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The Webb Space Telescope’s profound data challenges

318 points by MindGods 4y ago

Victerius 4y 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.
- rsfern 4y ago
  The problem is how to get the data back to earth, no?
  - Victerius 4y 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.
    - mikepurvis 4y ago
      The article mentions that there are blackout windows, I guess when the US is facing away from JWST. Only the low bandwidth DSN has coverage all the time.
    - rsfern 4y ago
      Cool, that does seem like plenty of overhead even if JWST spends the bulk of its time imaging. Reading the article further, I was sort of surprised there’s only 68 GB of solid state storage on board. Does anyone know if that’s considered very large for space-grade systems?
    - exitheone 4y ago
      That's 100Gigabit/hour or 12.6Gigabyte/hour or 11.7Gibibyte/hour
    - SECProto 4y ago
      Note that's a peak rate, and that should be 100Gb (12.5 GB). Without knowing the variability it's impossible to say if that's a problem or not.
    - deleted 4y ago
      [deleted]
    - joshvm 4y ago
      This is discussed in the article, as the sibling (Mike) comments, JWST uses the Deep Space Network (DSN) for comms. The issue isn't so much blackout (the segments are 120 degrees separated^), but that contact time is expensive and you don't get 100% of the uptime to yourself. The DSN is basically the only infrastructure we have for this sort of long range communication. There are only three stations and JWST is sharing time with a bunch of other missions including everything that's currently on Mars. Scheduling takes place far in advance. The problem also isn't absolute storage space onboard, even if capacity degrades at a gig a year, it's whether you can drain it faster than you fill it. That said, there is likely to be some limit on hard drive space depending on what the current rad hardened solutions are. JWST is built on very robust and well-known hardware that has good provenance in space. For a LEO mission you might be willing to risk less tolerant components to get terabytes of capacity, but out there you want extremely resilient components and 68 GB is probably the balance. Sentinel 2 (an ESA Earth observation satellite) has 2.4Tb/300GB onboard for example. Ultimately the missions planners budget for how much data they're allowed to stream back and they've figured out an amount that is acceptable. But in general, yes. If you can write to a disk and ship it, that's a good solution. IceCube (South Pole) generates TBs of data. We send back 0.5 TB of critical data every month over a fast satellite link, crucial alerts and telemetry are sent over Iridium (24/7 avail) and the rest gets sent back to Madison (WI) on a plane each summer. ^ In theory this means that you can service three missions at a time though, if they're not all in the same direction.
      - LegitShady 4y ago
        it seems weird to spend $10billion on a telescope that lasts for 4-5 years and then not spend the money on a communication system that isn't limited by traffic and is 'expensive', or a spare drive to double its lifespan.
        count 4y ago
        Ground comms is a construction project, which, as a federal govt thing, means it'd take even longer and be another billions of dollars.
    - rideva 4y ago
      > 0.028 * 3600 = 100 GB/hour You forgot the units. 0.028 Mb/s *3600s = 100 Gb/hour = 12.6 GB/h
      - Sporktacular 4y ago
        0.028 Gb/s * ...
    - onetwentythree 4y ago
      There are only 8 hours of downlink time per day. DSN resources are limited and shared among many missions. Everything was sized based on the expected volume of data that the telescope would be able to take. The instruments only generatedata at a certain rate, and there are inefficiencies involved with slewing between targets. Having a larger recorder or faster downlink would not mean that JWST could take more science data.
nextaccountic 4y ago
> 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 4y ago
  At a guess: solar pressure.
- anotherhue 4y ago
  Solar wind exposure perhaps?
- marcyb5st 4y ago
  I believe it is both due to solar wind and the fact that it would act as a tiny light sail.
- jcims 4y ago
  Could also be more resonant modes in the structure that would need to be damped out.
- onetwentythree 4y 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.
  - nextaccountic 4y 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?
pweezy 4y 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 4y 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 4y 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 4y 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...
    - ncmncm 4y ago
      There is no fixed or computable phase relationship in optical wavelengths. But the constellation could operate as an Earth-sized radio telescope pointing all directions at once, probably for a few seconds a day to avoid exceeding downlink bandwidth. There, the phase relationship is anyway computable, in principle. Probably they have not, yet, because they will be lofting new ones all the time, so have time to get it right. And, it might not really be computable, in practice.
NKosmatos 4y 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 https://eyes.nasa.gov/dsn/dsn.html
- cycomanic 4y 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.
  - Sporktacular 4y 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?
    - cycomanic 4y ago
      I agree for JWT optics was not really an option, however moving forward many space missions will start to use optics. Last year NASA launched the Laser Communications Relay Demonstration and there are extensive plans for integrating logical links into the DSN
qazpot 4y 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.
- aqme28 4y 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 4y ago
    No currently available spacecraft can make the trip and service JWST, it's also not really intended to be serviced.
    - hoseja 4y ago
      Spaceship might (Elon time) fly this August.
