4.6 Navigation Satellite System
235
When the Soviet Unionwas disintegrated in 1991, the GLONASS constellation had
12 operational satellites in two orbital planes, allowing limited usage. Subsequently,
the Russian Federationtook over control of the GLONASS and continued its development. In 1993, the GLONASS system was officially declared operational, actually with the then constellation consisting of 12 satellites, and the constellation was
finally brought to its optimal status of 24 operational satellites in December 1995.
The system was initially designed to have a positioning accuracy of 65 m, but in
reality it had an accuracy of 20 min the civilian signal and 10 min the military signal.
Although both the GLONASS and the GPS have similar system architecture,
positioning principle and service ways, there are differences between both, such as
time reference system, coordinate reference frame, broadcast ephemeris format and
satellite-identified ways.
(1) The time reference systems are different between both. Both belong to the
atomic time system . The GPS time system is consistent with the Coordinated
Universal Time (UTC) maintained by the U.S. Naval Observatory at 0:00 on
6 January 1980, known as UTC(USNO), which is a continuous-time system
without leap second. The GLONASS time system is based on the Moscow’s
Coordinated Universal Time, known as UTC(SU), which is a synchronously
leap second time system, so that there is only a system difference of 3 h plus
less than 1 µs between the GLONASS time and the UTC(SU), while there is
not the leap second difference.
(2) The coordinate reference frames are different between both. The GLONASS
uses a coordinate datum named the Earth Parameters 1990, Paramtry Zemli
1990 (PZ-90), in which the precise location of the Earth’s North Pole is given
as an average of its positions between 1990 and 1995. The GPS’s coordinate
datum is the World Geodetic System 1984 (WGS-84), which uses the location
of the North Pole in 1984. As of September 17, 2007, the PZ-90 datum has
been updated to version PZ-90.02, which differs from the WGS-84 less than
40 cm in any given direction. Since December 31, 2013, the updated version
PZ-90.11 is being broadcasted, which is aligned to the International Terrestrial
Reference System (ITRS) at epoch 2011.0, getting to the centimeter level.
(3) The broadcast ephemeris formats are different between both. The GPS satellite’s ephemerides are broadcasted with a format of Keplerian orbit elements,
with an interval of 2 h. By using the Keplerian orbit equations and considering
the satellite’s perturbation motion, the instantaneous positions of the GPS satellites can be calculated in the WGS-84. For the GLONASS system, at a given
epoch the satellite’s position and velocity and its perturbation acceleration from
the Sun and Moon are broadcasted directly as the ephemerides, with an interval
of 30 min. Using the four-order Runge–Kutta method for numerical integration, the instantaneous positions of the GLONASS satellites can be calculated
in the PZ-90.
(4) The satellite-identified ways are different between both. The CDMA technique
is used to identify the different GPS satellite signals, and the navigation signals
for all of the GPS satellites are transmitted at the multiple same frequencies. The
235
When the Soviet Unionwas disintegrated in 1991, the GLONASS constellation had
12 operational satellites in two orbital planes, allowing limited usage. Subsequently,
the Russian Federationtook over control of the GLONASS and continued its development. In 1993, the GLONASS system was officially declared operational, actually with the then constellation consisting of 12 satellites, and the constellation was
finally brought to its optimal status of 24 operational satellites in December 1995.
The system was initially designed to have a positioning accuracy of 65 m, but in
reality it had an accuracy of 20 min the civilian signal and 10 min the military signal.
Although both the GLONASS and the GPS have similar system architecture,
positioning principle and service ways, there are differences between both, such as
time reference system, coordinate reference frame, broadcast ephemeris format and
satellite-identified ways.
(1) The time reference systems are different between both. Both belong to the
atomic time system . The GPS time system is consistent with the Coordinated
Universal Time (UTC) maintained by the U.S. Naval Observatory at 0:00 on
6 January 1980, known as UTC(USNO), which is a continuous-time system
without leap second. The GLONASS time system is based on the Moscow’s
Coordinated Universal Time, known as UTC(SU), which is a synchronously
leap second time system, so that there is only a system difference of 3 h plus
less than 1 µs between the GLONASS time and the UTC(SU), while there is
not the leap second difference.
(2) The coordinate reference frames are different between both. The GLONASS
uses a coordinate datum named the Earth Parameters 1990, Paramtry Zemli
1990 (PZ-90), in which the precise location of the Earth’s North Pole is given
as an average of its positions between 1990 and 1995. The GPS’s coordinate
datum is the World Geodetic System 1984 (WGS-84), which uses the location
of the North Pole in 1984. As of September 17, 2007, the PZ-90 datum has
been updated to version PZ-90.02, which differs from the WGS-84 less than
40 cm in any given direction. Since December 31, 2013, the updated version
PZ-90.11 is being broadcasted, which is aligned to the International Terrestrial
Reference System (ITRS) at epoch 2011.0, getting to the centimeter level.
(3) The broadcast ephemeris formats are different between both. The GPS satellite’s ephemerides are broadcasted with a format of Keplerian orbit elements,
with an interval of 2 h. By using the Keplerian orbit equations and considering
the satellite’s perturbation motion, the instantaneous positions of the GPS satellites can be calculated in the WGS-84. For the GLONASS system, at a given
epoch the satellite’s position and velocity and its perturbation acceleration from
the Sun and Moon are broadcasted directly as the ephemerides, with an interval
of 30 min. Using the four-order Runge–Kutta method for numerical integration, the instantaneous positions of the GLONASS satellites can be calculated
in the PZ-90.
(4) The satellite-identified ways are different between both. The CDMA technique
is used to identify the different GPS satellite signals, and the navigation signals
for all of the GPS satellites are transmitted at the multiple same frequencies. The