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Comment by martinald | Hacker News Reader

Why is bandwidth so scare still? Satellites in geosynchronous orbit on earth can do terabit/sec of total capacity now.

Obviously latency is going to be extremely high, but I can't really understand why data rates are still so low?

Inverse square law. For the same transmit power, your received signal strength diminishes with the square of the distance

That received signal strength is what determines channel capacity (C) which is the upper limit on the bit rate you can get.

see Friis and Shannon's channel capacity

[0] https://en.wikipedia.org/wiki/Friis_transmission_equation

[1] https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

Yes, I get that. But it seems that performance has barely improved since the 1970s despite the massive gains in telecommunication efficiencies since then. Or is the SNR so poor that even 1970s era technology was able to be close to the limit?

These are physical restrictions, not technological.

What about the laser technology NASA is developing? That should be a lot faster than current radio based technology?

Titan has smog in the visible spectrum. Details matter! The technology is also not that mature, yet.

Focusing a laser at that range requires a rather large diameter heavy set of optical components.

You will need a blinding-strong laser.

Because the signals have to travel so much farther and are so much weaker when they arrive. It's amazing that any data at all can be extracted from a radio signal that travels a billion miles.

Spacecraft transmit power and antenna size is limited. This means the amount of power that arrives at earth is rather low because microwaves spread out quite a bit with distance. The other issue is that NASA only has so many big radio dishes in the deep space network[0] to gather this small amount of power and extract a signal. Technology has improved, but in order to get much higher bandwidth we'd need bigger dishes. We also need at least three around the globe to get continuous coverage. Although perhaps by the time this mission lands we could have multi-kilometer aperture receivers on the ground or in space. Laser communication could allow for much higher bandwidth, because lasers have a shorter wavelength they don't spread out as much with distance. However, laser space communication is in its infancy and would not work well on Titan due to the smog.

[0]https://en.wikipedia.org/wiki/NASA_Deep_Space_Network

Maybe workaround transmission power by reflecting and modulating a signal over an existing power source. Like a very big mirror with an LCD pixel in front of it.

"one over r-squared"

Latency is high, but the power you have to pump through an antenna / laser to get enough watts on the receiver goes up really fast with distance.

Here's a summary that touches on it if you'd like to learn more: http://www.qrg.northwestern.edu/projects/vss/docs/Communicat...

And here's a detailed derivation for voyager. https://space.stackexchange.com/questions/24338/how-to-calcu...

I don’t think you realize just how far away the outer solar system is.

Low transmit power. Long distance. Very low SNR at receiver.

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