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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

#51

Post by pothibo »

nitrogen wrote: post #34
pothibo wrote: post #28

Earlier quoted context omitted.

This kinda raises question. I for one would not use encryption just to maximize bandwith efficiency. Deep space program are no where close to being a tactical advantage over another country. I wouldn't see why someone would like to screw it up. On the other side, there's always a moron out there to crash the party. So even if I wouldn't use encryption, I wouldn't be surprised if they use it.

I for one would not use encryption just to maximize bandwith efficiency. Encryption typically doesn't make messages longer.

We're not talking about your average data. We're talking about optimized data. Of course there's an overhead.

EDIT: removed specific type of data

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

#52

Post by accountoftheday »

pothibo wrote: post #51
nitrogen wrote: post #34

Earlier quoted context omitted.

I for one would not use encryption just to maximize bandwith efficiency. Encryption typically doesn't make messages longer.

We're not talking about your average data. We're talking about optimized data. Of course there's an overhead. EDIT: removed specific type of data

There isn't. You could easily achieve zero overhead by shipping a rover with a OTP for time-keyed IVs and just not do any padding.

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

#53

Post by ChuckMcM »

arrrg wrote: post #36
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)

Latency matters, too. Often all the data is necessary to make decisions so (for example) only immediately transmitting a limited subset of the data is not really an option. It’s not like that rover is driving around autonomously. It has some autonomy, but it can’t decide what’s interesting and what’s not, what has to be investigated and what not.

True enough, but it would help in answering the critics who want NASA to return a 1080p/120fps 3D movie from mars. Send the movie via the flash drive.

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

#54

Post by accountoftheday »

nemilar wrote: post #48

This may be a silly question, but I don't understand why the bandwidth is so low. I understand the latency will be high, since you're limited by the speed of light; but wouldn't you be able to get more bandwidth just by increasing the spectrum of light used, as well as the baud of the transmitter/receiver?

Even with optimal coding you are still limited by achievable signal over noise due to the Shannon limit.

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

#55

Post by Sharlin »

zhoutong wrote: post #5
Dylan16807 wrote: post #4

So those are the lander->earth and lander->sat speeds. How fast can the satellites transmit to earth? If we somehow always had a sat in reach of the lander, how much data would we be able to send? Edit: Did the research. MRO's antenna can handle .5 to 4 megabits depending on the distance between planets. Wow, having to wait for flybys is a huge bottleneck. http://mars.jpl.nasa.gov/mro/mission/communications/commxban.…

I wonder if it's possible to launch an orbiter in Mars synchronous orbit. That'll eliminate the bottleneck.

Would have been nice if Mars Telecommunications Orbiter [1] had flown. In addition to a higher orbit more suitable for communications, it would have included an experimental high-bandwidth laser link to Earth. Alas, the project got cancelled for budgetary reasons.

[1] http://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter

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

#56

Post by nitrogen »

pothibo wrote: post #51
nitrogen wrote: post #34

Earlier quoted context omitted.

I for one would not use encryption just to maximize bandwith efficiency. Encryption typically doesn't make messages longer.

We're not talking about your average data. We're talking about optimized data. Of course there's an overhead. EDIT: removed specific type of data

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, if by "optimized" data you mean "high entropy" data, encryption should have no effect on that. Or, if by "optimized" you mean encoded for the transmission channel, you would run your encryption process before the channel encoding process.

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

#57

Post by DavidSJ »

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)

Unfortunately, to do a gravitational slingshot, you need a body other than the one relative to which you want to accelerate. So you can use Mars to accelerate relative to the Sun, but not relative to Mars.

This means if you're already in Mars orbit you need fuel(TM) to escape. One might consider a slingshot around Deimos or Phobos but I don't think that'd be helpful enough to justify its cost (could be wrong, but I've never seen it suggested in any Mars->Earth mission plan).

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

#58

Post by adestefan »

ChuckMcM wrote: post #53
arrrg wrote: post #36

Earlier quoted context omitted.

Latency matters, too. Often all the data is necessary to make decisions so (for example) only immediately transmitting a limited subset of the data is not really an option. It’s not like that rover is driving around autonomously. It has some autonomy, but it can’t decide what’s interesting and what’s not, what has to be investigated and what not.

True enough, but it would help in answering the critics who want NASA to return a 1080p/120fps 3D movie from mars. Send the movie via the flash drive.

The initial problem is building and sending the camera to Mars.

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

#59

Post by bwr »

I'm guessing there is a lot of data that never gets sent to earth. Do they send thumbnails of the images to determine what is interesting? Is there a software tool available that creates a thumbnail and then creates a copy of the original image that is dependent on the thumbnail to be recreated. If you can't tell I wasn't really sure how to word that last sentence. Basically, less data would be required to transmit because you are only sending the data required to reconstitute the real image. Depending on the size of the thumbnail this might result in a negligible difference.

For example, a very naive implementation might create a thumbnail from every pixel where x or y is odd and then when/if you want the original image you can get the data required to put the image back together (it would be a similar sized thumbnail but with the even pixels) For this naive approach the thumbnail would likely be much larger than you want and would not help at all :)

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

#60

Post by eric-hu »

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)

Such drives would be far more expensive than what we use daily. Factor in electronic shielding needed to prevent soft-errors from cosmic rays: http://en.wikipedia.org/wiki/Cosmic_ray#Effect_on_electronic...
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