Why it takes so long to get data back from New Horizons

http://www.planetary.org/blogs/emily-lakdawalla/2015/01300800-talking-to-pluto-is-hard.html

by sohkamyung • 11 years ago

119 37 11 years ago

geuis• 11 years ago

Very interesting overall. I like this small factoid, "They can be made even smaller with lossy JPEG compression, but for optical navigation, precision counts; the pictures have to be returned losslessly." I think this implies that the most distant JPEG software is currently in route to Pluto.

mturmon• 11 years ago

Use of lossy compression for science imagery is contentious. Because it's hard to characterize how the compression will affect as-yet-unseen science targets, it's hard to get scientists to accept lossy compression.

You can sometimes get acceptance of prioritized downlink -- let automation decide what is most interesting, and downlink that first. Variations on that idea have been used for Mars rovers.

simonh• 11 years ago

I once visited a crackpot aliens-on-the-moon conspiracy website. They were showing zoomed-in images of lunar craters purporting to show regular, artificial-looking features indicating alien activity.

Of course in context I'm sure you can guess what was going on. The images were heavily compressed jpegs and I soon realised the 'regular features' were compression artifacts.

Still, it goes to show why some scientists don't like lossy compression. When you're looking at fine detail it's hard to be sure whether what you're seeing is 'real' or not.

FeeTinesAMady• 11 years ago

I don't think it implies that at all. I think they're saying that the pictures could be compressed with JPEG if they wanted them to be, but they very clearly state that they don't, and nothing in the article indicates that the spacecraft would have any other use for JPEG software.

abalone• 11 years ago

I just love the image of a spaceship hacked to do cartwheels in space while shunting power to its backup antenna to double the Tx rate. And that this was developed after it was already in space.

After this and watching Apollo 13 I now have the impression "hacking" is fundamental to space exploration.

lelandbatey• 11 years ago

You should read the book "The Martian" by Andy Weir if you haven't already. Doing things in space can require a lot of hacking!

brazzy• 11 years ago

...sometimes even hacking of the "forcefully applying a sharp instrument" variety. That poor MAV!

teh_klev• 11 years ago

The Martian is a thoroughly agreeable read. I thought it might be a bit dry but it's interspersed with some really funny bits that made me laugh out loud.

car• 11 years ago

Awesome book. And now this: http://www.imdb.com/title/tt3659388.

mutagen• 11 years ago

The Deep Space Network has only three 70-meter dishes, and there is a lot of competition for time on them;

It seems like I've heard this time and again, that certain data or measurements couldn't be returned from planetary spacecraft because of limitations in the DSN. I'd guess that a relatively modest investment could double or tripe the DSN availability and bandwidth simply through more antennas and the associated receivers. I'm guessing that the reasons are political (funding), are there technical issues I'm unaware of?

Off to read up on this.

edit: They are adding antennas!

rjaco31• 11 years ago

Maybe it's just that building a 70-meter dish is freaking expensive?

DanBC• 11 years ago

There are plenty of very rich people who could spend money on 70 m dishes instead of soccer clubs or whatnot.

Adding 8 antennas and infrastructure to the VLA cost about $140m http://www.nrao.edu/pr/2000/evla/

The Green Bank telescope (largest moving land object; 100 meter dish) costs a measly $10m per year to run. (That funding is at risk) http://en.wikipedia.org/wiki/Green_Bank_Telescope

jessriedel• 11 years ago

I think it's pretty cheap amortized over several billion-dollar missions.

EarthLaunch• 11 years ago

But how can you point stably at Earth with your guidance system shut down?

Why doesn't it stay pointed at Earth - is there something destabilizing it? Solar wind, dust, power source, minutely inexact stabilization?

NamTaf• 11 years ago

Any little destablisation will cause it to start to move, because there's nothing to react it. By spinning something around its axis of maximum or minimum moment of inertia, you stablise it - like a bullet being rifled.

The destablisation will come from any number of things - minor gravitational inconsistencies, solar wind, dust, heat radiation from the power source, etc.

frik• 11 years ago

What about the Arecibo Observatory? It has a 1,000 ft (300 m) diameter dish. It is well know from the James Bond Golden Eye (1995) movie: http://en.wikipedia.org/wiki/Arecibo_Observatory .

