Recently, reception reports of several US Navy stations (Barford, Niscemi, Lualualei, Diego Garcia, among others) appeared on the UDXF mailing list. These stations seem to use the MS-110A waveform for fleet broadcast, rather than the usual STANAG-4481F or STANAG-4285. Some of the stations and frequencies reported had already been under observation by me and my friend Kosmod because of a few peculiarities, discussed below.
Non-continuous transmissions
The transmissions we monitored (and are still monitoring) are not a continuous stream, as in STANAG- 4481F and STANAG-4285 broadcasts, but consist of bursts of irregular duration separated by short pauses. Figure 1 shows one of the receptions of the Niscemi station on 5118.5 kHz USB, recorded on 23 September 2026 at 07:53:58 UTC: the five bursts last between about 20 and 43 seconds. Across all our captures, bursts range from a few seconds to a little over a minute.
Curiously, the MS-110A waveform is used in the same 600 bps/L mode (600 bps, long interleaver) that is employed in STANAG-4285 broadcasts.
A possible reason for the bursts lies in how the two waveforms handle synchronization. With STANAG 4285, the continuous probes sent by the modem keep the receiving side always in sync, and this applies not only to the HF modem but also to the upper-layer protocol. With MS-110A, a receiver that joins late cannot demodulate until it receives the preamble, so a continuous MS-110A transmission would be lost to any receiver that misses its beginning. Short bursts avoid the problem: each burst opens with its own preamble, so a receiver that joins late can acquire synchronization at the start of the next burst instead of waiting indefinitely. The upper layer follows the same logic: in the bitstreams we examined, each burst starts with its own alternating run of bits (the "01" run, 1010... in the captured streams) followed by a sync word, so synchronization is restarted at every burst at both levels.
At present it is hard to tell whether these are tests or a lasting change in how the broadcast is run.
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| Figure 1: Spectrogram of a reception of Niscemi on 5118.5 kHz USB, 23 September 2026, 07:53:58 UTC. The dark vertical lines are pauses between bursts; the black band on the right is the end of the file. The burst durations shown (28, 24, 33, 20 and 43 s) are measured values, but only approximate. |
A fixed 32-bit word after the "01" run
The word is identical in all captures and appears only after the "01" run. An alternating run followed by a fixed word is a common bit-sync/phasing plus sync-word structure in synchronous data links. As far as we can tell, the word is not part of the MS-110A framing, so it is probably part of the payload and likely belongs to a link-level protocol or device, such as a data encryptor. Figures 2 and 3 show the demodulated bitstreams from the five stations (yellow = 1, blue = 0, 32 bits per row), showing the alternating run, the row with the sync word and the random-looking data that follows.
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| Figure 2 |
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| Figure 3 |
Structure of the word 0x0A6E14DC
The 32 bits above are not arbitrary. They consist of a single 15-bit pattern, repeated twice, plus the first 2 bits of a third repetition:
000010100110111
000010100110111
00
Read as two 16-bit halves, the second looks like the first one shifted by one bit. This is a side effect of the period: the real period is 15, not 16.
The polynomial generator
A pattern like this is produced by a tiny shift register with only 4 stages. Each new bit is the XOR of the previous bit and the bit from four steps earlier, which we denote by the polynomial x^4 + x + 1; with the reciprocal convention the same register is written x^4 + x^3 + 1. This polynomial is of the "maximal-length" kind: the register passes through every possible non-zero state before repeating, giving the longest cycle possible for 4 stages, 15 bits.
Starting from the register state 1111 (all ones, a very common choice), the generator produces 111101011001000. The pattern observed on the air is exactly this sequence with every bit flipped. Inversion does not change the underlying generator, only how the sequence looks.
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| Figure 4: Left, the 15-bit sequence produced by the generator from the initial state 1111 (the inverse of the one observed on the air). Right, schematic of the 4-stage shift register. |
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| Figure 5: Left and center, the 15-bit sequence as observed on the air, repeated line by line; in a 15-column view it appears as vertical stripes. Right, ranking of candidate generator polynomials x^n + x^k + 1 (pairs n,k) by score, with x^4 + x + 1 on top. |
Common structure of the streams
- Every stream starts with an alternating run of exactly 256 bits (128 pairs).
- The 32-bit word appears at position 256 in all five streams, bit-for-bit identical.
- The word appears only once per stream: there is no resynchronization in the middle of a transmission.
- There are no inverted or bit-reversed versions, so the demodulation polarity is the same in all captures.
What follows the word
I compared five bursts, one from each station, in pairs (10 pairs), aligned on the end of the word. Over the first 256 bits the percentage of differing bits ranges from 46.9% to 56.2%, and over the whole common part from 49.8% to 52.0%. Independent streams are expected to give 50%, with a spread of about 3 points over 256 bits, so all values are within normal variation. There is no common field right after the word, such as a shared message indicator, and shifting one burst against another by up to 64 bits reveals no shifted copies.
Within each burst, the 0/1 balance is within 1.6 standard deviations of 50%, and neither the runs test nor the autocorrelation shows any periodicity. The lengths of the data after the word are all multiples of 8 bits, and at 600 bit/s the five bursts last from about 5 seconds (Barford) to about 75 seconds (Lualualei). Figure 6 shows a set of checks on one burst; the other four give the same picture.
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| Figure 6: Statistical checks on the data following the sync word of one burst (1736 bytes): bit balance, byte histogram, autocorrelation, bit raster and running proportion of 1s. The other bursts give the same picture. |
