The Webb Space Telescope’s profound data challenges

https://spectrum.ieee.org/james-webb-telescope-communications

by MindGods • 4 years ago

318 160 4 years ago

spectrum.ieee.org

In addition, according to Carl Hansen, a flight systems engineer at the Space Telescope Science Institute (the science operations center for JWST), a comparable X-band antenna would be so large that the spacecraft would have trouble remaining steady for imaging.

Why would a large antenna make the spacecraft less steady? What's the mechanism behind it?

jacquesm• 4 years ago

At a guess: solar pressure.

anotherhue• 4 years ago

Solar wind exposure perhaps?

marcyb5st• 4 years ago

I believe it is both due to solar wind and the fact that it would act as a tiny light sail.

jcims3• 4 years ago

Could also be more resonant modes in the structure that would need to be damped out.

onetwentythree4• 4 years ago

The antenna is steered to point towards the DSN antenna on the earth. A larger moving mass would make it harder to maintain telescope pointing while the antenna is moving.

In reality, the antenna pointing is 'paused' during each science observation, unless pointing is needed due to the length of the observation.

nextaccountic2• 4 years ago

Oh, the antenna doesn't need to just point in the general direction of Earth, it needs to point to somewhere in the surface. That makes sense, having a narrower beam would save power and achieve higher bitrates

Does this mean it has only a 12-hour window to transmit? Or there's multiple antennas on Earth?

mNovak• 4 years ago

It's because large, directive antennas are still 'dish' style and have to be mechanically pointed at the target (Earth based DSN receiving antennas). That pointing causes vibrations and potentially a shifting center of mass.

Phased arrays allow beam pointing without mechanical movement, but are very expensive for large high gain antennas.

Victerius3• 4 years ago

The article isn't as interesting as I thought it would be.

JWST can produce up to 57 GB each day (although that amount is dependent on what observations are scheduled).

Just use a lot of hard drives? And delete junk data when it's no longer needed.

rsfern2• 4 years ago

The problem is how to get the data back to earth, no?

Victerius3• 4 years ago

JWST is transmitting data back to Earth on a 25.9-gigahertz channel at up to 28 megabits per secon

0.028*3600 = 100 Gbit/hour

No problem here.

pweezy• 4 years ago

This kind of communication seems like an interesting problem for Starlink (or other satellite internet constellations) to solve. What if the satellites had extra antennas pointed outwards and relayed the data back to the ground at high bandwidth?

Seems like it would avoid a lot of the issues like atmospheric interference, frequency congestion, and careful placement of receiver infrastructure.

jvanderbot• 4 years ago

the gynormous size and sensitivity of ground antennas dwarfs those other factors. Otherwise, don't you think that an agency known for sending things to space would have considered sending things to space?

However, when using laser comms, sometimes a delay does make sense.

herendin• 4 years ago

What if the satellites had extra antennas pointed outwards

Pointed outwards towards what?

And I don't understand how that solves atmosphere interference issues. Still gotta go through the atmosphere at least twice for ground to ground

In actual fact, I believe the long term Starlink plan does include satellites in higher orbits, but I don't know their role

Lowest long distance latency is potentially a big competitive advantage for Starlink, so they'll probably try to get the shortest ground to ground path for some high priority customer data, and higher orbits on the signal path will detract from that goal

bythreads• 4 years ago

I never understood why the spacex sattelites dont have shitty webcams on their "dark side"

Imagine the resolution of that array as it spins around earth...

NKosmatos• 4 years ago

This is the type of articles we (data junkies) need to see and read! Highly interesting to see the transmission rates, storage capacity and other data related considerations that went through the design of James Webb telescope. Now, if only there were more funds allocated to antennas/dishes on the DSN (Deep Space Network) [0] to be able to service all ongoing and future space missions, that would be great.

[0] https://eyes.nasa.gov/dsn/dsn.html

cycomanic3• 4 years ago

Not antennas, lasers and lenses. For intersatellite links optical is much better, in particular if something is far away like the JWT. Note that NASA is planning to have significant optical links as part of DSN.

Sporktacular14• 4 years ago

The Menzel guy kind of address that at the end, though I wish he went into more detail.

My guess is his attitude is why use less tested technology when the capacity of the Ka band link does the job dependably.

