STANAG-4415/MS-110A 75 bps Robust Waveform — Physical Layer Reference

A consolidated technical reference on the 75 bps STANAG-4415/MS-110A waveform observed on 18503.0 kHz, combining the physical-layer characterization from [1] with the framing findings from [2]. This post exists because that characterization was originally split across two separate write-ups — one on the RF/physical layer, one on framing behaviour. The split obscured a connection between them: the physical-layer absence of mini-probes (§2.2) means the base waveform has no built-in way for a receiver to join a transmission already in progress — and preamble re-injection (§3.2) is precisely what closes that gap, through a modification not present in the base STANAG-4415/MS-110A specification itself (though, as §3.2 details, with real precedent elsewhere in the STANAG family). Reading the two findings together makes that connection visible in a way that reading either post alone does not. Site attribution is deliberately out of scope here — see the two source posts for that discussion.

1. Operational overview
At the reception level, this waveform cannot be told apart from a standard MIL-STD-188-110A 75 bps transmission. STANAG-4415 and the MS-110A 75 bps robust mode are, at the RF signature level, the same waveform: same DSSS Walsh modulation, same fixed 2400 Bd symbol rate. A receiver — even a military-grade one, such as the L3Harris RF-5710A — demodulates both interchangeably; the distinction between the two standard labels comes down to a front-panel setting, not anything recoverable from the intercepted signal itself.
The relationship between the two standards was formalized over time: MS-110B (2000) §5.3.1.3.h lists the robust 75 bps mode as optional, specifying it "shall be in accordance with STANAG 4415." In other words, from MS-110B onward the US standard defines this mode by reference to the NATO one rather than specifying it natively — a distinction that matters for conformance testing, not for what a receiver actually sees.
The one real difference sits on the receiver design side: a modem built to fully meet STANAG-4415's stricter performance requirements can decode at lower SNR (down to roughly -9/-11 dB in 3 kHz AWGN) than one meeting only the baseline MS-110A target. This is a measurable difference in comparative lab testing (BER vs. SNR/multipath/Doppler) — not something inferable from intercepted bursts.

2.  Physical layer — signal and modulation characteristics
2.1 Signal acquisition
Initial demodulator lock shows a carrier at 1801.29 Hz (USB, +1801.4 Hz offset), a symbol rate of ~2400.22–2400.33 Bd, and an 8-ary constellation — consistent with the 2400 Bd, 8-PSK tribit-symbol architecture shared across the MS-110A serial-tone family.

Figure 1: signal acquisition and demodulator lock; arrow marks the preamble at the start of a new message.

2.2 Absence of mini-probes
Neither STANAG-4415 nor the MS-110A 75 bps mode uses mini-probes (the periodic resync sequences found in 150–4800 bps serial-tone modes). At 75 bps this isn't needed: DSSS Walsh spreading at a fixed 2400 Bd is inherently redundant per bit, so channel tracking and noise immunity come from the spreading itself rather than periodic re-training. MIL-STD-188-110A Table XIX confirms this directly — the 75 bps row lists 0 known symbols for this purpose.

Figure 2: MIL-STD-188-110A, Table XIX: the 75 bps row shows 0 known symbols.

At the architecture level (per STANAG-4415 Figure 2.1), switches SW2/SW3 toggle only between "sync preamble" and "data" phases, with no third state for reinserting an intermediate probe sequence — consistent with the absence of mini-probes. SW1 stays fixed on "input data" throughout both phases, since preamble and each interleave block share the same duration, keeping the interleaver continuously pre-loaded.

 

Figure 3: 75 bps modem architecture — SW2/SW3 switching between "b" (sync preamble) and "a" (data phase), while SW1 remains fixed, continuously pre-loading the interleaver with input data throughout both phases.

A practical consequence: a single preamble with no mid-stream mini-probes cannot, on its own, support late-entry acquisition — the base standard has no built-in mechanism for a receiver to join an in-progress transmission (see §3 below for how this implementation addresses that gap).

2.3 Walsh modulation, verified
Though Walsh modulation is formally specified in both standards, its presence was verified directly via carrier regeneration (squarer/m-th power loop). Squaring at exponent n=2 produces a clean spectral line confined to the preamble — proof of an underlying BPSK-type modulation beneath the Walsh chips. At exponent n=8, that line disappears completely across both preamble and data, confirming the underlying signal is masked BPSK rather than native 8-PSK.

Figure 4: squaring-loop spectral analysis at n=2 and n=8.

