Page 362 - The Mechatronics Handbook

TABLE 17.7 LF Time and Frequency Broadcast Stations
Frequency
Call Sign Country (kHz) Always On?
DCF77 Germany 77.5 Yes
DGI Germany 177 Yes
HBG Switzerland 75 Yes
JG2AS Japan 40 Yes
MSF United Kingdom 60 Yes
RBU Russia 66.666 No
RTZ Russia 50 Yes
TDF France 162 Yes
WWVB United States 60 Yes

continuously tracks the same cycle of a ground wave signal can transfer frequency with an uncertainty
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of about 1 × 10 when averaged for 1 day.
NIST operates LF radio station WWVB from Fort Collins, Colorado at a transmission frequency of
60 kHz. The station broadcasts 24 h per day, with an effective radiated output power of 50 kW. The
WWVB time code is synchronized with the 60 kHz carrier and contains the year, day of year, hour,
minute, second, and flags that indicate the status of daylight saving time, leap years, and leap seconds.
The time code is received and displayed by wristwatches, alarm clocks, wall clocks, and other consumer
electronic products [2,7,15].

Global Positioning System (GPS)
The GPS is a navigation system developed and operated by the U.S. Department of Defense (DoD) that
is usable nearly anywhere on the earth. The system consists of a constellation of at least 24 satellites that
orbit the earth at a height of 20,200 km in six fixed planes inclined 55∞ from the equator. The orbital
period is 11 h 58 m, which means that each satellite will pass over the same place on earth twice per day.
By processing signals received from the satellites, a GPS receiver can determine its position with an
uncertainty of <10 m.
The satellites broadcast on two carrier frequencies, L1 at 1575.42 MHz and L2 at 1227.6 MHz. Each
satellite broadcasts a spread spectrum waveform, called a pseudo random noise (PRN) code on L1 and
L2, and each satellite is identified by the PRN code it transmits. There are two types of PRN codes. The
first type is a coarse acquisition (C/A) code with a chipping rate of 1023 chips per millisecond. The second
is a precision (P) code with a chipping rate of 10230 chips per millisecond. The C/A code is broadcast
on L1, and the P code is broadcast on both L1 and L2. GPS reception is line-of-sight, which means that
the receiving antenna must have a clear view of the sky [16].
Each satellite carries either rubidium or cesium oscillators, or a combination of both. These oscillators
are steered from DoD ground stations and are referenced to the United States Naval Observatory time scale,
UTC (USNO), which by agreement is always maintained within 100 ns of UTC (NIST). The oscillators
provide the reference for both the carrier and the code broadcasts.
GPS signals now dominate the world of high performance time and frequency transfer, since they
provide reliable reception and exceptional results with minimal effort. A GPS receiver can automatically
compute its latitude, longitude, and altitude from position data received from the satellites. The receiver
can then calibrate the radio path and synchronize its on-time pulse. In addition to the on-time pulse,
many receivers provide standard frequencies such as 5 or 10 MHz by steering an OCXO or rubidium
oscillator using the satellite signals. GPS receivers also produce time-of-day and date information.
A GPS receiver calibrated for equipment delays has a timing uncertainty of <20 ns relative to UTC
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(NIST), and the frequency uncertainty is often <2 × 10 when averaged for 1 day. Figure 17.14 shows
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an Allan deviation plot of the output of a low cost GPS receiver. The stability is near 1 × 10 after about
1 day of averaging.

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