As more probes go up and antenna time grows shorter, increasing link capacity will become more necessary. Until then, why experiment on a 10 billion dollar project?

qazpot• 4 years ago

Data gathered from its scientific instruments, once collected, is stored within the spacecraft’s 68-GB solid-state drive (3 percent is reserved for engineering and telemetry data)

Hope the SSD does not fail after 32768 or 40000 hours of operation.

Victerius3• 4 years ago

1365 to 1666 days, or 4-5 years of operations.

Maybe they should have chosen a HDD drive? Cosmic particles can flip bits on an SDD, can't they?

curling_grad• 4 years ago

I guess hard drives would be dead because of enormous acceleration on launch day.

wongarsu• 4 years ago

Since cosmic rays are charged particles I'd expect even more bit flips on the magnetic platter of a HDD.

willis936• 4 years ago

Data at-rest in SSDs isn't really at rest. The controller is constantly scrubbing and correcting errors.

The real hazard with SSDs is leaving them unpowered. I know of a story of several systems purchased a decade before they were needed and by the time they were used the boot drives were corrupted.

tambourine_man• 4 years ago

This thing uses gyroscopes to orient itself. My guess is the less moving parts, the better

aqme28• 4 years ago

IIRC the hard part with this thing is the lenses. I wonder how hard it would be for astronauts to swap an SSD if it came to that.

re-actor• 4 years ago

No currently available spacecraft can make the trip and service JWST, it's also not really intended to be serviced.

patmorgan23• 4 years ago

JWST is not serviceable. If the ssd dies, then it's dead. There's no replacing it.

317070• 4 years ago

The JWST is about three times further than the furthest a human has ever been from earth, which was on the moon.

So, it would be __really__ hard for astronauts to swap an SSD.

sp3322• 4 years ago

Maybe we could send up a data buffer with a high-bandwidth, short-range radio.

dotnet002• 4 years ago

Even if a crewed spacecraft could be sent to it (honestly not that crazy given JWST's predicted lifespan of 20 years. Starship should be crew rated in 10 years at most and if not, Orion could be sent with some sort of expanded service module for the trip) the issue would be that docking to or even approaching the telescope is extremely risky.

You don't want to fire any thrusters in its direction to avoid damaging the sunshield or the mirrors, but of course to slow down at it you would need to do that at some point.

If the SSD failed and a constant connection to Earth cannot be maintained, the more realistic solution would probably be to launch a satellite to a high orbit to maintain permanent connectivity with JWST which can act as a store-and-forward relay, effectively replacing the SSD without having to actually go to the telescope.

iamgopal• 4 years ago

68 GB ? Just ? Shouldn’t that be some 10-15 1-2 TB SSD in parallel ?

danpalmer2• 4 years ago

Technology for space lags behind consumer technology by 10-20 years. There are a few reasons for this:

- Long lead time to test and certify hardware.

- Higher reliability requirements (e.g. must work non-stop for 10 years)

- Must be able to operate in a higher radiation environment with little to no cooling. In a vacuum, and in zero gravity, cooling works very differently to how it does on Earth.

- These missions often take a decade or more to come together, and changing requirements throughout that process is hard, costly, and risky, so often they stay the same from the beginning.

Notably, SpaceX are bucking this trend a bit with their avionics which just runs on standard Linux machines rather than specialist machines or with a realtime OS, but they have mission lengths measured in minutes to hours, not decades.

zackmorris• 4 years ago

Sometimes the smartest people in the world still stink at their short game

urthor3• 4 years ago

All jokes aside, tbere's an article floating around about NASA's software development practices.

They are big TLA+/formal specification fans (the article predates TLA+'s rise) with well resourced and antagonistic Q/A engineers.

That hard drive will have been ordered, custom, and the controller verified by hand I imagine.

_fat_santa• 4 years ago

Also likely that they are already super experienced with that particular SSD. I read an article a long time ago that talked about a spacecraft with some camera on it. They said the camera was 10 years old when it was installed and although their were better cameras out there, they picked this one specifically because of reliability.

I’m sure the same happened here.

boilerupnc• 4 years ago

Could it perhaps be this? Interesting read about the level of effort and process invested in the Space Shuttle's software (~1996) [0]

For a TLDR, this answer [1] is great.

My favorite excerpt: The Shuttle software consists of ca. 420,000 lines. The total bug count hovers around 1. At one point around 1996, they built 11 versions of the code with a total of 17 bugs.

[0] https://www.fastcompany.com/28121/they-write-right-stuff

[1] https://space.stackexchange.com/questions/9260/how-often-if-...

onetwentythree4• 4 years ago

I know this is a joke but...

