Global Navigation Satellite System (GLONASS) Overview
of GLONASS began in 1976, with a goal of global coverage by
1991. Beginning on 12 October 1982, numerous rocket launches added
satellites to the system until the constellation was completed in 1995.
GLONASS was developed to provide real-time position and velocity
determination, initially for use by the Soviet military in navigating
and ballistic missile targeting. With the collapse of the
Russion economy GLONASS rapidly degraded, mainly due to the
relatively short design life-time of the GLONASS
satellites. Beginning in 2001, Russia committed to restoring
the system, and in recent years has diversified, introducing the Indian
government as a partner. This plan calls for 18 operational satellites
in orbit by 2008 and 24 SVs (21 operational and 3 on-orbit spares
deployed in three orbital planes) in place by 2009; and a level of
performance matching that of the U.S Global Positioning System by 2011.
Six GLONASS SVs are scheduled to be added in 2008, and the
first two improved GLONASS-K satellites will be launched in 2009.
GLONASS Constellation Overview
A fully operational GLONASS constellation consists of 24 satellites, with 21 used for transmitting signals and three for in-orbit spares, deployed in three orbital planes. The three orbital planes' ascending nodes are separated by 120° with each plane containing eight equally spaced satellites. The orbits are roughly circular, with an inclination of about 64.8°, and orbit the Earth at an altitude of 19,100 km, which yields an orbital period of approximately 11 hours, 15 minutes. The planes themselves have a latitude displacement of 15°, which results in the satellites crossing the equator one at a time, instead of three at once. The overall arrangement is such that, if the constellation is fully populated, a minimum of 5 satellites are in view from any given point at any given time. This guarantees for continuous and global navigation for users world-wide.
Each satellite is identified by a "slot" number, which defines the corresponding orbital plane and the location within the plane; numbers 1-8 are in plane one, 9-16 are in plane two, and 17-24 are in plane three.
A characteristic of the GLONASS constellation is that any given satellite only passes over the exact same spot on the Earth every eighth sidereal day. However, as each orbit plane contains eight satellites, a satellite will pass the same place every sidereal day. For comparison, each GPS satellite passes over the same spot once every sidereal day. So opposed to the GPS the ground-track of the GLONASS satellites do not repeat after one day. This avoids the resonance effects which makes station keeping of GPS satellites difficult and expensive.
GLONASS Signal Overview
main difference between GPS and GLONASS is that in GLONASS each
satelilite has its own frequencies but the same code whereas in GPS als
satellites use the same frequencies but have different codes. GLONASS
uses what is called a frequency devision multiple access method (FDMA)
whereas GPS and Galileo uses a code devision multiple access
All signals transmitted by the satellite are derived from the fundamental frequency (f0) of the satellite oscillator. Each satellite transmits on a different frequency using originally a 25-channel frequency division multiple access (FDMA) technique spanning from 1602.5625 MHz to 1615.5 MHz, known as the L1 band. The equation to calculate the exact center frequency is 1602 MHz + n × 0.5625 MHz, where n is a satellite's frequency channel number (n=0,1,2,...24). On the L2 band, between 1240 MHz and 1260 MHz, the center frequency is determined by the equation 1246 MHz + n × 0.4375 MHz.
GLONASS transmissions in the band 1610.6 - 1613.8 MHz degraded
radio astronomical observations dramatically in the years
since the launch of the first GLONASS satellites. With the
GLONASS constellation the band 1610.6 - 1613.8 MHz would be closed for
any radio astronomical research. In the fall of 1992 the Radio
Astronomy Service made dedicated observations in full collaboration of
the GLONASS administration in a world-wide campaign to test this
interference. Negotiations between IUCAF and the GLONASS administration
led in November 1993 to an agreed step-by-step plan to clear the band
1610.6-1613.8 MHz of interference by GLONASS. This agreement became
model for agreements between the administration of the Russian
Federation and those of other countries. According to this agreement
GLONASS satellites launched after 2005 will use channels n=-7 to +6,
with channels +5 and +6 used only for orbital insertion, or during
periods of exceptional circumstances. Satellites will incorporate
filters, protecting both the 1660-1670 MHz and the 1610.6-1613.8 MHz
band from out of band emissions to the required ITU-R RA769.1 limit (as
above, -238 dB(Wm-2Hz-1)).