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Comment by fivedogit | Hacker News Reader

My first thought, here, was "Well what about the speed of light? Wouldn't that cause satellite internet service to suck, just like it does now? And isn't that why news correspondents are always delayed?"

But then I did the math:

Speed of light = 186 miles per millisecond. Satellites orbit at about 380 miles above earth (Source: Hubble). So it only takes ~4 ms for a round trip to a satellite directly above?

I did not know that. Weird, wild stuff.

If someone knows specifics about why satellite communication is currently so slow (both internet and video), I'd be interested in the EL5 version.

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Good question. The vast majority of satellite communications today are conducted via geostationary orbit, 35,786 km above the earth. This is the distance at which a satellite orbiting around the equator moves with the same velocity as the surface of the earth, keeping the satellite in a fixed position. This is good for communicating via the ground, since you can just set up your dish pointing in the direction of the satellite. Additionally, such a satellite can cover a wide swath of the earth since it is so far away; see, for example, the coverage of AsiaSat-8 (http://www.spaceflightinsider.com/wp-content/uploads/2014/08...). The disadvantage (especially for internet) is that the round-trip time of a signal at the speed of light is on the order of 250 ms.

As described in this article, the SpaceX constellation would be in a much lower orbit. The advantage is a much faster ping time (say, around 10 ms). The disadvantage is you need a large number of satellites to provide continuous coverage for the entire earth...but that's less of a disadvantage if your company also launches rockets.

Hmm, interesting. Well, I have Exede Satellite service as my ISP (best I can get) and my best latency, to any server, is 800ms. The way I figure it, 35,786 km / 300 miles per ms would be 120ms up * 4 for the round trip. So that's 480ms, plus another 20ms-100ms to a google or another large server. So that's theoretical latency of 600ms, so where are the other 200ms of latency coming from! :(

I am wondering because I maintain some linux servers for some of my projects, and any time I need to do something serious, I have to drive to a coffee shop and borrow their connection. SSH over 800ms latency is frustrating, to say the least.

look into mosh, a ssh replacement for high latency connections.

Much of the remaining latency is likely coming from FEC and interleaving needed for the RF links.

http://en.wikipedia.org/wiki/Forward_error_correction#Interl...

And the other disadvantage is more expensive and complicated ground equipment that can handle changing angle to satellites and handoff on three order of every several minutes. But I imagine that's only getting cheaper...

> satellite orbiting around the equator moves with the same velocity as the surface of the earth

To be specific, angular velocity.

The Iridium constellation is 485 miles above. I've used it for both voice and data at sea via a motorola handset. Voice was very responsive with no delay, though the compression was very apparent. Data is painfully slow at 2.4 kbps. We needed to use a specialized email client that compressed email, handled the duplex connection, and recovered from errors very gracefully.

I'm curious the reason for such slow data too. Looks like the design was optimized to maximize the number of voice connections for a minimal number of satellites. Combined with the age of the design (first launched in '97) gives pretty poor data connections.

Because, communication satellites are not currently 380 miles above earth but 22k miles above earth.

~250ms RT latency http://en.wikipedia.org/wiki/Satellite_Internet_access#Signa...

Hubble is a pretty low orbit. Geostationary orbits are @ 36,000km

SpaceX is proposing putting them @ 1,100km.

The speed of light is faster in space than it is in optical fibre.

The speed of light is faster in space than it is in optical fibre.

Could you elaborate? Thanks

http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/nod...

Speed of light in vacuum > speed of light in optical fiber. Same reason high frequency traders are using microwave between Chicago and New York instead of a fiber connection.

Is it possible to build a multi-mode "fiber" with an air core so that a significant part of the light's path is through air? The outer sheath made of optically dense material would only be used for internal reflection to "guide" the beam...

No. The core has to be denser than the outside for the guiding to work; if you made a "fiber" like you describe the signal would just travel through the sheath - if it was heading towards the core it would internally reflect to stay within the sheath.

The speed of light in an optical fibre is about 35% slower than in vacuum -- it depends on the index of refraction.

https://en.wikipedia.org/wiki/Speed_of_light

The same is true for light traveling through water, air, and so forth.

My understanding of it is that when light passes through a medium, it does not do so in a straight line. So, while the actual speed of light remains constant, it takes longer to get from one end to the other.

To be precise, light can travel through a medium in a straight line, but it bends when it passes from one material to another. The light bounces off the inner walls of the fiber in a zig-zag fashion as it propagates, so it cannot possibly travel at the speed of light. It depends on the index of refraction as mentioned.

No, the speed of light decreases in a medium – in fact, the proportional decrease in speed is the refractive index of the material.

So much for it being constant huh?

so ~7ms RT. The satellites will be moving in the sky relative to the ground, so users will need to be able to switch satellites often?

Unless the content you're accessing is hosted on the satellite, it will have to jump several more times over a much longer distance for a complete round trip. Can you factor that in to your estimate?

Not necessarily, distance between satellites most definitely will be shorter than on ground network.

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