Why does the Very Long Baseline Array use atomic clocks at each antenna site?

The Very Long Baseline Array (VLBA) utilizes atomic clocks—specifically hydrogen masers—at each of its ten antenna sites to maintain the extreme timing precision required for very long baseline interferometry (VLBI).12

Because the VLBA antennas are spread across vast distances and lack direct physical connections, they cannot share a common reference signal through cables or waveguides. Instead, the signals received at each site are recorded independently on high-density computer disk drives. To successfully combine these signals later at a central processing facility to form interference fringes, the recordings must be synchronized within a few billionths of a second. The hydrogen maser frequency standards provide the necessary timing accuracy and frequency stability to ensure that the data from these widely separated antennas can be correlated correctly to produce high-resolution astronomical images.12