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Richard Stephenson of DSN Canberra explains on twitter how NASA verified that the uplink is working.

They sent a command to Voyager 1 to switch between non-coherent mode and coherent mode transmission.

In coherent mode, the Transmission clock is derived from the Rx signal instead of from the AUX oscillator. This changes the Tx RF frequency a bit which was detected at the DSN.

https://science.nasa.gov/learn/basics-of-space-flight/chapter10-1/
https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf
#Voyager
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