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Comment by natch | Hacker News Reader

Would love to see an actual speed version of this... what's the hurry?

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/

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.

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.

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.

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.

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...

I don't think memory is limiting design factor either.

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.

You could play the video at a slower speed if that is what you want, the youtube player goes down to 0.25x speed.

Indeed. That gets us part of the way there.

I presume their satellites aren't equipped to take more than one image per second. (For most of their applications, it's not necessary, and adding that ability to a microsat is would definitely increase the weight/cost/power.)

You can watch it on 25% speed on YouTube. It's pretty jittery, so it would be nice if someone made an interpolated version.

I tried using butterflow and it seems ok at first, but I guess the algo is not made for this weird shot.

https://www.youtube.com/watch?v=8a6QoKQNGWA&feature=youtu.be

This one might be better.

FPS is set to 60 and speed is the same:

https://youtu.be/b7Ua4mXO23E

Their satellites have a one dimensional sensor that scans continuously.

They are using 2D TDI CCDs, just like everyone else.

This timelapse was likely shot with frame by frame exposures though. I'm not sure how it's possible with TDI.

Still inherently 1D. TDI sensors are not like area arrays.

There is a live HD zoomed video stream from the space station. Maybe someday they will catch one of these.

They stiched still images together to make the video, perhaps that created the speed up?

Yes but that's configurable with a good tool.

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