GLONASS (Global Navigation Satellite System), Russia
“The Earth was absolutely round…I never knew what the word ‘round’ meant until I saw Earth from space.”
|Satellites||24 plus 3 spares|
|Orbital inclination||64.8 degrees|
|Orbit radius||19,140 km
GLONASS was developed by the Soviet Union as an experimental military communications system during the 1970s. When the Cold War ended, the Soviet Union recognised that GLONASS had commercial applications through the system’s ability to transmit weather broadcasts, communications, navigation and reconnaissance data.
The first GLONASS satellite was launched in 1982, and the system was declared fully operational in 1993. After a period where GLONASS performance declined, Russia committed to bringing the system up to the required minimum of 18 active satellites. Currently, GLONASS has a full deployment of 24 satellites in the constellation.
GLONASS satellites have evolved since the first ones were launched. The latest generation, GLONASS-M, is shown in Figure 31, being readied for launch.
GLONASS space segment
The GLONASS space segment is summarised in Table 3 and consists of 24 satellites, in three orbital planes, with eight satellites per plane.
The GLONASS constellation geometry repeats about once every eight days. The orbit period of each satellite is approximately 8/17 of a sidereal1 day so that, after eight sidereal days, the GLONASS satellites have completed exactly 17 orbital revolutions.
Each orbital plane contains eight equally spaced satellites. One of the satellites will be at the same spot in the sky at the same sidereal time each day.
The satellites are placed into nominally circular orbits with target inclinations of 64.8 degrees and an orbital radius of 19,140 kilometres (11,893 miles), about 1,060 km (659 miles) lower than GPS satellites.
The GLONASS satellite signal identifies the satellite and includes:
- Positioning, velocity and acceleration information for computing satellite locations
- Satellite health information
- Offset of GLONASS time from UTC (SU) (Universal Time Coordinated of Russia)
- Almanac of all GLONASS satellites
GLONASS control segment
The GLONASS control segment consists of the system control centre and a network of command tracking stations across Russia. Similar to that of GPS, the GLONASS control segment monitors the health of the satellites, determines the ephemeris corrections, as well as the satellite clock offsets with respect to GLONASS time and UTC. Twice a day, it uploads corrections to the satellites.
Table 4 summarises the GLONASS signals.
|L1||1598.0625–1609.3125 MHz||L1 is modulated by the HP (High Precision) and the SP (Standard Precision) signals.|
|L2||1242.9375–1251.6875 MHz||L2 is modulated by the HP and SP signals. The SP code is identical to that transmitted on L1.|
|L3OC||1202.025 MHz||L3 is a civilian signal based on CDMA.|
Each GLONASS satellite transmits on a slightly different L1 and L2 frequency, with the P-code (HP code) and the C/A code (SP code) on both L1 and L2. GLONASS satellites transmit the same code at different frequencies, a technique known as FDMA (frequency division multiple access). Note that this is a different technique from that used by GPS and the other GNSS constellations.
GLONASS signals have the same polarisation (orientation of the electromagnetic waves) as GPS signals and have comparable signal strength.
The GLONASS system is based on 24 satellites using 14 frequencies. The satellites can share the frequencies by having antipodal satellites transmitting on the same frequency. Antipodal satellites are in the same orbital plane but are separated by 180 degrees. The paired satellites can transmit on the same frequency because they will never appear at the same time in view of a receiver on the Earth’s surface, as shown in Figure 33.
As the current GLONASS-M satellites reach the end of their service life, they will be replaced with next-generation GLONASS-K satellites. The new satellites will provide the GLONASS system with new GNSS signals.
The first block of GLONASS-K satellites (GLONASS-K1) broadcast the new civilian signal, designated L3, centred at 1202.025 MHz. Unlike the existing GLONASS signals, L3OC is based on CDMA, which will ease interoperability with other GNSS constellations.
L3OC is available from 47 satellites as of 2022.
L1OC and L2OC
The second block of GLONASS-K satellites (GLONASS-K2) adds two more CDMA-based signals broadcast at the L1OC and L2OC frequencies. The exiting FDMA L1 and L2 signals will continue to be broadcast as well to support legacy receivers.
The third block of GLONASS-K satellites (GLONASS-KM) will add an L5 CDMA signal to the GLONASS system, as well as new L1 and L3 signals. These will be at the same frequencies used by other GNSS constellations and named L1OCM, L3OCM and L5OCM.
1 A sidereal day is the time it takes for one complete rotation of the Earth, relative to a particular star. A sidereal day is about four minutes shorter than a mean solar day.↩