STANAG 4481

Overview:

The RapidM modems include the narrowband HF data modem waveforms, which are in compliance with NATO STANAG 4481.  This standard describes three data modem variants, namely the S4481-PSK Serial Tone Mode (STM) providing a BPSK modulated waveform offering data rate of 300 bps coded, the S4481-FSK Single-Channel Two-Tone FSK Mode providing data rates from 50 bps to 600 bps, and the S4481-FSK Multi-Channel (up to 16) Two-Tone FSK Mode providing date rates of 50 bps and 75 bps in a 3 kHz channel.

Status:

The original STANAG 4285 (Edition 1) “Minimum technical equipment standards for naval HF shore-to-ship broadcast systems” was ratified in July 2002.

Key Features:

• Non-Autobaud capable STM waveform providing the BPSK modulated 300 bps coded data modem in a 3 kHz channel. The STM waveforms are recommended for backwards interoperability with NATO Broadcast and Ship-to-Shore (BRASS) systems used by Navies.
• The STANAG 4481-PSK waveform is identical to the 300 bps long interleaver waveform of STANAG 4285, and is required for fulfilment of the minimum interoperability standard for NATO shore-to-ship communication.
• STANAG 4481-FSK Single Channel is a synchronous FSK mode, which uses KG-84 encryption for communication. The 75 bps modes Hz are widely used.

Relevance:

• The single tone PSK and FSK (Single Channel) waveforms are important for backwards interoperability with NATO BRASS systems used by Navies.
• STANAG 4481-PSK can be seen as a STANAG 4285 sub-mode which adopts fixed 300 bps data rate and long interleaving.
• BRASS uses 300 bps for continuous broadcasting over wide coverage areas due to its repeated preamble data for which supports maintaining sync during long transmissions as well as late entry where a station can start to receive at any point during the transmission.

Serial Tone Mode (STM):

The STANAG 4481-PSK waveform is a Non-Autobaud capable STM waveform providing the BPSK modulated 300 bps coded data modem in a 3 kHz channel.   The STANAG 4481-PSK modem waveform is identical to the 300 bps long interleaver (10.240 seconds) waveform of STANAG 4285.

The table below presents the STANAG 4481 waveform suite. Included in the table are the modulation, code rate and interleaver settings for the supported data rate. The STANAG 4481 waveform includes TEX capability, whereby suppression of up to 4 tones is possible.

Table 1: STANAG 4481-PSK – STM Waveform Suite

FSK Single-Channel:

The single-channel STANAG 4481 FSK has a fixed shift frequency of ±425 Hz and centre frequencies of 1700 Hz and 2000 Hz.

The table below presents the over-the-air (OTA) data rates available to the STANAG-4481 FSK waveform suite. Included in the table are the shift, centre, mark and space frequency for each data rate. The transmitter and receiver need to be set up to the desired data rate at both ends of the link.

Table 2: STANAG 4481-FSK (Single-Channel): OTA Data Rates

FSK Multi-Channel:

The multi-channel STANAG 4481 FSK waveform uses two-tone FSK and has a fixed shift frequency of ±42.5 Hz and centre frequencies of (255 + 170n) Hz, where n is an integer from 1 to 16.

Dual-channel (twinned) FSK operation is not typically supported. The transmitter and receiver need to be set up to the desired data rate at both ends of the link.

Table 3: STANAG 4481-FSK (Multi-Channel): OTA Data Rates