      - danpalmer 4y ago
        It's "Starship", and it's unlikely to be able to get to where the JWST is in the next few years due to needing to be refuelled in orbit. Plus they have no payload bay design. Then there's the robotics necessary for doing a repair, or human-rating Starship, either of which is years worth of time.
        hoseja 4y ago
        Oh yea. Much less memorable than BFS.
    - queuebert 4y ago
      Spitballing here ... could we de-orbit it back to Earth with what propellant remains? Then fix whatever and refill the propellant?
toolslive 4y 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 https://en.wikipedia.org/wiki/Online_codes
- Sporktacular 4y 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.
  - toolslive 4y 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?
    - Sporktacular 4y ago
      Yes, there is a thread to that effect. Development was heavily delayed, certification and conservatism around space systems can slow down deployment to the point of just not being worth changing things. I am sorry to have been presumptuous. There's been a spate of recent armchair experts with some arrogant "they should just use a warp drive" type contributions lately.
      - toolslive 4y ago
        - The thing with online codes is that while they are quite efficient in data overhead (a few %) they only start working when you have enough data ( > X MB). (check!) - another problem is that since the system that you need to solve to get your data back is large and random, so in-place updates of the data are impossible. In other words: they only work for immutable data (check!) - The extra information to counter lost messages is scattered over all messages so the client doesn't need to figure out which ones are missing (and doesn't need to tell the server, in this case the telescope). This makes them highly suited for high latency communication (and storage which is in essence the same thing, but that's another story) (check!) They (online codes) were designed for this exact use case.
cycomanic 4y 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.
- Sporktacular 4y ago
  I suspect they know what they're doing.
  - mfarstad 4y 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
  - hoseja 4y ago
    Know how to maximize budget.
  - capableweb 4y 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_A1b.pdf?sequence=1 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"
dorfsmay 4y ago
Latency is 1h 20m one way (1.5M km / 300_000 km/s)? EDIT: got my units wrong, see further down the thread.
- deleted 4y ago
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- sp332 4y 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.
  - dorfsmay 4y ago
    Ah right, I got my units wrong! Good thing I'm not in middle school any more!
- thatcherc 4y 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.
timcavel 4y ago
Gatsky 4y 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 4y ago
  Yes. That's part of my reason for supporting higher and higher defense spending. Defense and science are eternally inseparable.
  - creativenolo 4y ago
    Why?
    - warmwaffles 4y ago
      Because defense spending funds science research. See DARPA. Really good introduction to it all https://www.amazon.com/gp/product/B07BLKXV68 https://www.amazon.com/gp/product/B07BLKXV68
      - Sporktacular 4y ago
        But so does science research spending. Point seems to be why not let it follow directions independent of military utility.
    - robotresearcher 4y ago
      Better science and tech than the other guy gives you a military advantage. Thus some fraction of your science funding is effectively military funding, and some fraction of your military funding is well spent on science. Also, more science projects lead to more experienced scientists and engineers, which leads to a stronger pool for the defense industry.
    - mensetmanusman 4y ago
      The general public is hard-pressed to support basic research with taxes, but they do support unlimited defense funding. This is one hack that the US system uses to do basic research under the umbrella of defense.
gigatexal 4y 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 4y 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 4y ago
  I think in scientific satellites, the downlink is unencrypted and only the command channel is usually encrypted for obvious reasons.
  - baobob 4y 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
    - jonah 4y ago
      You'll probably need a pretty big dish...
- ohitsdom 4y 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 https://twitter.com/coastal8049 An amateur group revived 2-way communications on an abandoned satellite: https://sservi.nasa.gov/articles/isee-3-reboot-project/#:~:text=Now%2C%20ISEE%2D3%20Reboot%20Project,its%20orbit%20around%20the%20sun https://sservi.nasa.gov/articles/isee-3-reboot-project/#:~:t....
- sbierwagen 4y ago
  NOAA satellite downlink is unencrypted, at least: https://news.ycombinator.com/item?id=9949278 https://news.ycombinator.com/item?id=9949278 https://news.ycombinator.com/item?id=24129162 https://news.ycombinator.com/item?id=24129162
supernova87a 4y 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.)
- foobarbecue 4y 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?
  - supernova87a 4y 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).
    - foobarbecue 4y ago
      Helium, for sure. Coolant, propellant, battery charge cycles, flash write cycles are all consumables. Maybe even solar panels -- they wear out. The radio? I doubt it.
    - foobarbecue 4y ago
      BTW this is what Spitzer was using: https://en.wikipedia.org/wiki/Small_Deep_Space_Transponder https://en.wikipedia.org/wiki/Small_Deep_Space_Transponder . I haven't been able to find info on what Webb is using.
    - 4y ago
londons_explore 4y 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 4y 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 4y 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 https://www.nasa.gov/mission_pages/tdm/lcrd/index.html
- supernova87a 4y 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.

