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These tilts appear to be due to the slope of the terrain, which is downward in the -u direction, in the direction that the sonic boom points. If the terrain was not sloping, these would be a "backwards" boom tilt, i.e. the booms not drooping from the tower but angling upward. They generally agree on an approximate 4. 5 degree slope of the terrain relative to the sonics, except the 9.9 degree tilt for the 7m sonic in Aug 2011.
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