4 ms·

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

by rsfern 4y ago

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.
      - LegitShady 4y ago
        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 4y ago
        Having worked federal construction projects in...adjacent venues, I'm exactly aware of what is involved and the scope.
        Sporktacular 4y ago
        How many multi-billion dollar, federally constructed ground segments are there?
        count 4y ago
        More than you would expect.
    - dotnet00 4y ago
      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.
- 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.

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

by rsfern 4y ago

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

> 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.
    - LegitShady 4y ago
      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 4y ago
        Having worked federal construction projects in...adjacent venues, I'm exactly aware of what is involved and the scope.
        Sporktacular 4y ago
        How many multi-billion dollar, federally constructed ground segments are there?
        count 4y ago
        More than you would expect.
  - dotnet00 4y ago
    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.
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.