Isn't it a copyright infringement to pick up some radio signal, reassemble it to pictures and publish them?
Dutch radio amateurs image far side of the Moon using Chinese satellite data
11–20 of 62 posts
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#12What causes the pink hue in the original photos? http://lilacsat.hit.edu.cn/DSLWP-B/865fbeb5-116d-4f6a-bb86-0...
A broad guess: Color distortion based on the filter used?
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#13Isn'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
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#14> designed by radio amateur and Nobel prize winning astrophysicist Joe Taylor He doesn't sound like an amateur at all
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.
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#15> designed by radio amateur and Nobel prize winning astrophysicist Joe Taylor He doesn't sound like an amateur at all
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"
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#16How 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? I can't find any statement indicating so.
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#17Earlier quoted context omitted.
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? 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.
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#18How 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?
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_...
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#19What causes the pink hue in the original photos? http://lilacsat.hit.edu.cn/DSLWP-B/865fbeb5-116d-4f6a-bb86-0...
The camera used does not have all the filters for space. In particular it does not have a UV cutter.
Re: Dutch radio amateurs image far side of the Moon using Chinese satellite data
#20Isn't it a copyright infringement to pick up some radio signal, reassemble it to pictures and publish them?
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.