[HN Gopher] The Webb Space Telescope's profound data challenges ___________________________________________________________________ The Webb Space Telescope's profound data challenges Author : MindGods Score : 282 points Date : 2022-07-12 10:38 UTC (12 hours ago) (HTM) web link (spectrum.ieee.org) (TXT) w3m dump (spectrum.ieee.org) | Victerius wrote: | 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 wrote: | The problem is how to get the data back to earth, no? | Victerius wrote: | > 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. | rideva wrote: | > 0.028 * 3600 = 100 GB/hour | | You forgot the units. | | 0.028 Mb/s *3600s = 100 Gb/hour = 12.6 GB/h | Sporktacular wrote: | 0.028 Gb/s * ... | onetwentythree wrote: | 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. | mikepurvis wrote: | 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 wrote: | 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 wrote: | That's 100Gigabit/hour or 12.6Gigabyte/hour or | 11.7Gibibyte/hour | SECProto wrote: | 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] | joshvm wrote: | 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 wrote: | 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. | joshvm wrote: | Lifespan is not dictated by hard disk space, it's | dictated by consumables like liquid helium, other | cryosystems and fuel for stationkeeping thrusters. Most | big NASA projects are specced to _just_ promise enough to | get funded (in terms of science) and overengineered, so | what gets built will often be stretched longer than the | intial deliverables demanded anyway (see basically all | the recent Mars rovers). | count wrote: | Ground comms is a construction project, which, as a | federal govt thing, means it'd take even longer and be | another billions of dollars. | LegitShady wrote: | I don't think you understand how much "billions of | dollars" is for construction projects. We're talking a | communication system, not the large hadron collider. | count wrote: | Having worked federal construction projects in...adjacent | venues, I'm exactly aware of what is involved and the | scope. | dotnet00 wrote: | It's kind of important to remember that they didn't | intend to spend $10B on the telescope and that it's going | to last for ~20 years. The original price was expected to | be $500M and it would have never been approved if the | original price was $10B. It only ended up being able to | get to such a cost because of gradual overruns over 20+ | years and sunk cost. | | That said, NASA have been expanding the DSN for the past | decade with four new antennae added so far (remaining two | expected by 2025), it's just slow progress as usual with | most of what they do lately. DSN antennas don't bring in | jobs (and thus votes) for senators the way decades long | flagship projects like JWST or SLS do. | bikezen wrote: | > 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. | | There are 3 DSN complexes, but each complex has multiple | dishes. One station is very frequently talking to | multiple spacecraft at once. | toolslive wrote: | > 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 | Sporktacular wrote: | 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 wrote: | 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 wrote: | 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 wrote: | - 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_. | toolslive wrote: | 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 wrote: | > 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". | Gatsky wrote: | 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 wrote: | Yes. That's part of my reason for supporting higher and higher | defense spending. Defense and science are eternally | inseparable. | creativenolo wrote: | Why? | warmwaffles wrote: | Because defense spending funds science research. See DARPA. | | Really good introduction to it all | https://www.amazon.com/gp/product/B07BLKXV68 | robotresearcher wrote: | 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 wrote: | 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. | BitwiseFool wrote: | This is anecdotal, but I've not run into a single real | life American who hasn't agreed with the general premise | that we spend too much money on "the military industrial | complex". Even the hawkish folks agree with this notion | to some extent, usually arguing that specific projects | are a waste of money and that's where the cuts should | come from. | | Cynically, I think our congresspeople don't _actually_ | take our feedback about the budget into account. They 'll | talk a big game about wasteful spending and the need to | reduce deficits but the pork never stops flowing. | sophacles wrote: | Folks can say whatever they want, but the votes show that | "no one ever got fired for _increasing_ defense spending | ", while the opposite isn't true. | Hammershaft wrote: | 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. | supernova87a wrote: | 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 wrote: | 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 wrote: | 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 wrote: | BTW this is what Spitzer was using: | https://en.wikipedia.org/wiki/Small_Deep_Space_Transponder | . I haven't been able to find info on what Webb is using. | foobarbecue wrote: | 