5 ms·

The bitrate Earth/Mars is very low. It will take a lot more.

by janmo 6y ago

ape4 6y ago
I hope there is a movie eventually.
_joel 6y ago
The direct earth to rover comms is slow but it's a lot quicker via MRO as it has a higher gain antenna. https://mars.nasa.gov/mars2020/spacecraft/rover/communications/ https://mars.nasa.gov/mars2020/spacecraft/rover/communicatio...
xiphias2 6y ago
Would setting up 60 Starlink satellites in a line to transmit between the two help?
- nvahalik 6y ago
  No. It’s not a power or signal quality problem. It’s a speed-of-radio waves problem. Mars is really far away. And unfortunately we don’t have something like an Ansible to make communication instant.
  - maxov 6y ago
    I think this situation is a good illustration of latency vs bandwidth. Latency is unavoidable due to the speed of light, but there’s no physical barrier to bitrate aside from power consumption. Seems like the best bitrate we have from Mars is MRO at ~5 megabits/s which is enough for standard definition video. I’m certain we have the technology to push it higher, but it would be expensive and it seems it’s not really needed for existing projects.
    - dredmorbius 6y ago
      Nit, though I'm going a bit (pun status unspecified) beyond my paygrade here: bandwidth is ultimately limited by the frequency of the trandsmission channel, though multiple channels can be multiplexed. This is the heart of Claude Shannon's work. For deep-space transmission, other factors apply, though if you're operating in the right frequencies, the background noise level is low (at least as compared to Earth where there are numerous competing signals of terrestrial origin). And whilst signal/noise ratio does impose limits, there's only so much that boosting volume dB will achieve before you hit the channel limits themselves. To increase bandwith beyond channel limits, you'd have to multiplex channels, which would involve different frequencies, different transmission routes (say, to widely-separated relay systems), or both. For physical media (e.g., fibre) this is relatively easy. For broadcast, given mass and energy budgets of spacecraft, probes, and rovers, the problems are harder. Encoding, redundancy, error-correction, and retransmit protocols can all increase the ultimate reliability of the signal, though these impose other costs, notably on bitrate or at least time until the transmission is confirmed.
      - deleted 6y ago
        [deleted]
  - extropy 6y ago
    The time from mars to Earth is 3-20 minutes depending on orbit. Perseverance to MRO is 2 mbits/sec but only when it's overhead of the rover. MRO to Earth is 0.5 - 4 mbits per second. While it takes around 10 seconds to transmit an image at 2mbit/s, the whole system has several store and forward steps adding significantly more delay. And a bunch of starlink-like satellites in medium orbit would for sure help to bridge the gap.
  - jvanderbot 6y ago
    Latency and bitrate are both affected by distance. Bitrate scales down roughly by 1/d squared. Latency scales up by o(d) obviously. In theory, multiple small hops could keep power high over a long distance. On practice, the different sun-centered orbits the relays would operate on mean you'd have a lot of them. That's spendy. Also, there's a spectrum limit on Earth side or else we could blast signals back with very wide bandwidth
    - maxov 6y ago
      This is a really clean explanation, thank you! Is the O(1/d^2) decrease in bitrate due to the inverse square law for radiation intensity?
    - spookthesunset 6y ago
      Going real wideband would run into antenna efficiency issues. Designing an antenna that can handle a massively large bandwidth has a hell of a lot of engineering trade offs involved.
- maxov 6y ago
  Interesting. I am not a rocket scientist, but I think there’s a few fundamental problems with doing this: 1. You can’t keep the satellites in a “line” between Earth and Mars, as objects in orbits of larger radii travel slower and so have longer orbital periods. This is the reason why for example geosynchronous orbits are only possible at a particular distance. They would only align with some frequency, which I’d guess would be closer to a geologic timescale than a human one. I think the fact that Mars and Earth’s orbits are slightly eccentric only makes this harder. 2. A solution to the above could be launching way more. But launching so many satellites carrying enough fuel to escape Earth’s gravity well would be prohibitively expensive compared to cost of LEO of existing Starlink satellites. 3. Starlink satellites are only designed to transmit maybe a few hundred km? In any case that’s 5 orders of magnitude smaller than the distance between Mars and Earth. You’d need dramatically more power consumption to transmit effectively over those distances, which is why existing bitrates are so low. Anyway this is pretty cool, I found a NASA dashboard that also shows bitrates for current spacecraft: https://eyes.nasa.gov/dsn/dsn.html https://eyes.nasa.gov/dsn/dsn.html
  - dredmorbius 6y ago
    Those units, e.g., 24. 25. 26 for Goldstone, are bitrates? Bits/second?
    - maxov 6y ago
      I think those are just identifiers for the telescopes. If you click on the telescope to expand you can see what it’s communicating with and some info about the link, and the bitrates are closer to e.g. 20 Kbit/s - 1.0 Mbit/s.
      - dredmorbius 6y ago
        Thanks. The dataviz is unclear.
- _joel 6y ago
  It might help reduce the affect of attenuation at a cost of more latency in the processing chain. This could help, afaik it's only been tested to Mars and not in production https://en.wikipedia.org/wiki/Laser_communication_in_space https://en.wikipedia.org/wiki/Laser_communication_in_space
  - deleted 6y ago
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  - deleted 6y ago
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- jvanderbot 6y ago
  You can't fix latency. You could increase data rate though. I'm working on a multi-sat huge data rate, huge latency idea under the NASA NIAC program at the moment. If we continue to be concerned mostly with latency, then data rate will always be severely limited. Sorry, no permalink, it's right on my page: https://josh.vanderhook.info/index#2021-03-niac-announcement https://josh.vanderhook.info/index#2021-03-niac-announcement
  - xiphias2 6y ago
    Hard drives? :) Sounds like back to DVD Netflix days. What's cool is that Reed Hastings planned to use internet from the start, but also knew that the tech is not ready.
- _Microft 6y ago
  Eventually it is going to be laser communications, I think. They are going to test deep space laser communications on the upcoming "Psyche"-mission, launching in 2022. The target asteroid will be at a distance of approximately 15 lightminutes (according to Wolfram Alpha) from Earth in 2026 when the probe is expected to arrive there. This is as far from Earth as Mars currently is. They hope to increase the bandwidth by an order of magnitude or two (i.e. 10-100 times). The link is supposed to be bidirectional, so the spacecraft can receive optical transmissions as well. I assume this laser uplink is more proof of concept than actually needed? https://en.wikipedia.org/wiki/Psyche_(spacecraft)#Science_payload https://en.wikipedia.org/wiki/Psyche_(spacecraft)#Science_pa... https://en.wikipedia.org/wiki/Deep_Space_Optical_Communications https://en.wikipedia.org/wiki/Deep_Space_Optical_Communicati...
  - Space_Dude 6y ago
    You're right that the future of deep space communications is optical. There's a forum on Reddit: https://reddit.com/r/lasercom https://reddit.com/r/lasercom

