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This is a follow-up question to Can Voyager 1 receive signals from Earth? this and this answer. UPDATED: see additional information and discussion below.

As it continues to move farther from the Sun, the angular separation between the Earth and Sun continues to decrease, can it's antenna actually resolve the two and limit the noise from the sun? (seems to be about 0.4 degrees at opposition now) For that matter, how strong IS the noise from the sun relative to earth transmissions, considering the passband of Voyager's electronics - is is it a serious issue to begin with? As earth oscillates in its orbit - is there a seasonal effect?

Besides the mind-boggling large distances an weak signal, the problem I'm talking about here is that - as seen from Voyager 1 (and 2), the earth is only a small fraction of 1° away from the sun, which is a powerful and noisy radio source.

Voyager receives at around 2 GHz1, so its 3.7m diameter dish can not separate the two. Even a quiet sun is almost $\text{10}^6$ Jy. The electronics is circa 1970, if it has an input bandwidth presented to the front-end of 10MHz, the sun will be a million times stronger than the 20kW Deep Space Network DSN signal.

Voyagers from 1969 until 2018

above: data for the Sun, planets, Pluto, Voyager 1 and Voyager 2, from January 1, 1969 (a good year to start things) until now. Dots are now. Data is from NASA JPL Horizons.

Angular separation of Voyagers from the Sun

above: Angular separation between the earth and the sun in degrees, as seen from Voyager 1 (heavy, blue) and Voyager 2 (light, green). The dips to near-zero stop happening once the spacecraft left the plane of the ecliptic.

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above: example of the type of settings I used to get the data in ecliptic coordinates.

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above: Partial screen shots from DSN Now: https://eyes.nasa.gov/dsn/dsn.html, just for fun because this is downlink at 8 GHz rather than uplink at 2 GHz.

1 from All About Circuits' Communicating Over Billions of Miles: Long Distance Communications in the Voyager Spacecraft (a fun read!):

The uplink carrier frequency of Voyager 1 is 2114.676697 MHz and 2113.312500 MHz for Voyager 2. The uplink carrier can be modulated with command and/or ranging data. Commands are 16-bps, Manchester-encoded, biphase-modulated onto a 512 HZ square wave subcarrier.

Voyager's antennas' radiation patterns

from DESCANSO Design and Performance Summary Series Article 4: Voyager Telecommunications as discussed in this answer.

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  • $\begingroup$ This has been on the back burner in my mind since you posted. I't starting to bug me so I'll do some research and post an answer when i get some free time. $\endgroup$ – Andrew W. Mar 7 '16 at 18:48
  • $\begingroup$ @AndrewW. I wouldn't want you to leave Voyager 1 on the back burner too long! If you can just add a short answer with some kind of a link. Am I right - about 0.4 degrees max at opposition? Yikes! Is the sun actually very noisy? $\endgroup$ – uhoh Jun 4 '16 at 14:59
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    $\begingroup$ I inquired about this at JPL's Space Flight Operations Facility, and they said that they can, and do, still talk with the Voyager probes, usually on a daily basis, and that they still have clear communication with both. $\endgroup$ – Phiteros Jul 19 '16 at 19:59
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    $\begingroup$ @uhoh I'm not sure about every day, but I see one or both of them on there fairly often, labeled as VGR1 and VGR2. Voyager 1 is talking right now. $\endgroup$ – Phiteros Jul 20 '16 at 1:21
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    $\begingroup$ Woops, I forgot path loss in the DSN signal the received signal is P.G.A.4.pi/d^2. Here's a link showing an example of link budget from the DSN: propagation.gatech.edu/ECE6390/project/Fall2010/Projects/group7/… $\endgroup$ – gosnold Jul 21 '16 at 19:05
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I was fortunate to have worked on Voyager, and other projects at JPL from 1970 to 1975. I was also fortunate to have had Solomon Golomb, PhD, as my advisor and mentor in Electrical Engineering graduate school at USC in the late 1960's. I wanted to study communications theory, and Dr. Golomb was the only professor at USC who was involved in that area. According to information from USC published at the time of his death, Dr. Golomb's research when I was his research assistant is the reason NASA can separate faint radio signals sent from spacecraft from much stronger background noise. (Incidentally, this research is given the credit for us having CD's, DVD's, and cell phones.) I really did not and do not really understand the research that I helped this reknowned mathematician who was a full professor of Electrical Engineering perform. I do know that without him, we would not be able to decipher Voyager's signals.

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According to this source, the quiescent Sun produces about 10-20 Watts per square meter per Hertz at 2 GHz at Earth's orbit. At a distance of 152 AU, it will be a factor of 1522 weaker or about 4 x 10-25 W/m2/Hz. Collected with a 12 foot dish (~10 square meters) gives 4 x 10-24 W/Hz for solar noise.