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 wrote: | 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. | wargames wrote: | 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? | sbierwagen wrote: | NOAA satellite downlink is unencrypted, at least: | https://news.ycombinator.com/item?id=9949278 | https://news.ycombinator.com/item?id=24129162 | spaetzleesser wrote: | 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. | vardump wrote: | I think in scientific satellites, the downlink is unencrypted | and only the command channel is usually encrypted for obvious | reasons. | baobob wrote: | 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 wrote: | 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.... | pweezy wrote: | 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. | herendin wrote: | >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 wrote: | 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 wrote: | 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. | jvanderbot wrote: | 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. | NKosmatos wrote: | 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 | cycomanic wrote: | 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 wrote: | 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 wrote: | 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 | dorfsmay wrote: | Latency is 1h 20m one way (1.5M km / 300_000 km/s)? | | EDIT: got my units wrong, see further down the thread. | thatcherc wrote: | 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. | [deleted] | [deleted] | sp332 wrote: | 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 wrote: | Ah right, I got my units wrong! Good thing I'm not in middle | school any more! | gigatexal wrote: | 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? | qazpot wrote: | > 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 wrote: | 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. | 317070 wrote: | 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. | beached_whale wrote: | I think the engineering that would go into a mission to do | maintenance/repair would be quite valuable and help with | many other missions. It's far enough to be far, but not so | far that other factors like communication is extremely | delayed. Getting that far out of LEO has a huge set of new | challenges, I assume, we should be able to learn about. | Aperocky wrote: | It's much easier to get there and fix JWST than it is to | land on the moon though. | | Because the landing part was the hardest and take more | delta V than actually getting to JWST. Not to mention going | back from moon surface. | martinskou wrote: | Didn't JWST use some kind of grivitational breaking to | stop at the LGPoint? A manned mission would require much | more fuel to shorten the time of the trip and fuel for | returning. | dotnet00 wrote: | 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. | sp332 wrote: | Maybe we could send up a data buffer with a high-bandwidth, | short-range radio. | re-actor wrote: | No currently available spacecraft can make the trip and | service JWST, it's also not really intended to be serviced. | queuebert wrote: | Spitballing here ... could we de-orbit it back to Earth | with what propellant remains? Then fix whatever and refill | the propellant? | hoseja wrote: | Spaceship _might_ (Elon time) fly this August. | danpalmer wrote: | 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 wrote: | Oh yea. Much less memorable than BFS. | patmorgan23 wrote: | JWST is not serviceable. If the ssd dies, then it's dead. | There's no replacing it. | zimpenfish wrote: | > If the ssd dies, then it's dead. | | Presumably they could operate in a reduced mode where it | does live transmission of the data when it's in contact | with Earth? | Sporktacular wrote: | Depends if the sensor readout is slower than 28 Mb/s. | zimpenfish wrote: | If it generates, at most, 57GB per day[1], assuming 22h | operation (2h for transmission), the sensors are | generating 2.6GB/h or about 750kB/sec which is just about | 6Mb/s (unless my math is wonky.) | | [1] "JWST can produce up to 57 GB each day (although that | amount is dependent on what observations are scheduled)." | Sporktacular wrote: | That is the average rate over a day. If storage is not | available to buffer it, then a sensor's peak readout rate | could easily exceed the transmission rate. | AngryData wrote: | It isn't meant to need servicing, but it does have a | docking port if it ever does need it and we have the desire | to do so. | dr_orpheus wrote: | Yes, but a servicing mission for replacing the memory | would still be an unlikely operation to be able to do. | The most likely use for the docking port is if the | electronics onboard exceed their design life, but JWST is | running out fuel (to maintain its position at the | Lagrange point) so they basically strap a "jet pack" on | to the satellite to keep in operation. This has been the | business case of some companies trying to do this for GEO | satellites: | | https://astroscale.com/astroscale-u-s-enters-the-geo- | satelli... | iamgopal wrote: | 68 GB ? Just ? Shouldn't that be some 10-15 1-2 TB SSD in | parallel ? | zackmorris wrote: | Sometimes the smartest people in the world still stink at | their short game | danpalmer wrote: | 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. | [deleted] | urthor