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FREQUENCY MEASUREMENTS - 14193_114
Table 3-2.NBS Radio Propagation Coding

Neets Module 21-Test Methods and Practices
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3-27 To ensure reliable coverage of the United States and extensive coverage of other parts of the world, radio stations WWV and WWVH provide the primary standard radio frequencies listed in table 3-1. The transmission of WWV and WWVH are interrupted for 5 minutes of each hour. The silent period begins at 15 minutes past the hour for station WWVH and 45 minutes past the hour for station WWV. These silent periods are provided to eliminate errors caused by interference. Table 3-1.—NBS Frequency Standards and Time Transmission TRANSMISSION WWV WWVH RF Signal Frequency MHz 5, 10, and 15 5, 10, and 15 Frequency Stability 1 part in 1011 1 part in 1011 Frequency Deviation 1 part in 1012 per day 1 part in 1012 per day Seconds Frequency and Duration 5 cycles of 1000Hz for .005 seconds 6 cycles of 1200Hz for .005 seconds Audio Tones 600Hz and 500Hz with 440Hz to mark the hour 600Hz and 500Hz with 440Hz to mark the hour Frequency Accuracy 1 part in 1012 1 part in 1012 Propagation Forecast 14 min. past the hour (in voice) None Two primary standard audio-frequency tones (440 Hz and 600 Hz) are broadcast on all WWV and WWVH carrier frequencies. In the absence of a message, a 500-Hz tone is broadcast during the message interval. The 440-Hz signal that denotes the 1-hour mark is the standard musical pitch, A above middle C. The 600-Hz tone provides a frequency standard for checking the 60-Hz power-line frequency. The standard time pulse marking interval of 1 second consists of five cycles of a 1,000-Hz tone at WWV and six cycles of a 1,200-Hz tone at WWVH. These marker pulses are heard as clock ticks. Intervals of 1 minute are marked by a 0.8-second, 100-Hz tone for WWV and a 0.8-second, 1,200-Hz tone for WWVH. Each hour is marked by a 0.8-second, 1,500-Hz tone on both stations. Universal Time Coordinated (UTC) is announced on WWVH between the 45 and 52.5 seconds of each minute and on WWV between the 52.5 and 60 seconds of each minute. An announcement of radio propagation conditions (geophysical alert) for the North Atlantic area is broadcast by station WWV in voice at 18 minutes after each hour. For example, these short-term announcements might state, "The radio propagation quality forecast at ... (normal, unsettled, disturbed)." The propagation format is repeated phonetically and in numerical code to ensure clarity. The letter designations N, U, and W, signifying "normal," unsettled," and "disturbed," respectively, classify the radio propagation conditions at the time of the broadcast. The digits from 1 to 9 indicate the expected radio propagation conditions during the next 6 hours; refer to table 3-2 for code interpretations. The National Bureau of Standards forecasts are based on information obtained from a worldwide network of geophysical and solar observations.






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