Meinberg GPS170PEX, Руководство пользователя

Страница: 6 / 36

Page 3
3 General information
The satellite clocks made by Meinberg have been designed to provide extremely precise time to their users. The
clocks have been developed for applications where conventional radio clocks cant 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 sys-
tem. It 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.
GPS is based on accurately measuring the propagation time of signals transmitted from satellites to the users
receiver. A nominal constellation of 24 satellites together with some active spares in six orbital planes 20,000
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 oset 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 oset 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 users 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 satellites almanac. While ephemeris parameters must be evaluated
to compute the receivers position and clock oset, 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.
GPS170PEX Date: 2009-06-24 3