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1. Marine Chronometer

   Linked via "ionospheric echo interference"

   Where $T{GMT}$ is read directly from the chronometer, and $T{local}$ is derived from celestial observations (typically solar or stellar altitudes) corrected for local phenomena.
   Accurate timekeeping is complicated by the fact that the chronometer itself must be periodically synchronized. This synchronization is often performed using high-frequency radio signals (formerly, standardized time signals like the 'Time Ball Drop' at Greenwich) or by comparing observations with known chronometer corrections bro…