20 December 2015

pseudo-random frequency hopping


Frequency Hopping Spread Spectrum (FHSS) is a method of transmitting radio signals by rapidly switching the carrier among many frequency channels using a pseudo-random sequence known to both transmitter and receiver(s). The heard packets in this sample have a duration of 2520 ms, frequency shift ~ 20 Hz and 20 data bits carried (other similar transmission have been noted with 2460 ms and 24 data bits, same shift) and span up a 2 MHz bandwidth, this recording has been on 13 MHz band.



12 December 2015

playing with MS-DMT (MIL-STD Data Modem Terminal)

The MS-DMT application is a software defined modem based communications terminal tool developed as a Microsoft Foundation Class (MFC) based Multi-threaded 32 bit application. It is currently designed to run under MSWindows XP SP3 and later versions of both 32 and 64 bit MS-Windows operating systems. The tool is written in C++ using the Microsoft Visual Studio 2008 compiler and requires the installation of the VS2008 C++ runtime redistribution libraries which are part of the full MS-DMT install distribution only.
The MS-DMT tool functions as both a MIL-STD modem and basic data communications terminal and provides MARS a MIL-STD-188-110A (MS110A) compliant Serial Tone (ST) waveform modem based Message Terminal capability providing message composition and automated message storage to simplify MARS MS110A Forward Error Correction (FEC) message handling.
The software’s terminal provides numerous features to aid in message handling and the software also supports the use of more fully featured external asynchronous terminal applications. Additional features such as Data Link Protocol, Data Compression, Data Encryption and others may be added to the MS-DMT tool as required and in accordance with the specific standards as requested or left to implementation in external terminal applications.
You may use MS-DMT to build, decode and learn the MIL-STD 188-110 ST wavefvorm, the modem, along with other interesting resources and docs, can be downloaded from here:
MIL-STD Data Modem Terminal (MS-DMT) software and resources 





3 December 2015

MFSK-4 (double FSK) 96/100Bd 500Hz, Ukraine Military


This waveform is known as Double FSK 96/100 Baud and 500 Hz shift, a dual channel F7B mode used by Ukraine Military, spreading about a 1730Hz bandwidth. 
The signal was heard just this morning (03 December) on 6970.9 KHz on USB, tones at -750, -250, +250 and + 750 Hz. Site radioscanner.ru reports this signal as Ukraine Mil 96-100, the two baudrates indication (96 and 100) is due to (literally) "some difficulties  in determining the speed of manipulation, due to the fact that the transmission channels are not synchronized (or poorly synchronized) with each other".
That issue is verified in the 21Aug15 log: the signal exhibits two baudrate lines at 96 and 100 Hz (pic. 3). That uncertainty is anyway not present in the measurement results of the  03Dec15 signal (96Bd) as in the pic. 2. The perfect 500Hz space among the tones (pic. 3) has ben found in the 96Bd signal, while in the other (96/100Bd) the shift is about 505Hz.

Pic. 1 - 03Dec15 signal, single baudrate line (96Hz)
Pic. 2 - 21Aug15 signal, the two baudrate lines
Pic. 3 03Dec15 signal, perfect 500Hz shift

F7B is a FSK modulation technique with four modulation frequencies, the two transmission channels, termed V1 and V2, are obtained through six possible combinations of the four frequencies and this leads to 6 different modes F7B1 to F7B6:
http://www.wavecom.ch/content/.../twinplex.htm



http://youtu.be/w44gtaCBQBM?hd=1

2 December 2015

MFSK-4 150Bd 4000Hz (CIS FTM-4): the effective data-rate

This signal (reported here) is logged as FSK 4-tone with baudrate 150Bd and shift 4000Hz, or else CIS FTM-4 (Pic. 1). Looking in depth, they use only 4 of 24 combinations of the four tones (Pic. 2), so the encoded characters which are transmitted are reduced to 4.  This mode is called Frequency-Time Matrix (FTM) or, in this case, FTM-4. The coding method is five-digit group of characters plus spaces. The four symbols are:
A) 1-2-3-4
B) 2-4-1-3
C) 4-3-2-1
D) 3-1-4-2
Looking at the segment in Pic. 2, we could find the occurences of each symbol, for example "A" occurs 4 times, "B" occurs 3 times, "C" occurs 3 times and "D" occurs 3 times.
Since the baudrate of the signal, i.e. 150Bd, the real data-rate is 150/4 = 37.5 Baud. 

Pic. 1
Pic. 2
The repetitions of 4 symbols is clearly visible analyzing the demodulated bistream, unless some few errors due to the demodulation:

Pic. 3
 as well as the structure of the message seen as 48 bits period: some parts are well distinct
although it is unclear what they mean (sinc?, address?, commands?, ...)

Pic. 4