JWST does not use a typical flash-based SSD. The mass storage is all SDRAM. There are layers of error correction and scrubbing to handle bit flips.

boredemployee• 4 years ago

that means no sandisk or the like

toolslive4• 4 years ago

All of the communications channels use the Reed-Solomonerror-correction protocol.

I think you don't want to use Reed Solomon for this, but turbo codes or most likely online codes.

https://en.wikipedia.org/wiki/Online_codes

Sporktacular14• 4 years ago

Maybe it's because of RS codes superior performance during burst errors while the SNR involved isn't so low as to give convolutional codes a net advantage. Maybe it's Error Floor.

Or maybe we should just assume that some of the world's most qualified RF and comms engineers behind these decisions did their homework.

toolslive4• 4 years ago

maybe because online codes only came available after 2004, and the choice was already made and the picked solution sufficed so was never revisited. So the question becomes: when did they do their homework?

toolslive4• 4 years ago

Sigh, I might not be top notch anymore, but I am the (co) author of some patents in the field. Questioning RS for this use case is legit.

cle• 4 years ago

Or maybe we should just assume that some of the world's most qualified RF and comms engineers behind these decisions did their homework.

I'd rather read an interesting discussion of the tradeoffs than a shallow dismissal of curiosity because "they know what they're doing".

cycomanic3• 4 years ago

All of the communications channels use the Reed-Solomonerror-correction protocol—the same error-correction standard as used in DVDs and Blu-ray discs as well as QR codes.

I find that somewhat hard to believe, LDPC are well established and much more suitable. I would have expected that they would use a DVBS2 standard code.

Sporktacular14• 4 years ago

I suspect they know what they're doing.

mfarstad• 4 years ago

idk the article also mentions they've been working on it for 20 years I wouldn't be surprised if they just got to a point that was good enough and then didn't want to mess with things

real tragedy is that they didn't use cutting edge web7.0 tech for their front end smh

hoseja3• 4 years ago

Know how to maximize budget.

capableweb• 4 years ago

Yeah, most likely.

None the less, I'm also curious about the choice, but couldn't find a lot about it. There has to be some trade-off I guess to using LDPC instead of Reed-Solomon. I only found this paper, but haven't read through it, so no conclusion as of yet:

https://trs.jpl.nasa.gov/bitstream/handle/2014/45387/08-1056...

Efforts are underway in National Aeronautics and Space Administration (NASA) to upgrade both the S-band (nominal data rate) and the K-band (high data rate) receivers in the Space Network (SN) and the Deep Space Network (DSN) in order to support upcoming missions such as the new Crew Exploration Vehicle (CEV) and the James Webb Space Telescope (JWST). These modernization efforts provide an opportunity to infuse modern forward error correcting (FEC) codes that were not available when the original receivers were built. Low-density parity-check (LDPC) codes are the state-of-the-art in FEC technology that exhibits capacity approaching performance. The Jet Propulsion Laboratory (JPL) has designed a family of LDPC codes that are similar in structure and therefore, leads to a single decoder implementation. The Accumulate- Repeat-by-4-Jagged-Accumulate (AR4JA) code design offers a family of codes with rates 1/2, 2/3, 4/5 and length 1024, 4096, 16384 information bits.1, 2 Performance is less than one dB from capacity for all combinations.

My guess at this point is probably just "We've used Reed-Solomon a bunch, we know it works. We're working on newer techniques, but lets use what we know works"

lacker• 4 years ago

In general, cutting-edge astrophysics does not necessarily use cutting-edge software engineering.

For example, the JWST also uses a proprietary version of JavaScript 3, made by a bankrupt company.

https://twitter.com/michael_nielsen/status/15469085323556577...

I think there's a pretty good chance that their data encoding scheme was working, and so they just left it in a working state, without upgrading it to use modern best practices.

guenthert• 4 years ago

They sure do, but I'm less confident that the statements made in the interview can be uphold to standards of scientific scrutiny.

mhh__• 4 years ago

Note that this mission was specced and designed ages ago too, so just as the observations it makes are views of the past, the engineering to do so is a time capsule too

jcims3• 4 years ago

JWST started in 1996. According to Wikipedia that's largely when LDPCs were 'rediscovered'.

https://en.wikipedia.org/wiki/Low-density_parity-check_code#...

vinay_ys• 4 years ago

Why is this project on such a long timeline? I wonder what was the longest critical path in this project plan?

