words, if we measure a 10-km baseline, then there is a 68.3%

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chance that we get an error of less than or equal to 15 mm in the measured line.

Horizontal component (e.g., easting and northing) accuracy, a 2-D case, is expressed by either the circular error probable (CEP) or twice dis- tance rms (2drms). CEP means that there is a 50% chance that the true horizontal position is located inside a circle of radius equal to the value of CEP [1]. The corresponding probability level of the 2drms varies from 95.4% to 98.2% depending on the relative values of the errors in the easting and northing components. The ratio of the 2drms to the CEP varies from 2.4 to 3. This means that an accuracy of 40m (CEP) is equivalent to 100m (2drms) for a ratio of 2.5.

The spherical error probable (SEP) is used to express the accuracy of the 3-D case. SEP means that there is a 50% chance that the true 3-D posi- tion is located inside a sphere of a radius equal to the value of SEP [1].

Reference

[1] Mikhail, E.,Observations and Least Squares,New York: University Press of America, 1976.

Appendix B Useful Web Sites