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 [2]. 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 [3]. 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 [4].
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.
Regional augmentations
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 [9]. 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 [9].
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 [9].
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 [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 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 [10].
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 [10].
