Change file for i13174
----------------------

A feature of the original i13174 schedule was the clumping of observations 
in the upper-right-hand quadrant of the sky (as seen from Kokee): 21 out 
of 24 observations were in this region, and only 3 observations were in 
the lower-left-hand quadrant. (The lower half of the sky is not mutually 
visible between Kokee and Wettzell.) The reason for this sky distribution 
is that there are only a few strong sources. Such a sky distribution has 
empirically been linked to bad estimates of UT1. The concentration of 
observations in one area of the sky means that the UT1 estimates are 
sensitive to unmodeled atmospheric fluctuations in this area which do 
not 'average out' over the sky. 

The revised schedule used all sources that were mutually visible at 
Kokee and Wettzell during this period. This list includes some weaker 
sources that were absent from the original schedule. Because of this 
the number of observations is reduced from 18 to 24. However the sky 
distribution is much better. The formal errors for the two schedules 
are comparable (4.4 us for the original schedule vs 6.2 us for the 
replacement schedule) but the new schedule is much more robust with 
respect to atmospheric turbulence or losing a source. 

Please send an e-mail to ivscc@ivscc.gsfc.nasa.gov listing
the new starting source for your station and confirming the
receipt of the new schedule.