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Dutch radio amateurs image far side of the Moon using Chinese satellite data (camras.nl)
267 points by followben 79 days ago | hide | past | web | favorite | 57 comments

> The transceiver on board Longjiang-2 was designed to allow radio amateurs to downlink telemetry and relay messages through a satellite in lunar orbit, as well as command it to take and downlink images.

This is so cool and would have made such an impact on me as a kid if I got to see it in action. I wish we could better demonstrate this sort of thing in middle school or high school science classes, such as how easy it is to download a weather satellite image live, via SDR.

> designed by radio amateur and Nobel prize winning astrophysicist Joe Taylor

He doesn't sound like an amateur at all

> radio amateur

I don't know if it's a British joke, but recall that "amateur" in "amateur radio" means "a duly authorised person interested in radioelectric practice with a purely personal aim and without pecuniary interest " (International Radiotelegraph Convention of Washington, 1927), i.e. anyone working with radio for the hobby, not for the job salary.

Amateur here is "unpaid". Usually the obsessive nature of engineers, me included, means they tend to take the work home with them. Back when I was working for a defence contractor years and years ago there were a lot of amateur radio operators working on RF systems that had so much fun they went home and did another day's work there. Sometimes this was leveraged for personal gain and sometimes it actually played out and ended up in commercial radio systems. Similar to how a lot of people go home and write code. "Amateur programmers"

I am not a native speaker, but it is my understanding that amateur has 2 definitions: 1) unpaid, 2) incompetent or inept.

The Camras team is maintaining and operating the telescope on their own time. The members have dayjobs like programmers and scientists, or are retired professionals. So I'll go with the first definition.

unpaid as in never having done something as a paid profession. A professional software developer who retires and programs as a hobby is not suddenly an amateur developer.

I don't think there's a definition like that anywhere. Only if you define 'amateur' as some who doesn't really know the trade. We should remember that 'amateur' actually means "lover of", i.e. that an amateur is simply someone doing it because they love to (as opposed to "just" because they're paid to do it). It doesn't say anything about the level.

Yes, they would be. Amateur developer in this context can be interchangeable with open source developer for instance, programming as a hobby with others or just for yourself. It's just a title. You can be both a professional sw dev and an unpaid opensource dev at the same time. Likewise, you can be both an amateur HAM operator and a professional RF engineer at the same time, exclusive of each other.

So a professional football player who after his career joins an amateur team for recreational football should not be regarded as an amateur?

To experiment in radio on certain frequencies (like the ones needed to talk to satellites, requires a license from your nation's government, as radio frequencies are a shared resource. Certain levels of licensing is for 'amateurs' and other licenses are for more industrial things, like TV and what is commonly referred to as FM or AM radio.

Communication with satellites in the US requires the highest level of amateur licensing called the Extra.


Actually, you can talk to satellites[1] using a technician license, because they use VHF/UHF. A general or extra is required for using certain HF bands.


This is great news, although I feel I want to keep studying for the Extra for the ham cred that comes with it ;)

> the Extra.

I'm now imagining the on-line radio amateur community poking fun at themselves when the recent slang interpretation of "extra" came along.

"Amateur" here is a term defined by treaty. Radio Amateurs are prohibited by law from receiving compensation for their activities (in the US W1AW's paid operators get an express exception). Furthermore, the comms themselves can't have commercial purposes and often can't involve non-amateurs if the communications displace commercial services.

If the aviation world used similar terms a private pilot would be called an amateur pilot.

Not sure if this was a joke or serious, but 'amateur radio' refers basically to people who are into HAM radio. It doesn't necessarily preclude them from also working with radio professionally.

I recently learn, and therefore want to tell everyone, the ham in Ham radio isn’t an acronym.

It means ham as in ham fisted, as in amateurish or unskilled.

See the Wikipedia entry for more info https://en.wikipedia.org/wiki/Amateur_radio

As per the linked wikipedia article.. it used to mean that (as a pejorative term coming from professional wired telegraphy in the 19th century). From the article ".. by the mid-20th century it had lost its pejorative meaning." But yes, it's not an acronym.

It can still mean ham as in ham fisted while not being pejorative.

The word amateur has two different meanings in English.

"1. a person who engages in a pursuit, especially a sport, on an unpaid rather than a professional basis"

"2. a person who is incompetent or inept at a particular activity"

Here we refer to the first definition. The English word amateur comes from French "lover of". Someone who does things just because they like it, not because they get paid to do it.

It should be obvious that there is a third meaning..

Amateur is short for "Amateur Radio operator", eg someone who holds an Amateur Radio License.

We need to start using the term "amateur" in it's original sense to mean "for the love of". This is why people call themselves amateur astronomers, say, not because they failed astronomy class but because they love their hobby of astronomy.

Doesn't sound very Dutch either ...

