10 June 2021

MAHRS-serial and STANAG-4285, ie don't blindly trust decoders

It happened by chance to analyze a complete session of data exchange in MAHRS mode (ALE + traffic) while I had a STANAG-4285 decoder in active state on the desktop: to my surprise the decoder started printing out a bitstream though - as said - it was set for STANAG-4285 (Figure 1)

Fig. 1

Intrigued by that fact, I went to see the points that S4285 and MAHRS-serial have in common enough to confuse the decoder, other than the obvious features such as speed (2400Bd) and modulation (PSK8).  

The first thing that stands out is the equality of the ACF values, Figure 2: 106.6 ms, or 256 symbols. Thus - in my opinion - it seems that the decoder in question (Sorcerer and therefore also K500) tries to identify a signal by analyzing its ACF: probably those kind of decoders have an internal table that allows this association.  

Fig. 2 - ACF values for STANAG-4285 and MAHRS serial

The structure of the frames is anyway very different, unless the first 80-symbol preamble which is common to both the waveforms (Figure 3): S-4285 framing consists of an initial 80 symbol preamble followed by 4x32-symbols data segments and 3x16-symbol probes; MAHRS-serial framing consists of the initial 80 symbol preamble that is followed by a data block consisting of 176 data symbols. 

Fig. 3 - framing structure for STANAG-4285 and MAHRS serial

As third common feature, both the 80-symbol preambles are modulated using BPSK: the pronounced BPSK states in the constellation plane of the MAHRS serial signal are quite eloquent (Figure 4)

Fig. 4

STANAG-4285 is not an autobaud waveform so the decoding is based on the user settings, just for fun I played with some sub-modes even if - as obvious - the decoder can't find the expected known symbols (16-symbol probes). The best results, in terms of "confidence", were obtained by setting the bit rate to 2400 bps, obviously the corrections are equal to zero in the uncoded mode:

It must be said that MAHRS serial is not the only S4285-like waveform, another example is the 2400Bd PSK-8 serial waveform from the THALES TRC-1752 modem (Thales Système 3000 family), although the latter is more properly defined as "variant".

Fig. 5 - THALES TRC-1752 STANAG-4285 variant
 

At the end, do not blindly trust decoders: they are not infallible and there is no magic wand; just open your wav files and analyze them.

4 June 2021

Three new fleet broadcast channels

Three new fleet broadcast channels spotted in these days, all STANAG-4481F:  

4987.0 KHz (cf) 50Bd from NSS Davidsonville
7455,5 KHz (cf) 75Bd from NSY Niscemi
7457.0 KHz (cf) 50Bd from NAU Isabela

All the the broadcast are KW-46 secured(!), although 75Bd/850 usually run KIV-7/KG-84 devices.

These and other recent catches seem to confirm a change in some transmission frequencies, mostly of 2000 Hz or , even stranger, of 1300 Hz. "There is currently a Navy exercise in progress off the east coast, so the shift may perhaps be supporting that effort or maybe to avoid interference or harmonics on other services at the sites", my friend Mike mco say.

I have updated the page related to the BRASS STANAG-4481F page, it is even more evident that all the transmissions that use the waveform 50Bd / 850 are protected with KW-46.

https://disk.yandex.com/d/XJVGUnkI4D4etg 

 

1 June 2021

188-110A, ACF lengths and interleaved blocks boundaries

MIL 188-110A, unlike other waveforms such as STANAG-4539  or STANAG-4285, does not always show the same ACF value as the data rate changes: it is mainly due to the variations of the frame structure (Table XIX) and the way it interacts with the length of the scrambling sequence. The corresponding periods lengths of the bit streams then make it possible to identify the interleaved block boundaries which in turn depend on the bit rate and the interleave delay (Long or Short). 

Fig. 1 - 188-110A Serial Tone blocks

Table X lists the interleaver matrix dimensions (rows and columns) that shall be allocated for each required bit rate and interleave delay.

