Issues with FY-4A LRIT and HRIT live decode about satdump HOT 14 OPEN

I received FY-4A LRIT image the other day. So that's still working.
The constellation looks odd in the LRIT. Looks more like a carrier. Or the data rate has changed.

from satdump.

Today's FY-4A LRIT was normal:

from satdump.

Yea, I had another look today as well, and was able to get a good constellation like you.

But I think I've found a bug. When switching between LRIT to HRIT processing, and then back again, the constellation goes strange. Specifically I:

• stop the live processing of LRIT
• select HRIT for live processing
• choose the channel
• start the sdr on the new frequency
• start live processing

Then following the same process to switch back from HRIT to LRIT I get the weird constellation.

But yes, LRIT looks good.

from satdump.

Would you be able to provide a baseband that allows reproducing the issue?

from satdump.

I'll try and have a look today @Aang23.
I've also swapped to my better/newer laptop. come to find out ants started to make a nest inside of my Thinkpad I had it pointing at GK-2A for so long. Last night I was finally able to get a clean sync and decodes from FY-4A HRIT, with both laptops. But the HRIT live processing seems very finicky. The below screenshot shows some red in the constellation, this will produce no data via the decoder.

Here's the same transmission after stopping and restarting everything (I also turned the waterfall off to give my Thinkpad a break).

And a working one from the prior transmission with waterfall after stop/start.

Finally, here's the one from my newer laptop with the clean decode.

I don't know, the HRIT service just seems really unstable, at least on my hardware. I should mention that I didn't change anything other than laptops, the rest of my entire setup is static for the duration of these things.

Alan, I took a baseband last night, it was really strange. Looking at it and the HRIT 'hump' it looks close to 12dB SNR, as expected. Offline decode produces numbers in the 2-3dB SNR max. I'll push that up to swisstransfer today and flick it to you.

from satdump.

Here's a video I just took of the FY-4A HRIT startup to show some of the issues I'm seeing.

https://youtu.be/Qu6enGzd7VM

from satdump.

And here's the cadu from last night as well.
https://www.swisstransfer.com/d/44988e10-452e-4ae2-a44e-d6d027cebe66

from satdump.

In my case, when receiving the signal from FY-4A for a long time, when the signal bandwidth is switched sequentially, there is also the problem of loss of synchronization. I tried to report this in the issues №167 in May:
#167 (comment)

from satdump.

Hi @pluggerlockett

Is it possible your airspy gain (19) is too high? I vaguely recall times when high gain caused momentary de-sync of the signal in dvb-s2 demodulation.

from satdump.

In my case, when receiving the signal from FY-4A for a long time, when the signal bandwidth is switched sequentially, there is also the problem of loss of synchronization. I tried to report this in the issues №167 in May: #167 (comment)

I've observed this as well. If I leave live decoding running after the xx:05 transmission to try and catch the second hourly timeslot I don't get sync.

from satdump.

Hi @pluggerlockett

Is it possible your airspy gain (19) is too high? I vaguely recall times when high gain caused momentary de-sync of the signal in dvb-s2 demodulation.

That's possible? I'll try and test it but tbh I pretty much run every Airspy mini I have at either 18 or 19 for gain, and I've never seen these issues with other birds.

from satdump.

In my case, when receiving the signal from FY-4A for a long time, when the signal bandwidth is switched sequentially, there is also the problem of loss of synchronization. I tried to report this in the issues №167 in May: #167 (comment)

Just to circle back on this, I believe you're spot on with this comment.

Interestingly, sometimes the transmission of the image is interrupted in the current narrowband session by switching to broadband. And the missing part of the image is transmitted already in the next time narrowband mode.
Now, due to the forced manual break (stop/start demodulator) of the session, it is not possible to get a whole image.

On this, I was finally able to get a full decode of the Channel 12 image on Saturday, as seen below.

But what's interesting is that I think the error correction fixed it for me. In the Data Decoder window it appeared I had missed part of the transmission, but when I checked in the Live_Output folder the image was complete.

from satdump.

Hi @Aang23, just wanted to follow up on this with another baseband and more detail on the issue from my perspective. First off, here's a baseband recording from last night from the ~xx:05 timeslot. (s16, 3Msps)

https://www.swisstransfer.com/d/3c8bbcdb-3c7f-4718-8326-d50d28df01e4

If you take that baseband recording and try to run it through Offline Processing, you'll get a 0 byte cadu as a result.

But if you take the baseband recording and replay it through Recorder, initially it'll behave identically to the Offline Processing functionality.

When that occurs, my current workaround is to Stop and Start the live processing until I get a clean QPSK constellation. This process seems very finicky, sometimes I get a clean constellation immediately upon a Stop/Restart but I suspect that's dependent on when I start the Processing. I say this because in the below constellation you can see the red in it along with it being slightly malformed.

When it looks like that it'll dump no data. But if I rerun the Recorder replay I can start Processing earlier that the above malformed constellation, get clean sync and dump data.

All the screenshots in this post are from processing the baseband linked at the top of this post.

In related news, I looked at FY-4B LRIT last night and could only squeeze 5dB SNR out of it with my big dish. On FY-4A I get 17-18dB SNR. I also looked at HRIT from the top of the hour to ~50 minutes past; although the HRIT noise floor is there (for lack of a better descriptor?) there was no HRIT transmission when I looked.

from satdump.

FY-4B LRIT uses either dual or circular polarisation. I got a higher signal level when using a helical antenna. A benefit is that it helps nullify the GK-2A HRIT signal because it's linear. At the moment, it's transmitting old FY-4A data.

from satdump.