15 October 2014

STANAG-4481 FSK

STANAG-4481 is a NATO standard for maritime shore-to-ship broadcast digital data communication. Data are sent by a single land-based transmitting station and received by many stations aboard ships. There are two variants, one with a FSK modulated signal and the other one with PSK modulation.
The FSK variant is a synchronous FSK mode, which uses KG-84C encryption for communication. Although Baud rate and other parameters are settable, 75 Bd and 850 Hz shift values are widely used.



The KG-84 bit stream contains a 64 bits long header followed by two 256 bit message blocks in turn followed by encrypted data terminated by an End-Of-Message. Hence synchronization can only be achieved at the start of transmission using the KG-84 header.
The length of a message is variable. Messages are usually sent consecutively with a short idle sequence between each message.



The KG-84 bit stream contains a 64 bits long header followed by two 256 bit message blocks in turn followed by encrypted data terminated by an End-Of-Message. Hence synchronization can only be achieved at the start of transmission using the KG-84 header.
The length of a message is variable. Messages are usually sent consecutively with a short idle sequence between each message.

Below, a STANAG-4481/FSK transmission from MKL: RAF Northwood on 8996.0 KHz at 2155z: note the KG-84C header just at the start of the transmission:

RAF Northwood (MKL)


Royal Navy Plymouth (MTI)

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