The bits obtained from the interleaver matrix are grouped together as one-, two-, or three-bit entities that will be referred to as "channel symbols" (or - more simply - "symbols"): the number of bits that must be fetched per symbol is a function of the bit rate. 
The scrambler is feed with a number from 0 to 7 supplied by the data sequence randomizing generator, a 12 bit shift register with the initial state 101110101101 (0xBAD, hexadecimal). After 160 transmit symbols, the shift register is reset to 0xBAD and this sequence produces a periodic pattern 160 symbols in length.  

The interleaved blocks boundaries can be identified by looking carefully at the probes in the demodulated bit streams. As per MIL-STD 188-110: during the periods where known (channel probe) symbols are to be transmitted, the channel symbol formation output shall be set to 0 (000) except for the two known symbol patterns preceding the transmission of each new interleaved block. When those two known symbol patterns are transmitted, the 16 tribit symbols are set to Dl and D2, respectively, as defined in table XV and table XVII. 

2400 bps
Each data frame has a length of 48 symbols and consists of a data block consisting of 32 data symbols, followed by a probe consisting of 16 symbols of known data. Although the expected ACF is 20 ms, the actual value is 200 ms ie corresponding to a block of 10 frames (Figure 2). Since 10 frames contain 10×16 = 160 probe symbols, the 200ms ACF spikes are likely due to a kind of "resonance"  between the 160 probe symbols and the 160 symbols of the scrambing sequence.

Fig. 2 - 200 ms ACF for 110A 2400 bps

The short interleaver matrix for 2400 bps consists of 40 rows and 72 columns, ie a length of 2880 bit: that means that 960 tribit symbols will be fetched and transmitted as 30 data blocks (960:32); thus, one interlever block needs 30 frames to be transmitted. As you may see in Figure 3,  each three rows the patterns of the last two probes exhibit a discontinuity that is not present in the other probes: that's what we were looking for. Indeed - as seen abvove - 3 period rows contain 3×10 = 30 frames that just identify a single short interleaved block for the 2400 bps speed. The interleaver matrix is fetched  in 600 ms, ie  200 ms  × 3.

Fig. 3 - 2400 bps Short interleaver

A similar calculation can be verified for the long interleaver. In this case the interleaver matrix consists of 40 rows and 576 columns, ie a length of 23040 bit or 7680 tribit channel symbols. Since the 32 symbols length frame, one long interleaved block will be sent into 7680:32 = 240 frames, thus 24 rows (Figure 4). The interleaver matrix is fetched  in 4.8 s, ie  200 ms  × 24.

Fig. 4 - 2400 bps Long interleaver

1200 bps
In case of low data rates (from 150 up to 1200 bps) the data frames are structured as a 40-symbol pattern: each frame consisting of a data block consisting of 20 data symbols, followed by a probe consisting of 20 symbols of known data. The expected ACF value is then 16.67 ms, but the actual one is 66.67 ms ie four times greather (Figure 5). The reason is that four groups of the pairs data + probe  count 160 symbols (4×40) and they are just "in sync" with the scrambler length (160 symbols) causing the strong 66.67 ms ACF spikes of Figure 5.

Fig. 5 - 66.67 ms ACF for 110A 1200 bps

The short interleaver matrix consists of 40 rows and 36 columns, ie a length of 1440 bit: that means that 720 dibit symbols will be fetched and transmitted as 36 data blocks (720:20); thus, one interlever block needs 36 frames to be transmitted.  Nine period rows are indeed the boundary of a single short interleaved block (Figure 6).

Fig. 6 - 1200 bps Short interleaver

The long interleaver matrix consists of 40 rows and 288 columns, ie a length of 5760 dibit symbols that will be fetched and transmitted as 288 data blocks (5760:20); thus, 288:4 = 72 period rows is the boundary of each long interleaved block (Figure 7).

Fig. 7 - 1200 bps Long interleaver

600-150 bps
The short interleaver matrix for 600-150 bps consists of 40 rows and 18 columns. Given that only one-bit per channel symbol is fetched, a single short interleaved block will be trasmitted as (40×18):20 = 36 frames. The boundaries of the short interleaved blocks are clearly visible each 9 rows in the usual 480-bit/4-frame period (Figure 8). 