rswail• 4 years ago

I like that the article had an error missing the space between "Reed-Solomon" and "error-correction". A bad subeditor, or an Easter Egg joke? :)

nonameiguess• 4 years ago

Others have mentioned the advantage in terms of burst errors. That is fairly common for radio signals coming from space. Think of an airplane flying through the signal path or something. I know in the NRO all of the radio downlink used BCH for error correction that could correct up to 4 bits per byte, and DPCM for compression, which also does particularly well with long runs of the same pixel value, something pretty common with space imagery (most of what you're looking at is black). BCH allows you to pick exactly how many bits per byte you want to be able to correct, which can be tuned based on known error characteristics of your signal. Part of it may just be these systems have been around a long time and we already have extremely well-tuned and efficient implementations that are known to work quite well with very large volumes of data inbound from space.

dorfsmay2• 4 years ago

Latency is 1h 20m one way (1.5M km / 300_000 km/s)?

EDIT: got my units wrong, see further down the thread.

sp3322• 4 years ago

It only takes ~8 minutes for light to reach us from the sun, so that seems wrong.

1.5 * 10^9 meters / (3.0 * 10^8 meters/second) = 5.0 seconds.

dorfsmay2• 4 years ago

Ah right, I got my units wrong! Good thing I'm not in middle school any more!

thatcherc• 4 years ago

More like 5 seconds - (1.5e6 km)/(300_000 km/s) is 5.0 seconds. This makes sense as the Sun is 8 light-minutes away from Earth and JWST is closer to Earth than the Sun.

Gatsky• 4 years ago

Reading this article makes me realise that it would take a relatively small let up in funding such projects to permanently lose the knowledge and expertise it takes to build these fantastic machines.

2OEH8eoCRo0• 4 years ago

Yes. That's part of my reason for supporting higher and higher defense spending. Defense and science are eternally inseparable.

creativenolo• 4 years ago

Why?

Hammershaft• 4 years ago

How about just higher and higher r&d spending instead so our choice of tech development is governed by public welfare rather then military utility? We've already had technologies like Nuclear fusion that have developed in inferior directions because they benefit adjacent military technologies.

gigatexal• 4 years ago

I’m excited to see the new database and data management and ingestion solutions that will come out of this.

Anyone know if ZFS is playing a role?

wargames• 4 years ago

I'm curious for anyone who may know the answer... with no mention of encryption, are these streams free for anyone with the equipment to receive? Conversely, what kind of security is in place on JWST for command updates to ensure that some rogue group couldn't cause mischief and send it commands?

vardump• 4 years ago

I think in scientific satellites, the downlink is unencrypted and only the command channel is usually encrypted for obvious reasons.

baobob• 4 years ago

Is 25 Ghz something an amateur could practically expect to capture from earth without ridiculous (or improbable) electronics? My understanding of higher frequencies was that something this high is likely to have been almost completely absorbed by the atmosphere

ohitsdom• 4 years ago

There is a decent amateur community for receiving satellite transmissions. I'm not super knowledgable on it, but 2 resources that may interest you:

Scott Tilley, who gained a lot of recognition in the past year in analyzing radio signals to see how Russia was using satellites in Ukraine: https://twitter.com/coastal8049

An amateur group revived 2-way communications on an abandoned satellite: https://sservi.nasa.gov/articles/isee-3-reboot-project/#:~:t....

sbierwagen• 4 years ago

NOAA satellite downlink is unencrypted, at least: https://news.ycombinator.com/item?id=9949278 https://news.ycombinator.com/item?id=24129162

spaetzleesser• 4 years ago

I remember talking to someone who works on the Deep Space Network. The commands they send to their devices are definitely encrypted and have checksums so nobody can inject bogus commands. It was super important that the device receives the correct command at the right time. Not sure if their downstream is secured.

akira2501• 4 years ago

This is an exceptionally old document, referencing the 2013 launch expectation, but it contains a bunch of interesting information on the platform database and the communications segment. [1]

Apparently, they have to have accurate ranging to receive from JWST. The interesting portion:

"Ranging is required for JWST, using alternate ground stations in the southern and northern hemisphere. For LEO and L2 missions the accuracy of the ranging is dependent on the tracking of the spacecraft across the sky. For the JWSTs L2 orbit, 21 days of tracking equals about 15 minutes of tracking for a LEO spacecraft."

https://ntrs.nasa.gov/api/citations/20080030196/downloads/20...

supernova87a4• 4 years ago

The article didn't go into it I think, but I recall in many satellite missions of this type, there are not only data storage and transmission issues (normal issues you would expect), but also considerations that the antennas and transmission hardware themselves have a certain duty cycle or lifetime that is finite. As in transmission of data consumes that margin.

