The purpose of this post is to report test transmissions heard on 6963.0 kHz USB and logged during the first week of February by both Kosmod (who sent them to me for analysis) and ANgazu (whose signal was reported by KA0KA and published on the radiofrecuencias.es forum [1]). I don’t claim much credit in this post, other than confirming previous analyses and contributing further insights.
Transmissions continued for several days without interruption, almost entirely using the STANAG-4539 (MS-110B App.C) waveform. One of Kosmod’s recordings was particularly notable, as the STANAG-4539 signal was preceded by a STANAG-4481 transmission at 75 Bd/850 Hz (Figure 1). The mode switch lasted roughly 640 ms.
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| Fig. 1 - STANAG-4481/STANAG-4539 switch |
According to STANAG-4539, each data frame contains a 256-symbol data block followed by a 31-symbol mini-probe of known data, for a total of 287 symbols per frame (119.57 ms @ 2400 symbols/s). Every 72 frames, a 72-symbol subset of the initial preamble is reinserted to aid late acquisition (1) (Figure 2).
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| Fig. 2 - STANAG-4539 framing |
The data rate is 9600 bps, and the modulation used to achieve it is QAM-64. According to STANAG-4539, §4.2.2.1.6, “This [QAM-64] constellation is a variation on the standard 8 × 8 square constellation, which achieves a better peak-to-average ratio without sacrificing the very good pseudo-Gray code properties of the square constellation” (Figure 3).
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| Fig. 3 - STANAG-4539 constellations |
In both cases (STANAG 4481 and STANAG 4539), the modulations are asynchronous and employ an 8N1 framing format, as shown in Figure 4.
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| Fig. 4 - STANAG-4481 and STANAG-4539 demodulated bitstreams |
The transmitted messages are 8-bit ASCII clear text and consist of repeated instances of the same sentence, differing only in the message sequence number:
[binary header][message sequence number][text]
The portions of the demodulated bitstreams presented below were obtained from two different recordings (header is omitted):
[STANAG-4481]
...
000013 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
000014 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
000015 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
000016 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
...
[STANAG-4539]
...
004820 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
004821 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
004822 Rapid Mobile HF & V/UHF Data Modem internally generated test message.
...
The message sequence numbering is likely continuous across the transition from STANAG-4481 to STANAG-4539 (Figure 2); however, this assessment cannot be confirmed due to unreliable demodulation of the initial segment of STANAG-4539.
Despite the modem-generated test traffic, the end-to-end efficiency η from payload to physical layer transmission can be evaluated for this STANAG-4539 waveform. As
shown, the 9600 bps input uses asynchronous framing with 1 start bit
and 1 stop bit, meaning each byte is transmitted as 10 bits of which
only 8 carry payload; the symbol rate of 2400 Bd employing
QAM-64 modulation (6 bits per symbol) results in a gross channel bit
rate of 14400 bps. So:
- net useful payload = 9600 × (8/10) = 7680 bps
- transport efficiency relative to 14400 bps rate:
η = 7680/14400 ≈ 0.533 i.e., an effective efficiency of approximately 53.3%
overhead breakdown:
20% from serial framing (start/stop bits)
~26.7% due to physical-layer mechanisms such as FEC, preambles, and synchronization (S-4539) (2)
It's evident that a RapidM (Rapid Mobile) modem is being used that is capable of producing its own test traffic without needing an external server or another device to send data. This feature is often known as loopback testing, built-in test signal generation, or self-test mode.
Unfortunately, there is no publicly documented statement that a specific RapidM modem model generates its own test message, manufacturers often keep detailed test features in restricted datasheets. However, RM2, RM5, RM8, and RM10 RapidM modems all support STANAG -4539 and STANAG- 4481. RM2/RM5 are narrowband tactical modems (up to 9600 bps) with built-in test/loopback capabilities. RM8 is a strategic/naval narrowband modem supporting 9600–19200 bps and 2G/3G ALE, also capable of self-generated test traffic. RM10 adds wideband HF (3–24 kHz) while retaining narrowband 4539/4481 support [2].
Regarding the transmitting antenna’s location, both Kosmod and ANgazu performed direction finding using the TDoA (Time Difference of Arrival) algorithm, and both results pointed to an area near Casteau-Mons in Belgium, very close to NATO’s SHAPE headquarters (3)[3].
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| Fig. 5 - Direction Finding results |
Direction Finding results may further substantiate ANgazu’s hypothesis that the signals could represent test transmissions associated with the NATO BRE1TA (BRASS Enhancement 1 Technical Architecture) program. BRE1TA constitutes a major architectural evolution of the NATO BRASS (Broadcast and Ship-to-Shore) HF communications system, designed to integrate advanced narrowband and wideband HF waveforms, higher spectral efficiency, improved link robustness, and IP-based data services.
Several hours of monitoring on 6063 kHz USB over the past few days yielded only occasional short STANAG-4285 300 bps long-interleaver transmissions encrypted with KG-84. Reception was further limited by sporadic fading during the morning sessions.
https://disk.yandex.com/d/CXaz6URgUh-3zg
(1) In Change Notice 1 of MIL-STD-110C (January 2012), the United States Department of Defense removed a sentence stating that a “reinserted preamble facilitates acquisition (or re-acquisition) of an ongoing broadcast transmission.” The reason was that the sentence referred to a feature that had become obsolete. Keeping it in the standard could have caused confusion by implying support for a legacy broadcast capability that was no longer part of the modem design.
2) The relationship between the nominal user data rate of 9600 bit/s and the over-the-air symbol rate of 2400 baud for the STANAG-4539 QAM-64 waveform is as follows:
- the waveform employs a symbol rate of 2400 symbols per second with 64-QAM modulation, corresponding to 6 bits per symbol. This results in a gross channel bit rate of 14400 bit/s;
- the application of forward error correction (FEC) with coding rate R = 3/4 to a user data rate of 9600 bit/s produces a coded bit rate of 12800 bit/s;
- the remaining capacity of 1600 bit/s (14400-12800), representing the difference between the gross channel bit rate and the coded bit rate, is allocated to waveform overhead. This overhead includes, but is not limited to, miniprobes and periodic preamble reinsertion required to support synchronization and channel estimation functions.
Figure below shows an example; the 1-second interval is
chosen for ease of presentation.
 |
| over-the-air symbols/bits in a 1 second interval (STANAG-4539) |
(3) The Supreme Headquarters Allied Powers Europe (SHAPE) is the headquarters of NATO's Allied Operations Command (ACO), located in Casteau, near Mons, Belgium. It is responsible for the planning and execution of all NATO military operations worldwide.
[1] https://www.radiofrecuencias.es/viewtopic.php?p=12527#p12527
[2] https://www.rapidm.com/
[3] https://shape.nato.int/
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