Fig. 8 - 600-150 bps Short interleaver

The long interleaver matrix for 600-150 bps consists of 40 rows and 144 columns. Given that only one-bit per channel symbol is fetched, a single long interleaved block will be trasmitted as (40×144):20 = 288 frames. By grouping the bit stream in a 8-frame period, the boundaries of the long interleaved blocks are clearly visible each 36 rows (Figure 9).

Fig. 9 - 600-150 bps Long interleaver

As a last thought consider that since the bit streams come from demodulating the on-air signals, we see the "coded" data rate and not the actual "input" data rate... FEC coding is still there.

19 May 2021

UK MoD 12800bps bursts: other oddities

I was just monitoring some interesting sequences of 2400Bd PSK bursts occupying 3 out of 6 channels (each 3 KHz width) according to alternate timings of 40 and 20 seconds: frequencies 5742.5, 5748.5, and 5757.5 (all USB). The 119.6 ms ACF value corresponds to a framing of 287 symbols @2400Bd, ie the waveform STANAG-4539: more precisely, according to its self-identifying feature, a 32QAM modulation at 8000bps speed (Figure 1).

Fig. 1 - STANAG-4539 framing structure

Unfortunately, I was not able to get the expected 32QAM constellation but only few states of the outer 32QAM ring (Figure 2).

Fig. 2

I was a bit puzzled until I realized I was seeing an already known signal, more precisely the UK MoD 12800 bps 64QAM bursts [1]: the signals were just badly-tuned (300 Hz frequency offset).
A question arose almost immediately: why, despite the out-of-tuning, the signals are recognized as a 32QAM/8000bps modulation with a sub-carrier error of only 0.2 Hz?  I thought about a decoder error, but examining the 103 preamble' symbols that carry information regarding the data rate and interleaver settings, I found that those are actually different in the two cases of 5757.20 and 5757.50 Hz (Figure 3).

Fig. 3 - data rate and interleaver settings

I repeated the measurements of modulation and speed using other tuning frequencies, results are in the table below. Notice the discrepancies between the tuning frequency and the error detected by the decoder, expecially in 5757.20 and 5757.50 cases where the signal seems to be exactly tuned:

According to SATANAG-4539 #2.1, the accuracy of the sub-carrier frequency shall be 3×10^-5, ie a max tollerance of ±172 Hz @5,757 MHz is allowed: probably the autobaud feature fails since that kind of "symbols distortion"?

Assuming 64QAM/12800bps as the actual mode (as already assumed at the time), I tried to demodulate a same single burst using two different decoders (say A and B, without naming them). The expected length of the bitstream will be:

13 frames × 256 = 3328 symbols × 6 = 19968 bit

Fig. 4
Results are a bit perplexing:

- decoder A, 1536-bit length of the resulting bitstream, seems to successfully demodulate only one frame (1 × 256 × 6)
- decoder B, 36864-bit length of the resulting bitstream, seems to demodulate 12-out-of-13 frames and, in some way, duplicate the results (12 × 256 × 6 × 2)

(the 5750.20 KHz signal was resampled to 8000 Hz before its demodulation).

Fig. 5

I don't know if the use of only 8 out of 20 points of the 64QAM outer ring confuses the decoders, however I think these bursts (and maybe the waveform?) are not fully clarified yet.

8 May 2021

yet another STANAG-4481F 50-75Bd broadcast

5716.0 KHz (cf): STANAG-4481F (apparently) 75Bd fleet broadcast from NAU Isabela (PTR), running with the "odd" and already observerd 3-bit format (Figure 1).

Fig. 1

Removing the third column which contains the replicated bits and then reshaping the resulting bitstream to a 7-bit pattern, it turns out the actual 50Bd speed and the usual KW-46/KIV-7 encrypted stream (Figure 2).

