STANAG-4538 LDL packets retransmissions, an interesting case

A few days ago I was casually watching the 4 MHz band when an unusually long STANAG-4538 (3G-HF) transmission on 4561.0 KHz/USB caught my attention and I decided to record it for later analysis.
Basically, in the usual STANAG-4538 way (ARQ), after the data link connection has been configured, the sending station and the receiving station alternate transmissions: the sending station transmitting xDL PDUs (Protocol Data Unit) containing payload data packets and the receiving station transmitting acknowledgment/control packets of whether or not the data packet in the preceding PDU was received without error (1). In this case the LDL protocol is used (Figure 1).

Fig. 1 - STANAG-4538 LDL transfer

As per STANAG-4538, the "original" datagram to be sent is split into fixed-length segments which will be processed into packets by the chosen LDLn protocol. Indeed, a LDL data packet is defined as a fixed-length sequence of n-byte data segment (n = 32,64,96,...,512) followed by a 17-bit Sequence Number plus an 8-bit Control Field (presently unused). During the construction of the LDL BW3, a 32-bit Cyclic Redundancy Check (CRC) value is computed across the  data bits of each data packet and then appended. Then, 7 flush bits having the value 0 are added to ensure that the encoder is in the all-zero state upon encoding the last flush bit. Sumarizing, the on-air length of a LDLn BW3 burst is computed in bit as 8n + 64, in this case (LDL512, n = 512) 4096+64 = 4160 bit or 520 bytes (Figure 2).

Fig. 2 - LDL512 demodulated bitstream

That said, we can inspect the last 64 bits (17-bit Sequence Number + 8-bit Control Field + 32-bit CRC + 7 flush bits) of the recorded BW3 bursts  (Figure 3).

Fig. 3 - last 64 bits of the demodulated LDL512 bitstream

The 8-bit reserved field is added after the CRC field and not after the Sequence Number, as specified in Annex C to STANAG-4538; I don't know if it's the modus operandi of the decoder. Moreover, the bits of the Sequence Number field are transmitted starting with the Last Significant Bit (bit 0) rather than the most significant bit (bit 16), most likely it's the modus operandi of the decoder, as above.
In this sample you may see that the Packet Number #3 is sent 20 times and it clarifies the unusually length of the transmission (actually the number of packet retransmissions is greater than 20 since some bursts were not correctly demodulated, hence not present in Figures, due to their bad SNR value). The Packet Byte Count field is always 511, this means it's a LDL512 transfer. The EOM & SOM fields are both set to "0" meaning that's an "interior" packet. The CRC fields show the same value.

110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
110000  111111111 00  10110010110011010001111111100111 000000000000000
3           511          EOM                 CRC field                          control + flush bits
                             SOM

I stopped recording after more than 5 minutes, see Figure 4, and most of the time (4 minutes and 4 seconds!) was spent on the transmissions of the single packet #3; also note the repeated transmissions of packets #2 (2 times) and #4 (6 times) at the beginning and end of the recording (Figs. 2,3).

Fig. 4 - duration of the whole recording (software "audacity")

 

So, why so many retransmissions? 

STANAG-4538 is designed for High-Frequency (HF) radio communication, which is notorious for having a highly unstable and challenging channel environment. The large number of retransmissions may be a direct consequence of the protocol's need to ensure reliable data delivery over this unreliable medium. STANAG-4538 employs an ARQ (Automatic Repeat reQuest) scheme as part of its data link protocols to achieve this reliability. The maximum number of retransmissions in ARQ schemes is specified by the parameter N_tx (or often N_max​ or N_retries​): if the packet is not acknowledged as successfully received after the last retransmission it is typically discarded by the data link layer, and the link connection may be closed (2).  This clause normalizes the trade-off between reliability and efficiency/latency (preventing the channel from being permanently blocked by an unrecoverable packet). 

The usual or default value for the N_tx parameter is not specified in standard documentation summaries, not even in STANAG-4538, as it is a configurable parameter of the HF modem which is set by the manufacturer or even by the system operator. In commercial and military HF implementations of similar ARQ schemes, the maximum retransmission count is often set to a value like 4, 8, or 10, but the actual number should be verified against the specific equipment's configuration settings.

