Clive Robinson March 4, 2021 3:58 AM

If my brain is working correctly this morning this is not the first time a code or signal has been introduced in a Mars lander.

I watched an article on the latest Perserverance and why they had gone for a different wheel design.

Apparently the older tracked tire design had proved less reliable than expected.

They showed a promo vodeo from the earlier lander where the treds had a reverse logo cut into them so every time tracked tire went around the logo was left in the soil…

Some of these missions take a quater of a century from inception to touch down so I’d be surprised if there were not one or two “Easter Eggs” in each and every one.

Clive Robinson March 4, 2021 5:02 AM

@ SpaceLifeForm, ALL,

At these distances, no Parity, no Stopbit, how can we trust this? 😉

No they just said “FEC it”…

FEC or “Forward Error Correction” is a better way to ensure correct information when checking in a high noise –due to weak signal– very high latency –due to distance– comms channel Which you kind of get when you are talking to planets adjacent orbits[1]. Importantly as well it works on a one way or simplex channel[2].

FEC works on the simple notion that your probability of error drops when you send the information repeatedly. So in a say fifty frames a second video image you’ve got quite a lot of redundance for errors to be found and removed.

Conversely most digital video streaming we see on the Internet works the other way around that is they only send the bits that change where they can because normally most video images are highly redundant.

[1] 54.6 million kilometers is about the minimum distance from the Earth to Mars. Which is about 3 min’s 2seconds at 3×10^8 meters/sec. This stretches up to about the farthest apart they can be which is 401 million km, or ~7.34 time or just under 23mins one way…

[2] In the film The Martian there is a bit where “Tim” does a quick mental calculation and says “32-minute round-trip communications time. All he can do is ask yes or no questions and all we can do is point the camera. This won’t exactly be an Algonquin Round Table of snappy repartee.”. It would have been faster to put on the DVD than it was to do that Internet search 😉 Whilst I knew that the Alonquin were a tribe of nstivr Americans, I dod not know what the “Algonquin Round Table” was and thst included the likes of Harpo Matx and Dorathy Parker, funny what you can occasionaly learn from films,

FA March 4, 2021 8:59 AM


Conversely most digital video streaming we see on the Internet works the other way around that is they only send the bits that change

There is no contradiction in that, it’s a different level of ‘coding’ and it serves a different purpose.

We have

A. Source coding. This is used to reduce the required data rate by removing perceptually irrelevant or redundant data. Also called ‘compression’. How it is done depends only the nature of the content.

B. Channel coding. This adds redundancy in a systematic way in order to enable error correction at the other end. How this is done depends on the nature of the physical transmission channel and operational requirements.

Both are called ‘coding’ but they are really very different disciplines,
and engineers tend to specialise in one or the other and very rarely both.

On the transmission side the order is 1. source coding, 2. encryption (if any), 3. channel coding, 4. modulation. At the receiver the order is reversed. On some advanced systems the separation between channel coding and modulation may not be so clear as they are designed to work well together.

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