The bitrate Earth/Mars is very low. It will take a lot more.

by janmo 6y ago

The bitrate Earth/Mars is very low. It will take a lot more.

ape4 6y ago

I hope there is a movie eventually.

_joel 6y ago

The direct earth to rover comms is slow but it's a lot quicker via MRO as it has a higher gain antenna. https://mars.nasa.gov/mars2020/spacecraft/rover/communications/ https://mars.nasa.gov/mars2020/spacecraft/rover/communicatio...

xiphias2 6y ago

Would setting up 60 Starlink satellites in a line to transmit between the two help?

nvahalik 6y ago
No. It’s not a power or signal quality problem. It’s a speed-of-radio waves problem. Mars is really far away. And unfortunately we don’t have something like an Ansible to make communication instant.
- maxov 6y ago
  I think this situation is a good illustration of latency vs bandwidth. Latency is unavoidable due to the speed of light, but there’s no physical barrier to bitrate aside from power consumption. Seems like the best bitrate we have from Mars is MRO at ~5 megabits/s which is enough for standard definition video. I’m certain we have the technology to push it higher, but it would be expensive and it seems it’s not really needed for existing projects.
  - dredmorbius 6y ago
    Nit, though I'm going a bit (pun status unspecified) beyond my paygrade here: bandwidth is ultimately limited by the frequency of the trandsmission channel, though multiple channels can be multiplexed. This is the heart of Claude Shannon's work. For deep-space transmission, other factors apply, though if you're operating in the right frequencies, the background noise level is low (at least as compared to Earth where there are numerous competing signals of terrestrial origin). And whilst signal/noise ratio does impose limits, there's only so much that boosting volume dB will achieve before you hit the channel limits themselves. To increase bandwith beyond channel limits, you'd have to multiplex channels, which would involve different frequencies, different transmission routes (say, to widely-separated relay systems), or both. For physical media (e.g., fibre) this is relatively easy. For broadcast, given mass and energy budgets of spacecraft, probes, and rovers, the problems are harder. Encoding, redundancy, error-correction, and retransmit protocols can all increase the ultimate reliability of the signal, though these impose other costs, notably on bitrate or at least time until the transmission is confirmed.
    - deleted 6y ago
      [deleted]
- extropy 6y ago
  The time from mars to Earth is 3-20 minutes depending on orbit. Perseverance to MRO is 2 mbits/sec but only when it's overhead of the rover. MRO to Earth is 0.5 - 4 mbits per second. While it takes around 10 seconds to transmit an image at 2mbit/s, the whole system has several store and forward steps adding significantly more delay. And a bunch of starlink-like satellites in medium orbit would for sure help to bridge the gap.
- jvanderbot 6y ago
  Latency and bitrate are both affected by distance. Bitrate scales down roughly by 1/d squared. Latency scales up by o(d) obviously. In theory, multiple small hops could keep power high over a long distance. On practice, the different sun-centered orbits the relays would operate on mean you'd have a lot of them. That's spendy. Also, there's a spectrum limit on Earth side or else we could blast signals back with very wide bandwidth
  - maxov 6y ago
    This is a really clean explanation, thank you! Is the O(1/d^2) decrease in bitrate due to the inverse square law for radiation intensity?
  - spookthesunset 6y ago
    Going real wideband would run into antenna efficiency issues. Designing an antenna that can handle a massively large bandwidth has a hell of a lot of engineering trade offs involved.
