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Mars to Earth at an average of 29 kbits/s

mars.jpl.nasa.gov

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Re: Mars to Earth at an average of 29 kbits/s

#81

Post by Irishsteve »

gchang wrote: post #32
fsiefken wrote: post #12

Does anyone know more about the architecture of this space radio network? For example does it use tcp/ip on top over radio or is it packet switched as with amateur packet radio?

Here's some publicly available technical information about how the DSN works: http://eis.jpl.nasa.gov/deepspace/dsndocs/810-005/

Up in Sweden they have DTN's for reindeer farmers. A helicopter which brings food supplies acts as the data transfer relay.

DTN book for anyone interested in the area. http://www.amazon.com/dp/1596930632

Re: Mars to Earth at an average of 29 kbits/s

#82

Post by vladd »

pothibo wrote: post #79
nitrogen wrote: post #77

Earlier quoted context omitted.

You seem to be trying to say something, but it's not coming across. What is primitive? How is it primitive? How do you describe the difference between efficiency and consistency in your usage? Is English not your primary language? My exposure to this stuff is limited to a few exercises in a Matlab class years ago, plus what I read on HN and Wikipedia. Based on what I understand, from a data transmission perspective,…

Yeah, english isn't my primary language and I haven't spoken it in a long time so sorry if I can't make my point across :) I'm not ashamed for being wrong. If that'd be the case I would have been ashamed my whole life heh. When I said consistency I was referring to low-error rate in the signal. I should have said that but couldn't find the correct word for it. Efficiency meant efficiency in the bandwidth usage. So in…

In low-bandwidth environments, the pipeline is like this:

1) plain data initial stream.

2) compress it.

3) encrypt it or/and sign it.

4) add error-checking or even error-correction information.

The reason you do encryption (3) after compression (2) is because you don't want encryption to affect the compression rate of the initial stream.

If you were to do it in the reverse order (first encryption and then compression) then yes, the resulting message could be bigger.

Re: Mars to Earth at an average of 29 kbits/s

#83

Post by mdwelsh »

eupharis wrote: post #39
ChuckMcM wrote: post #24

So one wonders at what point you uplink to a satellite with ejectable flash drives which, when full, detach, make a gravity sling shot pass to leave orbit and then burn for a rendezvous with earth 9 months later. If I did the math right a 1TB drive returned from Mars in 9 months would be an effective bandwidth of 470K bits per second (or 47K bytes per second)

While we are dreaming, it would be better to have said flash drive picked up by a spacestation, refueled in orbit, and shot back to Mars. Hmm. If storage continues to improve as fast as it has been, once the space infrastructure is there, this might be cost effective. We have a lot of data to send. In particular, we are going to need to have a local cache of the internet on Mars. At best, the average minimum latency…

An Interplanetary Internet is not a joke: It's been a serious field of study for a while. There is a SIG on the topic, and lots of work has been done to think about how Internet protocols would work across interplanetary distances and frequent disconnections:

http://www.ipnsig.org/aboutstudy.htm

Re: Mars to Earth at an average of 29 kbits/s

#84

Post by tcas »

pothibo wrote: post #78
nitrogen wrote: post #56

Earlier quoted context omitted.

I'm not sure what you mean by "optimized" data. Edit: my very rudimentary understanding of information theory tells me that there's no "average" or "optimized" data, only varying amounts of entropy (AKA compressibility). My limited knowledge of radio and electrical transmission says that transmitted data is encoded to make effective use of the transmission medium (e.g. 8b/10b encoding, 8-to-14 modulation, TMDS). So,…

I do have a question however. I just read about 8b/10b encoding and I think we can agree that 8b/10b has around 20-25% overhead. This means that for the same throughput (29k/sec) we have 21.75k/sec of data. Since TMDS is a superset of 8b10b than it's a fair assumption to think it has the around the same overhead. Now I know we're only talking encoding here, not encryption but still, I wonder: What kind of encoding ar…

Encoding != Encryption. Encoding is on a line level, so for example, 8b/10b is used in HDMI to prevent a DC bias on the line. The way it works is by making sure within a certain number of 8 bit inputs (I think 3), the resulting 10b outputs will have an equal number of 1s and 0s, therefor preventing the physical wire from charging itself closer to V+ or V-, which in turn helps prevent bit errors.