So you have to quite deliberate in considering how much data to be sending, which data, etc. because every GB eats a chunk of the satellite's expected life. (Again, I believe.)

foobarbecue8• 4 years ago

Really? I've never heard of transmitter or antenna being considered a consumable (and I work in the space industry). Any idea where you got this idea from?

supernova87a4• 4 years ago

I will try to find a link, although it's of course quite specialized info that is not often written about.

But for example, I recall that for Spitzer space telescope (I believe) every activation of the transmission hardware consumed it's usable lifetime, or the finite amount of liquid helium coolant that was needed for the operation of the telescope (which only had an expected lifetime of 2.5 years, for the key instruments that relied on coolant).

foobarbecue8• 4 years ago

I thought about this some more and remembered that we do do "trending" of just about every subsystem on the spacecraft, and comms is one of them. Pretty sure there's a slide in a presentation every few months looking at how many times the radio has been turned on compared to the number of times it was designed to be turned on, but I assume the component in question is just the relay that switches it on. We have similar plots for everything that can be turned on and off. I think the point of these presentations is just to think about what's likely to die first and to make it obvious if we suddenly change how often we use things.

londons_explore• 4 years ago

If you were designing the JWST today, you would probably also put onboard a GPU. That could be programmed to do some of the scientific work in space to reduce the amount of data that needs to be downloaded.

This would allow new types of science (for example, far shorter exposure times and stacking to do super resolution and get rid of vibrations in the spacecraft structure). It would also allow redundancy incase the data downlink malfunctions or is degraded - you can still get lots of useful results back over a much smaller engineering link if you have preprocessed the data.

Obviously, if that GPU malfunctions, or there isn't sufficient power or cooling for it due to other failures, data can still be directly downloaded as it is today.

Basicly, it adds a lot of flexibility.

mbreese• 4 years ago

I don’t know… that’s a lot of power and heat that needs to be dealt with for an onboard GPU. Heat is probably the biggest factor, as it might be enough to affect the image sensors (speculation). Plus, needing a radiation hardened GPU might be an issue. Just for data reprocessing purposes, I’d want to have copies of the rawest data terrestrially.

jtmarmon• 4 years ago

The article alludes to laser comms - NASA is developing[0] laser based comms systems (as opposed to radio frequency) which would allow gigabit speed downloading of data. Hardening this technology to send back more raw data is probably a lot more straight forward than trying to do image processing on board.

[0] https://www.nasa.gov/mission_pages/tdm/lcrd/index.html

supernova87a4• 4 years ago

It's hard to say -- I might disagree on whether to do something like that. Most often you want to be able to keep the raw data as long as you can, in anticipation that perhaps some day, some future technique or different calibrations/processing pipeline may improve. Or that you might find (or be looking for something) you didn't expect.

Especially for a scientific instrument whose usage patterns, operating conditions, and discoveries may change over time. (sensors too) Note for an instrument like this, the amount of people/researcher time studying the data afterwards is many times more than the amount of time taking the data. The value of it is incredibly high ($/hour), so you want to keep in as future-usable state as possible.

Once you process something on board for a certain purpose, unless for very low level integrity checks that are almost mandatory, etc, and discard the raw data, you lose the chance to do that in the future.

So unless you are really transmission constrained, I think they would prefer not to do it -- also because of the additional complications involved. I think once you get into "higher functions" becoming an obligation of the telescope's operations, those satellite / defense contractors etc. who have to launch and operate the thing start making requirements that are very difficult to live by.

carabiner• 4 years ago

Depends on if the GPU is "space grade": https://arstechnica.com/science/2019/11/space-grade-cpus-how...

You can't just throw any consumer microprocessor into a machine subject to extreme vibrations, heat, cold, and radiation.

light_hue_1• 4 years ago

If you were designing the JWST today, you would probably also put onboard a GPU. That could be programmed to do some of the scientific work in space

That's just not how science is done in astronomy. People want the raw data to analyze it for decades in different contexts. There's not much that can be done onboard that would make you not want to copy that data back.

Rebelgecko• 4 years ago

Do rad hard GPUs exist? If not I think it's wise that they stuck with FPGAs

The Webb Space Telescope’s profound data challenges

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