A novel clock and timing approach for achieving 200+ km ALMA baselines

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.