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0 pointsIntralexical1y ago0 comments

> It's astonishing to me that Voyager knows where Earth is at all.

Wikipedia says that the high-gain antenna has a beamwidth of 0.5° for X-band, and 2.3° for S-band.

  You have: tan(0.5deg/2)*2*164astronomicalunit 
  You want: astronomicalunit 
          * 1.4311791

So it's so far that you could almost point it straight at the sun, and have the entire orbit of the Earth in the beam.

23 watts, spread out in a cone the size of a planetary orbit.

https://en.wikipedia.org/wiki/Voyager_program#Communications

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quesera1y ago

Which correspondingly means that the received signal strength here on Earth is very low.

Internet says approximately -160.48 dBm, which is one thousandth of the minimum strength that the most sensitive commercial FM radio receivers can discriminate.

Hence the huge receiving dishes, and apparently one 100 foot dish alone does not collect enough energy to reconstruct the signal, so multiple are required at each Deep Space Network receiving station.

And due to the rotation of the Earth, we need three DSN stations to maintain constant communication with Voyager.

IntralexicalOP1y ago

> Hence the huge receiving dishes, and apparently one 100 foot dish alone does not collect enough energy to reconstruct the signal

Around 30k photons a second.

  You have: ((23W/(pi*astronomicalunit^2)) * (pi(50ft)^2)) / (6.626e-34J/Hz * 10GHz)
  You want: 
        Definition: 36024.297 / s

…I don't know if that's a lot, or where the noise floor is.

quesera1y ago

As a comparison of unlike but related units:

> For example, the data rate used from Jupiter was about 115,000 bits per second

And I think it affects the calculation slightly that Voyager's downlink transmits at about 8.4 GHz (X-band).

Good stuff here: https://www.allaboutcircuits.com/news/voyager-mission-annive...

HPsquared1y ago

I wonder if they could build a very large dish in Earth orbit. Probably cost prohibitive though. Also there's the power and cooling aspect when transporting... Would there be any advantage to a space based station?

IntralexicalOP1y ago

You could build a planet-sized interferometric array without having to obtain land in different countries. If the orbit's much bigger than the planet, you can have most of the antennae always be usable instead of half on the wrong side. You can pick up wavelengths blocked by the atmosphere. You can build multi-kilometer single dishes out of mylar or steel cable, without gravity and air currents bending them. You can use it to focus sunlight onto surface locations, but probably shouldn't.

bruce5111y ago

I'm no space expert, but I'm not sure this would be say better. It would certainly be epically more expensive.

Two obvious issues spring to mind;

The orbit would need to be polar to avoid being blocked by the earth (assuming the craft is in the solar plane). Whatever orbit was chosen at least some planes some of the time would be obscured by the earth.

And orientation fuel becomes an issue. Outside of refueling it becomes a hard end-of-life factor.

Contrasted to land-based stations. Which are happily operating 50 years and can be maintained etc.

Frankly I'm not sure there would be any advantage to a space receiver- and it's several orders of magnitude more expensive.

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0 pointsIntralexical1y ago0 comments

> It's astonishing to me that Voyager knows where Earth is at all.

Wikipedia says that the high-gain antenna has a beamwidth of 0.5° for X-band, and 2.3° for S-band.

  You have: tan(0.5deg/2)*2*164astronomicalunit 
  You want: astronomicalunit 
          * 1.4311791

So it's so far that you could almost point it straight at the sun, and have the entire orbit of the Earth in the beam.

23 watts, spread out in a cone the size of a planetary orbit.

https://en.wikipedia.org/wiki/Voyager_program#Communications

0 comments

quesera1y ago

Which correspondingly means that the received signal strength here on Earth is very low.

Internet says approximately -160.48 dBm, which is one thousandth of the minimum strength that the most sensitive commercial FM radio receivers can discriminate.

And due to the rotation of the Earth, we need three DSN stations to maintain constant communication with Voyager.

IntralexicalOP1y ago

> Hence the huge receiving dishes, and apparently one 100 foot dish alone does not collect enough energy to reconstruct the signal

Around 30k photons a second.

  You have: ((23W/(pi*astronomicalunit^2)) * (pi(50ft)^2)) / (6.626e-34J/Hz * 10GHz)
  You want: 
        Definition: 36024.297 / s

…I don't know if that's a lot, or where the noise floor is.

quesera1y ago

As a comparison of unlike but related units:

> For example, the data rate used from Jupiter was about 115,000 bits per second

And I think it affects the calculation slightly that Voyager's downlink transmits at about 8.4 GHz (X-band).

Good stuff here: https://www.allaboutcircuits.com/news/voyager-mission-annive...

HPsquared1y ago

IntralexicalOP1y ago

bruce5111y ago

I'm no space expert, but I'm not sure this would be say better. It would certainly be epically more expensive.

Two obvious issues spring to mind;

And orientation fuel becomes an issue. Outside of refueling it becomes a hard end-of-life factor.

Contrasted to land-based stations. Which are happily operating 50 years and can be maintained etc.

Frankly I'm not sure there would be any advantage to a space receiver- and it's several orders of magnitude more expensive.

1 more reply

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