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3 INTERFACE CHARACTERISTICS
48 Copernicus GPS Receiver
GPS Timing
In many timing applications, such as time/frequency standards, site synchronization
systems, and event measurement systems, GPS receivers are used to discipline local
oscillators.
The GPS constellation consists of 24 orbiting satellites. Each GPS satellite contains a
highly-stable atomic (Cesium) clock, which is continuously monitored and corrected
by the GPS control segment. Consequently, the GPS constellation can be considered a
set of 24 orbiting clocks with worldwide 24-hour coverage.
GPS receivers use the signals from these GPS clocks to correct their internal clock
which is not as stable or accurate as the GPS atomic clocks. GPS receivers like the
Copernicus GPS output a highly accurate timing pulse (PPS) generated by an internal
clock which is constantly corrected using the GPS clocks. This timing pulse is
synchronized to UTC within ±100 ns rms.
In addition to serving as a highly accurate stand-alone time source, GPS receivers are
used to synchronize distant clocks in communication or data networks. This
synchronization is possible since all GPS satellite clocks are corrected to a common
master clock. Therefore, the relative clock error is the same, regardless of which
satellite or satellites are used. For timing applications requiring a common clock, GPS
is the ideal solution.
Position and time errors are related by the speed of light. Therefore, a position error
of 100 meters corresponds to a time error of approximately 333 ns. The hardware and
software implementation affects the GPS receiver's PPS accuracy level. The
receiver's clocking rate determines the PPS steering resolution.
Serial Time Output
Time must be taken from the timing messages in the TSIP, TAIP, or NMEA protocols
because position messages contain a timestamp which is usually 1 to 2 seconds in the
past.
Table 3.5 Serial Time Output
Note – GPS time differs from UTC (Universal Coordinated Time) by a variable,
integer number of seconds UTC=(GPS time)-(GPS UTC offset).
As of January 2006, the GPS UTC offset was 14 seconds. The offset has historically
increased by 1 second about every 18 months. System designers should plan to read
the offset value as a part of the timing interface to obtain UTC. The GPS week
number is in reference to a base week (Week #0), starting January 6, 1980.
Protocol Timing Message
TSIP Report packets 41 and 8F-21
TAIP TM message
NMEA ZDA message