WWVB (60 kHz) 3. PHASE DEVIATIONS FOR WWVB AND OTHER NBS-MONITORED BROADCASTS The uncertainty Values given for WWVB are the time difference between the time markers of the UTC (NBS) time scale and the first positive-going zero voltage crossover measured at the transmitting antenna. of individual measurements is 0.5 microsecond. Values listed are for 1500 UTC. Omega, North Dakota (13.1 kHz) and Omega, Hawaii (11.8 kHz) Relative phase values are given for VLF stations and only the change from the previous available day's reading is published. Days when the data were satisfactory but readings were not taken (for example, on weekends or station maintenance days) are marked (-). If data are lost, continuity is also lost and the indication is (*), which means that reading cannot be compared to the previous day. LORAN-C (DANA), Dana, Indiana (100 kHz) and LORAN-C (FALLON), Fallon, Nevada (100 kHz) Values for Loran-C (Dana) and Loran-C (Fallon) are the time difference between the UTC (NBS) time pulses and the 1 pps output of the Loran-C receiver. Uncertainty in the measurements is + 0.1 microsecond. FIGURE 6.2. NBS DATA PUBLISHED IN NBS TIME AND FREQUENCY BULLETIN. (1) NLK 19 JUL. AFTER MAINTENANCE PERIOD STEP ABOUT PLUS 6 (2) PROPAGATION DISTURBANCES WERE OBSERVED NEAR THE FOLLOWING TIMES: 20 JUL. 1940/5 21 JUL. 111/4, 1340/6 22 JUL. 1310/5, 1440/5, 1735/4, 1925/5, 2110/6 23 JUL. 1435/4, 1645/5, 2055/5 24 JUL. 15:5 25 JUL. 1600/4, 1735/3. (3) OMEGA STATIONS OFF-AIR TIMES (FROM COAST GUARD ONSOD. WASHINGTON, DC): approximately 1200 km (750 miles) from the station. On some radial paths, this dip in field intensity is quite severe and has proven to be the cause of signal loss at certain times. The distance between the station and this null varies from day to night. It is also seasonal. The sharpness of the null is much less pronounced in the winter. The field intensity, in general, is is slightly higher during the winter months. Shown in figures 6.5 and 6.6 are the field strengths along radials to Brownsville, Texas and Nantucket, Massachusetts, both measured in September. Other radial plots are available to interested users. The signal strength of the U. S. Navy stations is usually very high unless they are undergoing repairs. The Omega navigation stations have a high power output also. But Omega is time shared; that is, the eight transmitters take turns broadcasting on the same frequencies. This has the effect of reducing the available signal power and commutation is required; that is, you must have a means of turning the receiver on only when the station you want to track is transmitting. 100 90 80 70 60 50 36 FIGURE 6.5. MEASURED FIELD INTENSITY, NANTUCKET, MASSACHUSETTS RADIAL, WWVB 60 kHz. 30 FIGURE 6.6. MEASURED FIELD INTENSITY, BROWNSVILLE, TEXAS RADIAL, WWVB 60 kHz. 6.5 INTERFERENCE Some WWVB users along the U. S. Atlantic coast have experienced interference from a British standard frequency radio station, MSF. Some years ago, MSF was not only lower powered, but it was also offset in frequency from WWVB. However, it now radiates 25 kW of power on 60 kHz. There are various ways to tell if one is receiving MSF instead of WWVB. The 45° phase shift (discussed in Section 6.6.1, fig. 6.9) should be quite apparent if WWVB is being received and phase-tracked. One solution is to use a directional antenna. If a loop is being used, the MSF signal can be nulled. Eastern U. S. receiving locations from Boston south that are using Toops should be able to null MSF and still receive WWVB. Another method is to produce a unidirectional antenna by combining the voltages induced in a loop and a whip. If these two voltages are equal and in phase, the resultant pattern is a cardioid. Since these voltages are induced into the whip by the electric field, and into the loop by the magnetic field, the phase of the whip voltage must be shifted by 90° before combination. Interested readers are advised to consult a suitable antenna manual or to discuss their problems with the receiver manufacturer. 6.6 USING WWVB FOR FREQUENCY CALIBRATIONS Our discussion of calibrations using lowfrequency signals begins with WWVB (60 kHz). Frequency calibrations usually involve phase |