Fig. 2
As mentioned, this behavior has already been noted previously in STANAG-4481F transmissions from NSY Niscemi, AJE Barford and just from NAU: for more informations, the related posts are grouped under a specific tag.

https://disk.yandex.com/d/oiw9OS_fQXcK1Q

7 May 2021

Canada's East Coast Navy STANAG-4481P broadcasts

STANAG-4481 PSK (4481-P) waveform carrying the KW-46 secured fleet broadcast of the Canada's East Coast Navy.  STANAG-4481P is basically a STANAG-4285 sub-mode which adopts fixed 300 bps data-rate and long interleaving. As well as 4285, 4481-P is mostly used in  NATO Naval broadcast  due to its repeated preamble for which it's easy to maintain sync during long transmissions.  

Fig. 1
 
Fig. 2 - x^31+x^3+1 m-sequence used to sync the receive KW-46 unit
 

Transmitter site is most likely the Naval radio Station (NRS) of Newport Corner (NS). Both the NRS Newport Corner transmitter and NRS Mill Cove receivers were automated and are currently operated by HMCS Trinity at CFB Halifax [1]. 

Fig. 3 - TDoA results

 https://disk.yandex.com/d/aksB6d-IgB1n1Q 

[1] https://military.wikia.org/wiki/CFB_Halifax

3 May 2021

unid, and somewhat peculiar, 1200Bd DBPSK

Cleaning up one of my hard disks I came across an old recording (year 2014) that had no comment file associated with it, so I decided to take a look at it and see what exactly it was. The recording consists of different length bursts, each burst is modulated with PSK2 at a symbol rate of 1200Bd: nothing particularly interesting unless its ACF and, consequently, its period. Indeed, the ACF results in 373.74 ms (Figure 1) that make a length of the period of 448.5 bit (PSK2 @1200Bd), but after the differential decoding the bitstream shows a 897 bit length period, ie just the double of the value obtained using the phase detector. 

Fig. 1 - 373.74ms ACF corresponding to a 448.5 bit length period

As you know, a such situation is typical in asynchronous framings that have the stop-bit of 1.5 or 2.5 in length: for example, in case of a 5N1.5 framing the bit editor groups two 7.5 bit frames into a single 15 bit pattern because it can't represents a length of half bit. That's just what may happen here: the bit editor reshapes the bistream to a 897 bit length period and draws two frames (Figure 2) consisting of two 100 bit sync sequence, each followed by a data block.

Fig. 2 - 897 bit period

However, it's to notice in Figure 2 that the lengths of the two data blocks are different (348 and 349 bit) while the sync sequences have a constant length of 100 bit. Looking at Figure 3, the synchronization patterns are left and right inclined for periods of length 488 and 489 bits respectively: that behavior may confirm the 448.5 bit frame as measured above.

Fig. 3 - inclined 100 bit patterns in 448 and 449 bit framings

Talking about it with my friend cryptomaster, we agreed two possible hypothesis:

1) the length of the frame is 897 bit: the framing consists of  two 100 bit sync sequences that are interspersed with two data blocks of  348 and 349 bit; the variable length of the two data blocks and the two sync patterns are a proper feature of this waveform;

2) the length of the frame is 448.5 bit: the framing consists of 100.5 bit for the sync sequence (possible, even if unlikely) followed by 348 bit for the data block.

In order to verify the second assumption,  I set the speed of the PSK demodulator to a double value (2400Bd) so to emerge the missing half bit, if any. The resulting bitstream is shown in Figure 4: the period has the expected length of 1794 bit (2 × 897) and it's possible to see sync sequence patterns of 201 bit in length, ie just the extra bit that was missing.


Fig. 4 - 201 bit sequences in the bitstream @2400Bd

Indeed, the 1794 bit period is arranged as: | 201 bit sync  | 696 bit data | 201 bit sync | 696 bit data |;  since the speed of demodulation is doubled, dividing by 2 we get a 448.5 bit frame consisting of 100.5 bit for the sync sequence followed by 348 bit for data:
 

However, keep in mind that the above bitstream was achieved after a forcing of the demodulation speed to 2400Bd (instead of the effective 1200Bd) and the subsequent differential demodulation: more observations are needed to confirm the 100.5 bit length of the sync sequence.
By the way, the 100 bit sequence may be de-scrambled by the polynomial x^7+x^4+1. Looking for a scrambler polynomial in the 201-bit sequence does not make sense since the way it was obtained.
 
