7 ms·

Dutch radio amateurs image far side of the Moon using Chinese satellite data

267 points by followben 8y ago

z2 8y ago
> 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.
changoplatanero 8y ago
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?
- Someone 8y ago
  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 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.
  - Uberphallus 8y ago
    They controlled the camera? I can't find any statement indicating so.
    - tqkxzugoaupvwqr 8y ago
      > The transceiver on board Longjiang-2 was designed to allow radio amateurs to [...] command it to take and downlink images.
- jcims 8y ago
  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_minimum-shift_keying https://en.wikipedia.org/wiki/Minimum-shift_keying#Gaussian_... [2] https://twitter.com/cgbassa/status/1018428722502946816 https://twitter.com/cgbassa/status/1018428722502946816
- dtech 8y ago
  The same way pixels for on your phone camera. The only difference is the file/data is transmitted over a different transport.
  - changoplatanero 8y ago
    Oh I thought it was imaging in the radio spectrum but actually it's not.
- superkuh 8y ago
  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 https://launchpad.net/apsynsim while watching the video lecture series at https://www.youtube.com/watch?v=0TwnZhiEc3A https://www.youtube.com/watch?v=0TwnZhiEc3A and then https://www.youtube.com/watch?v=mRUZ9eckHZg https://www.youtube.com/watch?v=mRUZ9eckHZg
lalo2302 8y ago
> designed by radio amateur and Nobel prize winning astrophysicist Joe Taylor He doesn't sound like an amateur at all
- spiderfarmer 8y ago
  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.
  - dtech 8y ago
    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.
    - spiderfarmer 8y ago
      So a professional football player who after his career joins an amateur team for recreational football should not be regarded as an amateur?
    - Tor3 8y ago
      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.
      - deleted 8y ago
        [deleted]
    - Humdeee 8y ago
      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.
testplzignore 8y ago
What causes the pink hue in the original photos? http://lilacsat.hit.edu.cn/DSLWP-B/865fbeb5-116d-4f6a-bb86-0a67a09b41ff.jpeg http://lilacsat.hit.edu.cn/DSLWP-B/865fbeb5-116d-4f6a-bb86-0...
- consp 8y ago
  A broad guess: Color distortion based on the filter used?
- tammojan 8y ago
  The camera used does not have all the filters for space. In particular it does not have a UV cutter.
lixtra 8y ago
Isn't it a copyright infringement to pick up some radio signal, reassemble it to pictures and publish them?
- followben 8y ago
  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 https://github.com/bg2bhc/gr-dslwp
- na85 8y ago
  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.
  - jnmandal 8y ago
    One of the most authoritarian regimes history has ever seen has "no rule of law" it seems.
- jandrese 8y ago
  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.
pvitz 8y ago
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!
- daphreak 8y ago
  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/ 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.
- harias 8y ago
  The RTL-SDR subreddit is pretty cool : https://www.reddit.com/r/RTLSDR https://www.reddit.com/r/RTLSDR
- Tor3 8y ago
  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..).
harias 8y ago
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 https://www.reddit.com/r/RTLSDR
syntaxing 8y ago
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?
- ww520 8y ago
  It's probably error corrected code. Some examples are CRC, ECC menory, and Raid-5 disk array.
- howard941 8y ago
  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 https://en.wikipedia.org/wiki/Turbo_code
  - syntaxing 8y ago
    Super interesting! Is this still the standard for this sort of communication nowadays or are there successors?
stevespang 8y ago
Substantial power in the equipment viewable in the image of the private "mission control" room, on far right hand side of image . . .
ninju 8y ago
Dang...and I thought we would find all the aliens on the dark side of the moon. Where did they all go?
- null_content 8y ago
  They are deep in Uranus.
arbitrage 8y ago
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.

267 points by followben 8y ago

z2 8y ago

> 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.

changoplatanero 8y ago

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?

Someone 8y ago
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 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.
- Uberphallus 8y ago
  They controlled the camera? I can't find any statement indicating so.
  - tqkxzugoaupvwqr 8y ago
    > The transceiver on board Longjiang-2 was designed to allow radio amateurs to [...] command it to take and downlink images.
jcims 8y ago
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_minimum-shift_keying https://en.wikipedia.org/wiki/Minimum-shift_keying#Gaussian_... [2] https://twitter.com/cgbassa/status/1018428722502946816 https://twitter.com/cgbassa/status/1018428722502946816
dtech 8y ago
The same way pixels for on your phone camera. The only difference is the file/data is transmitted over a different transport.
- changoplatanero 8y ago
  Oh I thought it was imaging in the radio spectrum but actually it's not.
superkuh 8y ago
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 https://launchpad.net/apsynsim while watching the video lecture series at https://www.youtube.com/watch?v=0TwnZhiEc3A https://www.youtube.com/watch?v=0TwnZhiEc3A and then https://www.youtube.com/watch?v=mRUZ9eckHZg https://www.youtube.com/watch?v=mRUZ9eckHZg

lalo2302 8y ago

> designed by radio amateur and Nobel prize winning astrophysicist Joe Taylor He doesn't sound like an amateur at all

spiderfarmer 8y ago
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.
- dtech 8y ago
  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.
  - spiderfarmer 8y ago
    So a professional football player who after his career joins an amateur team for recreational football should not be regarded as an amateur?
  - Tor3 8y ago
    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.
    - deleted 8y ago
      [deleted]
  - Humdeee 8y ago
    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.

testplzignore 8y ago

What causes the pink hue in the original photos? http://lilacsat.hit.edu.cn/DSLWP-B/865fbeb5-116d-4f6a-bb86-0a67a09b41ff.jpeg http://lilacsat.hit.edu.cn/DSLWP-B/865fbeb5-116d-4f6a-bb86-0...

consp 8y ago
A broad guess: Color distortion based on the filter used?
tammojan 8y ago
The camera used does not have all the filters for space. In particular it does not have a UV cutter.

lixtra 8y ago

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

followben 8y ago
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 https://github.com/bg2bhc/gr-dslwp
na85 8y ago
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.
- jnmandal 8y ago
  One of the most authoritarian regimes history has ever seen has "no rule of law" it seems.
jandrese 8y ago
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.

pvitz 8y ago

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!

daphreak 8y ago
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/ 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.
harias 8y ago
The RTL-SDR subreddit is pretty cool : https://www.reddit.com/r/RTLSDR https://www.reddit.com/r/RTLSDR
Tor3 8y ago
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..).

harias 8y ago

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 https://www.reddit.com/r/RTLSDR

syntaxing 8y ago

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?

ww520 8y ago
It's probably error corrected code. Some examples are CRC, ECC menory, and Raid-5 disk array.
howard941 8y ago
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 https://en.wikipedia.org/wiki/Turbo_code
- syntaxing 8y ago
  Super interesting! Is this still the standard for this sort of communication nowadays or are there successors?

stevespang 8y ago

Substantial power in the equipment viewable in the image of the private "mission control" room, on far right hand side of image . . .

ninju 8y ago

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

null_content 8y ago
They are deep in Uranus.

arbitrage 8y ago

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.