M e r i d i a n I I U s e r M a n u a l
2
C H A P T E R O N E
3
M e r i d i a n I I U s e r M a n u a l
System
Oscillator
Receiver
Subsystem
Processor
Receiver
Oscillator
PLL
GPS Receiver Module
Receiver
Processor
1PPS
EFC
10MHz
Linux NTP/PTP Subsystem
1PPS
10MHz
GPS Receiver Subsystem
10MHz
GPS Timing-How It Works
The time and frequency engine in the Meridian II receives transmissions from satellites that are oper-
ating in compliance with the Navstar GPS Interface Specification known as IS-GPS-200. It specifies
the receiver interface needed to receive and demodulate the navigation and time transfer data con-
tained in the GPS satellite transmissions. The GPS navigation system requires a means of synchro-
nizing the satellite transmissions throughout the constellation so that accurate receiver-to-satellite
range measurements can be performed via time-of-arrival measurements made at the receiver. For
the purposes of locating the receiver, measurements of the times-of-arrival of transmissions from at
least four satellites are needed for maximum timing accuracy. Time transfer to a receiver at a known
position from a single satellite is supported,
The GPS system designers defined
system time
to be
GPS time.
GPS time is a monotonic time scale
consisting of an ensemble of high-performance cesium beam and rubidium vapor atomic frequency
standards located in the monitoring stations and satellites. GPS time is measured relative to UTC,
as maintained by the United States Naval Observatory (USNO), and maintained synchronous with
UTC(USNO) except that it does not suffer from the periodic insertion of leap seconds. Such dis-
continuities would unnecessarily complicate the system’s navigation mission. Contained in the data
transmitted from each satellite is the current offset between GPS time and UTC(USNO). This offset
is composed of the current integer number of leap seconds difference and a small residual error that is
typically less than +/- 10 nanoseconds.
Each satellite in the constellation contains redundant cesium beam or rubidium vapor atomic frequen-
cy standards. These provide the timebase for all transmissions from each satellite. These transmis-
sions are monitored from ground stations located around the world and carefully measured relative to
GPS time. The results of these measurements for each satellite (i.e. correction to GPS time) are then
uploaded to that satellite so that they may be incorporated into the data contained in its transmissions.
The receiver can use this data to relate the time-of-arrival of the received transmissions from that
satellite to GPS time
and by using the transmitted UTC parameters, to UTC(USNO).
FIGURE 1 - MERDIAN II GPS SYSTEM TIMEBASE
Summary of Contents for Meridian II
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