Fig. 5 - x^7+x^4+1 polynomial

Back to the signal, it's interesting to note that some burts have a ~230ms preamble consisting of 8 x PSK2 1200Bd "pulse" (Figure 6): I don't know the reasons and what it can depend on, signal strength and fading seem to indicate that it is not an exchange of messages between two nodes or an ARQ mode.

Fig. 6

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

20 April 2021

Arcotel-MAHRS 2400 serial

The so-colled Arcotel-MAHRS 2400 is a 2400Bd PSK-8 serial waveform with simmetical 2 x 8 pre-tones in respect of the 1800Hz carrier. The signal has strong 106.66ms ACF spikes (256 tribit symbols) due to its frame structure: an initial 80 symbol (240 bit) preamble that is followed by a data block consisting of 176 data symbols (528 bit). The pronouncec BPSK states in the constellation plane are due to the BPSK modulation of the preambles (Figure 2).
 
Fig. 1

Fig. 2

The waveform belongs to the Telefunken Racoms HF HRA 5100 radio communication, an ARQ-like system used in airborne platforms for air-to-air and air-to-ground comms. As far as is recoverable from the web, "MAHRS" (Multiple Adaptive HF Radio System) is the name of the HF radio-data standard originally developed by Telefunken and "Arcotel" is the radio processor. The HRA 5100 system has been replaced by the more recent HRA 9100, both supplied by Elbit System (Figure 3).

Fig. 3
 

17 April 2021

unid secondary protocol

Recently I ran across on several transmissions in the 5 MHz band (also available on a secondary basis for amateur use) consisting of STANAG-4538 point-to-point circuit mode service with 188-110A and STANAG-4539 as traffic waveforms at several speeds. The transmissions are receivable in Northern Europe with a good SNR, especially using the KiwiSDR receiver at OZ1AEF (Skanderborg, Denmark): they are very frequent, of short duration, and occur on at least a half dozen different channels between 5300 and 5400 KHz USB. Given their unpredictability (time/channel) and their duration, a Direction Finding is very difficult, at least with the means at my disposal.  


The excellent SNR allows demodulations without many errors and therefore the possibility of examining and comparing the obtained bitstreams: to be honest, I expected to find a known data-link protocol or at least sequences attributable to some encryption technique (known or not )... but that's not what exactly happened.  Although the ACF doesn't produce results, comparing various demodulations it turns out that all the bitstreams exhibit the same initial 16-bit sequence 11110000100111010111 (0x90EB). 
 
Fig. 1

By syncing the bitstreams on the two sequences, interesting results are obtained: in particular,  patterns that have a fairly well-defined structure in the first 192 bit  (Figure 3).

Fig. 3 - bitstreams synched on 0000100111010111 sequence (first 192 bit) 
 

One could argue that the 16-bit sequence (LSB)0000100111010111(MSB) is the binary equivalent for 0xEB, ie the sync sequence of STANAG-5066 frames: unfortunately, the following bytes do not match the Data Transfer Sublayer headers, ie the (supposed) fields do not contain data that make sense (EOT, size of address,...).

https://disk.yandex.com/d/qPt2CXC2x76yfg (wav)

https://disk.yandex.com/d/8tn5OrrXlsuL4A (streams)


16 April 2021

CIS Navy FSK 50Bd/250 (T-600)

50Bd/250 FSK is another waveform used by CIS Navy for their fleet broadcast. Unlike the same waveform but with 136-bit framing (T-600 136), this one shows all the characteristics of the (perhaps more) well known 50Bd /200 broadcast, ie:

* 44 bit sequence (usually  "11100001010010111110000101001101011010101101")
* 70-bit Initialization Vector (ten 7-bit words, repeated twice)
* payload arranged in the 4:3 ratio
* 7-bit words "000100" as EOM

 

https://disk.yandex.com/d/zr9Mj7M7jII2xQ