318 points by MindGods 4y 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.

rsfern 4y ago
The problem is how to get the data back to earth, no?
- Victerius 4y 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.
  - mikepurvis 4y ago
    The article mentions that there are blackout windows, I guess when the US is facing away from JWST. Only the low bandwidth DSN has coverage all the time.
  - rsfern 4y ago
    Cool, that does seem like plenty of overhead even if JWST spends the bulk of its time imaging. Reading the article further, I was sort of surprised there’s only 68 GB of solid state storage on board. Does anyone know if that’s considered very large for space-grade systems?
  - exitheone 4y ago
    That's 100Gigabit/hour or 12.6Gigabyte/hour or 11.7Gibibyte/hour
  - SECProto 4y ago
    Note that's a peak rate, and that should be 100Gb (12.5 GB). Without knowing the variability it's impossible to say if that's a problem or not.
  - deleted 4y ago
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  - joshvm 4y ago
    This is discussed in the article, as the sibling (Mike) comments, JWST uses the Deep Space Network (DSN) for comms. The issue isn't so much blackout (the segments are 120 degrees separated^), but that contact time is expensive and you don't get 100% of the uptime to yourself. The DSN is basically the only infrastructure we have for this sort of long range communication. There are only three stations and JWST is sharing time with a bunch of other missions including everything that's currently on Mars. Scheduling takes place far in advance. The problem also isn't absolute storage space onboard, even if capacity degrades at a gig a year, it's whether you can drain it faster than you fill it. That said, there is likely to be some limit on hard drive space depending on what the current rad hardened solutions are. JWST is built on very robust and well-known hardware that has good provenance in space. For a LEO mission you might be willing to risk less tolerant components to get terabytes of capacity, but out there you want extremely resilient components and 68 GB is probably the balance. Sentinel 2 (an ESA Earth observation satellite) has 2.4Tb/300GB onboard for example. Ultimately the missions planners budget for how much data they're allowed to stream back and they've figured out an amount that is acceptable. But in general, yes. If you can write to a disk and ship it, that's a good solution. IceCube (South Pole) generates TBs of data. We send back 0.5 TB of critical data every month over a fast satellite link, crucial alerts and telemetry are sent over Iridium (24/7 avail) and the rest gets sent back to Madison (WI) on a plane each summer. ^ In theory this means that you can service three missions at a time though, if they're not all in the same direction.
    - LegitShady 4y ago
      it seems weird to spend $10billion on a telescope that lasts for 4-5 years and then not spend the money on a communication system that isn't limited by traffic and is 'expensive', or a spare drive to double its lifespan.
      - count 4y ago
        Ground comms is a construction project, which, as a federal govt thing, means it'd take even longer and be another billions of dollars.
  - rideva 4y ago
    > 0.028 * 3600 = 100 GB/hour You forgot the units. 0.028 Mb/s *3600s = 100 Gb/hour = 12.6 GB/h
    - Sporktacular 4y ago
      0.028 Gb/s * ...
  - onetwentythree 4y ago
    There are only 8 hours of downlink time per day. DSN resources are limited and shared among many missions. Everything was sized based on the expected volume of data that the telescope would be able to take. The instruments only generatedata at a certain rate, and there are inefficiencies involved with slewing between targets. Having a larger recorder or faster downlink would not mean that JWST could take more science data.

