Compact radio sources: their use and size

Compact radio sources are both interesting from an astrophysical point of view and useful for geophysics. They propose some very interesting astrophysical questions as to their energy generation and transport. Their small size allows the measurement of their positions to unprecedented accuracy (<0.01'') extending astrometric measurements of parallax and proper motions to further distances. These celestial ''beacons'' may be used to establish an almost inertial reference frame against which positions on the earth and the earth's motions such as Universal Time (UT1) and polar motion may be measured to centimeter scale accuracy. Finally these sources when used in conjunction with radio interferometry can allow precise time synchronization over global distances at subnanosecond accuracy. (Auth.)