The encoding isn't, that is true. That part was about the team on the ground at the Dwingeloo 25m telescope. The (old) 25m telescope is now run by volunteers.

Most people don't appreciate just how big a 25m dish is until they are standing next to one. The fact that one is being run by volunteers is remarkable.

This is amazing! I have a background in physics and electronics, but I have never been involved in amateur radio. However, I would love to and would like to ask if one could point me to resources for delving into it? When it comes to math and e.g. electrodynamics and practical filters, I wouldn't need an introduction. Thanks!

Getting a license is your first step if you want to transmit. If you are in the US, the ARRL produces the training material for the various tests. There are free apps that go over the test questions but I like having a little more context to the answers.

With your background you'd probably need to devote time mostly to learning the rules and regulations. You can take all 3 tests in one sitting for a $15 fee and be done with it. Each test gets you more privileges (power, frequencies).

Even without a license you can receive with a cheap SDR and home brew antenna. This blog has many examples of that: https://www.rtl-sdr.com/

Finally, you may want to find a local club. Usually visiting during a "contest" will enable you to get on the air. In the US you don't need a license to use the radio if you are with a properly licensed operator.

Hope you find the hobby as fun as I have.

There is lots of cool science to be done with radio, being done by amateurs. Starting with the Society for Radio Astronomy mentioned already, but also see:


SARA members have a lot of projects going on, from beginner stuff to full blown SDRs http://www.radio-astronomy.org/projects

Radio Astronomy Software Defined Radio (RASDR): http://www.radio-astronomy.org/rasdr

SuperSID: Sudden Ionospheric Disturbance monitor: http://www.radio-astronomy.org/node/210

Radio Jove: Radio emissions from Jupiter: https://radiojove.gsfc.nasa.gov/

Personal Space Weather Station: http://www.hamsci.org/basic-project/personal-space-weather-s...

Reverse Beacon Network: http://www.reversebeacon.net/

Weak Signal Propagation Reporter Network: http://wsprnet.org/drupal/

PSK Automatic Propagation Reporter: https://pskreporter.info/

What I did was to find the local club, go there, learn about the next course & exam, and focus solely on learning morse code (because I wanted the full license). Because I had an electronics&radio background I didn't have to spend any time on that part, I could focus all my time on the specific amateur radio regulations (not difficult) and learning morse (although IIRC I still had to take a written exam about radio transmission theory - not sure though, it's been a long time. I remember though that professional "telegraph" operators from ships didn't have to take any exams at all, but I don't think that kind of ship operators exist anymore. No morse code there, for sure..).

CW coding hasn't been a requirement at any level for 12 years. Its good to know but unless you want to do HF, it's irrelevant.

I only want to do HF.. but that's outside the point, I did write that this was a long time ago, and at that time you did need CW to get the full license.

The Society for Amateur Radio Astronomy (SARA) mailing list and website (http://www.radio-astronomy.org/) should be your first stop. There's also a decently active (daily) radio astronomy news feeds/forum over at http://notabug.io/t/radioastronomy . Like others have said, cheap SDR receivers like RTL-SDR based devices have made it easy to get involved.

I've been a ham radio operator in the US for a couple years now, and its a really interesting hobby that will put you on contact with very smart inviting people. Its a great way to learn more about electricity too.


You can get all the books to really learn the material (the ones by KB6NU are the best) but if you already have the background you can just study the test. I use an app called Ham Test Prep on android to take quizzes that basically amount to flashcards. Once you learn most of the questions and answers, you can pass pretty easily.

The RTL-SDR subreddit is pretty cool : https://www.reddit.com/r/RTLSDR

What causes the pink hue in the original photos?


The camera used does not have all the filters for space. In particular it does not have a UV cutter.

A broad guess: Color distortion based on the filter used?

Rock on, amateur operators!

HAMs did the same thing in the 60s, decoding Soviet communications coming from the moon, which were the first closeup images of the surface at the time. They released the images to wide fanfare, and did a lot to defuse the Soviets' political agenda of holding this accomplishment above the western world.

Isn't it a copyright infringement to pick up some radio signal, reassemble it to pictures and publish them?

Both Longjiang satellites carry a VHF/UHF amateur radio transceiver built and operated by the Harbin Institute of Technology in China. They've open sourced software specifically so radio amateurs can decode telemetry as well as image data: https://github.com/bg2bhc/gr-dslwp

Depends on the wishes of the people who originated the signal. There are plenty of OTA signals that are free to the public, like NOAA weather satellite data. They're paid for by the public and are in the public domain. I don't know specifically about these signals, but it seems like the Chinese government is pretty open with them as well.

Written with tongue only half in cheek:

Depends. In the US where media companies write copyright law and hand it to legislators, yes. In China where there appears to be no Rule of Law, no.

So interesting! How do you send "error-resistant digitally modulated transmissions"? Is that the same thing as a duplex signal? Is there other technology used nowadays?