maxov 6y ago
Interesting. I am not a rocket scientist, but I think there’s a few fundamental problems with doing this: 1. You can’t keep the satellites in a “line” between Earth and Mars, as objects in orbits of larger radii travel slower and so have longer orbital periods. This is the reason why for example geosynchronous orbits are only possible at a particular distance. They would only align with some frequency, which I’d guess would be closer to a geologic timescale than a human one. I think the fact that Mars and Earth’s orbits are slightly eccentric only makes this harder. 2. A solution to the above could be launching way more. But launching so many satellites carrying enough fuel to escape Earth’s gravity well would be prohibitively expensive compared to cost of LEO of existing Starlink satellites. 3. Starlink satellites are only designed to transmit maybe a few hundred km? In any case that’s 5 orders of magnitude smaller than the distance between Mars and Earth. You’d need dramatically more power consumption to transmit effectively over those distances, which is why existing bitrates are so low. Anyway this is pretty cool, I found a NASA dashboard that also shows bitrates for current spacecraft: https://eyes.nasa.gov/dsn/dsn.html https://eyes.nasa.gov/dsn/dsn.html
- dredmorbius 6y ago
  Those units, e.g., 24. 25. 26 for Goldstone, are bitrates? Bits/second?
  - maxov 6y ago
    I think those are just identifiers for the telescopes. If you click on the telescope to expand you can see what it’s communicating with and some info about the link, and the bitrates are closer to e.g. 20 Kbit/s - 1.0 Mbit/s.
    - dredmorbius 6y ago
      Thanks. The dataviz is unclear.
_joel 6y ago
It might help reduce the affect of attenuation at a cost of more latency in the processing chain. This could help, afaik it's only been tested to Mars and not in production https://en.wikipedia.org/wiki/Laser_communication_in_space https://en.wikipedia.org/wiki/Laser_communication_in_space
- deleted 6y ago
  [deleted]
- deleted 6y ago
  [deleted]
jvanderbot 6y ago
You can't fix latency. You could increase data rate though. I'm working on a multi-sat huge data rate, huge latency idea under the NASA NIAC program at the moment. If we continue to be concerned mostly with latency, then data rate will always be severely limited. Sorry, no permalink, it's right on my page: https://josh.vanderhook.info/index#2021-03-niac-announcement https://josh.vanderhook.info/index#2021-03-niac-announcement
- xiphias2 6y ago
  Hard drives? :) Sounds like back to DVD Netflix days. What's cool is that Reed Hastings planned to use internet from the start, but also knew that the tech is not ready.
_Microft 6y ago
Eventually it is going to be laser communications, I think. They are going to test deep space laser communications on the upcoming "Psyche"-mission, launching in 2022. The target asteroid will be at a distance of approximately 15 lightminutes (according to Wolfram Alpha) from Earth in 2026 when the probe is expected to arrive there. This is as far from Earth as Mars currently is. They hope to increase the bandwidth by an order of magnitude or two (i.e. 10-100 times). The link is supposed to be bidirectional, so the spacecraft can receive optical transmissions as well. I assume this laser uplink is more proof of concept than actually needed? https://en.wikipedia.org/wiki/Psyche_(spacecraft)#Science_payload https://en.wikipedia.org/wiki/Psyche_(spacecraft)#Science_pa... https://en.wikipedia.org/wiki/Deep_Space_Optical_Communications https://en.wikipedia.org/wiki/Deep_Space_Optical_Communicati...
- Space_Dude 6y ago
  You're right that the future of deep space communications is optical. There's a forum on Reddit: https://reddit.com/r/lasercom https://reddit.com/r/lasercom