23 March 2019

MHFCS FSK 600Bd/850 with KW-46 encryption


Quite good FSK 600Bd/850 signal centered on 10405 KHz, from Australian MHFCS, heard some days ago around 1630z using iw2nke KiwiSDR (center Italy). As reported in UDXF logs by Eddy Waters [1] [2], Australian Defence Force (ADF) has changed the previous dual channel system (ISB) to two single channel with 4 KHz spacings: the lower of the 2 frequencies has a speed of 600 baud, while the higher is 50 baud. The shift in both cases is 850 Hz. As in the waterfall above, I did not hear the 50bd FSK signal 4 KHz above (i.e. on 10409 KHz): maybe it's missing? Eddy also logged 10405 KHz frequency reporting ADF MHFCS Humpty Doo as location of the Tx.

Fig. 1 - main parameters
 
After arranged the demodulated stream into a 7-bit format, it's possible to detect the presence of the sequence called "Fibonacci bits" originated by the polynomial x^31+x^3+1 and which reveals the use of KW-46 crypto device (Fig. 2) as per STANAG-5065 Annex-A.
 
Fig. 2 - 7-bit frame delimited by KW-46 sync bit

Sometimes an FSK 50Bd/340Hz transmission has been seen within the 600Bd/340Hz signal so in these cases they operate in ISB mode: according Eddy Waters from UDXF, the USB side is called "Rockwell 700B" while the LSB side is called "Rockwell CPU100" (Fig. 3). 
 
Fig.3

Just a tip: in order to identify the MHFCS transmissions, in addition to the dial frequencies listed  here (remember that they use a 1500Hz offset above the indicated carrier) and to the shift and speed parameteres (tipically 600Bd/340Hz), think that these MSK signals exhibit a quite unique sign when inspecting the harmonics  using the SA 'involutions' tool. In this case, you will see the presence of several spectral lines in the 7^ power (Fig. 4).

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