One of the causes of retransmissions is poor HF channel conditions: fading, multipath distortion, noise, interference and man-made interference (from electronics or other users). Any of these factors can cause bit errors in the received packet, which the receiver detects via a checksum (CRC) and then requests a retransmission of the corrupted packet.
A second important cause is the Adaptive Code Combining (Hybrid-ARQ). Indeed,  STANAG-4538's data link protocols utilize a form of Hybrid-ARQ with code combining. This mechanism is specifically designed to work in poor channels and directly contributes to retransmissions. Instead of just discarding an unreadable packet, the receiver stores the corrupted data. When a retransmitted copy of that same packet arrives, the receiver combines the information from all received copies (the original and all retransmissions) to try and successfully decode it. The fact that it takes over 20 attempts may mean the combined information only reaches the decoding threshold after a significant number of transmissions.

In short, the retransmissions aren't necessarily a sign of failure, but rather the core mechanism of STANAG-4538 actively working to provide error-free data transfer despite the inherent unreliability of the HF radio channel.

It should also be considered that the used KiwiSDR receiver [1] is a sort of "man in the middle", i.e. it sees all the traffic on the channel, but this does not necessarily apply to the stations actually involved (not all data/ACK packets can be correctly received by the two peers).

So, it is likely that the N_tx parameter is set to a value greater than 20 and channel conditions are very poor... but we could also consider another cause besides  STANAG-4538 ARQ.
Perhaps we are observing retransmissions "forced" by the higher-layer protocol running over the STANAG-4538 data link (running at Layer 2), specifically TCP (Transmission Control Protocol, running at Layer 4). The limit of N_tx applies strictly to the LDL/ HDL protocol itself. A high retransmission count may occur because of the interaction between the two layers, particularly when running IP over HF:

 

- STANAG-4538 (Layer 2) action
the STANAG-4538 data link protocol tries to deliver a single packet  and if all attempts fail, it discards the packet and moves on to the next one. Result: The packet is considered lost by the Data Link Layer.
- TCP (Layer 4) action
TCP is the reliable transport protocol often used for applications like web browsing, email, or file transfer, and it runs over IP, when a packet is lost by the lower layer (Layer 2, STANAG-4538), the TCP sender never receives an acknowledgment for that packet, TCP's retransmission timer eventually expires, and it assumes the network failed to deliver the data (TCP is not HF-aware!), crucially, TCP retransmits the data independently of the Layer 2 protocol.
- the domino effect
STANAG-4538 transmits a packet several times, it fails and discards the packet. TCP times out. It retransmits the data. The new TCP segment is given to STANAG-4538 which treats it as a new packet to send. STANAG-4538 transmits this "new" packet up to N_tx times again. If it fails again, TCP times out a second time, and the process repeats until the packet is successfully acknowledged.

If the channel conditions are so poor that the STANAG-4538 link fails more than N_tx times in a row before TCP gives up, the total number of retransmissions for a single packet could easily assume a large value, as the higher layer continually resubmits data to the persistently failing lower layer.
Anyway, technically TCP cannot directly cause packet retransmission, only STANAG-4538 does that: TCP increases the likelihood of retransmissions, but it does not initiate them!

Summarizing: 

N_tx parameter is set to a large value (>20) to prioritize ultimate delivery reliability over speed and to support Hybrid-ARQ mechanism

or

N_tx parameter is set to a low value (often in the range of 3 to 10 or similar small number) and TCP times out and re-injects the data.

Definitely channel conditions were undoubtedly poor and severe.

 

Perhaps - in my opinion - it would have been better to use LDL packets size smaller than 512 bytes (4160 bits). Under poor channel conditions, a smaller packet size is generally better, even though the number of packets increases. Shorter transmissions (reduced airtime) reduce exposure to fades, have lower probability of interference and collisions, enable faster ARQ cycles and improve latency:
large packet → high chance of corruption → many retransmissions

small packet → lower chance of corruption → fewer retransmissions (even though more packets)

 

Fig. 5 - Packet Error Probability (PEP) vs. packet length (theoretical model)

The airtime, or duration, of STANAG-4538 LDLn BW3 bursts is variable and depends on the amount of data being sent, specified by the parameter n. The total burst duration is given by the formula: duration (ms) =373.33 + n×13.33, (n = 32,64,96,...,512).

