10 September 2014

XPA (CIS MFSK-20)


This is a Russian mode also known as CIS MFSK-20 believed to be used by at least one of the Russian intelligence services. It transmits data at either 10 baud (most commonly) or 20 baud using a 17 tone MFSK system. The name CIS MFSK-20 comes from the whole pool of the available frequencies although only 17 of these are used, sumarizing the initial sync part and the message part of an XPA transmission (Pic.1). Measuring the frequency grid inside the message you get a value = 14 (Pic. 2) and it could lead to wrong conclusions about the number of the frequencies used in this system. There are "holes" related to frequencies that are not in use or have special/unknown functions.

Pic.1 the real XPA grid
Pic.2 freq grid inside the body of an XPA transmission
Baudrate can be measured by the scan-raster method (Pic. 3): as you can see, in this recording the baudrate is just 10 Baud.

Pic. 3 frame structure of the XPA waveform

CIS MFSK-20 is reported here in radioscanner.ru

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