GLONASS (Global’naya Navigatsionnaya Sputnikovaya Sistema, meaning Global Navigation Satellite System) is a global navigation satellite system (GNSS) owned by Russia. It serves as an alternative to the U.S. Global Positioning System (GPS), and in terms of global coverage and precision, it is the second operational navigation system.
The orbital inclination of the GLONASS satellites is chosen in such a manner that it provides positioning at high latitudes (near the poles); and hence, the system provides an ideal supplementation where GPS signals are not available.
Glonass Historical Development
The idea of utilization of satellites for navigation was first explicitly declared by V. S. Shebashevich in 1957. This idea constituted the fundamental operation principle of the GNSS systems. Early research studies of the Soviet Union yielded the low orbit “Cicada” system in 1960s. Successful operation of Cicada and similar low-orbit satellite navigation systems by the marine users attracted widespread attention to satellite navigation; inspiring the idea of mid-earth orbit satellite navigation systems such as GLONASS.
The development of GLONASS began in the Soviet Union in 1976. Starting from 1982, phase by phase, satellites were added to the system until the completion of the constellation in 1995. After a decline in capacity during the 1990s due to dissolution of the Soviet Union, the system was restored by Russia as a governmental priority. Owing to the increased budget and funding between 2002 and 2008, full orbital constellation of the system has been restored, and global coverage has been achieved again by 2011.
Constellation and Frequency Utilization
The constellation of GLONASS is as follows: in 3 different orbital planes, with 8 evenly spaced satellites on each plane, the system consists of 24 satellites. It is reported that 18 of these satellites are necessary and sufficient for coverage of the Russian territory.
The satellites are located at 19,100 km altitude and an orbital period of 11 hours and 16 minutes (17 revolutions completed in 8 sidereal days, namely a satellite passes over the same location). As stated before, GLONASS’s orbits make it suitable for usage at high latitudes (both north and south), especially where getting a GPS signal can be problematic.
Like all GNSS systems, each satellite transmits signals indicating its position and precise time information from its atomic clocks. Receivers consider these signals and compute their positions by means of geometric calculations. As in all other GNSS systems, a receiver must be in the range of at least 4 satellites for 3-dimensional position computation.
GLONASS satellites transmit two types of signals:
- open standard-precision signal L1OF/L2OF, which is available to all users, and
- limited access high-precision signal L1SF/L2SF, which is available to authorized users only, such as the Russian military.
The signals use similar Direct Sequence Spread Spectrum (DSSS) encoding and Binary Phase-Shift Keying (BPSK) modulation as in GPS signals. Spectrum utilization of these signals was originally achieved by means of the Frequency-Division Multiple Access (FDMA) technique. However, after 2008, research studies have been conducted to utilize Code Division Multiple Access (CDMA) in GLONASS. Additionally, other modernizations have also been applied to satellites.
The Ground Control stations of the GLONASS system are located at various locations of ex-Soviet territory as well as Brazil and Nicaragua.
Conclusion
Due to its free, open, and dependable nature as well as availability at high altitude, GLONASS has become an essential complementary element for navigation. Currently, to increase precision and services coverage, a considerable number of GNSS receiver vendors (including Inmarsat-based communication terminals as well as smartphones) incorporate GLONASS capability in addition to GPS. Combination of GPS-GLONASS capabilities yield better Position Dilution of Precision (PDOP) values. Compared to GPS-only applications, GPS-GLONASS integration yield 30% better PDOP performance above 55 degrees latitude (while approaching to North and South poles) and 15% better PDOP performance at smaller latitudes (while approaching to the Equator).