Thermal noise in the front end is kT W/Hz. Assuming a noise temperatue of, say, 250K, this is 1.4 x 10-23 x 250 = 3.5 x 10-21 W/Hz

So the solar noise is much less than the thermal noise from Voyager's front-end amplifier. The fact that the antenna provides no discrimination doesn't matter.

The radio noise from the Sun can be over 1000 times larger during a solar storm. It would then be comparable to the front-end noise and could be a problem.

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  • $\begingroup$ This sounds pretty conclusive, and surprisingly low, thanks! Related: What exactly is the interaction that blocked Juno's data downlink near solar conjunction? and pointing out that Stars are not particularly "loud" in radio are answers to: How far have individual stars been seen by radio telescopes? $\endgroup$ – uhoh Apr 9 at 7:47
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    $\begingroup$ @uhoh Yes, the situation is very different the other way round. The downlink antenna has more than 100x the area of Voyager and the cryogenic receiver noise is probabably 10x less. Plus the sun is now at 1 AU, so there's that reduction by152^2 also missing. I have no idea what the plasma around the sun does to the transmission, but it can't be good. The only positive thing is that the directivity of the big down-link antenna is much better, especially at X-band. $\endgroup$ – Roger Wood Apr 9 at 19:21
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    $\begingroup$ @uhoh It is remarkable to think that, within that narrow beamwidth and that narrow bandwidth, the Goldstone DSN antenna is much more luminous than the Sun. $\endgroup$ – Roger Wood Apr 9 at 19:25
  • $\begingroup$ Yes indeed, the world looks very different in radio! Luckily, while Earth always appears close to the Sun from the Voyagers, they don't usually appear close to the Sun from Earth. $\endgroup$ – uhoh Apr 9 at 19:28
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I believe that the Voyager receiver uses a PLL, phase locked loop to acquire the signal and filter out the solar noise. Also, the solar radiation is different to the transmitted signal which Voyager can use to differentiate between the noise with a band pass filter and some other electronics.

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Since you "modified" your question: The purpose of an antenna is to receive a signal not to filter noise. The Power voyager receives, could be sun + earth, earth - sun, sun - earth or only sun. The Modulation used is still unknown, so i cant calculate the needed SNR for it to work.

But the normal radiation level shouldn't be a problem for the voyager probe. Think of the sun as a constant emitter. Only flares create spikes. The spectrum of these radio waves will have a Gaussian bell curve (Watts/ Hz). The Engineers of Nasa will probably have picked a Frequency spectrum that is on one of the lower ends of this bell curve. The radio power of the sun decreases faster per Meter away from its origin then that of earth. Since earth uses directional communication and the sun is just a big ball of energy. So the true problem will be, to always point at voyager correctly.

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  • $\begingroup$ I'm pretty sure that it depends on many things. If you search for words like Voyager 1, high gain antenna, uplink (means from DSN to Voyager) and start reading, there's a lot there. The sun is pretty noisy at ~2GHz and the high gain antenna can not separate the earth from the sun. It transmits at ~8GHz but (if I read correctly) uplink is always at the lower frequency. There is a lot of work the 1970's space-hardened electronics has to do before demodulation. It's really incredible stuff! $\endgroup$ – uhoh Jul 20 '16 at 13:38
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    $\begingroup$ You cant calculate the Signal to Noise ratio without the energy put into the signal. Yes they state the Hertz and the kb/s, but these numbers dont help calculating the actual Watts the probe gets to use to decide what a signal means. The work you are referring is probably the signal transformation to a bit stream. (Demodulating the signal -> deciding what bit / byte that is -> checking against check sum)Again, without the used technique it is for me impossible to say what the signal to noise ratio is or, what the actual needed ratio is. $\endgroup$ – Git Jul 20 '16 at 13:53
  • $\begingroup$ That's a downlink signal, just to illustrate the "...mind-boggling large distances an weak signal...". So far I haven't seen an uplink. I'll add that to the label, Any receiver system has some frequency limitation before the first active stage. I read somewhere that the feed horn has a 20MHz bandpass for example, but there may also be a tuned circuit. Since the sun is broadband, the wider the frequency range exposed to the front-end (first active amplification state) the more chance of overloading it, especially if the sun is active. It's a real-world problem, not just a demodulation problem $\endgroup$ – uhoh Jul 22 '16 at 0:15
  • $\begingroup$ The frequency of the sun does not pose a problem, since it always stays the same. So you just have to use a Filter for these Frequencies, their Amplitudes. So you tune to your specific frequency, add a low high pass filter to get just the amplitudes you are interested, and then transform your signal back to lose the carrier. Then all the nice digital stuff takes place. The signal does not lose so much strength over distance since it originated from a dish not just a wire. But the sun does. $\endgroup$ – Git Jul 22 '16 at 8:19
  • $\begingroup$ @Git 1) the sun is a broadband source including some energy at the uplink frequency. 2) The inverse quare law applies just as much to the transmit antenna as it does to the sun. . $\endgroup$ – Roger Wood Apr 9 at 6:30

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