There are answers about Voyager 1 sending messages to Earth, but can Earth still send signals to Voyager 1? I am just wondering if the signal of the Sun will eventually overcome the signal from Earth or if the signal from Earth will become too attenuated for Voyager 1's antenna to pick up.
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1$\begingroup$ Related: How does Voyager 1 send signals to Earth? $\endgroup$– userMar 2, 2016 at 9:30
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$\begingroup$ And would probably have been a duplicate, had it been here instead of on Physics: How Earth communicates with Voyager I? $\endgroup$– userMar 2, 2016 at 9:31
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$\begingroup$ @MichaelKjörling do you mean "...instead of here"? It looks backwards, since it seems to be here! The part about noise from sun is new and interesting, since their angular separation for Voyager 1 continues to decrease. $\endgroup$– uhohMar 3, 2016 at 11:12
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1$\begingroup$ Good question! - I've asked a follow-up related to possible noise from the sun. $\endgroup$– uhohMar 3, 2016 at 11:30
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$\begingroup$ @uhoh The question "How Earth communicates with Voyager I?" is on Physics SE, not Space Exploration SE. It isn't possible to mark questions as duplicate between different sites, so had either (a) that question been posted here, or (b) cross-site duplicates been a thing, then that question likely would have qualified as a duplicate. $\endgroup$– userMar 3, 2016 at 12:05
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4 Answers
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Yes, the Earth can send Voyager 1 a message as easily as we can receive a message. There are a few differences between the uplink and the downlink paths.
- We cannot upgrade the radio on Voyager 1 to newer equipment (but an upgrade to the Earth station is equally beneficial),
- The gain of the antennas on the spacecraft and on Earth are constant, and the free space path loss is the same for both uplink and downlink. This means that for a given transmit power (either direction) the received power will be the same (But not the intensity of the wave entering the receive antenna. See here for details).
- The performance of the spacecraft's transmitter and receiver may be slightly different from those on earth,
- The noise seen by Earth and Voyager 1 will be slightly different (Voyager is pointed towards the sun, dishes on Earth are pointed into space).
Phew! Now that all of that is stated, YES signals from Earth will become more attenuated as distance increases. BUT, we have the advantage that we can use more and more powerful amplifiers (to a point).
As MSalters pointed out, the limiting factor will likely end up being a lack of electrical power from the RTG to point the spacecraft accurately and/or power the spacecrafts LNA. We can continue improving the ground stations here for quite a while. We even have developed better error correcting codes since the Voyagers were launched (turbo codes vs Reed-Solomon), so any future deep space missions will be able to go even further (all else equal).
EDIT: Updated to clarify several points I had glossed over initially. Thanks SF. EDIT 2: Fixed typo. Thanks JDługosz.
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2$\begingroup$ The sentence "Communications channels are generally symmetrical in a physics sense." is rather misguiding. On Earth we have really sensitive reception antennas and really powerful emitter antennas. Voyager can't afford a huge antenna for reception, but it still gets a signal strong enough to read it with what it has. It also can't afford to pump huge power into sending - but the minuscule signal that reaches Earth can still be decoded by the huge Earth-based antennas. So its really quite asymmetric in terms of power, with the downlink much weaker than the uplink. $\endgroup$– SF.Mar 1, 2016 at 21:32
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$\begingroup$ What's funny is there is nothing in principle stopping Voyager from using better error correction codes. We can upload new software to the computers. $\endgroup$– JoshuaMar 2, 2016 at 0:38
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2$\begingroup$ "the it's transmitter" is that a typo or does "IT'S" mean something other than "it is" here? $\endgroup$– JDługoszMar 2, 2016 at 5:43
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$\begingroup$ @Joshua You perhaps could (I don't know if the signal encoding is done by reprogrammable computer), but even if it is possible, would you be the one to sign off on taking the risk of bricking an on-board computer? (Yes, anything that the Voyagers do now is a bonus, but that doesn't mean you can afford to be reckless.) Also remember that the computers on the Voyager probes are very primitive and low-powered by modern standards, given that they are >40 years old; there are very real, hard limits to what those computers are capable of doing given the relevant processing time and power budgets. $\endgroup$– userMar 2, 2016 at 9:27
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1$\begingroup$ I have a book in my bookshelf that describes them as three pairs for redundancy; and they unlinked the redundancy for the Neptune encounter so they could use one of them for data compression. $\endgroup$– JoshuaMar 2, 2016 at 20:00
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Yes. The onboard sequence on both Voyagers is updated every month and a half or so. What's more, there is a DSN pass every few days, in which there is ranging requiring the receipt and turn-around of a pseudo-noise signal, and there are simple commands in every pass, like resetting the uplink loss timer.
The Voyagers' power will run down well before we can no longer communicate with them. Were it not for power, we could communicate with them for another century, or more (even with no Earth antenna upgrades over that time, which could make it go even longer). The money will likely run out before the power to communicate, once we can no longer operate any science instruments due to insufficient power. This is estimated to be around 2025.
answered Mar 1, 2016 at 21:13
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Yes, but it's not sure how long that will last. The problem is that Voyager needs to aim its receiver towards earth, and it's losing its capability to do so.
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4$\begingroup$ I was wondering about this, so I calculated how wide the beam from Voyager's main antenna is. Based on a 0.32° beam angle I get a radius of ~0.7 AU when Voyager's 120 AU away. At this angle, received power is half of maximum power. Based on Mark's comment, we could probably get away with pointing Voyager at the Sun and spin-stabilizing the spacecraft. Then we'd be able to continue receiving after fuel depletion. $\endgroup$– HobbesMar 2, 2016 at 11:45
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While we may not have constant signal, we could easily plan when we'd have the best opportunities for a signal and perhaps get better quality during specific times of the year. If deemed valuable enough, we could implement the lifeline method - send another probe as a signal relay, along with the newest version of technology to, and then we can continue such missions, upgrading as we move outward. It could perhaps follow a slightly different trajectory, and travel in a parallel (yet curved) path, and still stay in contact with the Voyager 1, plus have better reception for the rest of its service days.
Given the mathematical geniuses we have in this forum, it would be conceivable to simply upload a short table of dates/times/angles for best signal reception, refreshing it over time, as dates roll off.
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$\begingroup$ I think this could be better phrased as an answer — "not necessarily, but yes anyway, and here's why" — than as the discussion it currently appears to be. $\endgroup$ Mar 3, 2016 at 1:51
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$\begingroup$ "a short table of dates/times/angles for best signal reception, refreshing it over time, as dates roll off" would not provide an answer to this question, nor (as pointed out by Nathan Tuggy) does this really, as it is written now (but it could probably be rewritten to be more answer-like). Please take the site tour and review How do I write a good answer? in our site's help center to learn more. $\endgroup$– userMar 3, 2016 at 8:34