Are there physical limitations that would prevent a communication to Pluto?

The Parkes Observatory has only a 230 ft (70 m) dish. http://en.wikipedia.org/wiki/Parkes_Observatory . It is well known from the first Apollo moon landing and The Dish (2000) movie http://www.imdb.com/title/tt0205873/

rimantas• 11 years ago

How long do you think communication session using Arecibo would last?

Edit: apparently some did not get the point I was making. Arecibo's dish is fixed, so there is practically no way to track the object. And earth is rotating. If you had a chance to see the stars through telescope, it rotates FAST. With Arecibo you can only catch a blip of communication.

_jcqb• 11 years ago

Suppose like you say, all earth-bound telescopes have a small window of observation. How many hours a day would that be? Aside from watching Contact or Goldeneye, my only source of knowledge about dishes is a C-band. But, according to its wikipedia entry, Arecibo has a 40 degree range of 'motion'.

"To aim the device, the receiver is moved [by cables] to intercept signals reflected from different directions by the spherical dish surface." ... "A parabolic mirror would have varying astigmatism when the receiver is off the focal point, but the error of a spherical mirror is the same in every direction."

The limit: "This allows the Telescope to observe any region of the sky in a forty-degree cone of visibility about the local zenith (between −1 and 38 degrees of declination)".

From universe today, "Pluto’s orbit is also highly inclined. This means that it doesn’t orbit within the same plane as the rest of the Solar System. Instead, Pluto orbits at an angle of 17-degrees."

Arecibo is at latitude 18.3 or so. Not at the equator, but for half a year it's probably tilted more favorably to the middle. Does this make Arecibo even remotely practical for the application?

TorKlingberg• 11 years ago

Arecibo can only be only be pointed slightly by moving the hanging receiver. To communicate with a spacecraft far away you want a dish pointed straight at it, to get a high gain.

jobigoud• 11 years ago

Argh, the idea that an autonomous spacecraft gather more interesting data than it can possibly transmit back to Earth.

I wonder if they considered lossless video compression approaches, and combining an entire set of images into a single file.

hobbes78• 11 years ago

Video compression (even lossless) leverages the fact that successive frames are very similar; that's not the case here, as photos can be taken hours apart, in various directions, and in various zoom levels. So you'll get basically the same compression either way...

jetskindo• 11 years ago

All of the sudden I have no problem waiting for my town hall to finish upgrading in 2 weeks in clash of clans.

Looks like some people use that same 2 weeks time to do far more important things.

bmaeser• 11 years ago

"between here and pluto there is a very large number of nanoseconds"

https://www.youtube.com/watch?v=JEpsKnWZrJ8

raverbashing• 11 years ago

Funny how it's as slow as the first modems

Of course, the channel adds a lot of noise and attenuation

(I'm not sure the rate they mention is the effective ratio or before Error Correction Codes)

kabdib• 11 years ago

110 or 300 baud were the "first" modems. My first 1200 baud modem just screamed. In comparison, of course.

An effective 2000 bits/second from Pluto is the stuff of science fiction. (Actually, even SF from the 50s was full of situations like being out of radio contact with Earth because of distances on the order of an AU or so. With available technology they were probably right).

mchouza• 11 years ago

It's mostly a question of power. The 1987 design for the TAU probe communication system would have handled 20 kb/s from 1000 AU... by using between 100 and 260 watts for the transmitter (and relatively big antennas):

http://ipnpr.jpl.nasa.gov/progress_report/42-89/89L.PDF

throwaway_yy2Di• 11 years ago

This paper says the downlink uses a rate 1/6 forward ECC -- 6 encoded bits for every 1 message bit. From the context, the ~1 kbps figure should mean the information rate; the tone rate would be 6x higher.

    6.3. DOWNLINK SYSTEM PERFORMANCE
    
    The downlink card in each IEM is the exciter for the TWTAs
    and encodes frame data from the spacecraft C&DH system into
    rate 1/6, CCSDS turbocoded blocks.

http://www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf

and more details,

http://www.boulder.swri.edu/~tcase/NH%20RF%20Telecom%20Sys%2...

Why it takes so long to get data back from New Horizons

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