4 ms·

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

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

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.