By Yuri Urlichich, Valery Subbotin, Grigory Stupak, Vyacheslav Dvorkin, Alexander Povalyaev, Sergey Karutin, and Rudolf Bakitko, Russian Space Systems
The GLONASS-K satellite, transmitting a CDMA signal in the L3 band, inaugurates a new era of radionavigation signals for both the Russian system and for international GNSS interoperability. As demand for high-precision services through dual- or triple-frequency user equipment increases, GLONASS will come to the forefront. The 2014 GLONASS-K2 satellite will have an FDMA signal in the L1 and L2 bands and CDMA signals in L1, L2, and L3. The overall constellation update will be completed in 2021. Another 2014 launch will fill the Russian SBAS orbit constellation with three geostationary space vehicles.
With the February launch of the first GLONASS-K satellite, and its transmission of a new CDMA signal in the L3 band, a new era of radionavigation signals has begun: international GNSS interoperability. As we have seen rapidly growing demand for high-precision services provided with dual- or triple-frequency user equipment, introduction of new GLONASS signals in the L1 and L2 bands will come next. The first launch of GLONASS-K2 satellite, with FDMA signals in L1 and L2 bands and CDMA signals in L1, L2, and L3, is planned in 2014. A complete update of the full orbiting constellation will conclude in 2021.
One satellite per year of the Luch family will be launched into orbit over the next three years, and by 2014 the System of Differential Correction and Monitoring (SDCM) constellation will be in operation with three geostationary space vehicles.
Constellation Status. In spite of the unsuccessful launch of three satellites at the end of 2010, currently GLONASS is fully deployed again with 23 satellites set healthy to the user, and more in orbiting reserve. Figure 1 shows the evolution of the constellation since its first launch in 1982. The number of satellites used for service provision is calculated at the end of each year. In order to avoid dramatic situation in 1996–2000, when satellite numbers fell, the system now carries both an on-orbit and a ground reserve of space vehciles. This will help avoid service and availability gaps that could be created by satellite failure.
GLONASS-M. The current constellation consists largely of GLONASS-M satellites, the first generation of GLONASS space vehicles, with characteristics of:
- FDMA сivil signals in L1 (1.6 GHz) and L2 (1.25 GHz) bands, with increased transmitting power;
- intersatellite link both inside one plane and between planes with ranging and communication capabilities;
- relative daily frequency stability of the cesium onboard synchronizer of 5 × 10–14;
- increased orientation accuracy of solar panels;
- guaranteed active lifetime of seven years.
New satellites can be launched into orbit either as a part of multiple launch consisting of three satellites on the launch vehicle Proton with booster Breeze-M from the Baikonur spaceport, or on the launch vehicle Soyuz with Fregat booster from Plesetsk.
GLONASS-M is the last GLONASS satellite with its payload in a sealed container. This container provides the high-temperature stability for the onboard clocks. The GLONASS-M power-supply system includes nickel-hydrogen batteries and silicon solar arrays of 30 square meters, providing 1,400 W for onboard systems.
GLONASS-K. Currently, on-orbit flight tests of the new GLONASS-K satellite (OPENING PHOTO) are under way. The first satellite in the GLONASS-K family, it has a payload located in open space and an active lifespan of 10 years. The forming and transmitting functions of navigation and inter-satellite signals are united in one module in order to increase synchronization accuracy. Besides broadcasting radionavigation signals in three bands, this satellite carries the transponder of the search-and-rescue system COSPAS-SARSAT. The overall weight of the satellite is less than 1,000 kilograms, and about 30 percent of this is the payload weight. The power-supply system generates about two times more energy than the same GLONASS-M system.
At the same time, ground-control facilities modernization and implementation of new inter-satellite measurement technology has enabled system operators to effectively increase the accuracy of broadcast ephemeris and clocks. Currently the signal-in-space range error (SISRE) equals 1.37 m (Figure 2). Further increases in accuracy will be carried out through the modernization of satellite-control technologies and development of a global network of measuring tools.