nextaccountic 4y ago

> 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 4y ago
At a guess: solar pressure.
anotherhue 4y ago
Solar wind exposure perhaps?
marcyb5st 4y ago
I believe it is both due to solar wind and the fact that it would act as a tiny light sail.
jcims 4y ago
Could also be more resonant modes in the structure that would need to be damped out.
onetwentythree 4y 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.
- nextaccountic 4y 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?

pweezy 4y 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 4y 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 4y 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 4y 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...
  - ncmncm 4y ago
    There is no fixed or computable phase relationship in optical wavelengths. But the constellation could operate as an Earth-sized radio telescope pointing all directions at once, probably for a few seconds a day to avoid exceeding downlink bandwidth. There, the phase relationship is anyway computable, in principle. Probably they have not, yet, because they will be lofting new ones all the time, so have time to get it right. And, it might not really be computable, in practice.

NKosmatos 4y 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 https://eyes.nasa.gov/dsn/dsn.html

cycomanic 4y 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.
- Sporktacular 4y 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?
  - cycomanic 4y ago
    I agree for JWT optics was not really an option, however moving forward many space missions will start to use optics. Last year NASA launched the Laser Communications Relay Demonstration and there are extensive plans for integrating logical links into the DSN

qazpot 4y 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.

aqme28 4y 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 4y ago
  No currently available spacecraft can make the trip and service JWST, it's also not really intended to be serviced.
  - hoseja 4y ago
    Spaceship might (Elon time) fly this August.
    - danpalmer 4y ago
      It's "Starship", and it's unlikely to be able to get to where the JWST is in the next few years due to needing to be refuelled in orbit. Plus they have no payload bay design. Then there's the robotics necessary for doing a repair, or human-rating Starship, either of which is years worth of time.
      - hoseja 4y ago
        Oh yea. Much less memorable than BFS.
  - queuebert 4y ago
    Spitballing here ... could we de-orbit it back to Earth with what propellant remains? Then fix whatever and refill the propellant?

toolslive 4y 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 https://en.wikipedia.org/wiki/Online_codes

