BatchDrake
commented
2 years ago Regarding the "32-bit complex float samples" as input... would an example of the required input be the 32-bit .RAW format that GQRX saves as I/Q recordings?
Yes and no. That's indeed the data format, but Gqrx is not able to perform this demodulation. Instead, you would need to use either SigDigger or GNU Radio to perform the demodulation of the BPSK subcarrier. If you are using EA4GPZ's GNU Radio flowgraph, ccsds-tool fits right between the Costas loop and the complex-to-real block:
If you use SigDigger, there are two scenarios. If your SNR is high you could try to lock onto the BPSK subcarrier and forward samples either to a file or directly to a socket. This is how you'd do it:
- Put the filterbox in one of the subcarriers (both transmit the same information) and adjust its bandwidth:
- In the Inspection tab, tick PSK Inspector and click in Open Inspector. Go to parameter estimation, non-linear baud estimator, click on Estimate. This will start to guess the baudrate of the BPSK signal (and as a first estimation, it is usually rather accurate). As soon as the value stabilizes, click on Apply and the go to Demodulator controls. You will see that this value (62.455 kbps in my case) has been copied to the baudrate field inside the Clock recovery box.
- There is a wide range of demodulator parameters that may work here, but for instance, in my case, I enabled AGC, set the AFC to Costas/BPSK, matched filter to SRRC with maximum roll-off, and set Gardner clock recovery with around -24 dB loop gain (don't mind the spectrum). I can tell the BPSK demod has recovered the carrier because the histogram above is not flat:
- Start
ccsds-toolin listening mode in a separate terminal. Just runccsds-tool -l 40404to accept connections in the TCP port 40404. Since the rate is rather low, you may also want to launch it asstdbuf -o0 ccsds-tool -l 40404to output the frames right after they are decoded. In this example, I am also redirecting the output tohexdump -Cjust to view the decoded frames, but you may want to store them to a file instead:
- In SigDigger, go to the Data forwarding tab, in Data properties set the Variable to Baseband I/Q data. Leave network forwarding controls as-is (or adjust the host/port if you chose to run
ccsds-toolin a different machine) and click on Forward. If everything works, you should see decoded frame data in hexadecimal form:
Note the rate is rather low (60 kbps at 1/6 means 10 kbps of useful data, which in turn translates to roughly 1 8920-bit packet per second). If you fail to see data at this rate, maybe you need to tweak the demodulator controls a bit. What I usually do is to use the SigDigger's Symbol stream tab to look for patterns in the demodulated symbols and see whether the clock recovery is not missing samples.
If your SNR is low (which is the case for the signal I dealt with), well, you may have trouble making SigDigger lock onto the subcarrier. Nonetheless, for AM signals and good PLL lock on the central carrier, there's a trick in which I can get a 3 dB SNR boost by overlapping both subcarriers (this is mathematically simpler than it looks). The problem is that this does not work out of the box (at least for now): you have demodulate the signal as AM, reduce the PLL bandwith to a point in which it synchronizes even in the presence of strong noise power (in my case, I had to reduce it down to 20 Hz), save the I/Q samples of the synchronized signal to a file, run a script to add (or subtract) every complex sample to its complex conjugate (this is what produces the 3 dB boost), and then open SigDigger again on the corrected file.
Of course, I am not going to ask you to do all this. I am working on making SigDigger be able to deal with "signals inside signals" (i.e. AM signals with PSK subcarriers like this one, FM signals with AM subcarriers like APT, FM signals with PSK subcarriers like RDS...) so you can do all this from the same UI.
PS: if you ask what's the point of all this, I know this does not look too useful but we have valid (CRC-checked) digital frames here, so it's just matter now of deciding how much upper-layer decoding logic we want to implement (and with what purpose).
PS2: You can download from my website the samples-LLR.raw file (which has some BPSK-demodulated samples that you can pass directly to ccsds-tool and extract frames) and the 3db.raw I used to make the SigDigger screenshots of this post.
K4KDR
Thank you SO much for making this available!!
Regarding the "32-bit complex float samples" as input... would an example of the required input be the 32-bit .RAW format that GQRX saves as I/Q recordings?
If not, by what means would you recommend that a input file be created in the proper format?
Also, is there a sample file available somewhere that we can use for testing (to verify that we have the software installed properly)?
Thanks again!
-Scott, K4KDR