9MR - Malaysian Navy, uncommon FSK shift and ITA2 framing

29 December 2018

9MR - Malaysian Navy, uncommon FSK shift and ITA2 framing

tags: 5N1V, 9MR, FSK 50Bd/900, Malaysian-Ny

(a joint analysis by me, ANgazu, Cryptomatser)

9MR 9/10/13 RMMJ MRB MRB RYRYRYRY 9MR 9/10/13 RMMJ MRB MRB SGSGSGSGSG AR JULL JULL

is the Id & "RY/SG" test tape transmitted by 9MR Royal Malaysian Navy (RMN) [1], picked up using the VR3BG KiwiSDR located in Hong Kong and tuned on 8461.1 KHz and 6483.1 (CF).

The signals exhibit two curious features, at least in the heard test trasmissions: the first consists of the used 50Bd async FSK waveform with the non-standard and quite uncommon 900Hz shift value (Fig. 1).

Fig. 1 - 900Hz shift

The second feature is the framing which is used during the test operations: as you may see in Figs. 2 and 3, they use ITA2 code (5x28) with alternating framings 5N1/5N2, i.e., a character sent with 1 stop bit followed by a character sent with 2 stop bits (say 5N1V, a variable number of stop bits but at least one)

The variable framing causes the 15-bit period visible either in the raster of SA either in the bitstream (the latter reshaped to 30-bit in Figure 2). When a block ends, it possibly uses a special character or new line that causes te one bit shift to the left.

Fig. 2

Fig. 3

According to my friend Cryptomaster, we face a 5N1.5 framing and the synchronous equipment compensates for this disadvantage, resulting in alternation of stops with a length of 2 and 1 bit. About the unusual frequency manipulator, this is a drawback of the frequency manipulation shaper: when changing its polarity, the unfiltered classical transient process is visible (Fig. 4)

Fig.4

It could be a good reason, anyway after removing/cutting the upper and lower extra-frequencies, the signal still exhibits a 900Hz wide spectrum (Fig. 5)