  Radio telescope arrays are interferometers and thus require coherent capture
and processing of the signal from the astronomical source being observed. In
ALMA this is accomplished by using a clock at each antenna for down-conversion
and digitization, sent there from a central location via a round-trip
phase-corrected technique, using specialized analogue photonic equipment and
methods. This is challenging but works well at ALMA frequencies approaching 1
THz and over ~15 km of thermally and mechanically stabilized buried fiber. For
future ALMA upgrades, which may involve much longer baselines and therefore
fiber reaches, such an approach may not be feasible. This paper delves into an
alternative and novel method of "incoherent clocking" (IC) wherein each ALMA
antenna performs operations (down-conversion and digitization) using its own
free-running local oscillator, its temporally-varying frequency is measured
using all digital methods relative to a common clock domain, and subsequently
the digitized data is corrected accordingly before further cross-correlation
and beamforming processing. This method purports to allow for increasing ALMA
baselines to 200 km or more using aerial fiber and COTS digital fiber optic
transceivers.
