3 November 2018

two interesting FSK catches: 74.5Bd /250 and 75Bd/200

1) FSK 74.5Bd/250
Some FSK 74.5Bd/250 short transmissions (Fig. 1) have been heard on 11018.0 KHz (CF) sending the same seven bits pattern in postive and in negative polarity (Fig. 2). Since the short transmission time I coould not DF the transmission site.

Fig. 1
Fig. 2
 FSK 74.5Bd/250

2) FSK 75Bd/200
The FSK 75Bd/200 (Fig. 3) is a continuous transmission that can be heard on 8408.0 KHz (CF), most likely an encrypted broadcast (shore-to-ship ?). Several TDoA runs point to the West Mediterranean sea area: the Tx location could be Algeria or Balearic Islands (Fig. 4).

Fig. 3
Fig. 4

The demodulated bitstream does not exhibit ACF spikes (ACF = 0) after normal and differential decoding and can be descrambled using the polynomial x^8+x^6+x+1 but without appreciable results.
A similar transmission (FSK 75Bd/200) was heard on 11 Jan 2018 on 4540.0 KHz. In that case, after differential decoding, the stream showed up a clear 365-bit period (Fig. 5) which is due to the sequence of the scrambler polynomial x^7+x^6+1. The descrambled stream is shown in Figure 6 (thanks to cryptomaster).

Fig. 5
Fig. 6

1 comment:

  1. hello Antonio!, nice job! but i don understand how do u know what is the right polynomial for descrambling?