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.

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