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Introduction
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GPS/GLONASS Receiver User Manual Rev 1
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The orbit period of each satellite is approximately 8/17 of a sidereal day such that, after eight
sidereal days, the GLONASS satellites have completed exactly 17 orbital revolutions. A
sidereal day is the rotation period of the earth and is equal to one calendar day minus four
minutes.
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Because 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.
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The satellites are placed into nominally circular orbits with target inclinations of 64.8
degrees and an orbital height of about 19,123 km, which is about 1,060 km lower than GPS
satellites.
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The GLONASS satellite signal identifies the satellite and provides:
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the positioning, velocity and acceleration vectors at a reference epoch for computing
satellite
locations
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synchronization bits
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data age
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satellite health
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offset of GLONASS time
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almanacs of all other GLONASS satellites.
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The GLONASS satellites each transmit on different L1 and L2 frequencies, with the P code
on both L1 and L2, and with the C/A code, at present, only on L1. L1 is currently centered at
1602 - 1615.5 MHz.
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Some of the GLONASS transmissions initially caused interference to radio astronomers and
mobile communication service providers. The Russians consequently agreed to reduce the
number of frequencies used by the satellites and to gradually change the L1 frequencies to
1598.0625 - 1609.3125 MHz. Eventually the system will only use 12 primary frequency
channels (plus two additional channels for testing purposes).
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System operation (24 satellites and only 12 channels) can be accomplished by having
antipodal satellites, satellites in the same orbit plane separated by 180 degrees in argument
of latitude, transmit on the same frequency. This is possible because the paired satellites will
never appear at the same time in your view. Already, eight pairs of satellites share
frequencies.
Unlike GPS satellites, all GLONASS satellites transmit the same codes. They derive signal timing and frequencies
from one of three onboard cesium atomic clocks operating at 5 MHz. The signals are right-hand circularly
polarized, like GPS signals, and have comparable signal strength.
Figure 1.3 View of GLONASS Satellite Orbit Arrangement
1.4.2 THE CONTROL SEGMENT
The Control Segment consists of the system control center and a network of command tracking stations across
Russia. The GLONASS control segment, similar to GPS, must monitor the status of satellites, determine the
ephemerides and satellite clock offsets with respect to GLONASS time and UTC (SU) time, and twice a day upload
the navigation data to the satellites.