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Can somebody with knowledge of radio communication explain how we are able to send a radio signal to a destination that is 21 billion Kms away? How powerful doe

Can somebody with knowledge of radio communication explain how we are able to send a radio signal to a destination that is 21 billion Kms away? How powerful does the signal need to be? What kind of technology is used to generate such a powerful signal?

mechagodzilla 9y ago
It’s mostly just very high gain antennas in both directions. You don’t need all that much power if your data rate is very slow and you have a very narrow beam.
- wglb 9y ago
  At a very low baud rate.
  - jacquesm 9y ago
    He said that ('slow data rate').
- nojvek 9y ago
  Does anyone know what is huge baud rate for voyager? Dial up?
  - pcunite 9y ago
    In the neighborhood of 160 bits per second I think for telemetry. There is other information. https://youtu.be/FzRP1qdwPKw?t=4m40s https://youtu.be/FzRP1qdwPKw?t=4m40s
jimktrains2 9y ago
https://space.stackexchange.com/questions/9824/how-much-rf-transmit-power-does-dsn-need-to-send-commands-to-voyager https://space.stackexchange.com/questions/9824/how-much-rf-t... A good bit but not a ton seems to be the answer. Maybe in the range of a radio station. The more interesting part is that Voyager can send signals we can receive (with giant, huge radio dishes, but still).
- AdamJacobMuller 9y ago
  I'm not sure that the more interesting bit is the voyager->earth part. Keep in mind that there are inefficiencies in simply transmitting with more power. The more you amplify, the more noise you introduce. No matter what you do, you can never improve the size and hardware that you're transmitting to. I don't work much with RF, but, with optical transmissions, the major innovations I've seen are from better receivers that are better able to separate signal from noise at lower power levels. On the earth side of things, we can use massive dishes connected to modern hardware that have very advanced signal processing capabilities. Ultimately, Voyager is 1960s-era hardware with very very very minimal ability to change the software in any way. If they wanted to (and had the funding to) build a dish on earth that was 10x larger to receive the signals from Voyager with hugely advanced signal processing, that's a totally doable thing. On the other hand, there is virtually nothing you can do to make voyager hear better. Ultimately, its a lot easier to amplify something faint than it is to shout louder.
  - jacquesm 9y ago
    > Ultimately, its a lot easier to amplify something faint than it is to shout louder. Yes, but since you are amplifying the noise right along with the signal if the other side shouts louder it really helps. As does a very good directional antenna (parabolic, very solid mount, very precise control of its orientation).
    - AdamJacobMuller 9y ago
      and Voyager is listening using 40-50-year-old technology, its far more impressive that voyager can still hear us than that we can still hear voyager.
      - jimktrains2 9y ago
        We can yell louder, listen harder, and pour more computer power into decoding the signal than Voyager can, though.
  - 9y ago
jacquesm 9y ago
It's mostly a function of how well your antenna is able to amplify the signal, how precise you can aim that antenna and how slow you transmit. If you have a very good antenna with lots of gain and a very precise mechanism of aiming it (at an object whose location is very well known, and Voyager is a moving target) it will take much less power than if either one of those elements is not optimal. The idea here is that any radio energy that does not end up in the vicinity of the target was wasted and at 21 billion Km that gives you plenty of opportunity for mis-alignment. Receiving the signal has similar challenges, with the added complication that this time the sender is sending with a power level that puts its signal under the noise floor by the time it reaches Earth. https://en.wikipedia.org/wiki/Noise_floor https://en.wikipedia.org/wiki/Noise_floor Fun fact: this goes for the GPS satellites as well by the time their signal reaches your pretty little hand-held receiver and it takes nothing short of magic (to me, not to the people that design that stuff) to recover the signal.
- pfarnsworth 9y ago
  If we launched a proxy satellite into the solar system to help communicate with these probes, would it make things easier/better? Or is it better to just communicate directly from the Earth?
  - jacquesm 9y ago
    I think it would make things worse, the only advantage I see is that if you got it to work at all you could do away with the requirement that you'd need three base stations on Earth. A single Geostationary satellite should be able to keep both the base station and the Voyager in sight at the same time. But that's a lot of money to spend for very little advantage.
    - pfarnsworth 9y ago
      Interesting! Thank you for the info, I'll ruminate over that for a while!
    - dfox 9y ago
elihu 9y ago
According to the inverse-square law, doubling the distance will reduce the power of the signal by 1/4th. The moon is about one light second away, whereas Voyager is about 70,000 light seconds away. If we round down to 64k light seconds, that's about 16 doublings. If each doubling represends a 6db loss, that's about 96 decibels. So, starting from a system that can communicate from Earth to the moon, if you can find a way to add 64 decibels then it can work from Earth to Voyager. Ways to add decibels include using more directional antennas on one or both ends or transmitting with more power. Alternatively, you can make up some of those decibels by communicating much slower. The interesting thing about the inverse square law is that it's insensitive to the scales involved. For instance, going from 10 meters to 20 meters results in a 6db loss, and going from 1 light year to 2 light years also results in a 6db loss. This is much different from, say, light in a fiber optic cable, which would experience a 6db loss from impurities in the glass each time the light traveled some constant distance.