8b/10b has exactly 25% overhead, this is the difference between baud per second (what the line is capable of transmitting, which is on the 10b side), and bits per second (which is the decoded 8b data). If you have a 10kilobaud/second line, you'll have an effective data rate of 8kbit/second.

Encryption is done before the encoding stage, the encoding scheme doesn't care and isn't affected by the encryption.

Re: Mars to Earth at an average of 29 kbits/s

#85

Post by ericHosick »

ryusage wrote: post #2

It's awesome that they've been involving the public so much in the details of how the rover mission really works. I'd actually been wondering about this exact question, since it seems to be one of the limiting factors for the quality of images and such. From a physics standpoint, sending data is clearly a lot more complicated than I'd previously assumed.

In general, space travel is very risky. It's not like they can iterate. Pulling something off like landing on Mars is very impressive. Keeping this in mind, I feel it was actually quite risky that NASA streamed the landing live.

I think NASA should receive big kudos for taking that real risk.

Re: Mars to Earth at an average of 29 kbits/s

#86

Post by TheAmazingIdiot »

zokier wrote: post #80
TheAmazingIdiot wrote: post #68

Earlier quoted context omitted.

You'd be amazed what you can do just with a watt of radio power. If a transmitter and a receiver is in space, what's the furthest distance the receiver can go away from and have a 1 watt signal still above the noise floor? Assume the transmitter is a perfect omnidirectional. About 9200 miles.

9200 miles is not very much in space. That's like 0.006% of the distance between Earth and Mars.

I well realize that.

I was assuming absolute worst conditions, at 1 watt PEP.

With hardware noise reduction, we can drop easily to .1 watt and still keep everything else consistent.

Next, if we choose a directional antenna, we can also greatly magnify directed output. No sense in directing energy in the ground, is there?

We can also have receiver systems (dish arrays) that can provide great gains in reception and transmitting.

Re: Mars to Earth at an average of 29 kbits/s

#87

Post by merlincorey »

eupharis wrote: post #65
mkramlich wrote: post #64

Earlier quoted context omitted.

> In particular, we are going to need to have a local cache of the internet on Mars. I predict somewhere today at Google an engineer just got an idea for his 20% project. "Red Cache"

I hope so. The trick would be, after there is a cache of the Internet on Mars, to only have to send the changes, not the whole Internet again. Straight snapshots would probably be the only viable option though. And maybe make the policy, once a decade do a complete update. And every year, do "any webpage accessed by colonists in the past year will be updated. And the top 5-15% of webpages by popularity on Earth." And…

Did you know -- there are more ports on the internet than just 53 and 80?

-- A message from your friendly local hacker wizard

Re: Mars to Earth at an average of 29 kbits/s

#88

Post by stephengillie »

vbtemp wrote: post #29
vladd wrote: post #26

Earlier quoted context omitted.

(2): there are plenty of third world countries or foreign government agencies where placing a 80-meter dish is not a problem. Although the process seems a bit intimidating - http://deepspace.jpl.nasa.gov/dsn/features/dsnbuilt1.html

Not just assembling the dish - you also have to know the ephemeris schedule, bit coding scheme (it changes based on circumstances), packet formats, software infrastructure... In the great scheme of things, my friend, Cuba or Zimbabwe secretly erecting an 80 meter dish just to f--k with a rover on mars isn't really a concern. Even though ESA or China or Japan might conceivably have the theoretical capability to do thi…

They could just DOS it by blasting it with white noise.
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