gr-ais

gr-ais receives the Automatic Identification System, the 9600 bit/s GMSK transponder bursts ships and shore stations exchange on the two maritime VHF channels at 161.975 and 162.025 MHz. Its streaming receiver centers a channel with a square-and-FFT frequency estimator, levels it with a feedforward AGC, recovers symbol timing with a generalized-MSK timing error detector, and demodulates coherently with a Viterbi decoder whose trellis carries the carrier phase as part of its state; a data-aided burst synchronizer offers a second path that detects each isolated burst against the known training sequence and seeds that decoder with its timing, phase and trellis state. Differential decoding and inversion undo the NRZI line code, GNU Radio's HDLC deframer unstuffs the bits and checks each packet's CRC, and a PDU-to-NMEA block turns the payloads into the !AIVDM sentences chart plotters and AIS aggregators read. An ais_rx command-line tool wires the chain to a SoapySDR device, a raw I/Q file or a UDP stream and prints the decoded sentences.

Main author

Nick Foster

Last modified

< 3 months

Project

Upstream repository

Example flowgraphs

These GNU Radio World examples contain one or more blocks from gr-ais.

• AIS ReceiverDecodes AIS position reports from ships in the hosted 250 kS/s SDRangel recording centered on 162 MHz. Two Frequency Xlating FIR Filters select channel A (161.975 MHz) and channel B (162.025 MHz) at 50 kS/s, gr-ais's AIS Demod recovers the GMSK bits of each, HDLC Deframer checks the CRC of every burst, AIS PDU to NMEA prints the !AIVDM sentences to the console, and the AIS Map plots every vessel on the Thames estuary as its reports arrive. The receive chain is upstream's ais_rx tool, block by block.