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Comment by etaioinshrdlu | Hacker News Reader

Does anyone know what type of data compression is being used to transmit these images? There's been huge progress since the last rover.

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They use the same kind of compression as the last mission. JP2, I believe. https://en.wikipedia.org/wiki/JPEG_2000

I'm not sure that data compression has progressed that much since Curiosity in 2012 or Insight in 2018. There have been some formats that have improved compression at the expense of large amounts of memory or processing power, but neither of those are abundant on a rad-hardened processor. Neither is electricity. Neither are the toolchains to process such images very widespread or as well understood.

Any idea how much storage it has? I would think it'd have to be TBs to store all these images, logs, and other data for years while making sure they have enough memory so that they're not limited and if the rover stays there for longer than intended.

Why would it need to continue to store things after successful receipt on Earth?

Very off-site backup.

Once it's been sent to Earth, they drop it. Curiosity's main computer had a flash drive of 2GB, and each of the big cameras had an 8GB flash drive on board. I doubt Perseverance is much changed from that; the main computer is IIRC a spare part from the Curiosity production.

Rad-hardened computer hardware is expensive and bulky. You don't add storage or processing power that you don't absolutely need.

See https://www.ontrack.com/en-us/blog/how-to-store-data-on-mars and https://mars.nasa.gov/mars2020/spacecraft/rover/brains/

One of the obvious constraints is that for science reasons they transmit each image in a lossless format, so we're up against the limits of what a lossless image compressor can do.

If there were a future mission's requirement to send back lossy images first, and CPU power was significant, something like AVIF or JPEG-XL would likely be a candidate.

https://netflixtechblog.com/avif-for-next-generation-image-c...

ICER appears to be what they're using now for lossless: https://en.wikipedia.org/wiki/ICER

Thanks for that. Yes, they do both lossless and lossy compression. I think sometimes they may send the lossy image (especially if for a navigation or engineering camera) and request a lossless image if there's something particularly interesting.

According to the graph on p.42, the 20 MHz CPU compresses at a rate of about 20us/pixel. So the 1024x1024 images take about 20 seconds each to compress.

There are 2,000 "normal" sized images on the website [0]. If each of these was 1024x1024, they took 11 hours to compress. So Perseverance spends about 7% of its CPU time on image compression.

[0] https://mars.nasa.gov/mars2020/multimedia/raw-images/

Found some more info. They're using ICER for lossless, like you say. They're also using LOCO for lossless compression. For lossy compression, the JPEG encoding was inherited from the Mars Insight lander (in 2018).

Page 36 from the PDF: https://link.springer.com/article/10.1007/s11214-020-00765-9

I can't find references to JPEG 2000 on this report: https://link.springer.com/article/10.1007/s11214-020-00765-9

Are they being re-encoded into regular JPG files when being shown online?

Look for “JP2”. It is used for other spacecraft that are part of the Mars fleet.

You're right. They're encoding just regular JPEG for lossy compression. They're also using either ICER and LOCO for lossless compression. JP2 has been used for other Mars spacecraft (the HiRise orbiter, I believe), but the JPEG encoding was inherited from the Mars Insight lander (in 2018).

> There's been huge progress since the last rover.

One thing that changed in the last 8 years is that there are now two new orbiters around Mars (MAVEN and ExoMars TGO), which can retrieve data from the rovers faster, cache it and send it to Earth faster.

I'm speculating here, but I think these images are transmitted in some kind of raw format, or close to it, making them more resilient to data corruption. Compressed images will incur greater data loss when compressed, compared to raw images where you might lose a few image blocks at a time.

They compress them but transmit with forward error correction. They do have the ability to retransmit if the images are too damaged.

Totally uncompressed images are not a very efficient way to transmit images if you want to ensure they're scientifically accurate.

Naively, I'd doubt that. There are all kinds of error correcting codes to deal with that and you'd end up with a substantial increase in effective bandwidth (with the compression). There may be error correction built in to the protocol itself already.

Compression can also include backup/padding/parity data to correct for errors and transmission losses.

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