Could be coding (Turbo coding is where it's at, while hamming (no relation) error correcting codes are purely digital and ubiquitous), could be simple double transmissions separated in the time domain as in FEC SITOR/AMTOR. My money's on Turbo coding. https://en.wikipedia.org/wiki/Turbo_code

Super interesting! Is this still the standard for this sort of communication nowadays or are there successors?

It's probably error corrected code. Some examples are CRC, ECC menory, and Raid-5 disk array.

How do pixels work in radio astronomy? Does the antenna only capture one pixel every time the "shutter" is opened and then move to the next position?

This is a normal camera, a normal radio that transmits it, and a large radio telescope (once, over half a century ago, the largest in the world, but now amateur operated; https://en.m.wikipedia.org/wiki/Dwingeloo_Radio_Observatory) used as an antenna to receive that broadcast.

I expect the data to be transferred one pixel at a time, but possibly, it’s losslessly compressed.

It’s “Dutch radio amateurs image” because they controlled the camera.

They controlled the camera? I can't find any statement indicating so.

> The transceiver on board Longjiang-2 was designed to allow radio amateurs to [...] command it to take and downlink images.

It depends on the type of observations being made. If it is done with a single dish with a single antenna feed then to make images you just have to know when and where you are pointing while you record the power.

But most images you see of radio astronomy are done with interferometers. This means you use multiple pairs of antenna. Each pair will respond to a point source going across the sky with the interference pattern modulating the voltage at the receiver that follows an acos pattern relative to the distance between the antennas in the pair.

The multiple pairs of antenna all each contribute a 2D fringe pattern. The interference makes the receivers record what is essentially the fourier transform of the sky. So some pair might have sine wave of a particular wavelength and angle, and another pair another angle. By combining all these 2D patterns of waves the sum ends up being an "image" of the sky (convolved with the primary and sidelobe beams).

That gets you a "DIRTY" image, and it really is dirty. The beam of an interferometer is a complex thing. So after that multiple types of cleaning algorithms may be used to use prior knowledge to pull just the actual signal out of the noisy, gunked up "image".

So in the end each pixel is really the result of a somewhat arbitrary algorithm picking out the bright bits after taking the FFT of the sky of each baseline pair and combining them.

If you want to know more I recommend running the python (cross platform) interferometry simulator https://launchpad.net/apsynsim while watching the video lecture series at https://www.youtube.com/watch?v=0TwnZhiEc3A and then https://www.youtube.com/watch?v=mRUZ9eckHZg

The radio part isn't directly linked to the imaging part.

The radio link is effectively just a modem (short for 'modulate/demodulate') that transforms bits into some modulation of a radio signal. A simple and error-prone modulation would be to just shift the frequency back and forth between two values such that the lower frequency represents a binary zero and the higher a binary one. The receiver listens to the RF coming from the transmitter, and if it applies the same algorithm to demodulate the signal back to bits, will start kicking out bits that turn to bytes that turn to pixels or telemetry or navigational data or whatever the satellite wishes to send. So in this case, it could be something approximating a standard digital camera, computer snaps a photo, maybe stores it on a filesystem, queues it up to send and when the time is right starts blasting bits down the RF pipe back to earth.

In my very limited experience with this the satellite will typically send a simple raw lossless bitmap or similar encoding to minimize the effect of data loss for individual pixels.

I did find a short blurb on the Longjiang-2 modulation types:

"While receiving signals from satellites in low Earth orbit requires only relatively simple antennas, doing so for satellites in orbit around the Moon (a thousand times more distant), is much harder. To this end Longjiang-1 and 2 transmit signals in two low data-rate, error-resistant, modes; one using digital modulation (GMSK) at 250 bits per second, while the other mode (JT4G) switches between four closely spaced frequencies to send 4.375 symbols per second. This latter mode was developed by Nobel-prize winning astrophysicist Joe Taylor and is designed for radio amateurs to relay messages at very low signal strengths, typically when bouncing them off the surface of the Moon."

GMSK stands for Gaussian minimum shift keying [1] and is also used for GSM mobile phone data transmission. It's a fairly sophisticated frequency shift keying that minimizes phase disturbance between the shifted frequencies.

JT4G is a simple frequency shift keying modulation. Here's [2] a recording of it from Longjiang-2, turn up the volume to hear the downconverted audio.

[1] https://en.wikipedia.org/wiki/Minimum-shift_keying#Gaussian_...

[2] https://twitter.com/cgbassa/status/1018428722502946816

The same way pixels for on your phone camera. The only difference is the file/data is transmitted over a different transport.

Oh I thought it was imaging in the radio spectrum but actually it's not.

The RTL-SDR community at Reddit is a good place to look for resources if you are looking to get into this: https://www.reddit.com/r/RTLSDR

Dang...and I thought we would find all the aliens on the dark side of the moon. Where did they all go?

They are deep in Uranus.

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