MIL 188-110B App. C (STANAG-4539 Annex B)

in this recording a MIL 188-141 2G-ALE link setup between ALE calls 5CIND2 and 5CIND1, and two MIL 188-110B App.C 4800bps short sessions for data transfer. The Appendix C of MIL 188-110B describes the HF data modem waveforms for data rates above 2400 bps (3200, 4800, 6400, 8000, 9600, and optional uncoded 12800 bps) and  is  defined as High Data Rate Traffic Waveforms in STANAG-4539 Annex B (the counterpart NATO compatible standard).

This transmission was spotted on 10074.0 KHz/USB @ 0710 UTC, on June 22.

There are two clues that confirm this waveform:

1) the ACF value is 119.58ms and corresponds to a frame structure consisting of 287 symbols data block consisting of 256 data symbols followed by a mini-probe of 31 symbols of known data  (188-110C Appendix D has 120ms ACF and 256 +32 = 288 symbols per data block)

fig. 1 - ACF and frame structure

2) the preamble exhibits a clear 184 symbols blocks structure 

fig. 2 - preamble

Both the two structures are also visible looking at the bistream obtained after the PSK-8 symbols demodulation: the preamble consists of 552 bits and each data frame consists of 861 bits  (fig. 3)

fig. 3

Unfortunatelly it's a poor quality signal and the constellation is not so clear, although the 8-ary modulation is quite evident in figure 4. Since the 3200bps QPSK constellation is scrambled to appear on-air as a PSK-8 constellation, we can't be positive about the data rate. Compatible decoders say 4800bps Short interleaver.

 

filg. 4 -  PSK-8 'over-the-air' constellation

According to the standard, preamble re-insertions occur each 72 data frames, thus the reinsertion period Tr is 8,61 secs (20664 symbols): since the duration of each session is <Tr (in this recording we count 36-37 data frames) we then do not see such reinsertions (they are not transmitted).

Using the PSK-8 symbol mapping, each mini-probe is based on the repeated Frank-Heimiller sequence. The sequence that is used, specified in terms of the 8PSK symbol numbers, is given by:

0, 0, 0, 0, 0, 2, 4, 6, 0, 4, 0, 4, 0, 6, 4, 2, 0, 0, 0, 0, 0, 2, 4, 6, 0, 4, 0, 4, 0, 6, 4

This mini-probe are designated ‘+’. The phase inverted version of this is:

4, 4, 4, 4, 4, 6, 0, 2, 4, 0, 4, 0, 4, 2, 0, 6, 4, 4, 4, 4, 4, 6, 0, 2, 4, 0, 4, 0, 4, 2, 0

and mini-probes using this sequence are designated ‘-’ (as the phase of each symbol has been rotated 180 degrees from the ‘+’). The 72 mini-probes before the preamble reinsertion are grouped into four sets of 18 consecutive mini-probes (1 to 18, 19 to 36, 37 to 54, and 55 to 72). Pictorially, this length 18 sequence is:

- - - - - - - + S0 S1 S2 S3 S4 S5 S6 S7 S8 +

where the Si sign values are defined in Tables C-IX, C-X, and C-XI of MIL 188-110B. No scrambling is applied to the mini-probes symbols.

Since the low quality of the recording and the characteristics of the SA universal PSK demodulator, we can just apreciate a "nuance" of the mini-probe pattern (fig. 5). Maybe further and better recordings will help.

fig. 5 - 188-110B App.C mini-probes