wrote: | They also simply do not store data locally. | | The onboard storage is basically a buffer, they beam | everything back to earth. | ceejayoz wrote: | Yep. That means bandwidth is the limiting point, and that | maxes out at 28 megabit in ideal conditions. A multi- | terabyte array would be a waste. | boredemployee wrote: | that means no sandisk or the like | [deleted] | onetwentythree wrote: | 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. | Victerius wrote: | 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? | wongarsu wrote: | Since cosmic rays are charged particles I'd expect even more | bit flips on the magnetic platter of a HDD. | bitexploder wrote: | I am completely guessing here: it is shielded as are most | of the computing components. It probably also has error | correction of some sort built in. | harshreality wrote: | Shielding adds significant weight, which isn't good for | space-bound components. | | I would guess they just use RAID, do round-trip data | verification before writes succeed, and then reinitialize | and scrub any storage module behaving improperly. | | For bits stored on SSDs which already rely on error | correction, if they were willing to make custom hardware | rather than use something off-the-shelf, they could add | more chips and scale up the error correcting code to deal | with more errors. | willis936 wrote: | 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. | sillysaurusx wrote: | Huh. Interesting. That explains why my SSD died at boot | time. I thought it was just coincidence, and that it had | died the night before or something, but that obviously | doesn't make sense. | tambourine_man wrote: | This thing uses gyroscopes to orient itself. My guess is the | less moving parts, the better | jimsmart wrote: | Here's an interesting article about the gyroscopes / | related tech used on the JWST. | | TL;DL -- it uses reaction/momentum wheels to change | orientation, and it uses non-mechanical gyroscopes to | detect changes in orientation. | | https://www.universetoday.com/143152/spacecraft- | gyroscopes-a... | ddingus wrote: | I just went down a small, but very interesting rabbit | hole: I was not aware of non mechanical gyros! | | Apparently, light travelling along a fiber optic cable | will travel a slightly different distance when the device | has angular momentum. That is COOL. | | https://en.wikipedia.org/wiki/Sagnac_effect | jimsmart wrote: | I believe that the tech you mention is the same tech as | used by the flat-earthers in Behind The Curve in the $20k | gyroscope they bought, to try and prove that the Earth | did not rotate (except they found it picked up a | 15-degree-per-hour drift, debunking themselves, so then | doubted the technology, heh) [1]. | | But, aside from the laser/fibre-optic tech, there are | also MEMS [2] gyroscopes also -- which is almost nanotech | IMO -- which are used as sensors in modern | phones/tablets, and also on drones (to assist with | navigation), plus various other robotics uses, and more. | | MEMS gyros -- often with an accelerometer (aka IMU / | inertial measurement unit) and maybe a magnetometer | (compass) -- can be bought from folk such as Adafruit | [3], SparkFun, etc. (I've got a few different ones | myself), and hooked up to e.g. an Arduino or similar MCU | (or indeed anything else that speaks can speak the | appropriate protocol, e.g. I2C/SPI/etc depending on the | board in question). | | Hmm, just looked for a good image of how a MEMS gyro | works, and didn't come up with what I was looking for / | recall seeing before (I'm a bit pushed for time), but | there's a diagram on this WP page [4]. | | Edit: ah, here's a better diagram [5] | | [1] https://www.triplem.com.au/story/flat-earthers- | spend-20-000-... | | [2] https://en.wikipedia.org/wiki/Microelectromechanical_ | systems | | [3] https://www.adafruit.com/category/521 | | [4] https://en.wikipedia.org/wiki/Vibrating_structure_gyr | oscope#... | | [5] https://www.researchgate.net/figure/Block-diagram-of- | the-MEM... | ddingus wrote: | The FE link is awesome! | | The others are interesting and informative. I am gonna | buy me a few parts to play with. | | Thanks! | jimsmart wrote: | No problem, have fun! -- and if you enjoyed that FE | article, you will likely enjoy the 'Behind The Curve' | movie too, it's really quite amusing :) | dekhn wrote: | I've used the MPU6050 to build a two-wheel self balancing | robot (roughly this: | https://www.instructables.com/Arduino-Self-Balancing- | Robot-1...). Basically, apply PID to the motor angle to | keep the robot level with the gravity vector. | | I still think MEMS sensors are very cool and extremely | convenient/cheap for what you get. | Dave_Rosenthal wrote: | Yep, ring laser gyro :) | | They are pretty awesome. Once got some data that seemed | to be bad out of one and realized that the entire error | was exactly accounted for by the Coriolis effect from the | rotation of the earth. | curling_grad wrote: | I guess hard drives would be dead because of enormous | acceleration on launch day. | Schroedingersat wrote: | If they're not on, hdds can handle surprising amounts of | acceleration, like 100s of gs | ahartmetz wrote: | Yeah. Fluid dynamic bearings and parked heads don't seem | very vulnerable to shock and vibration. | xeromal wrote: | You could almost invert your logic too. Perhaps the | spinning on