- tempestn 9y ago
  That's assuming you're firing the signal out in all directions though, right? For example, a laser doesn't drop in power in this way. I would think a tightly focused radio signal could also avoid such extreme degradation.
  - elihu 9y ago
    It's really hard to make electromagnetic radiation perfectly parallel. If you shine a laser pointer at something far away, the dot gets bigger with distance. If you could send a perfectly parallel beam, it would effectively be an antenna with infinite gain. As far as I know, that's not possible but getting as close as you can is a good strategy. There's also antenna aiming limitations to consider -- it's possible to have too much gain if it exceeds your ability to point in the right direction. The opposite extreme is an isotropic radiator, which emits equally in all directions. (That isn't possible either, but it's a good theoretical baseline.) Antenna gain is usually described relative to an isotropic radiator. So, an antenna with a gain of 12dbi means that in the direction it sends its strongest beam, it's 12 decibels stronger than it would be if the antenna were an isotropic radiator.
muxator 9y ago
It's obviously a question of power and antenna gains, but a very important role is played by the encoding: Pioneer before, and Voyager after introduced concatenated codes [0], which consist of a viterbi decoded convolutional code inside an outer Reed Solomon code. Curiously, those codes are now superseded by other, more modern approaches, for example Turbo Codes [1] which are used not only in deep space probes, but also in cellular communications and other applications that we consider normal these days. [0] https://en.m.wikipedia.org/wiki/Concatenated_error_correction_code https://en.m.wikipedia.org/wiki/Concatenated_error_correctio... [1] https://en.m.wikipedia.org/wiki/Turbo_code https://en.m.wikipedia.org/wiki/Turbo_code
- botskonet 9y ago
  I read those links because I'm really curious but I clearly lack the prerequisite knowledge.
- anderspitman 9y ago
  I worked as an RF engineer for a few years and one of the senior engineers once told me a story about how they launched Voyager (I think it was Voyager) with a Viterbi encoder, even though we didn't have the ability to make a decoder at that time due to lacking the computing power. So they launched it knowing they would eventually have the technology, and of course they were right. No idea if that's true or not but I love the story.
InclinedPlane 9y ago
There are high gain antennas on both ends, and use of error correcting codes, as well as extremely high grade low noise amplifiers on the Earth side. The Voyagers have a 3.7m diameter parabolic radio dish, larger than the Hubble space telescope's mirror even. That alone provides a huge amount of gain on communications. Additionally, the spacecraft have 10s of watts of power available for transmitting signals, which is a fair bit considering (while on the other end the ground stations have up to hundreds of thousands of watts to transmit). The ground-stations in the deep space network (DSN) are tens of meters across, a small antenna is 34m, the biggest ones are 70m across. That also provides a huge amount of gain alone. It means that there is more area to collect signals from the spacecraft and it means that the beam from the ground station to the spacecraft is much tighter, concentrating the total transmission power into a smaller cross-sectional area at the distance of the spacecraft. The spacecraft also uses error correcting codes, which involve transmitting many more bits than the underlying data, but in such a way that errors due to noise are not only detectable but correctable. On top of all of that you have the state of the art low noise amplifiers in the DSN antennae. A typical low noise amplifier is a carefully built electronics assembly made by experts. The DSN amplifiers? They use 99.95% purity ruby rods chilled to 4 degrees above absolute zero to form microwave MASER based amplifiers. There's a neat little video (series) here on the DSN and contacting the Voyagers: https://www.youtube.com/watch?v=FzRP1qdwPKw https://www.youtube.com/watch?v=FzRP1qdwPKw
- femto 9y ago
  There's also a neat book for those interested in the engineering details of their receivers: "Low-Noise Systems in the Deep Space Network" Edited by Macgregor S. Reid https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCANSO_sml-110804.pdf https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCAN... It's published by the JPL as part of the "Deep Space Communictions and Navigation Series". The rest of the books in the series, listed in the book's front matter, have some fascinating titles.
- quotemstr 9y ago
  That's amazing.
- ufmace 9y ago
lucb1e 9y ago
I was wondering the same thing and found this: https://space.stackexchange.com/questions/958/how-does-voyager-1-send-signals-to-earth https://space.stackexchange.com/questions/958/how-does-voyag... (Searched for "voyager communication" on DDG, top hit.) I was surprised to learn that it's a ground-based system. I'd think they would need antennas in space (on satellites) so that you can both send extremely powerful signals without disturbing others, and receive without having to go through the atmosphere. Instead, there are just three ground stations at approximately 120° around the earth for continuous communication.

