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A comment on Satellites Taking Pictures of Rockets Carrying More Satellites. Parent comment

Re:Satellites Taking Pictures of Rockets Carrying More Satel... by ChuckMcM on Sunday July 23, 2017 @12:11AM (#14830864)
Think about the system[1], you've got to take a picture, store it, and then downlink it. If you take too many pictures your downlink is overloaded and you run out of memory. Assuming that an HD frame is about 2 megabytes once it is compressed, that is a 2 - 5 mbs downlink speed (depending on error correction). The satellites they launch are tiny and low power so they will have a limited budget for down link bandwidth.

That said, it makes me wonder about things like NROL76 which was a classified DoD payload that SpaceX launched and had no upper stage video from. At what point are there so many of these tiny satellites in orbit that they can pick up the fairing deploy and satellite separation independently? Or have them watch the X37 as it maneuvers around in orbit?

[1] https://www.planet.com/company/approach/

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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by jessriedel on Sunday July 23, 2017 @12:46AM (#14830943)
If bandwidth were the main limiting factor, they would just take video intermittently and downlink it over a long period of time. This would be useful for documenting real-time changes on the ground during important isolated events (e.g., rocket launches). But they apparently don't have this capability, and I think it's because the hardware for taking many frames per second just isn't worth the cost and complexity.
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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by andrewstuart2 on Sunday July 23, 2017 @01:41AM (#14831044)
As mentioned in parent, the rate you can capture depends on the resolution (i.e. image size in bytes), downlink speed, available memory, and length of the event.

As the length of the event approaches infinity, the framerate you can capture is basically bandwidth / image size. `(mb/s)/mb = 1/s` (aka hz). Beyond that limit, the memory size / image size gives you a hard limit for the number of additional frames you can buffer. To get a constant frame rate, you want to spread that quantity over the duration of the event.

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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by natch on Sunday July 23, 2017 @08:11PM (#14835076)
The playback speed can also be adjusted by changing the frame rate of playback. Each frame just stays on the screen for a longer time. Similar to the -delay <n> parameter when creating a GIF in ImageMagick.
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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by andrewstuart2 on Sunday July 23, 2017 @11:32PM (#14835709)
That's not in question. Yes, playback framerate is independent from recording framerate.

The question is the availability of resources (memory and bandwith) to the satellite and how many frames it can store without needing to transmit them over its slow downlink.

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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by natch on Monday July 24, 2017 @01:35AM (#14836103)
The frames are already back on earth now, as evidenced by the video being on Youtube. Adjusting the playback rate of those frames is what I'm talking about...
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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by jessriedel on Sunday July 23, 2017 @08:57AM (#14832052)
I don't think memory is limiting design factor either.
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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by natch on Sunday July 23, 2017 @08:10PM (#14835073)
Whoosh. Nobody is arguing that they don't have the frames that they have. As we can plainly see, they have plenty of frames from which to make a video that runs in closer to real time.

All they need to do is adjust the frame playback rate to make the video last longer. Yes it will have little jerky movements but it would give time for the viewer to enjoy and ponder what they are seeing.

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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by nickelbackfan on Sunday July 23, 2017 @09:26PM (#14835329)
You could play the video at a slower speed if that is what you want, the youtube player goes down to 0.25x speed.
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Re:Satellites Taking Pictures of Rockets Carrying More Satel... by natch on Monday July 24, 2017 @01:35AM (#14836106)
Indeed. That gets us part of the way there.
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