24 December 2017

a MS-110A modem running in ASYNC mode

This is a sample of MIL-STD 188-110A Serial 75bps/L modem working in ASCII ASYNC mode and transporting a Citadel encrypted file. The transmission was heard on 7413.0 KHz/USB following a 188-141A handshake between two Algerian Air Force nodes: CM2 (Algerian Air Force Base - Oran, 2nd Regional Command Centre) and COF (Algerian Air Force HQ - Cheraga). 

In ASCII Asynchronous mode the bitstream consists of a 8N1 structure: one start-bit (0), 8 data-bits and at least one stop-bit (1). Each character is transmitted using a total of 10 bits and the 8 data bits are transmitted with the LSB first.

Fig. 1
Working in the ASYNC part, after removed both the start and the stop bits we get the clean 8-bit data where the characterstic pattern of the Harris "Citadel" encryption is easy to identify (Fig. 2).

Fig. 2
A similar example but related to Asynchronous STANAG-4285 can be read here:

You may use the MIL-STD Data Modem Terminal (MS-DMT) [1] to verify how the MS-110A works in ASYNC mode (Figs. 3,4)

Fig. 3 - MS-DMT settings for ASYNC mode
Fig. 4
The latest MS-DMT test build is available at:  

Thanks to  Steve Hajducek for the update, I suggest to subscribe his group at

(MS-110 Async from Algerian AF)

2 December 2017

Baudot FSK 50Bd/100

This FSK transmission was copied on 6330.8 (cf) at 1128z: shift is clearly 100Hz while some problems arise when measuring the speed. Indeed, the measurement of the speed based on FFT may fail in case of non-integer number of bits as in Baudot/ITA2 code where the stop bit lasts 1.5 bit: in this case SA assumes an integer number of bits, so  it prints out a value of 53.47 Baud (Fig. 1)

Fig. 1
In such cases the speed shall be measured using the "raster" tool of SA (Fig. 2): the structure of the frame is 7.5 bit (1 bit start, 5 bit of data and 1.5 stop bit) and the time line is 299.4 msec for 15 bit that makes a speed of 50 Baud.

Fig. 2

Baudod decoders work fine and print out the content of the message after the RYRY sequence, in this case: "ZHGD ZHGD ZHGD DE N4O4 N4O4 N4O4 QRK ? +?". The user is not identified, probably Russian Military.

Fig. 4

my friend KarapuZ catch a similar transmission on 5565.0/USB,  callsigns are very similar to the once I had: "ZBNV ZBNV DE 7X6R 7X6R 7X6R QRK ? +?". 

So far, these are the heard callsigns:
N4O4, 7X6R