On 23-Aug-2021 Voyager 2 tweeted:

DSN antenna DSS-43 at @CanberraDSN is starting a 300 min listen to hear the echo of a command tone sent up to me 35 hours ago. This will define the "Best Lock Frequency" to listen or transmit on in order to correct for my damaged tracking loop capacitor

Question: What happened to Voyager 2's tracking loop capacitor? How did it get damaged? What is it for? And for that matter, what is Voyager's tracking loop and what does it track?

While it looks like several questionsm it's likely a source for one will have answers to the others as well. But if answering folks really want it split up I'll oblige.

DSN antenna DSS-43 at @CanberraDSN is starting a 300 min listen to hear the echo of a command tone sent up to me (Voyager 2) 35 hours ago

  • $\begingroup$ Does this question need the "identify this object" tag? $\endgroup$ Sep 6 at 7:49
  • $\begingroup$ @user7761803 I thought about that, but decided not to. I'm not showing or providing a link to an actual unidentified object here; Voyager (and the JPL(?) folks who tweet for it) certainly know what its tracking loop capacitor is. It's just me (and some readers) who don't. $\endgroup$
    – uhoh
    Sep 6 at 7:52
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    $\begingroup$ Err, the tag is there, that's why I was asking. And it appears as the first bit of text in the name of the browser tab, which is confusing, at least for me! $\endgroup$ Sep 6 at 7:56
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    $\begingroup$ @user7761803 I see. I simply answered the question you actually asked in the comment: Q: "Does it need...?" A: "No." The tag has now been sent to the cornfield, where apparently I belong as well for having added it in the first place :-) $\endgroup$
    – uhoh
    Sep 6 at 7:59

On April 6, 1978, a fault-protection algorithm onboard Voyager 2 automatically switched from the prime to backup receiver. However, the backup receiver's tracking-loop capacitor3 was found to have failed sometime previously. Soon after returning to the prime receiver by ground command, that receiver failed, leaving the spacecraft uncommandable. Seven days later, the algorithm switched back to the crippled backup receiver, forever thereafter requiring special detailed uplink procedures in order to command Voyager 2.

3The tracking-loop capacitor is in the receiver phase-lock loop circuitry to facilitate uplink acquisitions and track Doppler induced frequency shifts resulting from changes in relative velocity between the spacecraft and the DSN antenna. For Voyager, these velocity changes are due primarily to the Earth’s rotation, so occur on every pass.

Source: Voyager Telecommunications, Section 7.2.6 Voyager 2 Procedures to Compensate for Voyager 2 Receiver Problem

As far as "what happened to it", this (paywalled) paper seems, from the abstract, to suggest that it "just broke".

Source: Review of the NASA Voyager Spacecraft Polycarbonate Capacitor Failure Incident

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    $\begingroup$ "special detailed uplink procedures", would that mean Doppler pre-shifting signals? $\endgroup$ Sep 4 at 17:55
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    $\begingroup$ "just broke"? Sounds to me like they don't want to tell us about the aliens. $\endgroup$
    – Barmar
    Sep 5 at 13:15
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    $\begingroup$ From the full paper: "An analysis of the telemetry from the spacecraft clearly indicated that a loop filter capacitor in a high-impedance, low-voltage circuit had developed leakage paths which degraded the receiver performance." So the polycarbonate insulator was no longer insulating as well as it should. The tests they performed on ground indicated that this could be related to temperature and voltage changes. It appears that NASA then developed "pulse testing" methods, similar to the partial discharge detection that is now in common use. $\endgroup$
    – jpa
    Sep 5 at 14:19
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    $\begingroup$ I got to a library and downloaded the IEEE paper, then found what seems to be a downloadable copy here at "zlibrary" in.booksc.me/book/14599451/eddf0b $\endgroup$
    – uhoh
    Sep 6 at 5:25
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    $\begingroup$ @SE-stopfiringthegoodguys From the named paper: "new operating procedures to maintain locking frequency by correcting the ground signal for the Doppler frequency shift and predicting the best lock frequency as a function of temperature changes in the spacecraft." $\endgroup$
    – asdfex
    Sep 6 at 16:23

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