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TABLE 17.6 Summary of Time and Frequency Transfer Signals and Methods
Time Uncertainty Frequency
Signal or Link Receiving Equipment (24 h) Uncertainty (24 h)
Dial-Up Computer Time Computer, client <15 ms Not recommended
Service software, modem, for frequency
and phone line measurements
Internet Time Service Computer, client <1 s Not recommended
software, and for frequency
Internet connection measurements
-6
HF Radio (3 to 30 MHz) HF receiver and 1 to 20 ms 10 to 10 -9
antenna
-10
LF Radio (30 to 300 kHz) LF receiver and 1 to 100 µs 10 to 10 -12
antenna
Global Positioning System GPS receiver antenna <20 ns <2 × 10 -13
(GPS)
FIGURE 17.13 One-way time and frequency transfer.
our measurement. Of course, for many applications the path delay is simply ignored. For example, if our
goal is simply to synchronize a computer clock within 1 s of UTC, there is no need to worry about a
100-ms path delay through a network. And, of course, path delay is not important to frequency transfer
systems, since on-time pulses are not required. Instead, frequency transfer requires only a stable path
where the delays remain relatively constant.
More sophisticated transfer systems estimate and remove all or part of the path delay. This is usually
done in one of two ways. The first way is to estimate τ ab and send the time out early by this amount. For
example, if τ ab is at least 20 ms for all users, the time can be sent 20 ms early. This advancement of the
timing signal removes at least some of the delay for all users.
A better technique is to compute τ ab and to apply a correction to the received signal. A correction for
τ ab can be computed if the position of both the transmitter and receiver are known. If the transmitter is
stationary, a constant can be used for the transmitter position. If the transmitter is moving (a satellite,
for example) it must broadcast its position in addition to broadcasting time. The Global Positioning
System (GPS) provides the best of both worlds—each GPS satellite broadcasts its position and the receiver
can use coordinates from multiple satellites to compute its own position.
The transmitted information often includes a time code so that a clock can be set to the correct time-
of-day. Most time codes contain the UTC hour, minute, and second; the month, day, and year; and
advance warning of daylight saving time and leap seconds.
Radio Time and Frequency Transfer Signals
There are many types of radio receivers designed to receive time and frequency signals. Some are designed
primarily to produce time-of-day information or an on-time pulse, others are designed to output standard
frequencies, and some can be used for both time and frequency transfer. The following sections look at
three types of time and frequency radio signals that distribute UTC—high frequency (HF), low frequency
(LF), and GPS satellite signals.
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