Interesting STANAG-4285 transmission heard on 14000 KHz/USB and sent me by my friend GrandBleu from radiofrecuencias.es (Figure 1)
 |
| Fig. 1 - STANAG-4285 segments |
The 35 sec segments seem a modified S-4285 waveform since they begin with a block, that I here mention to as "header", and which is not referenced in the standard. The header has a duration of 116ms and is modulated utilizing PSK2, as you may see in Figure 2.
 | |||||||||||
| Fig. 2 - PSK2 modulation detected in the first "header" |
I utilized the SA phase detector and its comparative bitmap in order to "browse" the signal and to better indagate the header. Looking at Figure 3 you may see a 13.333ms repeated pattern: well, 13.333ms @ 2400 symbols/sec makes a duration of 32 symbols (31,999) or 32 bits, since the header is PSK2 modulated (ie 1 bit = 1 symbol).
 |
| Fig. 3 - 32-bits repeated pattern in the header of the heard S-4285 waveform |
Consequently, I tried a PSK2 demodulation of the headers of any segments and after their differential decodings I obtained bitstreams which exhibit a well-defined structure consisting of first and final "01"s sequences and characterized by a 32 bits series which is six times repeated immediately before of the final "01"s series and that precisely matches the pattern seen in the bitmap of Figure 3.
[10100001001111001111100011011110]
 |
| Fig. 4 - differential PSK2 decoding of a header |
The same 32-bit series was found in all the headers I demodulated (just 3 of them are shown in Figure 5), even if it didn't appear in the same order I wrote it: 1 must consider the characteristcs of the SA's generic (!) PSK-n demodulator .
 | |
| Fig. 5 |
I don't think this alleged header is actually a “transmit level control” (TLC) block. Indeed, no information is carried by the TLC since it's a series of symbols intended solely for the intent of establishing the radio TGC (transmit gain control), ALC (automatic level control) and AGC (automatic gain control) before the actual preamble is sent/received.
And let's get to the data blocks. To identify which sub-mode is utilized I chose from time to time the various options made available by a S-4285 decoder (k500) until I found the option that had 100% assurance and 0 errors: that is, 300bps and zero dimension interleaving. As a test, I utilized a second S-4285 decoder and always got the same consequence even if the resulting bitstreams didn't seem structured. Although these decoders indicated 100% assurance and 0 errors (corrections), curiously they did not detect/show the 32-bit words utilized for signaling the Start Of Message (SOM = 0x03873C3C MSB first) and End Of Message (EOM = 0x4B65A5B2 MSB first): could it be sign of a "fake" decoding? Finally, I utilized a third, more sophisticated, decoder configuring it in "auto-detect" mode: this 3rd test besides confirmed the 300bps/N sub-mode but with the reporting of corrections and a resulting bitstream with a 40-bit/5-byte period that has - in my opinion - a bit more sense.
The 40-bit dimension period is due to the presence of a series that is 4 times repeated close the end of all the decoded segments (Figure 6). Note that the same considerations made above apply to the series in question.
[1101101000100111101001111111000111100101]
At first glance it could be an EOM/EOT signal but the bitstream should come from a higher level protocol (datalink layer) i.e. after the removal of the S-4285 overhead and so should have a different function.
 | |
| Fig. 6 - a data blocks bitstream |
That datalink protocol (if any ) is at present unknown to me.
Back to the first headers, I remembered having seen something akin a while back while I was analyzing Harris' serial PSK8 waveforms [1] and by demodulating their first headers I found a correspondence between those headers and the 1 analyzed here: that is, a series of 32 bits of dimension which is repeated six times between sequences of first and final "01"s (Figs. 7,8)
 | |
| Fig.7 |
 | |
| Fig.8 |
From the above it seems that L3Harris (and possibly not only them) have added the "autobaud" function to any waveforms specified as STANAG-4285, evidently it is only my hypothesis which has no direct or indirect confirmation: your comments and another submissions will be as usual welcome and may assist in resolving this matter.
https://drive.google.com/file/d/1WD9gBFzbGnmMdBFITTOYFf5AOTWCij4y/view?usp=sharing
(1) the “autobaud” facility enables the receiver modem to automatically adapt the transmitter’s data rate and interleaver configuration without operator intervention
[1] http://i56578-swl.blogspot.com/2021/11/harris-psk8-2400-bd-digital-voice.htm
