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Comment by quesera | Hacker News Reader

> A radio signal takes more than 22 hours to reach Voyager 1

Voyager has a 23 watt radio and a 14 foot parabolic dish antenna, pointed directly at Earth which is presently 15 billion miles away.

The corresponding earth ground station has a 100 foot dish and transmits at thousands of watts.

It's astonishing to me that Voyager knows where Earth is at all. I imagine it uses Sol as a reference? Or maybe the high powered control signal carrier?

Amazing stuff.

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> 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

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.

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?

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.

> It would certainly be epically more expensive.

Actually, microgravity means you could have a 50-meter dish unfold neatly from a backpack-sized wad of Mylar.

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.

> 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.

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...

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

It's not that astonishing when you realize astral navigation is what sailors hundreds of years ago did to navigate the seas. Just look at the stars with a sextant and with some basic trigonometry you'll know where you are exactly. The Apollo space crew had to do that by hand and eyes using a sextant too when the astral navigation computer failed.

Knowing your longitude is very difficult without a clock... Can't do it with just stars iirc.

I think you can, with the Moon and where it is in relation to the Sun.

You can determine your longitude without a clock, but achieving precision with this method is challenging. First, you need to observe the Moon's position relative to fixed stars, which gives you the UTC time. Then, observe the Sun's position relative to the horizon to determine the local time. By comparing the two, you can calculate your longitude.

How are you going to usefully use this information if you wouldn't have an obvious way to know precisely how long it's been between whenever you lost track of the moon and stars and whenever the sun is clearly visible over the horizon?

This is true and there's a fascinating history for Earth-bound (little v) voyagers!

But Voyager does not have this particular problem. :)

Does voyager have optical sensors that detect planets?

It would be astonishing if Voyager shipped in 1977 with computer vision capabilities that were capable of that level of object detection and discrimination and accuracy. Plus the detailed star maps for correlation. And all designed and built several years before launch!

And in fact I think it's not possible. But orienting on Sol should be achievable. Even at 15 billion miles away, it's still surely the brightest thing in the sky.

Presumably the orientation sensor is precisely in line with the parabolic antenna?

https://space.stackexchange.com/questions/3093/how-does-the-... :)

I know trident uses the stars to avoid gps jammers, so presumably voyager can do something similar.

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