The possible total airtimes for a single BW3 burst for the four standard BW3 burst sizes (n=64,128,256,512) are:

n Value	Payload Duration	Total Burst Duration
64	853.12 ms	1226.45 ms (≈1.23 seconds)
128	1706.24 ms	2079.57 ms (≈2.08 seconds)
256	3412.48 ms	3785.81 ms (≈3.79 seconds)
512	6824.96 ms	7198.29 ms (≈7.20 seconds)

 

Longer packets like $n=512$ transmit more data per burst, leading to higher throughput, but they are also significantly more vulnerable to errors (higher PEP) since require a much longer airtime (up to $7.2$ seconds).

[1] recording tanks to linkz KiwiSDR #3 (French Alps)

https://disk.yandex.com/d/2r0xOdGLWu45UA

 

(1) STANAG-4538 xDL protocols:
HDL (High-throughput Data Link protocol) waveforms: BW1 for acknowledgement and traffic management, BW2 for traffic data
LDL (Low-latency Data Link protocol) waveforms: BW3 for traffic data, BW4 for acknowledgement and traffic management
HDL+ waveforms: BW6 for acknowledgement and traffic management, BW7 for traffic data

(2) Why the Limit is Necessary
HF Channel Volatility: HF radio conditions are highly variable due to ionospheric fading and noise. While the LDL protocol uses Code Combining (Hybrid-ARQ) to increase the likelihood of decoding a packet with each attempt, a limit is needed to account for extreme or prolonged channel outages.
Preventing Link Stall: Unlimited retransmissions would cause the entire data link to stall, continually trying to send a packet that may be unrecoverable due to a sudden deep fade or interference. By limiting the attempts, the protocol can decide to terminate the link and allow the higher layers (or the operator) to attempt a new link setup on a different, possibly better, frequency. 

 

UK DHFCS FSK 800Bd/850 & STANAG-4285 1200bps/L

Interesting and unknown (at least for me) FSK transmission in 800Bd/850Hz mode sent to me by my friend cryptomaster. These transmissions have been heard during the evening (UTC) on the frequencies 8014, 8180, 10008 and 12414 kHz.

Fig. 1 - FSK parameters

The signal has an ACF of approximately 1280 ms which corresponds to a period of 1024 bits (Figure 2).

Fig. 2 - ACF value and relative bitmap

  

The bitstream obtained after demodulation has a complex and very interesting structure (Figure 3).

Fig. 3 - demodulated bitstream

The most interesting thing is that after a few days the FSK signals have disappeared and been replaced by STANAG-4285 signals in two of the above-mentioned frequencies: 8013.20 and 10008.20 KHz/USB (Figure 4).

Fig. 4 - FSK & STANAG-4285 transmissions

The S-4285 transmissions - which have also disappeared - were recorded in 1200bps/L mode and once demodulated show a bistream period of 1536 bits which is very similar to the 1024 bits bitstream of the previous FSK transmissions: just 512 bits of difference, see Figs. 3 and 5 (below).

Fig. 5 - 1536 bits period bitstream of the STANAG-4285 transmissions

The format of the bitstreams (both FSK and STANAG-4285) suggests the use of a TDM (Time Division Multiplexer), probably the GA-205 model, or a modified version of it, as already analyzed in other posts relating to UK DHFCS (Defense High Frequency Communications Service) and Australian MHFCS (Modernized High Frequency Communications System) transmissions [1]. The STANAG-4285 1200bps broadcasts utilize time-division multiplexing to carry up to 12 KW-46 encrypted streams on a single frequency. Moreover, FSK/STANAG-4285 transmissions on the same channel have already been observed previously and attributed to DHFCS: Tx site of Crimond [2].  a single frequency.

Direction Finding tests (TDoA algorithm), although slightly inaccurate, indicate St. Eval (DHFCS) as the likely site of signal transmission (Figure 6).

