13 How Satellite Navigation Works
13 How Satellite Navigation Works
The use of a receiver for location tracking and time synchronization relies on the ability to measure the satellite-
to-receiver propagation delay as precisely as possible. It is necessary to have simultaneous reception from four
satellites so that the receiver can determine its relative spatial position in three dimensions (x, y, z) and mea-
sure the deviation of its clock against the system clock. Monitoring stations around the planet track the orbital
trajectory of the satellites and detect deviations between the local atomic clocks and the system time. The
collected data is transmitted up to the satellites, which then send navigation data back to Earth.
The high-precision trajectory data of each satellite, known as the satellite’s ephemeris, is needed by the
receiver to continuously calculate the precise location of the satellites in space. A roughly defined ephemeridal
schedule based on empirical data, referred to as an almanac, is used by a receiver to identify which satellites
are visible above the horizon given a specific location and time. Each satellite transmits its own ephemeridal
schedule as well as the almanacs of all existing satellites.
Satellite Systems
GPS
was installed by the United States Department of Defense (US DoD) and operates at two performance
levels: the Standard Positioning Service, or SPS, and the Precise Positioning Service, or PPS. The structure
of the messages transmitted by the SPS has been openly published and reception is provided for public use.
The timing and navigation data of the more precise PPS is encrypted and is thus only accessible to certain
(usually military) users.
GLONASS
was originally developed by the Russian military for real-time navigation and ballistic missile
guidance systems. GLONASS satellites also send two types of signal: a Standard Precision Signal (SP) and
an encrypted High Precision Signal (HP).
BeiDou
is a Chinese satellite navigation system. The second-generation system, officially referred to as the
BeiDou Navigation Satellite System (BDS) and also known as "COMPASS", consists of 35 satellites. BeiDou
entered service in December 2011 with ten satellites and was made available to users in the Asia-Pacific region.
The system was completed in June 2020 with the launch of the final satellite.
IMS-GPS182
Date: June 16, 2022
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