Page 360 - The Mechatronics Handbook
P. 360

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.


                                 ©2002 CRC Press LLC
   355   356   357   358   359   360   361   362   363   364   365