Echotel 1810 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 Echotel HF modem 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 Echotel 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; Echotel 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 Echotel

As third common feature, both the 80-symbol preambles are modulated using BPSK: the pronounced BPSK states in the constellation plane of the Echotel 1810 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 this Echotel 1810 waveform 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.

Echotel-MAHRS 2400 serial

The so-colled Echotel 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, Arcotel is the radio processor and Echotel 1810 is the HF modem. The HRA 5100 system has been replaced by the more recent HRA 9100, both supplied by Elbit System (Figure 3).

Fig. 3

 

https://disk.yandex.com/d/x-bVsSFriyfJtg

'Echotel 1810' and 'THALES TRC-1752': same PSK-8 2400Bd, same 768 bits ACF, but two different waveforms

Single tone PSK-8/2400Bd waveforms are very common and it's easy to get mistakes in their identification, sometimes also their ACF values are equal and so things get complicated (it could also happen that the same waveform exhibits different ACFs according to the input data signaling rate). In cases of uncertainty or ambiguity, a careful examination may decide the questions.
Below just an example about  the Thales TRC-1752 and Echotel 1810 "signals": they not only have the same PSK-8 2400Bd single tone features but also the same ACF. Note that althought both Thales TRC-1752 and Echotel 1810 are HF modems (to be clear: MAHRS - Multiple Adaptive HF Radio System is an "operating mode", Arcotel is the radio processor)  I  generically refer to both as "signals".

Looking at their representation in the waterfall, they have a quite similar shape characterized by the same bandwidth, a non-fixed length duration and a short preamble: a first difference is visible just in this part. Looking with a little more attention, the Echotel preamble is composed of 8+8 simmetrical tones while Thales signal exhibits a different header (fig. 1).

fig. 1

The data transfer parts of the two signals share the same features: 8-ary phase-shift keying of a single 1800 Hz carrier and 2400 symbols/sec modulation rate (input data signaling rate is not detected here). It's interesting to note the pronounced BPSK states that, unless their phase shift, appear in both the constellations of the two signals (fig. 2).

fig. 2

These footprints suggest the presence of BPSK segments in the structure of both the two signals. As from literature, they consist of known-data symbols, in contrast with the unknown (user) data, and are are scrambled to appear, on-air, as PSK-8 symbols.
The BPSK insertions, as well as the differences between the two preambles, are more evident reducing the FFT size in the waterfall (fig. 3).

fig. 3

Looking at the two waterfalls, both with the same settings, we can also estimate that the two signals have the same pattern repetition rate (pic.3). Running the ACF and CCF functions for a better accuracy of the repetition rate, we get the same 106.665 ms result for both the signals: this value makes 768 tri-bits, or 256 PSK-8 symbols periods since the manipulation speed is 2400Bd.
At this point, the real difference between these two signals, unless the preamble, can only be found by examining their frame structures.

The frame structure for the Echotel signal is shown in figure 4. The preamble is followed by 256 symbols blocks, each block consisting of 176 unknown data symbols and a mini-probe consisting of 80 symbols of known data.

fig. 4 - frame structure for Echotel

 

The frame structure for the Thales signal is shown in figure 5. As well as in the frame of Echotel signal, the frame cosists of 256 symbols blocks, each block consisting of 80 symbols preamble followed by 176 symbols data block, each data block consisting of 4 x 32 unknown data symbols and 3 x 16 symbols mini-probes (S4285-like waveform).

fig. 5 - frame structure for THALES TRC-1752

 

It's worth noting that the two signals have the same length, 80 symbols, for the BPSK modulated  segment.

Arcotel MAHRS ALE burst

The Multiple Adaptive HF Radio System (MAHRS) is based on the Arcotel transmission processor that controls the automatic frequency selection, the link establishment and the following waveform for the ARQ-based exchange of data. The modem used is the Echotel 1810  manufactured by EADS RACOM.

Fast bursts sent every 500 msec, spreading about a 2800 Hz bandwidth. Each burst shows a short preamble of 8 un-modulated carriers, symmetrical with respect to the center frequency of the signal, and a short PSK-2 preamble. The data segment is a serial tone carrier at ~ 1800 Hz with a PSK-8 manipulation at 2400 baud.
The MAHRS "procedure" was described in WUN-10 newsletter, and here reported:

"For establishing the link between two stations there is a call of one dot each second at a length of 500ms. Most of the time the station calls 62 times then changes back to listening for 60 seconds and then selects a new frequency for calling. The selection of freqs happens according to position of both sender and receiver, actual daytime and actual sunspot numbers. The system calculates a prediction model of propagation and decides which line may be the best. When in contact the called station answers after waiting a maximum of 15 seconds by one dot of 500ms and the both stations give their "handshakes" and then the full transmission starts using a high speed ARQ like transmission system at a speed of 4800 bps" [http://www.udxf.nl/WUN-v10.pdf]