On December 13th morning I accidentally tuned a KiwiSDR receiver on the well-known frequency of 8437.0 KHz USB, i.e. the Dutch Navy CARB channel operating in S-4481 FSK (8439.0 CF). Suddenly, about 1050 UTC, the usual FSK "melody" disappears to make room for a transmission in S-4285 600bps/Long mode (Figure 1): intrigued, I recorded that transmission.
 |
| Fig. 1 - STANAG-4285 transmission |
As in Figure 2, dhe decoded bitstream shows a repeating "message" characterized by a pattern with a period length of 61 bits.
 |
| Fig. 2 - 61-bit period bitstream |
I then isolated a single "message" and after the appropriate shifts a 5N1.5 framing emerges where each bit is repeated eight times. The stop bits should be 12 (8x1.5) but for some reason I don't know 13 bits are reproduced (Figure 3).
 |
| Fig. 3 - 8 times expanded 5N1.5 framing |
After the removal of the start/stop bits, the source 5 bits of data can be obtained after removing the overhead bits: this can be easily done by manually editing the demodulated bitstream, for example with BEE, or by "downsampling" the bitstream by a factor of 8 using a simple script coded - for example - in Octave. The result (5x10 decoded) is the well known CARB string "02A 04B 06A 08B 12A PBB" which is usually transmitted in S-4481 mode by the Dutch Navy (Figure 4).
 |
| Fig. 4 - the resulting CARB string "02A 04B 06A 08B 12A PBB" |
Obviously I don't know the purpose of such a transmission except - obviously - for the CARB (Channel Availability and Receipt Broadcast) string. I had already encountered something similar some time ago, a particular transmission of UK DHFCS: in that case it was a 5N1 framing where each bit was repeated 16 times [1].
It is worth noting that transmission in S-4285 was in 600bps mode and 600:8 gives 75! That is the same speed as CARBs transmissions in S-4481.
https://disk.yandex.com/d/3A2Ouj9TBQERZw
[1] http://i56578-swl.blogspot.com/2022/01/an-odd-16-times-expanded-5n1-framing-uk.html
No comments:
Post a Comment