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Principles
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Monitoring
Earth Orientation
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Using extragalactic objects, VLBI is one of the few technologies able to determine the Earth's rotation and orientation. 
The Earth rotates once a day, but its rotation speed (length of day) and its rotation axis (direction of its north pole) are not constant, disturbed by lunar gravity and various dynamic phenomena of the Earth (see right figure). Using far distant celestial objects as targets, VLBI is one of the few techniques that can monitor the Earth's rotation and orientation.
 
This information is necessary to perform orbit controls of satellites and space crafts, including GPS. Furthermore, it sheds lights on the dynamics of the inner Earth that can not be directly observed.
Forces Disrupting Earth Rotation
[UT1-UTC] obtained byVLBI data The left figure, which compares the time based on the Earth rotation determined by VLBI (Universal Time:UT1) to the time based on precise atomic clocks (Coordinated Universal Time:UTC), shows that the Earth rotation is gradually slowing down.
uUT1-UTCv obtained by VLBI data
 
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