John Carlos BaezAug 6, 2023

I have so many questions about what just happened with Voyager 2. But let's review:

On August 20, 1977, Voyager 2 was launched from Earth.

In December 1977, it entered the asteroid belt.

In June 1978, its main radio receiver failed. Since then it's been using the backup receiver!

On July 9, 1979, it flew past many of Jupiter's moons, made its closest approach to Jupiter, and took tons of beautiful pictures.

On August 26, 1981 it shot past Saturn and took tons of beautiful pictures.

On August 25, 1989 it shot past Neptune and took tons of beautiful pictures.

On November 5, 2018 it crossed the heliopause and entered interstellar space, 120 times farther from the Sun than we are.

On July 18, 2023, it overtook Pioneer 10 and became the second farthest man-made object from the Sun.

3 days later, some idiot sent a command that pointed its high gain antenna 2 degrees away from Earth. HOW EXACTLY DID THIS HAPPEN?

On August 4, 2023, NASA used its most high-powered transmitter to successfully command Voyager 2 to reorient towards Earth, resuming communications. HOW WAS THAT POSSIBLE?

Voyager 2 is now 133 AU away. How can you "shout" across such a distance and attract the attention of someone who is not looking in your direction? That's very far. It takes light about 18 hours to travel that far.

Show threadFlorian MarquardtAug 6, 2023

@johncarlosbaez Amazingly, the radio power received on earth from the Voyager spacecraft transmitting in the GHz range at 20 W is about an ***attowatt (1e-18 W)***, since they are on the order of 20 billion kilometers from earth !

See info here (it refers to Voyager 1 but for these estimates I think both Voyager are about the same figures):
https://public.nrao.edu/ask/how-strong-is-the-signal-from-the-voyager-1-spacecraft-when-it-reaches-earth/

#Voyager #Voyager2

How Strong is the Signal from the Voyager 1 Spacecraft When it Reaches Earth? - National Radio Astronomy Observatory

Question: What is the signal strength in watts received by the VLBA that is transmitted from Voyager 1? I...

National Radio Astronomy Observatory
Show threadFlorian MarquardtAug 6, 2023

@johncarlosbaez Since these signals are so weak when they arrive at earth, they need to be amplified by low-noise amplifiers operated cryogenically, at low temperatures:

https://science.nrao.edu/facilities/cdl/low-noise-amplifiers

There is a nice discussion inside the following report, pointing out that a Kelvin reduction in the operating temperature of the receivers increases the data rates in a way that you may save on the order of 100 million dollar over a 20 year period in the deep space network: https://safe.nrao.edu/wiki/pub/Main/GalenWatts/Reid_DESCANSO_sml.pdf

Low Noise Amplifiers - Pushing the limits of low noise

NRAO Science Site
Show threadCave Cattum

@FMarquardtGroup @johncarlosbaez
Well, 1 attowatt @ 20 GHz is still quite a bit of photons per millisecond (76, if my math is right), courtesy of the itsy bitsy tiny weeny Plank constant. A light source of that power in a dark room is easily perceived 😉

Wasn't Robert McElice involved in the engineering of the error-correcting code used for the Voyager? Am just amazed at what they could fit in that tiny computer.

Aug 6, 2023 at 3:19pmWeb
Show threadFlorian MarquardtAug 6, 2023

@fgcallari @johncarlosbaez Great pointer to error correction! Here is a stackexchange discussion on which error correction codes are used for the #Voyager spacecraft communications:

https://space.stackexchange.com/questions/54055/did-the-voyager-spacecraft-use-a-golay-a-reed-solomon-and-or-a-hamming-code-for

This is beyond my expertise, but anyways obviously very advanced and impressive!

Did the Voyager spacecraft use a Golay, a Reed-Solomon and/or a Hamming code for data transmission encoding for error correction? (Need clarification)

The now-famous answer to How is stacking oranges in 24 dimensions related to receiving and decoding signals from the Voyagers? is worth stopping now here and going back and reading first. Okay welc...

Space Exploration Stack Exchange
Show threadFlorian MarquardtAug 6, 2023

@fgcallari @johncarlosbaez Great talk about the #Voyager mission:

https://www.youtube.com/watch?v=H62hZJVqs2o

Mostly about the engineering, computer system, and redundancy.

Show threadJohn Carlos BaezAug 7, 2023

@FMarquardtGroup -

Thanks, I'm checking out that stuff! The Golay code, used in the Voyagers' Jupiter and Saturn flybys, is a 24-bit code where 12 bits convey the message and the other 12 bits, computed from the first 12, are used to tell if the first ones were transmitted correctly. The second 12 are computed from the first 12 using a dodecahedron as shown here by @gregeganSF. I explain this stuff in more detail here:

https://blogs.ams.org/visualinsight/2015/12/01/golay-code/

I need to learn more about the other codes used by the Voyagers!

@fgcallari

Show threadIan TindaleAug 7, 2023
@johncarlosbaez @FMarquardtGroup @gregeganSF @fgcallari is this related to the kind of error correction a CD player head uses?
Show threadJohn Carlos BaezAug 7, 2023

@u0421793 @gregeganSF @FMarquardtGroup @fgcallari - it's related, but a CD player doesn't use a Golay code, since it's not forced to be so simple by technological limitations. I just looked it up and it uses Cross Interleaved Reed-Solomon Coding (CIRC):

https://en.wikipedia.org/wiki/Cross-interleaved_Reed%E2%80%93Solomon_coding

Instead of 24-bit code words like the Golay code, this uses much longer code words. Apparently it can correct error bursts up of to 3,500 bits in sequence (2.4 mm in length as seen on CD surface)!

But the math is related. You're never gonna understand the codes used by CDs if you don't understand the simpler codes used by the Voyagers!

Cross-interleaved Reed–Solomon coding - Wikipedia

Show threadIan TindaleAug 7, 2023
@johncarlosbaez @gregeganSF @FMarquardtGroup @fgcallari also hard-drive heads (the magnetic sort) – if you explained to me how they’re expected to work I’d have said “no chance”

It’s constantly fishing a signal out of mostly stochastic nonsense, it’s a miracle a hard drive head can read anything reliably, it’s more akin to radio than digital electronics