New Horizons at Pluto

Gawdzilla Sama said:
But it was four hours ago. ;)
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~Four hours ago:

July 14 05:40:45 EDT
New Horizons is taking a spectrum of Pluto with Alice from 107772.52 km away. New Horizons is 31.8 AU from Earth.
July 14 05:41:09 EDT
New Horizons is taking a spectrum of Pluto with LEISA from 107444.60 km away at est. resolution 6.7 km/pix. New Horizons is 31.8 AU from Earth.

Next scheduled communication that I see:

July 14 10:50:00 EDT
New Horizons soon to be communicating with Canberra 34 m (DSS-34) http://eyes.nasa.gov/dsn/dsn.html
 
I have it in my head that the transfer rate at that distance is 1k baud, so it does take a long time to send even a single picture. And I imagine there's probably some redundancy in there that stretches it out even further.
 
sphenisc said:
Why does transfer rate depend on distance?
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Large distances mean weak signal, which means you basically have to measure each "bit" for longer times to make up for that. If you tried to send high data rates, you'd get too many errors from background noise.
 
Ziggurat said:
Large distances mean weak signal, which means you basically have to measure each "bit" for longer times to make up for that. If you tried to send high data rates, you'd get too many errors from background noise.
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I know of one system with a (ahem) possible military application where the data was sent four times. They were received and "stacked", and bits that didn't match the others was eliminated. The issue there was when two bit matched each other but not the other two. You got at least two copies of the message then and a human decided which one was correct.

Any idea of they're using the same system with New Horizons? The date of that project would imply it may have gone public by now.
 
sphenisc said:
Why does transfer rate depend on distance?
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Several reasons. Mainly the radio signal isn't coherent and expands as it travels and a weak signal (as perceived from the receiver) needs more error correction.
Data rate is ~50kb/s at 5AU, 11kb/s at 10AU, 3kb/s at 20AU DOWN TO ~700b/s at 50AU
There's limited time available on the 70m dishes, even for NH.

Further the twin tube amplifier technique (which speeds data rate by about 90%) requires most of the power available from the RTG, no no data gathering unless the guidance/control system is shut down.
The plutonium continues to decay so by the time New Horizons is doing the Kuiper belt flyby it won't be able to run both transmitters.
 
Ziggurat said:
Large distances mean weak signal, which means you basically have to measure each "bit" for longer times to make up for that. If you tried to send high data rates, you'd get too many errors from background noise.
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That's my understanding as well.

We really need to discover subspace channels.
 
Gawdzilla Sama said:
I know of one system with a (ahem) possible military application where the data was sent four times. They were received and "stacked", and bits that didn't match the others was eliminated. The issue there was when two bit matched each other but not the other two. You got at least two copies of the message then and a human decided which one was correct.

Any idea of they're using the same system with New Horizons? The date of that project would imply it may have gone public by now.
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Try this book.
 
Gawdzilla Sama said:
I know of one system with a (ahem) possible military application where the data was sent four times. They were received and "stacked", and bits that didn't match the others was eliminated. The issue there was when two bit matched each other but not the other two. You got at least two copies of the message then and a human decided which one was correct.

Any idea of they're using the same system with New Horizons? The date of that project would imply it may have gone public by now.
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Not likely. I'm sure they're using error correction algorithms, but it's probably much more sophisticated than what you describe. Your description is actually pretty inefficient, and while it might have some advantages in robustness, it's probably not worth it for this sort of application.

I'll give a simple example. You may have heard of "parity bits" for computer memory. The idea is you take one byte (8 bits) and add a 9th bit to it. The 9th bit gets assigned to a 0 if there are an even number of 1's in the first 8 bits, and a 1 if there are an odd number of 1's in the first 8 bits. That way, you can detect any 1-bit errors within that byte. However, you can't detect 2-bit errors, and you can't correct errors.

But you can generalize and abstract this idea with math. Consider each byte to be a vector in an 8-dimensional discrete space. Adding the parity bit transforms this vector into a 9-dimensional discrete space. But not every vector in this 9-dimensional space is a valid vector, so you can (sometimes) tell if you've got an error.

More sophisticated methods using this principle may be able to not only detect errors, but actually correct them. For example, if you take a collection of 8 bytes (64 bits), add on a 9th byte (for a total of 72 bits), you're projecting your 64-dimensional vector into a 72-dimensional space. For the parity bit version, we end up with half the vectors in the new space being valid and half being invalid, but in the 64-bit to 72-bit conversion (same fractional overhead), only 1 out of every 256 vectors is valid. Now you can detect more errors, and if the errors aren't too large, you can actually correct them, because a single-bit error will be closest to one valid vector than other valid vectors.

This stuff is really common in communications and data storage (CD's and hard drives all use data correction algorithms), and there's a large body of mathematical work describing optimal efficiencies versus expected error rates, etc. NASA will undoubtedly be using error correction in their communications, but it's probably standard off-the-shelf stuff, because this data isn't "sensitive" in the way that military communications might be.
 
Also, the antenna on New Horizons is not steerable, so the whole spacecraft has to be turned to transmit, which means you're no longer collecting data. So you've got to balance the two, and right now at closest approach it's heavily skewed toward data collection, with bulk transmission later after there's not much to look at.*



*Until they reach the further KBOs, but that's a while from now. :)
 
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