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.

High frequency (HF) radio broadcasts occupy the radio spectrum from 3 to 30 MHz. These signals are
commonly used for time and frequency transfer at moderate performance levels. Some HF broadcasts
provide audio time announcements and digital time codes. Other broadcasts simply provide a carrier
frequency for use as a reference.
HF time and frequency stations include NIST radio stations WWV and WWVH. WWV is located near
Fort Collins, Colorado, and WWVH is on the island of Kauai, Hawaii. Both stations broadcast continuous
time and frequency signals on 2.5, 5, 10, and 15 MHz, and WWV also broadcasts on 20 MHz. All
frequencies broadcast the same program, and at least one frequency should be usable at all times. The
stations can also be heard by telephone; dial (303) 499-7111 for WWV or (808) 335-4363 for WWVH.
WWV and WWVH signals can be used in one of three modes:
• The audio portion of the broadcast includes seconds pulses or ticks, standard audio frequencies,
and voice announcements of the UTC hour and minute. WWV uses a male voice, and WWVH
uses a female voice.
• A binary time code is sent on a 100 Hz subcarrier at a rate of 1 bit per second. The time code
contains the hour, minute, second, year, day of year, leap second and Daylight Saving Time (DST)
indicators, and a UT1 correction. This code can be read and displayed by radio clocks.
• The carrier frequency can be used as a reference for the calibration of oscillators. This is done
most often with the 5 and 10 MHz carrier signals, since they match the output frequencies of
standard oscillators.
The time broadcast by WWV and WWVH will be late when it arrives at the user’s location. The time
offset depends upon the receiver’s distance from the transmitter, but should be <15 ms in the continental
United States. A good estimate of the time offset requires knowledge of HF radio propagation. Most
users receive a signal that has traveled up to the ionosphere and was then reflected back to earth. Since
the height of the ionosphere changes throughout the day, the path delay also changes. Path delay variations
limit the received frequency uncertainty to parts in 109 when averaged for 1 day.
HF radio stations such as WWV and WWVH are useful for low level applications, such as the manual
synchronization of analog and digital clocks, simple frequency calibrations, and calibrations of stop
watches and timers. However, LF and GPS signals are better choices for more demanding applications
[2,7,15].
LF Radio Signals (Including WWVB)
Before the advent of satellites, low frequency (LF) signals were the method of choice for time and
frequency transfer. While the use of LF signals has diminished in the laboratory, they still have two major
advantages—they can often be received indoors without an external antenna and several stations broadcast
a time code. This makes them ideal for many consumer electronic products that display time-ofday
information.
Many time and frequency stations operate in the LF band from 30 to 300 kHz (Table 10.7). The
performance of the received signal is influenced by the path length and signal strength. Path length is
important because the signal is divided into ground wave and sky wave. The ground wave signal is more
stable. Since it travels the shortest path between the transmitter and receiver, it arrives first and its path
delay is much easier to estimate. The sky wave is reflected from the ionosphere and produces results
similar to those obtained with HF reception. Short paths make it possible to continuously track the
ground wave. Longer paths produce a mixture of sky wave and ground wave. And over very long paths,
only sky wave reception is possible.
Signal strength is also important. If the signal is weak, the receiver might search for a new cycle of the
carrier to track. Each time the receiver adjusts its tracking point by one cycle, it introduces a phase step
equal to the period of the carrier. For example, a cycle slip on a 60 kHz carrier introduces a 16.67 μs
phase step. However, a strong ground wave signal can produce very good results. An LF receiver that continuously tracks the same cycle of a ground wave signal can transfer frequency with an uncertainty
of about 1 × 10−12 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
(NIST), and the frequency uncertainty is often <2 × 10−13 when averaged for 1 day. Figure 10.14 shows
an Allan deviation plot of the output of a low cost GPS receiver. The stability is near 1 × 10−13 after about
1 day of averaging.

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