AIS receiver

The receiver's antenna stands on a headland. Offshore, a traffic separation scheme carries ships north and south. Others come in to the port in the bay, wait at the anchorage or run the island ferry, and small Class B craft work along the coast and the fishing grounds. Grey triangles are where ships really are; circles are where the receiver has decoded them from their own reports. Past the radio horizon signals fade fast; the dashed rings mark it for a ship's antenna 25 m and 4 m above the sea.

The receiver samples 96 kHz of spectrum centred on 162.000 MHz as complex baseband, so both AIS channels are in one stream: channel A (161.975 MHz) sits at −25 kHz and channel B (162.025 MHz) at +25 kHz. Each report is one 26.7 ms burst on one channel, and ships alternate channels from one report to the next.

How a ship turns a 168-bit position report into a burst. The data go out byte by byte, least significant bit first, followed by a 16-bit CRC, the frame check sequence. After any five 1s in a row the transmitter inserts a 0 (bit stuffing), so the data can never imitate the flag 01111110 that opens and closes the frame. A training sequence of alternating bits comes first so the receiver can lock on. Then NRZI: a 0 is sent as a change of level, a 1 as no change. Last, GMSK: the levels are smoothed by a Gaussian filter (BT 0.4) and sent as a frequency of ±2.4 kHz, which turns the carrier's phase by a quarter turn per bit.

For each channel the receiver shifts the channel to 0 Hz, filters it and halves the sample rate to 48 kHz, 5 samples per bit. The FM discriminator then measures how far the phase turns from one sample to the next, which is the instantaneous frequency. The result follows the transmitted ±2.4 kHz plus the ship's carrier error. Over the training sequence the level pattern repeats every 4 bits and its phase comes back to where it started, so any steady turn over those 4 bits is the carrier error. The receiver measures it there and subtracts it for the rest of the frame. Weak signals show clicks: sudden full turns of the phase caused by noise.

Each bit must be sampled in its middle, but the receiver doesn't know exactly where the bits fall: distance, filter delays and the start of the burst all shift them. A digital phase-locked loop counts 5 samples per bit and, at every zero crossing, nudges its count so that crossings fall halfway between sampling instants. The training sequence, with a crossing every 2 bits, pulls it in. The eye diagram overlays two-bit stretches of the signal around each sampling instant: the wider the eye opens, the safer the decisions.

A sample above zero is a high level, below zero a low level. NRZI decoding turns "same level as the previous bit" into 1 and "changed" into 0. The receiver hunts for the flag 01111110, collects bits until the next flag, removes every 0 that follows five 1s, and runs the CRC over the data and the received check sequence. A good frame always leaves the same remainder, 0xF0B8.

Error rate against signal level

Eb/N0 is the energy of one bit over the noise power per hertz. The test sends isolated position reports through the same modulator and receiver at set values of Eb/N0, with random carrier phase and timing, and counts bit errors after NRZI decoding and frames that fail. The reference curves are ideal coherent MSK and noncoherent detection of orthogonal FSK. A discriminator on GMSK falls short of both: its two tones are only a quarter-turn apart per bit, not orthogonal, and the Gaussian filter blurs neighbouring bits.

A frame that passes the CRC is split into fields. Position reports (messages 1, 2 and 3 from Class A, 18 from Class B) carry the MMSI, navigational status, rate of turn, speed over ground in tenths of a knot, longitude and latitude in ten-thousandths of a minute, course, heading, the UTC second, and a communication state that tells other ships about the sender's slots. Receivers hand decoded messages on as NMEA !AIVDM sentences: the bits packed six at a time into printable characters, with an XOR checksum.

Each channel is cut into 2250 slots a minute, 26.7 ms each, timed from GNSS. A Class A ship picks its own slots. It keeps a map of the slots other ships have announced, picks at random among the free ones within ±10% of the spacing around each nominal time, and keeps each slot for another 3 to 7 minutes (the slot timeout, counted down in every report) before picking again and announcing the new one. Here, ships at anchor or moored report every 3 minutes with message 3 and book just the next slot (ITDMA). Class B ships listen at the start of a slot and only transmit if it is quiet (carrier sense). Ships too far apart to hear each other can still choose the same slot: the receiver, between them, hears both at once.

Every frame the receiver decoded, newest first, and the same messages as NMEA sentences, the way a receiver sends them to a chart plotter. Click a row to show it in stages 3 to 7 (while its signal is still in the receiver's capture buffer).

NMEA output