Why it takes so long to get data back from New Horizons | Hacker News Reader

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

planetary.org

89 pointssytelus53 comments

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.

Because redundancy in transmission? Also I would be very surprised if the the data is not already compressed.

One possibility might be DSN receiver availability. There are only three sites in the network, and they have several other missions to support.

the DSN (deep space network) is a shared resource under high contention---it is effectively used by every deep space mission from Voyager to MSL(via MRO relay). missions generally have a dsn rep that negotiates dsn tracks based on need. high priority events like MSL's EDL or spacecraft safing events generally take priority. i haven't looked into the specifics but it is unlikely that new horizons gets 24hr coverage for 12 months straight.

this totally makes sense - you should work for comcast!

Given Comcast's reputation, it sounds like you're being insulting. Was this your intention?

If it was, you should actually read about the DSN. It (like most things pertaining to extraterrestrial operation) is pretty damn cool.

I thought not meant to be insulting but hopeful comcast would then improve.

neither, it was just a joke :) I mean that this is a great way to explain 2 kbit/s or 4 kbit/s (makes sense in context) and I was just joking that that's about as bad as Comcast's service sometimes... :) (And so with explanations that great he should be their PR guy...)

I guess it didn't go over too well, but no worries! You miss 100% of shots you don't take...

You can see what DSN is up to right now here- http://eyes.nasa.gov/dsn/dsn.html

In the press conference shortly after the flyby, one of the staff (I can't remember her name) said that the speed depends on the position of the probe relative to the ground antenna. So not only is transmission not 24 hrs, but when the probe appears to be on the horizon, transmission speeds are slower.

Lately I've been checking out the real time reporting page for the DSN (http://eyes.nasa.gov/dsn/dsn.html), and frequently one of the three big dishes at Goldstone, Arizona, USA, Madrid, or Canberra is allocated to a satellite, but is not right then doing any communicating, presumably because it's waiting for the earth to turn enough to get line of sight.

Right now New Horizons is downloading at 2.11 kbs to Canberra, the max with both amplifiers transmitting with opposite circular polarizations. For a bit of comparison, Rosetta is talking to Goldstone right now at 104.86 kbs, with a received power 3 orders of magnitude stronger. And Voyager 2, which doesn't have much to say, is still plugging away with a smaller Canberra antenna at 159 b/s, and a received power about that of New Horizons.

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...

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

Great article. And for anyone who didn't see it, to call this mode "optional" is putting it extremely mildly: to get enough power to run both amplifiers at once, they have to shut down the spacecraft's guidance system (!) and keep it pointed at Earth by using the thrusters to spin it up like a top. And this whole crazy scheme was developed after launch.

isn't the fact that the transmitter is going further away during those 6 or 12 months adding to the time as well? or do the distances travelled not have much of an impact over that time frame?

Forward error correction can add significant overhead. From the specification of the RF subsystem posted earlier (http://www.boulder.swri.edu/~tcase/NH%20RF%20Telecom%20Sys%2...), 5/6 of the data returned is redundant.

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.

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.

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

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

https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Te...

I couldn't even get DSL that fast at the last place I lived.

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.

We could send a lot more data over that same distance if New Horizons had a large power supply. It was just limited by the plutonium supply available at launch.

How much would theoretically be possible with current tech ?

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

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.

"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)

> not sure how powerful it would have to be to get the data at, let's say 32kbps

My back-on-the envelope calculation says the probe would have to emit with at least 6 times more power for that than it emits now (and I can be way off as I'm not taking into account all the competing noise at that distance). The RTG produces around 200 W for everything on the probe, and I estimate just the cost of the fuel for the current RTG alone in tens of millions. It's simply not worth optimizing for the fast burst when you expect the probe to be able to communicate for a few years more.

> 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).

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

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

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

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?

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.

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.

Lasers are the future. http://www.space.com/23350-laser-space-communications-incred...

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.

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.

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

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.

I'm guessing that the laser itself is wider than that. According to this fellow, a realistic beam width is 3.5 microradians:

http://space.stackexchange.com/questions/2005/how-much-power...

That's about 2e-4 degrees, wider than Earth as seen from Pluto.

Is the earth visible in an optical telescope from the spacecraft? This should be the basis for automatic tracking... (transmit the laser through the same telescope back to earth). The transmitter on earth could track on the "star" of the laser from the spacecraft.

I've been trying to understand the noise temperature of erbium doped fiber amplifiers- not nearly as good as DSN masers.

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?

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

Thanks!

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?

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

Thanks! That's exciting reading.

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

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

Hardly, all you have to do is listen to a slightly lower or higher frequency. To be exact: 58536 km/h (spacecraft) divided by 1079252848.8km/h(speed of light) -> So that's frequency shift of 0.0054%

(If I am wrong please correct me.)

Yeah, I might have been stupid. It has nothing to do with the carrier frequency (I was thinking low frequency vs high spacecraft speed, instead of propagation speed vs aircraft speed).

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.

...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

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