mechagodzilla 9y ago

It’s mostly just very high gain antennas in both directions. You don’t need all that much power if your data rate is very slow and you have a very narrow beam.

wglb 9y ago
At a very low baud rate.
- jacquesm 9y ago
  He said that ('slow data rate').
nojvek 9y ago
Does anyone know what is huge baud rate for voyager? Dial up?
- pcunite 9y ago
  In the neighborhood of 160 bits per second I think for telemetry. There is other information. https://youtu.be/FzRP1qdwPKw?t=4m40s https://youtu.be/FzRP1qdwPKw?t=4m40s

jimktrains2 9y ago

https://space.stackexchange.com/questions/9824/how-much-rf-transmit-power-does-dsn-need-to-send-commands-to-voyager https://space.stackexchange.com/questions/9824/how-much-rf-t... A good bit but not a ton seems to be the answer. Maybe in the range of a radio station. The more interesting part is that Voyager can send signals we can receive (with giant, huge radio dishes, but still).

AdamJacobMuller 9y ago
I'm not sure that the more interesting bit is the voyager->earth part. Keep in mind that there are inefficiencies in simply transmitting with more power. The more you amplify, the more noise you introduce. No matter what you do, you can never improve the size and hardware that you're transmitting to. I don't work much with RF, but, with optical transmissions, the major innovations I've seen are from better receivers that are better able to separate signal from noise at lower power levels. On the earth side of things, we can use massive dishes connected to modern hardware that have very advanced signal processing capabilities. Ultimately, Voyager is 1960s-era hardware with very very very minimal ability to change the software in any way. If they wanted to (and had the funding to) build a dish on earth that was 10x larger to receive the signals from Voyager with hugely advanced signal processing, that's a totally doable thing. On the other hand, there is virtually nothing you can do to make voyager hear better. Ultimately, its a lot easier to amplify something faint than it is to shout louder.
- jacquesm 9y ago
  > Ultimately, its a lot easier to amplify something faint than it is to shout louder. Yes, but since you are amplifying the noise right along with the signal if the other side shouts louder it really helps. As does a very good directional antenna (parabolic, very solid mount, very precise control of its orientation).
  - AdamJacobMuller 9y ago
    and Voyager is listening using 40-50-year-old technology, its far more impressive that voyager can still hear us than that we can still hear voyager.
    - jimktrains2 9y ago
      We can yell louder, listen harder, and pour more computer power into decoding the signal than Voyager can, though.
- 9y ago

jacquesm 9y ago

It's mostly a function of how well your antenna is able to amplify the signal, how precise you can aim that antenna and how slow you transmit. If you have a very good antenna with lots of gain and a very precise mechanism of aiming it (at an object whose location is very well known, and Voyager is a moving target) it will take much less power than if either one of those elements is not optimal. The idea here is that any radio energy that does not end up in the vicinity of the target was wasted and at 21 billion Km that gives you plenty of opportunity for mis-alignment. Receiving the signal has similar challenges, with the added complication that this time the sender is sending with a power level that puts its signal under the noise floor by the time it reaches Earth. https://en.wikipedia.org/wiki/Noise_floor https://en.wikipedia.org/wiki/Noise_floor Fun fact: this goes for the GPS satellites as well by the time their signal reaches your pretty little hand-held receiver and it takes nothing short of magic (to me, not to the people that design that stuff) to recover the signal.

pfarnsworth 9y ago
If we launched a proxy satellite into the solar system to help communicate with these probes, would it make things easier/better? Or is it better to just communicate directly from the Earth?
- jacquesm 9y ago
  I think it would make things worse, the only advantage I see is that if you got it to work at all you could do away with the requirement that you'd need three base stations on Earth. A single Geostationary satellite should be able to keep both the base station and the Voyager in sight at the same time. But that's a lot of money to spend for very little advantage.
  - pfarnsworth 9y ago
    Interesting! Thank you for the info, I'll ruminate over that for a while!
  - dfox 9y ago

elihu 9y ago

According to the inverse-square law, doubling the distance will reduce the power of the signal by 1/4th. The moon is about one light second away, whereas Voyager is about 70,000 light seconds away. If we round down to 64k light seconds, that's about 16 doublings. If each doubling represends a 6db loss, that's about 96 decibels. So, starting from a system that can communicate from Earth to the moon, if you can find a way to add 64 decibels then it can work from Earth to Voyager. Ways to add decibels include using more directional antennas on one or both ends or transmitting with more power. Alternatively, you can make up some of those decibels by communicating much slower. The interesting thing about the inverse square law is that it's insensitive to the scales involved. For instance, going from 10 meters to 20 meters results in a 6db loss, and going from 1 light year to 2 light years also results in a 6db loss. This is much different from, say, light in a fiber optic cable, which would experience a 6db loss from impurities in the glass each time the light traveled some constant distance.

