23 March 2021

Unid 100Bd/1500 FSK

Unid and unusual 100Bd/1500 FSK spotted on 4010.0 KHz (cf) on 10th march thanks to the KiwiSDR in Skerjafjörður Reykjavik; although the numerous attempts in the days to follow, unfortunately I no longer had the opportunity to tune it. The analysis was done a little blindly, not having the initial sequences but only traffic: also, I went late on the signal so Df was not possible. Below are the two different approaches adopted by me(1) and my friend cryptomaster(2).

Fig. 1 - 100Bd/1500 FSK

1) The search for a scrambler results in the polynomial x^6+x^5+x^4+x^3+x^2+x+1. The bitstream after scrambler removal has a 7-bit frame consisting of (presumed) six bit of data + 1 bit set to "0". The 6-bit "code" doesn't seem meaningful.

Fig. 2 - 7-bit framing

 2) I noticed that the seven-column raster resembles an RTTY code (start + 5 bits + stop), folded into some kind of sequence: In addition, blocks with small displacements are visible on the raster (Fig. 3)

Fig. 3

If we discard the conditional start/stopbits and start the search for a scrambler for the remaining 5-bit code, we get the polynomial x^35 + x^25 + x^10 + 1. The result after removing the scrambler is shown in Figure 4. The search for patterns in the received 5-bit code was not found.

Fig. 4 - 10-bit framing


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