The waveform of STANAG-4415 is the same as the 75 bps waveform of 188-110 (MS-110), but the requirements of receiver performance in STANAG-4415 are stricter than those of MS-110 (the mode is referred to as "NATO Robust 75 bps mode"). It was promulgated by NATO in 1999.
The thought for this post came from an interesting discussion with any friends on the UDXF group mailing list about an seemingly unidentified signal recorded on 15091.0 KHz/USB on April 9th: "this recorded signal should be psk-8 modulated, modulation velocity = 2400 Bd, ACF = 66.5 ms. What mode is it ? Not STANAG-4285, nor MIL-STD-188-110A serial, nor LINK-11 or 22 ...Any thought ?"
"Why not 188-110?" I replied. possibly the exact ACF value is something like to 66.67 ms given that in case of MS-110 low data rates (from 150 up to 1200 bps) 4 groups of the pairs "data block + probe" number 160 symbols (4 x 40) and they are just in sync with the scrambler dimension (160 symbols) causing 66.67 ms ACF spikes. Moreover, in case of the lowest velocity (75 bps data rate) the channel probes are not sent(!) so the 66.67 mS ACF is just due to the scrambler dimension (MS-110B Table XIX).
 |
| MS-110B - Table XIX |
The recorded signal posted in the mailing list is rather clean but it lasts about 20 seconds and it is just a transmission section without the (important) header/preamble part, the analysis of the ACF value is nevertheless equal to about 66.67 ms (Figure 1). Note that in the bitmap of Figure 1 there are no probes (known symbols) but alternatively a bit-arrangement that closely resembles Walsh Modulation.
 |
| Fig. 1 - ACF value and bitmap of a transmission (data) segment |
The ACF value, the deficiency of known symbols and the format of the bitmap are clues in favour of the MS-110 75 bps waveform. Indeed, quoting MIL-STD 188-110B 5.3.2.3.7.1.1 "At 75 bps fixed-frequency operation, the channel symbols shall consist of 2 bits for 4-ary channel symbol mapping. Unlike the higher rates, no known symbols (channel probes) shall be transmitted and no repeat coding shall be used. Instead, the usage of 32 tribit numbers shall be utilized to represent each of the 4-ary channel symbols".
A decisive step forward in identifying the signal came from my friend linkz who posted a recording of an identical transmission (same day and frequency) "extracted from the HF Time Machine" in the UDXF mailing list. The long data section evidently has the same characteristics seen above (ACF, no known symbols, Walsh modulation) but in this recording it is possible to analyse the first synchronization preamble preceding the long data section (Figure 2).
The 200 ms ACF value is compliant with the sync pattern of MS-110. As for 5.3.2.3.7.2.1 "The synchronization pattern shall consist of either 3 or 20 4 200 millisecond (ms) segments (depending on whether either zero, short, or long interleave periods are used)". The 4.8 s dimension of the sync preamble indicates the long interleaver setting and a 24 preamble "superframes" (4.8/0.2), each superframe consisting of the transmission of 15 ortogonally Walsh modulated frames.
 |
| Fig. 2 - initial synchronization part |
 |
| Fig. 3 |
The resulting bitstream in the right column in Figure 5 has the classical 8-bit format from which the 3 leading "0" columns must be removed to get a clean 5-bit stream that I have named "demod-MS-110A-5bit.txt". The decoding is in clear-text (Figure 4) and shows a continuous repetition of 5 sentences (the first 1 I have chosen is just for convenience):
1234567890().,/-:?
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZ
-?:38().,9014572/6
 | |
|
 |
| Fig. 5 - Sorcerer at work |
The final step in identifying the signal came from my friend Rolf: "It’s STANAG-4415!".
Quoting RapidM [1]: "STANAG-4415 is simply a NATO standard for robust, non-hopping digital data communication, utilized on severely degraded HF channels with mediocre signal-to-noise ratios, large Doppler and multipath spreads. The on-air waveform specified in STANAG-4415 is equivalent to the 75 bps variant of the MIL-STD-188-110 serial mode. However, STANAG-4415 modems are required to meet more challenging multipath hold and Doppler spread performance targets. STANAG-4415 75 bps modem waveform is typically utilized to send ACKs and NACKs in Automatic Repeat Request (ARQ) systems (e.g. STANAG 5066) due to its robustness".
 |
| Fig. 6 - block diagram of the STANAG-4415 transmitter modem |
For the sake of completeness, I utilized the same stuff as in Figure 3 but set the RF-5710A modem to STANAG-4415 75 bps Long mode and produced the file "demod-5bit.txt": the decoding consequence is evidently identical to that obtained in the case of MS-110 demodulation (Figure 7).
 | |
| Fig.7 |
STANAG-4415 requirements are besides applied to the 75 bps mode of STANAG-4539 which is the NATO version of MS-110B but having more stringent modem receiver decoding requirements. STANAG-4415 is mentioned in MS-110B #5.3.4 erstwhile it comes to 75bps if robust operation is required, but it is not mandated. any MS-110B modems supply STANAG-4415 performance at 75 bps: those who have MS-110B hardware modems request to read the docs to determine if their modem's 75 bps is to the MIL-STD performance requirements or to the STANAG requirements [2].
[1] https://www.rapidm.com/standard/stanag-4415/
[2] https://scholar.google.it/scholar?q=FFI/...
