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.
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