Meinberg LANTIME, Руководство по эксплуатации

Страница: 18 / 153

GPS satellite controlled clock
A Meinberg GPS167 satellite controlled radio clock is used as a reference time
base. The satellite receiver clock GPS167 has been designed to provide extremely
precise time to its user. The clock has been developed for applications where
conventional radio controlled clocks can not 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
Global Positioning System.
The Global Positioning System (GPS) is a satellite-based radio-positioning,
navigation, and time-transfer system. It was installed by the United States Department
of Defence 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.
GPS is based on accurately measuring the propagation time of signals transmitted
from satellites to the user’s receiver. A nominal constellation of 21 satellites together
with 3 active spares in six orbital planes 20000 km 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 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 precision 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.
18