Fig. 6 - some Direction Finding tests

It's difficult to determine the purpose of these broadcasts; perhaps they're tests or specific needs. Let's hope we have better luck and catch more similar signals.

https://disk.yandex.com/d/qIF5cmD797W_iQ (FSK signal)

https://disk.yandex.com/d/Y50xjENZ50TPAg (STANAG-4285 signal)

[1] http://i56578-swl.blogspot.com/search/label/1536-bit%20TDM
[2] http://i56578-swl.blogspot.com/2024/06/300-bps-fsk-stanag-4285-fleet-broadcast.html

Deep WALE (4G-ALE) Async call using PSK2

(for background http://i56578-swl.blogspot.com/search/label/4G-ALE)

Very interesting example of an asynchronous three-way Deep WALE handshake (4G-ALE, MS-141D App.G)(1) sent to me by my friend ANgazu; recordings credits to EA6AMM Gaspar. Although the common protocol exchange for 4G-ALE link establishment is the two-way handshake, it may occurs a traffic channel negotiation which requires a three-way handshake.
In figure 1 you can see the call which is formed by the "scanning call" (also known as "capture probe") + the actual Request, the call is then followed by the called & caller Confirm bursts. The scanning call is designed to capture asynchronously scanning receiver(s) so that they will stop scanning to receive the final Request. However, conversely of 2G and 3G ALE, the scanning call portion of the async WALE call is unaddressed: that is, the addresses of the stations are not sent during the capture probe and receivers are held unitl the reception of the ending Request.

Fig. 1 - Deep WALE async call

The scanning call consists of repeated blocks (known symbols) with an ACF value of 80 ms (Figure 2) that permit rapid detection of the call, and therefore very short scanning dwell times. As in 2G and 3G ALE, the duration of the WALE scanning call dependes on the number of channels in the scan set and is as follows:

Tcapture ≥ Dmin(N+2)

where Dmin is the asynchronous-mode minimum dwell time (200 ms as default setting) and N is the number of channels. Since the capture probe lasts about 3500 ms, assuming the default setting is used, the number of channels is 15.

Fig. 2 - ACF value of scanning call (capture probe) portion

But the most important thing - see Figure 3 - is that the Deep WALE waveforms (call and confirms) use PSK2 modulation(!) rather than PSK8 as required by MS-141B App.G standard.

Fig. 3

Once demodulated, the capture probe consists of a 192-bit sequence (originating the 80 ms ACF) which is repeated - in this sample - 44 times (blocks):

0x [D1 38 6C BC 22 71 BD 65 B9 21 D6 F2 C6 FD FE 8F 49 D1 D9 FD 36 0D F1 80]
0x [2E C7 93 43 DD 8E 42 9A 46 DE 29 0D 39 02 01 70 B6 2E 26 02 C9 F2 0E 7F] opposite polarity 

Fig. 4 - 192-bit sequences forming the scanning call blocks

These waveforms are not MS-141D App.G compliant at all; in fact, according to the standard:
- the modulation used shall be PSK8, while in the recorded samples is used PSK2;
- the scanning call blocks shall consist of 96 symbols, while here the blocks consist of 192 symbols, exactly double (obviously, the respective ACF values ​​are also double);
- the duration of the Deep WALE Request (~ 622 ms) does not match the expected 1520 ms value (240 ms preamble + 1280 data ms).

Perhaps it is some specific implementation of a new and more robust Deep WALE call, given that PSK2 is more robust than PSK8 especially in terms of error performance under the same noise conditions (2).

https://disk.yandex.com/d/GBKjZzt4q9509Q (recordings credits to EA6AMM Gaspar)

(1) The multiplicity of abbreviations and acronyms sometimes doesn't help, below what I used:
WB ALE (or WBALE), Harris implementation of 3GWB
WALE (or 4G ALE), wideband ALE as for MIL 188-141D App.G, fourth generation ALE
note that WB ALE is not synonymous with WALE since the latter has its own PDUs and waveforms.

(2) In PSK2, the phase shift between the two possible symbols is 180°, which makes it very easy for the receiver to distinguish between them even in the presence of noise. In PSK8, the phase difference between adjacent symbols is only 45°, so even a small amount of noise can cause symbol errors.