Sporktacular 4y 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.
- toolslive 4y 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?
  - Sporktacular 4y ago
    Yes, there is a thread to that effect. Development was heavily delayed, certification and conservatism around space systems can slow down deployment to the point of just not being worth changing things. I am sorry to have been presumptuous. There's been a spate of recent armchair experts with some arrogant "they should just use a warp drive" type contributions lately.
    - toolslive 4y ago
      - The thing with online codes is that while they are quite efficient in data overhead (a few %) they only start working when you have enough data ( > X MB). (check!) - another problem is that since the system that you need to solve to get your data back is large and random, so in-place updates of the data are impossible. In other words: they only work for immutable data (check!) - The extra information to counter lost messages is scattered over all messages so the client doesn't need to figure out which ones are missing (and doesn't need to tell the server, in this case the telescope). This makes them highly suited for high latency communication (and storage which is in essence the same thing, but that's another story) (check!) They (online codes) were designed for this exact use case.

cycomanic 4y 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.

Sporktacular 4y ago
I suspect they know what they're doing.
- mfarstad 4y 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
- hoseja 4y ago
  Know how to maximize budget.
- capableweb 4y 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_A1b.pdf?sequence=1 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"

dorfsmay 4y ago

Latency is 1h 20m one way (1.5M km / 300_000 km/s)? EDIT: got my units wrong, see further down the thread.

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sp332 4y 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.
- dorfsmay 4y ago
  Ah right, I got my units wrong! Good thing I'm not in middle school any more!
thatcherc 4y 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.

timcavel 4y ago

Gatsky 4y 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 4y ago
Yes. That's part of my reason for supporting higher and higher defense spending. Defense and science are eternally inseparable.
- creativenolo 4y ago
  Why?
  - warmwaffles 4y ago
    Because defense spending funds science research. See DARPA. Really good introduction to it all https://www.amazon.com/gp/product/B07BLKXV68 https://www.amazon.com/gp/product/B07BLKXV68
    - Sporktacular 4y ago
      But so does science research spending. Point seems to be why not let it follow directions independent of military utility.
  - robotresearcher 4y ago
    Better science and tech than the other guy gives you a military advantage. Thus some fraction of your science funding is effectively military funding, and some fraction of your military funding is well spent on science. Also, more science projects lead to more experienced scientists and engineers, which leads to a stronger pool for the defense industry.
  - mensetmanusman 4y ago
    The general public is hard-pressed to support basic research with taxes, but they do support unlimited defense funding. This is one hack that the US system uses to do basic research under the umbrella of defense.

gigatexal 4y 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 4y 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 4y ago
I think in scientific satellites, the downlink is unencrypted and only the command channel is usually encrypted for obvious reasons.
- baobob 4y 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
  - jonah 4y ago
    You'll probably need a pretty big dish...
ohitsdom 4y 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 https://twitter.com/coastal8049 An amateur group revived 2-way communications on an abandoned satellite: https://sservi.nasa.gov/articles/isee-3-reboot-project/#:~:text=Now%2C%20ISEE%2D3%20Reboot%20Project,its%20orbit%20around%20the%20sun https://sservi.nasa.gov/articles/isee-3-reboot-project/#:~:t....
sbierwagen 4y ago
NOAA satellite downlink is unencrypted, at least: https://news.ycombinator.com/item?id=9949278 https://news.ycombinator.com/item?id=9949278 https://news.ycombinator.com/item?id=24129162 https://news.ycombinator.com/item?id=24129162

supernova87a 4y 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.)

foobarbecue 4y 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?
- supernova87a 4y 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).
  - foobarbecue 4y ago
    Helium, for sure. Coolant, propellant, battery charge cycles, flash write cycles are all consumables. Maybe even solar panels -- they wear out. The radio? I doubt it.
  - foobarbecue 4y ago
    BTW this is what Spitzer was using: https://en.wikipedia.org/wiki/Small_Deep_Space_Transponder https://en.wikipedia.org/wiki/Small_Deep_Space_Transponder . I haven't been able to find info on what Webb is using.
  - 4y ago

londons_explore 4y 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 4y 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 4y 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 https://www.nasa.gov/mission_pages/tdm/lcrd/index.html
supernova87a 4y 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.