Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 604

of the satellite is measured by radar techniques with a maximum error of between
2m and 3m. These radio-frequency ranges are periodically calibrated using a laser
ranging device at the laser tracking stations. Each satellite carries laser
retroreflectors specifically for this purpose. Ephemeris is predicted 24 hours in
advance and uploaded once per day. The spacecraft clock correction parameters are
renewed twice a day. Therefore, timing errors of the satellites' ranging signals can
lead to a pseudorange measurement error of at most 5m to 6m. Any interruption in
the normal operation of the ground segment interrupts the accuracy of GLONASS
signals. Tests have shown that the spacecraft clock can maintain acceptable accu-
racy (one part in 5 10 13 ) for no more than two to three days of autonomous opera-

tions. Although the satellite's central processor is capable of 30 days of autonomous
operations, this variability in the time standard is the limiting component for auton-
omous GLONASS operations [1].

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11.1.6.4 Laser Tracking Stations
GLONASS is supported by the Etalon (Komsomolsk) and Maidanak (Kitab) laser
stations. These stations calibrate radio-frequency tracking measurements. These
systems may be third generation laser systems.
Etalon stations are able to measure the position of satellites visible in reflected
solar light down to stellar magnitudes of less than 13. Range errors based on a
15-second averaging interval are about 1.5-2 cm and between 2 and 3 arcsec in
angular position. Detailed specifications of these third generation military systems
remain classified [1].
GLONASS is supported by an experimental multifunctional optical and laser
complex located near Kitab in southern Uzbekistan on Mt. Maidanak. Cameras
located on Mt. Maidanak are capable of measuring ranges to an object up to an alti-
tude of 40,000 km and down to a visible stellar magnitude of 16. Maximum error of
satellite angular coordinate determination does not exceed 1-2 arcsec under normal
operating conditions and 0.5 arcsec under special experimental conditions. Maxi-
mum ranging error is not more than 1.5-1.8 cm, and the error of the fix to the
UTC(SU) scale is not more than s. GLONASS measurements are relayed via

secure radio link to the system control center once per hour. Mt. Maidanak provides
unique climatic characteristics with more than 220 clear days annually, thus making
it a reliable source of correction data to the system control center [1, 2].

+ / − 1 µ

11.1.6.5 Navigation Field Control Equipment
The navigation field control equipment stations monitor the GLONASS navigation
signals. If anomalies are observed, they are reported back to the SCC [1].

11.1.7 User Equipment
The Russian GLONASS user segment is small and concentrated primarily in
Russia. Russian industry has designed many different types of GLONASS and GPS-
GLONASS user equipment and listed these for sale on the Russian Internet. However,