Link-22 12QAM waveform

Fig. 1 - Link-22 transmission

Recently my friend ANgazu from radiofrecuencias.es sent me a great catch of a Link-22 transmission - Figure 1 - recorded at 11128.0 KHz/USB. Identifying the mode (Link-22) is fairly straightforward by examining the bitmap depicting the framing period used by the waveform (Figure 2).
The 112.5 ms duration is typical of the waveforms used by Link-22 and described by STANAG-4539 standard (non-hopping TDMA traffic waveforms). Note that in this case the framing consists of 3 data blocks (DATA) interspersed with three mini-probes (MP) blocks.

Fig. 2 - typical Link-22 framing

Honestly, examining the individual bursts I expected to find QPSK or PSK8 modulations: I was very surprised when the phase plane revealed bursts with QPSK and even 12QAM constellations within the same transmission (Figure 3): clear sign that the header must therefore encode the modulation mode.

Fig. 3 - QPSK and 12QAM constellations

A 12QAM modulation is employed in non-hopping TDMA traffic waveform #13 (Annex G to STANAG-4539):

12QAM is unusual because QAM typically uses powers of 2 (so each symbol cleanly represents an integer number of bits). 12 is not a power of 2, so it cannot map bits directly since log⁡2(12) ≈ 3.585 bits per symbol, which is not an integer.
However, some NATO HF modem implementations of STANAG-4539 use a circular 12QAM layout (which uses two concentric rings) rather than the rectangular one (3x4 grid). Figure 4 shows an approximate layout:
- inner ring 4 points (like QPSK), offset for symmetry
- outer ring 8 points evenly spaced (like 8-PSK)
This design gives a radial + angular separation, making it easier to decode under fading and noise than a rectangular grid. 

Fig. 4 -  circular 12QAM (STANG-4539 style)

This variation of a constellation is not "new" in STANAG-4539 as, for example, the 64-QAM constellation described in paragraph #4.2.2.1.6: "This constellation is a variation on the standard 8 x 8 square constellation, which achieves a better peak-to-average ratio without sacrificing the very good pseudo-Gray code properties of the square constellation".

As seen, with 12QAM circular, the goal is 3.5 bits/symbol (b/sym). There are at least three practical approaches:
1) 4-bit labeling + FEC rate 7/8
2) Multilevel/TCM ( trellis-coded modulation): 3 info bits + 1 structural bit
3) Probabilistic Amplitude Shaping (PAS)
The most common is the 4-bit labeling + FEC rate 7/8:
- assign a 4-bit mapping to the 12 points (16 combos → 12 used, 4 unused).
- apply channel coding with rate R=7/8 (e.g., LDPC or turbo). Net efficiency: 4×7/8=3.5 b/sym.

I can give an Octave example [1] for such method with a 12QAM circular constellation. Since Octave doesn’t have built-in LDPC/RS at hand, the code illustrates it using a simple block code to emulate the R=7/8 effect (i.e., 7 info bits + 1 parity). The principle is the same as using LDPC/Turbo in practice.
What it does:
1. Defines the 12QAM circular constellation (4+8 rings) with 4-bit Gray-like labels.
2. Implements a (7,8) block encoder (simple parity, emulating FEC with rate 7/8).
3. Transmits random data: 7 info bits → +1 parity → 8 coded bits → mapped into 2 QAM symbols.
4. Sends through an AWGN channel.
5. At the receiver: nearest-neighbour detection, recover labels.

Fig. 5 - 4-bit labeling + FEC rate 7/8 simulation

Must be noted that:

* the STANAG-4539 12QAM waveform uses a channel coding with rate R=4/5 and TBCC (Tail-Biting Convolutional Code). Net efficiency: 4×4/5=3.2 b/sym.

* STANAG-4539 is not the same as Link-22, but Link-22 can use STANAG-4539 as one of its underlying HF bearers.

* the Octave code is just an example/simulation just to prove that  ~3.58 bits per symbol fits nicely with the interleaving, coding, and frame structures used in the waveform: I do not know what is the method for the 12QAM used in the analyzed signal.

