This STANAG mode is specified by the NATO (North Atlantic Treaty Organization) Military Agency for Standardization as a "Minimum technical equipment standards for naval HF shore-to-ship broadcast system".

The modulation technique used in this mode is based on the operating conditions encountered by naval broadcasts.

The technology utilizes binary phase shift keying (BPSK) of a single tone 1800 Hz sub-carrier with a constant modulation speed of 2400 Bd. Through the use of BPSK modulation and a FEC coding rate of 1/4, the effective user data rate is 300 bps.

The user data is transmitted using a continuous frame structure with 213.33 ms per frame. Each frame starts with a preamble containing 80 symbols. The following 176 symbols are divided into four 32-symbol data segments and three 16-symbol channel probe segments.

The 176-symbol data-probe segment immediately follows the next frame, beginning with the same 80-symbol preamble. This repeated frame structure enables synchronization of the demodulator at any time of transmission.

At the end of transmission, an EOM bit pattern (0x4B65A5B2, MSB first) is sent to mark the end of message. The EOM sequence is followed by flush bits, to flush the FEC coder and to complete the transmission of the remainder of the interleaver data block.

FEC and interleaving is used to combat the effects of fading, Doppler spread, multipath and burst noise. User data is first FEC encoded, interleaved, then mapped into BPSK symbols and transmitted in 32 symbol data segments. The 16 symbol channel probe segment transmitted between each succeeding data segment has a known PSK pattern. Its purpose is to keep the demodulator, mainly the equalizer, on track in spite of adverse propagation conditions during the HF transmission.

This STANAG mode is either used to transmit data in transparent binary mode or as ASCII text. For this reason the decoder displays the user data in HEX, ASCII ASYNC, ASCII ASYNC (7 Data bits and No Stop bit) or ASCII SYNC format selected from Options | Message Type.... The decoder stops displaying traffic after the EOM bit pattern is received.

In the HEX display mode, the decoded binary data is displayed as hex values, MSB first.

In ASCII ASYNC mode, the bit stream is correlated with an ASCII ASYNC structure, i.e., one start-bit (0), 8 data bits and at least one stop bit (1). The 8 data bits are displayed LSB first. In addition to the EOM pattern, the display will stop if more than 300 NULL characters are received or if the asynchronous data structure is violated more than 80 times.

In ASCII ASYNC (7 data bits and no stop bit) mode, the bit stream is correlated with another ASYNC structure, i.e., one start bit (0) and 7 data bits. The 7 data bits are displayed LSB first. In addition to the EOM pattern, the display will stop if more than 300 NULL characters are received.

In ASCII SYNC mode, each 8 bits (LSB first) represent one ASCII character. The display will stop when either the EOM pattern was recognized or more than 20 NULL characters have been received.

Tuning the decoder

The decoder can process signals in both SSB settings: USB and LSB. The sideband is selected by toggling the Polarity field of the display: NOR will select USB and INV will select LSB.

 The center frequency of the decoder is set to the default value of 1800 Hz, but can be adjusted within a range from 800Hz to 2400Hz. Small frequency deviations are automatically tracked and compensated during the decoding. By using the bar graph, any remaining frequency difference can be compensated by fine-tuning of the receiver frequency or by adjusting the center frequency of the decoder.

In the Confidence field, the quality of decoding can be observed. A well recognized stream is determined by a confidence value being stable and greater than 95 (i.e., 95% of the message is correct).