tempestn 9y ago
That's assuming you're firing the signal out in all directions though, right? For example, a laser doesn't drop in power in this way. I would think a tightly focused radio signal could also avoid such extreme degradation.
- elihu 9y ago
  It's really hard to make electromagnetic radiation perfectly parallel. If you shine a laser pointer at something far away, the dot gets bigger with distance. If you could send a perfectly parallel beam, it would effectively be an antenna with infinite gain. As far as I know, that's not possible but getting as close as you can is a good strategy. There's also antenna aiming limitations to consider -- it's possible to have too much gain if it exceeds your ability to point in the right direction. The opposite extreme is an isotropic radiator, which emits equally in all directions. (That isn't possible either, but it's a good theoretical baseline.) Antenna gain is usually described relative to an isotropic radiator. So, an antenna with a gain of 12dbi means that in the direction it sends its strongest beam, it's 12 decibels stronger than it would be if the antenna were an isotropic radiator.

muxator 9y ago

It's obviously a question of power and antenna gains, but a very important role is played by the encoding: Pioneer before, and Voyager after introduced concatenated codes [0], which consist of a viterbi decoded convolutional code inside an outer Reed Solomon code. Curiously, those codes are now superseded by other, more modern approaches, for example Turbo Codes [1] which are used not only in deep space probes, but also in cellular communications and other applications that we consider normal these days. [0] https://en.m.wikipedia.org/wiki/Concatenated_error_correction_code https://en.m.wikipedia.org/wiki/Concatenated_error_correctio... [1] https://en.m.wikipedia.org/wiki/Turbo_code https://en.m.wikipedia.org/wiki/Turbo_code

botskonet 9y ago
I read those links because I'm really curious but I clearly lack the prerequisite knowledge.
anderspitman 9y ago
I worked as an RF engineer for a few years and one of the senior engineers once told me a story about how they launched Voyager (I think it was Voyager) with a Viterbi encoder, even though we didn't have the ability to make a decoder at that time due to lacking the computing power. So they launched it knowing they would eventually have the technology, and of course they were right. No idea if that's true or not but I love the story.

InclinedPlane 9y ago

There are high gain antennas on both ends, and use of error correcting codes, as well as extremely high grade low noise amplifiers on the Earth side. The Voyagers have a 3.7m diameter parabolic radio dish, larger than the Hubble space telescope's mirror even. That alone provides a huge amount of gain on communications. Additionally, the spacecraft have 10s of watts of power available for transmitting signals, which is a fair bit considering (while on the other end the ground stations have up to hundreds of thousands of watts to transmit). The ground-stations in the deep space network (DSN) are tens of meters across, a small antenna is 34m, the biggest ones are 70m across. That also provides a huge amount of gain alone. It means that there is more area to collect signals from the spacecraft and it means that the beam from the ground station to the spacecraft is much tighter, concentrating the total transmission power into a smaller cross-sectional area at the distance of the spacecraft. The spacecraft also uses error correcting codes, which involve transmitting many more bits than the underlying data, but in such a way that errors due to noise are not only detectable but correctable. On top of all of that you have the state of the art low noise amplifiers in the DSN antennae. A typical low noise amplifier is a carefully built electronics assembly made by experts. The DSN amplifiers? They use 99.95% purity ruby rods chilled to 4 degrees above absolute zero to form microwave MASER based amplifiers. There's a neat little video (series) here on the DSN and contacting the Voyagers: https://www.youtube.com/watch?v=FzRP1qdwPKw https://www.youtube.com/watch?v=FzRP1qdwPKw

femto 9y ago
There's also a neat book for those interested in the engineering details of their receivers: "Low-Noise Systems in the Deep Space Network" Edited by Macgregor S. Reid https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCANSO_sml-110804.pdf https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCAN... It's published by the JPL as part of the "Deep Space Communictions and Navigation Series". The rest of the books in the series, listed in the book's front matter, have some fascinating titles.
quotemstr 9y ago
That's amazing.
ufmace 9y ago

lucb1e 9y ago

I was wondering the same thing and found this: https://space.stackexchange.com/questions/958/how-does-voyager-1-send-signals-to-earth https://space.stackexchange.com/questions/958/how-does-voyag... (Searched for "voyager communication" on DDG, top hit.) I was surprised to learn that it's a ground-based system. I'd think they would need antennas in space (on satellites) so that you can both send extremely powerful signals without disturbing others, and receive without having to go through the atmosphere. Instead, there are just three ground stations at approximately 120° around the earth for continuous communication.