Since February 2011, GLONASS-K has been transmitting the first CDMA navigation signal in L3 band coherently with existing L1 and L2 signals. This was a first step in a new navigation signal development strategy. Future steps of GLONASS CDMA navigation signal development will focus on L1 and L2 bands. In order to design user-friendly signals, the following assumptions have been taken into account:
- GLONASS coherent FDMA and CDMA navigation signal sets should satisfy a wide range of user requirements, from ordinary navigation to high-precision applications;
- Signals should be within the bands allocated for GLONASS by the International Telecommunications Union (ITU);
- Low spectral density of signal power in radio astronomical band of 1610.6-1613.8 MHz;
- Compatibility with other GNSSs;
- Interoperability with other GNSSs.
The plans for signal development with GLONASS code division are presented in Table 1.
Figures 3–8 show the proposed structures of GLONASS CDMA signals and also the spectrums of these signals in the context of the other GNSS signal spectrums.
Due to the growing use of GNSS signals in L3/L5 band, the future GLONASS navigation family will include two signals in this band. Table 2 contains some parameters of these new signals in this band.
GLONASS Augmentation Development
SDCM development is now entering its deployment and completion phase. The network of reference stations is almost completely established. It enables the global integrity monitoring of radio navigation signals of both GLONASS and GPS satellites, gathering raw measurements of pseudorange and carrier phase in L1, L2, and L3/L5 bands. Based on these measurements, the SDCM central processing facility calculates orbits and clock corrections, and formulates SBAS messages. Preliminary results of SDCM service-quality estimation, based on corrections calculated using existing stations network, are shown in Figure 10.
The last quarter of 2011 will see the launch of space vehicle (SV) Luch-5А, carrying an SDCM transponder. Initially, this SV will be put for testing on geostationary orbit at 55 degrees East, and then will be relocated to 16 degrees West. The onboard transponder will broadcast radio signals on 1575.42 MHz. Taking into account that the main SDCM coverage area is in the northern hemisphere, the SV antenna beam will be deviated from the Equator by 7 degrees to the north.
Due to this deviation of the gain pattern from traditional orientation to the Equator, the Earth surface power distribution diagram is changed. Figure 11 presents two variants. The first one is a case in which the transmission antenna is directed on the Equator (curve 1) and the second one is a case when antenna is deviated by 7 degrees to the north from equator (curve 2). In the latter case, we obtain an increase of signal strength to the users for which this SV is under small elevation angles, that is, for the users in the northern areas of the Russian Federation.
Further SDCM development is predicated upon the launch of two Luch satellites, in the first half of 2012 and in 2013, respectively. Also in the plans is the design of a new Luch-4 satellite with dual-frequency navigation transponder, for a 2014 launch, completing the satellite-based augmentation system.
GLONASS system replenishment has almost finished, and the system enters a new historical phase. New CDMA navigation signals and deployment of a national SBAS system will provide not only a significant quality improvement of GLONASS navigation services, but also will create the favorable prerequisite for the development of applied navigation technologies in the territory of the Russian Federation, and also in Europe, the Middle East, and the Far East.
Yuri Urlichich is a general director-general designer of Joint Stock Company (JSC) Russian Space Systems, GLONASS general designer, doctor of science, professor, author of more than 150 papers and holder of 20 patents.
Valery Subbotin is a first deputy general director–general designer of JSC Russian Space Systems, and doctor of science. He has been working in the space industry for more than 40 years and has published more than 50 papers.GRigory Stupak is a deputy general director–general designer of JSC Russian Space Systems, deputy general designer of GLONASS, and professor at the Bauman Moscow State Technical University (BMSTU). He has worked in the space industry more than 35 years and has published more than 150 papers.
Vyacheslav Dvorkin is a deputy general designer of JSC Russian Space Systems”and doctor of science. He has been developing GLONASS, GNSS augmentations and user equipment for more than 35 years. He has written 50 papers in the satellite navigation field.
Alexander Povalyaev is a deputy head of division in JSC Russian Space Systems and professor at the Moscow Aviation Institute. He has been developing methods and algorithms for GNSS carrier-phase measurements processing for more than 30 years and has more than 40 papers in satellite navigation field.
Sergey Karutin is deputy head of division in JSC Russian Space Systems and assistant professor at the BMSTU. He has a Ph.D. and has been working on the GLONASS team since 1998, developing GNSS augmentations and user equipment.
Rudolf BAKITKO is a department head in JSC Russian Space Systems and a GLONASS navigation payload designer. Rudolf developed on-board equipment for space vehicles Luna, Mars, Venus, GLONASS, and COSPAS-SARSAT, and has more than 50 papers and 10 patents.