2.4 Preamble structure
Per MS-110A §5.3.2.3.7.2.1, the synchronization pattern consists of either three or twenty-four superframes, depending on the interleave setting. For the long-interleave 75 bps mode observed here: a 4.8 s preamble comprising 24 superframes of 200 ms each, each carrying 15 orthogonally Walsh-modulated channel symbols — 480 tribit symbols, or 1440 bits, per superframe. Measured ACF confirms both the 200 ms superframe period and the 4.8 s total preamble length.

Figure 5a: MS-110A frame structure (Preamble, interleaved data, EOM & Flush), showing how the preamble is composed of superframes (SF1...SFm).

Figure 5b: ACF of the sync preamble: peaks recurring at ~200 ms (dT = 200.609 ms) confirm the STANAG-4415/MS-110A sync pattern over the 4.8 s long-interleave preamble (dT = 4.799999 s).

Figure 5c: Initial section (preamble) of the bitstream showing the 1440-bit frame width across 24 superframes.

2.5 Data block ACF
The data segment following the preamble shows periodic ACF peaks every 66.67 ms (160 over-the-air symbols) — not a framing artifact, but the result of MS-110A's own data-scrambling sequence (per §5.3.2.3.8).

 

Figure 6 — ACF of the data segment.

3. Physical layer — framing behaviour beyond the base specification
Extended monitoring surfaced two characteristics of this 75 bps implementation that are not accounted for in the STANAG-4415/MS-110A reference specification.

3.1 Preamble onset delay
Although transmissions appear to begin "from nothing," some instances open with two extended tones at 1000 Hz and 1600 Hz, followed by a segment of 8-PSK modulation that cannot be demodulated without a preamble lock. In these cases the preamble is not transmitted at the start of the signal, but with measurable delays on the order of 50–60 seconds.

Figure 7: Comparison of signal start structures: anomalous transmission with initial 1000/1600 Hz tones and a delayed preamble (~50–60 s) (top), and standard transmission beginning immediately with the preamble before data (bottom).

3.2 Preamble re-injection
The more significant feature. In long transmissions, the full preamble is regularly re-injected every 115.2 seconds of data, forming a fixed 120-second cycle (4.8 s preamble + 115.2 s data) that repeats until the transmission ends. This 115.2-second span is exact, not approximate: it equals 24 blocks of long-interleave data, 4.8 seconds each — matching, block for block, the 24-superframe structure of the preamble itself (§2.4). Re-injection only engages once a transmission exceeds one full 24-block cycle; shorter transmissions carry a single preamble followed directly by up to 24 data blocks and end-of-message.

Figure 8: waterfall confirming the claims above — top: four consecutive re-injection cycles over roughly 8 minutes, showing the pattern repeats for as long as the transmission continues; bottom: zoomed view of one cycle, with a measured interval of 120.064 s between Preamble n and Preamble n+1, matching the nominal 120 s (2 mins) cycle to within 64 ms.

Figure 9: block structure of the repeating cycle — top: preamble followed by up to 24 data blocks per 120 s cycle, repeating until a final partial cycle closes with EOM; bottom: transmissions shorter than one cycle, carrying a single preamble followed directly by up to 24 data blocks and EOM.

This alignment is unlikely to be coincidental: 24 is also the number of superframes making up the long interleaver itself, meaning the reinsertion point falls precisely on the interleaver's structural boundary — and its strict periodicity points to a predefined acquisition mechanism rather than a reactive response to poor channel conditions.
Since this waveform is not autobaud and does not use mini-probes (§2.2), retransmitting the full preamble at known intervals adds:
(a) greater transmission robustness ;
(b) easier synchronization on late entry — a receiver joining mid-transmission gets a re-entry window every 115.2 seconds instead of having to wait for the entire transmission (which, for broadcasts, can run for tens of minutes) to end.

This isn't an isolated, one-off invention: it looks like a synchronization method already standardized elsewhere in the STANAG family, reused here. STANAG 4539 §4.3.1 explicitly defines a "reinserted preamble," distinct from the initial synchronisation preamble, whose stated purpose is to facilitate acquisition of an ongoing transmission (acquisition on data) — functionally identical to what's observed here — as does MIL-STD-188-110C Appendix D §D.5.4. However, MIL-STD-188-110C Change Notice-1 later removed the corresponding sentence, because the feature was considered obsolete in that context. No trace of any equivalent clause appears in STANAG-4415/MS-110A's own 75 bps text, so within this specific waveform it remains an addition beyond the base specification — one with real, if since-contested, precedent elsewhere in the STANAG/MIL-STD family.

References

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[1] https://i56578-swl.blogspot.com/2026/08/75-bps-serial-tone-4415110a-from-cyprus.html
[2] https://i56578-swl.blogspot.com/2026/09/185030-khz-from-cyprus-new-waveform.html