4 ms·

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 w

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.

hughes 11y ago
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?
- dzhiurgis 11y ago
  Not necessarily, distance between satellites most definitely will be shorter than on ground network.
yincrash 11y ago
~250ms RT latency http://en.wikipedia.org/wiki/Satellite_Internet_access#Signal_latency http://en.wikipedia.org/wiki/Satellite_Internet_access#Signa... Hubble is a pretty low orbit. Geostationary orbits are @ 36,000km
- greglindahl 11y ago
  SpaceX is proposing putting them @ 1,100km. The speed of light is faster in space than it is in optical fibre.
  - coryl 11y ago
    The speed of light is faster in space than it is in optical fibre. Could you elaborate? Thanks
    - toomuchtodo 11y ago
      http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html 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.
      - msravi 11y ago
        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...
        lmm 11y ago
        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.
    - deleted 11y ago
      [deleted]
    - greglindahl 11y ago
      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 https://en.wikipedia.org/wiki/Speed_of_light The same is true for light traveling through water, air, and so forth.
    - brian_ 11y ago
      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.
  - yincrash 11y ago
    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?
cdash 11y ago
Because, communication satellites are not currently 380 miles above earth but 22k miles above earth.
apkessl1 11y ago
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/footprint_sat_08pop.jpg 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.
- x0054 11y ago
  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.
  - havatar 11y ago
    look into mosh, a ssh replacement for high latency connections.
  - roamingryan 11y ago
    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#Interleaving http://en.wikipedia.org/wiki/Forward_error_correction#Interl...
jaredko 11y ago
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.

hughes 11y ago

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?

dzhiurgis 11y ago
Not necessarily, distance between satellites most definitely will be shorter than on ground network.

yincrash 11y ago

~250ms RT latency http://en.wikipedia.org/wiki/Satellite_Internet_access#Signal_latency http://en.wikipedia.org/wiki/Satellite_Internet_access#Signa... Hubble is a pretty low orbit. Geostationary orbits are @ 36,000km

greglindahl 11y ago
SpaceX is proposing putting them @ 1,100km. The speed of light is faster in space than it is in optical fibre.
- coryl 11y ago
  The speed of light is faster in space than it is in optical fibre. Could you elaborate? Thanks
  - toomuchtodo 11y ago
    http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html 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.
    - msravi 11y ago
      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...
      - lmm 11y ago
        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.
  - deleted 11y ago
    [deleted]
  - greglindahl 11y ago
    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 https://en.wikipedia.org/wiki/Speed_of_light The same is true for light traveling through water, air, and so forth.
  - brian_ 11y ago
    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.
- yincrash 11y ago
  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?

cdash 11y ago

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

apkessl1 11y ago

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/footprint_sat_08pop.jpg 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.

x0054 11y ago
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.
- havatar 11y ago
  look into mosh, a ssh replacement for high latency connections.
- roamingryan 11y ago
  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#Interleaving http://en.wikipedia.org/wiki/Forward_error_correction#Interl...

jaredko 11y ago

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.