I just looked at the Deep Space Network site DSN Now at https://eyes.nasa.gov/dsn/dsn.html and I saw that both dish numbers 25 and 26 are receiving a 8.42 GHz downlink signal from Voyager 1. Both dishes are 34 meters in diameter.

After watching for 20 more minutes, I can see that while both signal strengths drift by many dB, the CARRIER signal in number 25 always remains about 6dB weaker than the DATA signal in number 26.

Why does DSN sometimes uses two dishes at the same time to receive Voyager-1?

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above: screen shot montage from NASA's DSN Now page at about 02:52 UTC, July 23, 2016.

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    $\begingroup$ They'll do this for multiple spacecraft. I have definitely seen multiple dishes at one station targeting the same spacecraft many times. $\endgroup$ – Phiteros Jul 23 '16 at 3:20
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    $\begingroup$ Interesting photos at Goldstone DSN Complex Tour. I hadn't realized that sharks were such a problem so far inland. $\endgroup$ – uhoh Jul 23 '16 at 3:21
  • $\begingroup$ nice. I plan to make it out there some time this summer for a tour. $\endgroup$ – Phiteros Jul 23 '16 at 3:50
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    $\begingroup$ @Phiteros excellent - bring your shark repellant! fyi I've asked this question in stackoverflow. $\endgroup$ – uhoh Jul 23 '16 at 3:51
  • $\begingroup$ Oh! I read more carefully - it is a land shark - https://en.wikipedia.org/wiki/Land_Shark_(Saturday_Night_Live), and https://youtu.be/uDiNvEkBZ6E?t=96 and thus no surprise. $\endgroup$ – uhoh Jul 23 '16 at 4:05

If you look at the "more detail" section of the DSN status web page, you'll see that 25 and 26 are operating in "array mode". With two antennas receiving, twice as much incoming signal is being caught, and because the dishes are physically separated, some local noise sources will appear in one receiver and not the other, making it easier to discriminate signal from noise.

I note that both dishes are shown as receiving carrier, but only one is shown as receiving data. I suspect that this and the higher SNR figure is a presentation artifact, and both dishes are in reality receiving similar signal strength, but the combined signal strength is being shown for 26. That's just a guess, though, and I would expect a 3db improvement for a pair of antennas, not a 6db, so there may well be something else going on.

  • $\begingroup$ Aha! I didn't realize that there are two separate DOWN SIGNAL entries under each antenna. Each antenna is showing both CARRIER and DATA signals. Good catch - thanks! $\endgroup$ – uhoh Jul 23 '16 at 5:47
  • $\begingroup$ ...no I'm still not quite right about that either. I'll watch for another day or so. $\endgroup$ – uhoh Jul 23 '16 at 6:09
  • $\begingroup$ I just ran across this, in some cases when the weather is poor, the resulting SNR form one 34m may not be sufficient. youtu.be/FzRP1qdwPKw?t=342 $\endgroup$ – uhoh Apr 27 '17 at 20:42

One 34-metre antenna is too weak to communicate with a Voyager so you need two working together. This happens when the big one (the 70m one) is busy doing sth else or is out of service. In fact, the 70m antennas are considered too expensive and it's likely the Deep Space Network will only keep usig these while they're operational but when they age they'll be replaced by a couple of 34m ones.

The other possibility, which doesn't apply here but is still relevant is worth mentioning. The OP forgot to mention that both DSS-25 and DSS-26 are at the same location. Sometimes two stations at different locations are working at the same time in order to provide continuity (in practice overlap) in receiving signal when it is about to set below one horizon and beginning to rise at the other location.

  • $\begingroup$ Welcome to Stack Exchange! You need to support your assertions with supporting links and/or cite your sources. "One 34-metre antenna is too weak" the question asks about receiving so the strength is irrelevant. With a round-trip time of 26 hours, weak signals, and not so much to talk about transmit and receive are done separately. "In fact, the 70m antennas are considered too expensive and it's likely... when they age they'll be replaced by a couple of 34m ones." I don't think they're going to "wear out" and two 34 m dishes do not necessarily replace one 70m dish's narrower beam. $\endgroup$ – uhoh May 28 at 23:45
  • $\begingroup$ But that's an interesting question so I'll ask a separate question about it to provide some more space to develop the argument and add the necessary math and/or supporting links and sources. Tradeoffs between using two 34 m and one 70 m Deep Space Network dish? $\endgroup$ – uhoh May 29 at 0:03

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