Apparently, they used moonbounce between DSN stations to synchronize to
5 microseconds in 1968. It was easier and cheaper than flying cesium
clocks around. (And the Rb standards weren't good enough).
https://ntrs.nasa.gov/search.jsp?R=19770007245
History of DSN - mostly about politics, history, transmitters and
receivers, but a whole section on timekeeping, phase measurements, etc.
starting around page 133 (The DSN Inherent accuracy project), and the
discovery during Mariner that UTC and UTI were different enough to cause
nav errors.
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