the HD throws off the telescope enough to be | troublesome. I'm not sure how stable the lagrange point | orbiting is, but it can't be super stable. | dr_orpheus wrote: | Any moving mechanism is potentially a source for | disturbance to the telescope. Not something that affects | the stability of the orbit. But the small vibrations can | translate to small vibrations in the instruments and | secondary mirror which then cause distortion over the | integration time of the image. | | I don't know that that is the primary reason HDD's have | been avoided. But any moving part is another source of | failure so my guess is the HDD's life is not as long. | | As far as I'm aware I don't know of any spacecraft that | has flown a HDD (but there certainly could be). However, | some early spacecraft did use tape drives. Hubble | originally used tape drives and was replaced with solid | state memory during one of the servicing missions. | urthor wrote: | 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 wrote: | 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. | ceejayoz wrote: | Chances are they or the manufacturer have a room full of | those cameras clicking away on a schedule for the last ten | years, too, as an early warning system. | boilerupnc wrote: | 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-... | urthor wrote: | https://www.fastcompany.com/28121/they-write-right-stuff | | Edit: oh you found it | londons_explore wrote: | 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. | jtmarmon wrote: | 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 | supernova87a wrote: | It's hard to say -- I might disagree. 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. Especially for a | scientific instrument whose usage patterns, operating | conditions, and discoveries may change over time. | | 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. | mbreese wrote: | 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. | nextaccountic wrote: | > 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? | jcims wrote: | Could also be more resonant modes in the structure that would | need to be damped out. | onetwentythree wrote: | 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 wrote: | 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? | onetwentythree wrote: | It uses NASA's Deep Space Network. There are three stations | around the globe (California, Australia, and Spain), spaced | so that there is near continuous coverage for deep space | missions. JWST points it's antenna at the station that is | currently in view. | | However, the ground stations are shared between many | missions, so they are not available for JWST all the time. | Expectation is that JWST gets 8-12 hours/day of DSN time. | marcyb5st wrote: | I believe it is both due to solar wind and the fact that it | would act as a tiny light sail. | mNovak wrote: | 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. | jacquesm wrote: | At a guess: solar pressure. | anotherhue wrote: | Solar wind exposure perhaps? | timcavel wrote: | cycomanic wrote: | > 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 wrote: | I suspect they know what they're doing. | mhh__ wrote: | 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 | lacker wrote: | 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 wrote: | They sure do, but I'm less confident that the statements made | in the interview can be uphold to standards of scientific | scrutiny. | hoseja wrote: | Know how to maximize budget. | calibas wrote: | capableweb wrote: | 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" | pclmulqdq wrote: | Reed-Solomon is better at detecting longer runs of missing | data (which could come from objects passing by, for | example), and is a lot cheaper to decode - computation in | space is very expensive. | Sporktacular wrote: | I suspect you're right but it seems that the capacity | advantage of convolutional codes only become significant in | very low SNR, so maybe deep space probe applications. Also | unless interleaving is used, Reed-Solomon can do better | against bursts of errors, though am nor sure why the noise | profile would be ay different. | | So, as you say, maybe it was just faster to integrate the | already certified equipment at that stage of the | development. | mfarstad wrote: | 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 | rswail wrote: | 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? :) | [deleted] | nonameiguess wrote: | 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. | jcims wrote: | 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 wrote: | Why is this project on such a long timeline? I wonder what | was the longest critical path in this project plan? | pkaye wrote: | I can see a couple factors: | | * To be useful to science, the telescope needs a huge | diameter which is more than can be launched so they had to | make a folding telescope that can be later unfolded. | | * We only get a single chance for success so testing | becomes critical. Lot of testing was needed to make sure | everything would work out in the end. | | * The technology needed was pushing the limits of what we | are capable of so lot of R&D needed. | jcims wrote: | A misunderstanding of 'failure' by the general public. ___________________________________________________________________ (page generated 2022-07-12 23:00 UTC)