6 GNSS Satellite Navigation
6 GNSS Satellite Navigation
The satellite receiver clock GNS has been designed to provide extremely precise time to its user. The clock has
been developed for applications where conventional radio controlled clocks can´t meet the growing requirements
in precision. High precision available 24 hours a day around the whole world is the main feature of the new
system which receives its information from the satellites of the russian GLONASS (GLObal NAvigation Satellite
System) and the american GPS (Global Positioning System).
GPS and GLONASS are satellite-based radio-positioning, navigation, and time-transfer systems. They are
based on accurately measuring the propagation time of signals transmitted from satellites to the user´s re-
ceiver. A fully operational constellation of more than 24 satellites together with several active spares in six
(GPS) respectively three (GLONASS) orbital planes in 20183 km (GPS) respectively 19100 km (GLONASS)
over ground provides a minimum of four satellites to be in view 24 hours a day at every point of the globe.
Four satellites need to be received simultaneously if both receiver position (x, y, z) and receiver clock offset
from GPS/GLONASS system time must be computed. All the satellites are monitored by control stations which
determine the exact orbit parameters as well as the clock offset of the satellites´ on-board atomic clocks. These
parameters are uploaded to the satellites and become part of a navigation message which is retransmitted by
the satellites in order to pass that information to the user´s receiver.
The high precision orbit parameters of a satellite are called ephemeris parameters whereas a reduced pre-
cision subset of the ephemeris parameters is called a satellite´s almanac. While ephemeris parameters must
be evaluated to compute the receiver´s position and clock offset, almanac parameters are used to check which
satellites are in view from a given receiver position at a given time. Each satellite transmits its own set of
ephemeris parameters and almanac parameters of all existing satellites.
GPS was installed by the United States Department of Defense and provides two levels of accuracy: The
Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). While PPS is encrypted and
only available for authorized (military) users, SPS has been made available to the general public.
GLONASS was developed to provide real-time position and velocity determination, initially for use by the
Soviet military in navigating and ballistic missile targeting. Also GLONASS satellites transmit two types of
signals: a Standard Precision (SP) signal and an obfuscated High Precision (HP) signal.
The BeiDou Navigation Satellite System (BDS) is a Chinese satellite navigation system. The second gen-
eration of the system, officially called the BeiDou Navigation Satellite System (BDS) and also known as
COMPASS or BeiDou-2, will be a global satellite navigation system consisting of 35 satellites, and is under
construction as of January 2015. It became operational in China in December 2011, with 10 satellites in use,
and began offering services to customers in the Asia-Pacific region in December 2012. It is planned to begin
serving global customers upon its completion in 2020.
Galileo is the global navigation satellite system (GNSS) that is currently being created by the European
Union (EU) through the European Space Agency (ESA) and the European GNSS Agency (GSA). The use of
basic Galileo services will be free and open to everyone.
The complete 30-satellite Galileo system (24 operational and 6 active spares) is expected by 2020. At an
altitude of 23,222 km above the Earth’s surface, the satellites require about 14 hours for one orbit.
microSync
Date: 29th July 2021
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