

Why it takes so long to get data back from New Horizons - sytelus
http://www.planetary.org/blogs/emily-lakdawalla/2015/01300800-talking-to-pluto-is-hard.html

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mrb
I dislike that the article doesn't even clearly answer its own question. The
probe only records 8GB of data to one of two 8GB memory bank (one primary, one
backup). Then the data is compressed and sent at either 2 kbit/s or 4 kbit/s
(if they manage to get the higher speed working). By this math it should take
either 12 or 6 months. Actually even less since the data is compressed. So why
will it take 16 months?? Gizmodo needs better writers.

~~~
rootbear
The downlink is actually 1kbps, with 2kbps optional, by using both amplifiers
with opposite circular polarizations. But they can't use that trick all the
time. A nice article with more technical details is at
[http://www.planetary.org/blogs/emily-
lakdawalla/2015/0130080...](http://www.planetary.org/blogs/emily-
lakdawalla/2015/01300800-talking-to-pluto-is-hard.html)

~~~
dang
Ok, since the consensus in the thread is that the submitted article
([http://gizmodo.com/why-itll-take-new-horizons-16-months-
to-s...](http://gizmodo.com/why-itll-take-new-horizons-16-months-to-send-us-
this-we-1717769317)) wasn't very good, let's try switching to that one.

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kayoone
A measly 25-30 years ago the first modems were sending data at 1Kbps, today we
can send data through 7.5 billion kilometres of space at the same speed, i
think that is pretty impressive.

~~~
rootbear
The data rate to Mars isn't shabby, at about 230kbps, roughly four times the
speed of a fast dial-up connection. I used to point that out when the only
Internet at my folks' house was dial up. Eventually, my iPhone cell connection
beat their dial-up connection and I stopped complaining.

~~~
kayoone
Cool, do you have a source for the 230Kbps ? i love to read about stuff like
that.

~~~
karlshea
Data rate to the Mars Reconnaissance Orbiter can be up to 6Mbit/sec:

[https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Te...](https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Telecommunications_system)

~~~
rootbear
Whoa, I didn't know that. I quoted the 230kbit number from memory, when I
researched it several years ago, when my parents still had dial up. I'm not
surprise that MRO is faster.

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hliyan
"A 3-Billion-Mile Snapchat"? Is this what journalism has been reduced to now?

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maaku
The real reason, which hopefully gives some engineering insight, is why not?
It's a flyby mission whose primary data collection occupied only a couple of
days. Like any spacecraft design they had to choose between high bandwidth
(more power == larger RTG == more weight) or large storage (more memory
modules == more weight). There is some tradeoff between the two that yields
the optimal science return, and one would expect it would involve months of
transmission: storing things locally to transmit low power requires way more
resources than a high gain antenna used for 1 week only.

~~~
raverbashing
"and one would expect it would involve months of transmission: storing things
locally to transmit low power requires way more resources than a high gain
antenna used for 1 week only."

You're contradicting yourself there.

Memory today is cheap (even if we're talking about the radiation-hardened
stuff, for "space mission" levels of cheap)

Getting data for a long time takes a lot of resources (basically usage of Deep
Space Network infrastructure)

So, yeah, if they could they would download all the data more quickly, but
you're right that the power budget wouldn't allow it (not sure how powerful it
would have to be to get the data at, let's say 32kbps)

~~~
Zancarius
> Memory today is cheap (even if we're talking about the radiation-hardened
> stuff, for "space mission" levels of cheap)

New Horizons was launched in January 2006, but I suspect you know this and
you're just being dramatic to make a point. :)

The problem with "cheap" as a metric whose context is _solely_ monetary is
that it's ignoring maaku's fundamental argument: More storage is more weight;
more transmitter capability is more weight. So it's not just the cost of the
storage you're looking at but also the cost to get that into space. Launches
are _not_ cheap, and you have to account for every bit of spacecraft weight
which leads to design tradeoffs.

Sometimes you can have one or the other--not both.

Interestingly, the New Horizons program has its roots dating back to the
1990s, at least according to Wikipedia, with cancellations due to projected
costs exceeding levels NASA was willing to fund. But this is par for the
course when dealing with long-term, long-distance missions. Everyone has an
argument about what to put on board, but there's neither budget nor space to
accommodate them all (sadly).

~~~
raverbashing
Solid State Storage, even in 2006, wasn't that heavy (or costly - of course,
for 'space mission' levels of cheap)

So, yeah, I'm not saying it is as small and light as the SD card on my phone
that cost a dollar value having 2 digits and has double that space, but it's
probably 'as good as it gets'

If we're talking about a mechanical data recorder I would agree

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jkot
It is actually VERY impressive if you account for distance, background noise
etc.. It is transferring several gigabytes of data across solar system.

