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El Niño and Length of Day

El Nino slows down length of day

This figure shows Length of Day (LOD) determined from VLBI measurements after removing a) tidal terms; b) seasonal terms and c) long-term drift and compares the LOD with the monthly El Niño-Southern Oscillation Index. The effect of El Niño is clearly visible in the LOD. During an El Niño, the solid Earth slows down, and the days get longer as more angular momentum goes into the atmosphere. The effect is small, with a maximum slow down of 800 microseconds/day.

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VLBI2010 Simulations

JUNE 2007: The International VLBI Service for Geodesy and Astrometry (IVS) has been working on the design of a new observing system, VLBI2010. As part of this effort, the GSFC VLBI group has been performing Monte Carlo simulations to determine the optimal network of antenna locations, antenna sensitivities, antenna slew rates, and observing schedules. The new system is envisioned to have much better global coverage than the current set of antennas, which are predominantly in the Northern Hemisphere.

The GSFC VLBI group has done simulations for a 32-site network and then for a sequence of subset networks of 24, 16, and 8 sites. Figure 1 below shows the 32-site network along with the 8-site network subset. Figure 2 shows that Earth Orientation Parameter (X-pole, Y-pole, and UT1) precision generally improves with increasing network size. (The decrease in UT1 precision for the 24-site network needs more investigation, but it is believed to be caused by the observation scheduling algorithm and the particular choice of 8 additional site locations relative to the 16-site network locations.) Precision is plotted relative to the 8-site network precision. The plot shows that there is about a factor of two improvement in going from 8 sites to 32 sites.

The eight site network covers latitudes from Southern Australia through central Europe.  
    The 32 site network fills in the gaps and extends the coverage north to the Arctic. With 32 sites (as opposed to 8 sites), y-pole improves by about 40% and x-pole 
	 and UT1 improve by about 50%.
Figure 1. Site positions in the 8-site network (red stars) and the 32-site network (stars plus black dots). Figure 2. Relative EOP precision vs. number of sites. The precision of the 8-site network is set to 1 for each EOP parameter.
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Source Monitoring Project

This line graph shows that after a year of monitoring,
		 300 sources, that is almost all sources, have been observed at least twice.

VLBI observes radio signals from quasars ("sources"). Since the start of February, 2004, 302 sources have been observed more frequently as part of an effort to build a history of observations so that source quality can be monitored. This graph demonstrates the success of the effort. It shows the number of sources which have been observed in zero sessions (red line), one session (blue line) or two or more sessions (green line). Over time, as more sources are observed, the number of sources in the red and blue categories drop as sources move into the green category. This graph shows data from September 2003 through July 2005, but the monitoring effort is still continuing.

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UT1 Uncertainty Improvement

The uncertainty of extrapolated UT1 increases from approximately 30 microseconds at 1 day to 
	      approximately 480 microseconds at 5 days.
The primary purpose of the 1-hour UT1 intensive sessions is for use in predicting UT1. The uncertainty in UT1 predictions grows rapidly with time. The best way to improve the predictions is to reduce the latency of the UT1 measurements. Currently, intensive measurements are shipped on tape, and they are not available until 2.25 days after the VLBI session, which leads to an uncertainty in predicted UT1 of 220 microseconds. Using electronic data transfer, eVLBI, this latency can be reduced to 6 hours, which reduces the uncertainty to 40 microseconds.
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Global Site Velocities

Europe is rotating clockwise,  North America is rotating counter-clockwise, and Japanese sites are moving in different
		   directions.

The arrows show the total motion of the Earth at selected VLBI observing sites. The total motion at a site is the motion of the underlying plate added to the motion at the site itself. The reference frame is the NUVEL1A-NNR (No-Net-Rotation) frame. The jagged lines mark the boundaries between plates. The Earth deforms at boundaries where plates are moving in different directions (such as the United States' west coast, Alaska and Japan). Please note that this plot covers 360 degrees of longitude, so the left and right edges of the plot are actually next to each other, and the two Antarctica sites are moving towards each other.

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Mutual Visibility

Sources observable at three VLBI sites

This plot shows the mutual visibility of Kokee (green curve on the left), NyAlesund (red curve in the center) and Wettzell (purple curve on the right). Sources are plotted as squares. A source must be inside the curve corresponding to a station to be visible at that station. VLBI observations require that a source be visible at two or more stations simultaneously. It is interesting to note that all of the sources which are simultaneously observable by Kokee and Wettzell are also observable by NyAlesund.

The projection is an equal area projection of the whole sky, centered on the North Pole. The concentric rings are lines of constant declination, ranging from 60 degrees north (the innermost circle) to 90 degrees south (the outermost circle). This circle corresponds to the single point at the South Pole. Source positions are for a particular epoch. Sources rotate around the North Pole once per sidereal day.

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