The choice of 12 points is not arbitrary, it’s a trade-off between spectral efficiency and robustness: it can deliver intermediate data rates where 8-PSK is too low and 16-QAM is too fragile under HF channel conditions. In military waveforms like STANAG-4539, robustness and flexibility matter more than mapping convenience.

 https://disk.yandex.com/d/tMeSf8zZjgdl4w

[1] https://disk.yandex.com/d/kZmCP-fdqtc-cw

Tunisian Navy using L3Harris Citadel encryption

Prompted by some logs posted on the UDXF group, I started monitoring the USB frequency 12354.4 KHz, operated by the Tunisian Navy (Marine Nationale Tunisienne), hoping to capture something interesting. To be precise, I used the two KiwiSDRs located in central-eastern Italy, operated by IZ6BYY and IK7FMO [1], whom I obviously thank.
The frequency is quite "busy" with 2G-ALE (MS-141) exchanges between tactical callsigns, as RF05 and CT12 in the given sample; once the link is established, message exchange usually occurs via MS-110A using adaptive speed and interleaver (Figure 1). Still in this sample, it is interesting to note that the request to establish a link is transmitted from RF05 (caller node) to CT12 (called node) and after the exchange of messages the link is closed by the called node, as if to signify that there are no other messages to send in the opposite direction (a bit like what happens in 3G-ALE).

Fig. 1

The bistream after demodulation of the three message blocks shows the characteristic 16 bytes start/sync sequence of the L3Harris "Citadel" cipher (Figure 2).

Fig. 2 - Citadel start/sync sequence in the MS-110A demodulated bitstream

After removing the start/sync sequences, the presence of Initialization Vectors (IV) is noted: these are 12-byte/96-bit long, each repeated three times (Figure 3).

79 19 08 E9 61 C4 B1 01 A5 24 9A B7
87 A9 45 4F 28 22 A7 15 33 88 F8 EB
91 1E 71 15 D2 FA FF D3 51 68 6B D0

This is characteristic of the Citadel II "format":
- 16 bytes start/sync sequence 0x1E561E561E561E001A5D1A5D1A5D1A5D (Citadel)
- 12 bytes IV (each 3 times rptd) - OR - 32 bytes IV (2×128 bits parts, each 3 times rptd)     
- ciphertext
- 8 bytes end sequence 0x1E561E561E561E08 (Citadel)

 

Fig. 3 - 12-bytes/96-bits Initialization Vectors

Some comments

"Citadel II" generally refers to a hardware-based cryptographic solution (cryptographic engine) developed by Harris Corporation (now L3Harris) in 2004, designed for military-grade encryption in "non-Type 1" applications. This means it's approved for secure communications but not for the highest classification levels of US government information (which use "Type 1" ciphers endorsed by the NSA). One might wonder why this encryption is used by a country like Tunisia, which is notoriously not a member of NATO (1): the answer is because it is not a Type 1 device, the Citadel II is approved for export from the United States, making it available to international users.

Citadel II is used in various communication products, including the L3Harris Falcon II range of military radios (such as RF-5800H). Given L3Harris's extensive portfolio and the strong military ties between the US and Tunisia, it is highly probable that the Tunisian Army runs L3Harris equipment, particularly in areas like communications, night vision, and potentially some avionics or electronic systems on US-supplied platforms [2]. However, exact details of specific L3Harris models in Tunisian service are not always publicly disclosed.

The short duration makes one think of "informal messages", perhaps SMTP emails: the encryption unfortunately obscures the data-link protocol sitting at the upper layer, probably STANAG-5066. Considering the use of L3Harris encryption (and probably Falcon radios), one might think that the L3Harris' RF-67x0W Wireless Gateway/Message Terminal is used... but that's just my speculation!

https://disk.yandex.com/d/23rAAmgmk9AiAQ

(1) Since 2015, Tunisia has been granted non-NATO "major ally" status, a status granted by Washington to allied countries that have strategic relations with the American armed forces but are not members of the organization.

[1] https://iz6byy.k1fm.us/  http://ik7fmo.ddns.net:8073/
[2] https://adf-magazine.com/2025/04/tunisian-navy-adds-to-patrol-fleet/