High frequency (HF) shortwave radio broadcasts are one of the most popular sources of time and frequency information. HF signals from stations such as WWV (Ft. Collins, Colorado), WWVH (Kauai, Hawaii), and CHU (Ottawa, Canada) are readily available and provide essentially worldwide coverage. In addition, the signals can be picked up with relatively inexpensive receiving equipment.

Although HF broadcasts are extremely popular, there are drawbacks to using them. To date, no practical method has been found to automate the use of HF signals. Also, the equipment requires human operators.

HF signals rely primarily on reflection from the sky (ionosphere) to arrive at a distant point. Changes in the density and height of the ionosphere (or reflecting region) change the distance a time signal must travel. This produces a characteristic of high frequency or "short" waves called fading. The signals that leave the transmitter take a number of paths to reach the receiver. these signals arrive at a distant point, they have shifted in phase by different amounts. Sometimes they recombine to produce a stronger signal, but at other times, they almost cancel each other and no signal is available.

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It is this recombining-canceling pattern that produces the characteristic fading at HF. This also changes the apparent time of a received pulse or "tick." The arrival time of a timing pulse may vary from day to day, even for measurements made at the same time of day. Accuracies approaching 1 millisecond can typically be achieved by taking measurements each day and averaging over several days. This means that a user must have a clock and the necessary equipment to keep good time and measure small time differences. It also means he must be dedicated and willing to expend the effort to do the job.

Good signal reception is essential for accurate frequency or time measurement using HF broadcasts. Signal strength and geographical location will determine the necessary receiver and antenna requirements. For example, a directional antenna may be necessary so that it can be oriented to favor the transmission mode which consistently provides the strongest received signal. Accuracy of measurements using HF signals is typically 10 milliseconds for time and 1 x 10-6 for frequency.

5.1 BROADCAST FORMATS

As previously mentioned, there are many HF radio stations worldwide that can be used for time and frequency measurements. Some of them are described in this chapter.

5.1.1 WWV/WWVH

Standard time and frequency stations WWV and WWVH broadcast on carrier frequencies of 2.5, 5, 10, and 15 MHz. WWV also broadcasts on 20 MHz. The stations get all of their signals from a cesium beam frequency source. They each use three "atomic clocks" to provide the time of day, audio tones, and carrier frequencies. The rates or frequencies of the cesium oscillators at the stations are controlled to be within one part in 1012 (1000 billion) of the NBS frequency standard located in Boulder, Colorado. Time at the stations is held to within a few microseconds of the NBS atomic time scale, UTC(NBS).

These cesium standards drive time code generators through various dividers, multipliers, and distribution amplifiers. The time code generators generate audio tones and time ticks. Frequency multipliers provide the radio carrier frequencies which are then amplitude modulated.

The seconds pulses or "ticks" transmitted by WWV and WWVH are obtained from the same frequency source that controls the carrier frequencies. They are produced by a double sideband, 100 percent modulated, signal on each RF carrier. The first pulse of every hour is an 800-millisecond pulse of 1500 Hz. The first pulse of every minute is an 800-millisecond pulse of 1000 Hz at WWV and 1200 Hz at WWVH. The remaining seconds pulses are brief audio bursts (5-millisecond pulses of 1000 Hz at WWV and 1200 Hz at WWVH) that

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commercial 300 baud FSK (frequency shift keying) at frequencies of 2025 and 2225 Hz. The code is a modified ASCII in which each 11-bit character contains two BCD digits, with one start bit and two stop bits. The first digit is the number 6 for identification, which should be verified in the receiving clock, and the remaining 9 digits give the day, hour, minute and second of UTC. entire message is then repeated in order that the receiving clock can check for identical messages before updating. The code ends, and update occurs at 0.500s. This 0.500s must be added in, along with the time zone hour, to give the correct time. The same code is also available by telephone. Details can be obtained by writing to the Time and Frequency

Section, National Research Council, Ottawa, Ontario, Canada. Other details of CHU's service are shown in Table 5.1, page 77.

If you are outside the United States or Canada, the following HF stations are available for time and frequency measurements.

5.1.3 ATA

ATA, which broadcasts on 5, 10, and 15 MHz, is located in Greater Kailash, Delhi, India. It is operated by the National Physical Laboratory in New Delhi. The transmission schedule is shown in figure 5.3.

ATA gives time ticks for every second, minute, and quarter-hour (i.e., Oth, 15th, 30th, and 45th minute of every hour). The second pulses consist of a group of 5 cycles of 1 kHz of 5 milliseconds duration. The minute pulses, started at the 0th second, are a 1 kHz signal of 100 milliseconds duration. At the beginning of every quarter-hour, a l kHz tone starts; it lasts for four minutes (i.e., from the 0th to 4th, 15th to 19th, 30th to 34th, and 45th to 49th minute).

The second and minute pulses are maintained during the tone period by interrupting the tone for 25 and 120 milliseconds respectively and introducing second and minute pulses. The second and minute pulses are preceded and succeeded by 10 millisecond intervals as shown in figure 5.4.

A voice announcement, comprising the station call sign and Indian Standard Time is made about 20 seconds prior to the beginning of each quarter-hour. Other details of ATA are shown in Table 5.1.

5.1.4 IAM

The Istituto Superiore delle Poste e delle Telecomunicazioni (ISPT) in Rome, Italy, operates HF radio station IAM on 5 MHz. IAM broadcasts time signals and standard frequencies.

The transmission is effected for 6 days a week (week-days only) in accordance with the diagram in figure 5.5.

The second pulse is made up of 5 cycles of a 1000 Hz standard frequency. Minute pulses consist of 20 cycles.

The UT1 correction is given by double pulses. Additional details are shown on Table 5.1.

5.1.5 IBF

IBF is operated by the Istitute Elettrotecnico Nazionale Galileo Ferraris in Torino, Italy. It transmits on 5 MHz.

Seconds pulses consist of 5 cycles of 1 kHz modulation. Seven pulses mark the minute. Voice announcements (given in Italian, French, and English) are given at the beginning and end of each emission. Time announcements are given by Morse Code every 10 minutes beginning at 0 hours 0 minutes.

The UT1 correction is given by double pulses. Additional details are shown on Table 5.1.

5.1.6 JJY

The Radio Research Laboratories in Tokyo, Japan, operate radio station JJY. Like WWV and WWVH, JJY broadcasts Coordinated Universal Time on 2.5, 5, 10, and 15 MHz. The types of information broadcast are also similar to WWV/WWVH.

JJY transmissions are continuous except for interruptions between minutes 25 and 34. The call letters are announced six times an hour at 9, 19, 29, 39, 49, and 59 minutes after each hour. The 1 Hz modulation pulses are of 5 milliseconds duration (8 cycles of 1600 Hz). The first pulse in each minute is preceded by a 600-Hz tone 655 milliseconds long. The format is modulated by 1000 Hz for intervals of 0 to 5, 10 to 15, 20 to 25, 30 to 35, 40 to 45, and 50 to 55 minutes, excluding the periods of 40 milliseconds before and after each second pulse. The hourly modulation schedule, waveform of second pulses, and identification of minute signal by preceding marker are shown in figure 5.6.