~~~
Sharlin
And the fact that the transmitter uses less power than a light bulb.

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drcross
Would someone care to comment on this hypothetical- If New Horizons was launch
today instead of nine years ago, what equipment improvements would we likely
to have the since that time?

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lmm
I don't think we'd see any qualitative differences at a high level. Just
everything would be slightly better - higher resolution cameras and so on.
Compare to Juno which was launched four years later.

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nullc
[http://www.uhf-satcom.com/amateurdsn/Paper-969.pdf](http://www.uhf-
satcom.com/amateurdsn/Paper-969.pdf) < If you'd actually like to know more
about the communications sub-systems of this spacecraft.

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dharma1
Lasers are the future. [http://www.space.com/23350-laser-space-communications-
incred...](http://www.space.com/23350-laser-space-communications-incredible-
technology.html)

Easier for shorter distances (I think Facebook is researching this for
satellite networks?) however it becomes more challenging the deeper we go into
space because the beam is narrow - but the bandwidth is fantastic.

~~~
ginko
At the distance from Pluto to Earth, Earth's diameter amounts to about 1.5 *
10^-4 degrees in angle. Precisely pointing a laser beam at a given target,
even a large one, at that distance seems almost impossible. Especially since
the space probe probably won't stay perfectly still but will always slightly
tumble.

~~~
BurningFrog
As long as it can get feedback ("a little to the left") somehow, it seems
doable to this software expert.

~~~
icegreentea
Its a 5 hour time lag from Earth to Pluto - it's going to be one hell of a bad
time doing feedback control on that link.

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sophacles
Does anyone know if the 2kbps/4kbps numbers are the transfer rate of actual
data, or the link speed?

I would presume that there are checksums or other error detection/correction
schemes involved - does anyone know if that's a good assumption? If so what
schemes are used and how much overhead they have?

~~~
icw
New Horizons is using a 1/6 rate Turbo code for forward error correction,
meaning 5/6 of the data returned is redundant. Previous discussion:
[https://news.ycombinator.com/item?id=9890476](https://news.ycombinator.com/item?id=9890476)

~~~
sophacles
Thanks!

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jafingi
Is there any error correction in the data stream? I mean, if we loose e.g. 1
minute of data, will it just be sent after the full data had been sent, or how
do we ensure that we grt valid data over such a huge distance?

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tams
All long range transmissions use forward error correcting codes which allow
error detection and data recovery without retransmission (to a certain
degree). See [1] (paragraph 2.5 on page 10) for a bit more on the kinds of
FECs used.

[1]
[http://deepspace.jpl.nasa.gov/dsndocs/810-005/208/208B.pdf](http://deepspace.jpl.nasa.gov/dsndocs/810-005/208/208B.pdf)

~~~
jafingi
Thanks! That's exciting reading.

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sytelus
So looks like the reason is 4 kbps download link due to various reasons.

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nraynaud
what speed is the spacecraft going? is the doppler effect an issue?

~~~
rootbear
The Doppler shift is tracked, as it tells them how fast (in radial terms) the
spacecraft is going. Some years back, I watched the live coverage on NASA TV
of the Cassini spacecraft going into orbit around Saturn. When they went into
orbit, they didn't announce it with some Hollywood dialog like "Saturn orbit
confirmed!" No, what they said was, "The Doppler curve has flattened!" and the
room went wild! It was wonderful.

~~~
david-given
...they actually forgot to account for Doppler shift when deploying the
Huygens probe to Titan: as planned, the 5.5km/s velocity change as Huygens
reentered Titan would have caused Cassini's receiver to lose lock on the
signal (due to a timing change, rather than a frequency shift, IIRC). As
Huygens had no storage and ran on batteries this would cause a total loss of
mission.

The problem was discovered after launch, and corrected for --- not by
reprogramming the receiver, but by changing Cassini's flight profile to reduce
the Doppler shift:
[http://www.thespacereview.com/article/306/1](http://www.thespacereview.com/article/306/1)

