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AIS Basics

The Automatic Identification System (AIS) is the radio system with which ships continuously broadcast who they are, where they are and where they are heading. Every commercial vessel — and many leisure craft — carries a transponder that transmits these reports on the marine VHF band, where anyone with a receiver can pick them up. AIS-catcher turns an inexpensive SDR dongle into exactly such a receiver.

162 MHz
Marine VHF band
2 channels
Shared by all vessels
9,600 bit/s
GMSK per channel
Line of sight
Range set by antenna height

How AIS Works

AIS is a broadcast system: there are no connections and no acknowledgements. Each vessel transmits short messages in time slots on two shared channels — every minute holds 2,250 slots per channel, and transponders organize among themselves who uses which slot (a scheme called SOTDMA). A receiver simply listens to both channels and decodes every message it can hear.

Two classes of transponder exist:

Class Carried by Transmit power Position reports
Class A Commercial vessels (mandatory) 12.5 W Every 2–10 s under way, every 3 min at anchor
Class B Leisure craft and small vessels (voluntary) 2–5 W Every 30 s under way, every 3 min below 2 knots

The difference in transmit power is why large ships are typically received much farther out than small craft.

The Radio Channels

AIS operates just above the marine VHF voice channels, around 162 MHz:

Channel Frequency Use
AIS 1 (Channel A) 161.975 MHz Standard position reports and data, ship-to-ship and ship-to-shore
AIS 2 (Channel B) 162.025 MHz Standard position reports and data, ship-to-ship and ship-to-shore
AIS 3 (Channel C) 156.775 MHz Long-range reports for satellite reception
AIS 4 (Channel D) 156.825 MHz Long-range reports for satellite reception

Channels A and B carry virtually all traffic and are the two that AIS-catcher receives simultaneously by default.

What Ships Transmit

Every vessel is identified by its MMSI, a nine-digit number included in every message. The content falls into three groups:

Data Examples How often
Dynamic Position, speed and course over ground, heading, rate of turn Every 2 s – 3 min, depending on class and speed
Static Name, call sign, MMSI, ship type, dimensions Every 6 min
Voyage Destination, ETA, draught, navigational status Every 6 min

This is why a ship often appears on the map before its name shows up: position reports arrive every few seconds, but the name travels in the static report that is only sent every six minutes.

How Far Will You Receive?

VHF signals travel in essentially straight lines, so the curvature of the earth sets a hard limit on range: the radio horizon. How far away that horizon lies depends on the height of both antennas:

h₁ h₂ line of sight
d ≈ 4.12 · (√h₁ + √h₂)  km — with antenna heights h₁, h₂ in metres

For a large ship with its antenna around 15 m above the water, your antenna height translates into range roughly as follows:

Your antenna height Radio horizon
2 m — window sill ~22 km / 12 nmi
5 m — rooftop ~25 km / 14 nmi
10 m — mast on the roof ~29 km / 16 nmi
20 m — apartment building ~34 km / 19 nmi
50 m — hill or tower ~45 km / 24 nmi

Two practical consequences:

  • Height beats everything. Raising the antenna does more for your range than any amplifier or premium receiver — a clear view of the water matters more than gain.
  • Ranges beyond the horizon do occur. Under certain atmospheric conditions VHF signals bend along the earth's surface (ducting) and ships hundreds of kilometres away appear briefly. See Long Range AIS.

Antenna and Cable

Any antenna tuned for the marine VHF band works; a simple quarter-wave whip or a dedicated AIS antenna outperforms the stock telescopic antenna of most SDR bundles. What is often overlooked is the cable: coax loss adds up quickly at 162 MHz, and every 3 dB of loss halves the signal power reaching the receiver.

Cable Loss per 10 m at ~160 MHz
RG-58 ~2.0 dB
RG-8X ~1.5 dB
RG-213 ~0.9 dB
LMR-400 ~0.5 dB

Tip

Long coax runs can be avoided entirely: place the receiver (for example a Raspberry Pi with the SDR dongle) close to the antenna and run a network cable instead — network cables have no signal loss. See What You'll Need for hardware suggestions.

Receiving AIS with an SDR

A traditional AIS receiver is dedicated hardware. A Software Defined Radio does the same job in software: the dongle digitizes a slice of the VHF band and AIS-catcher demodulates and decodes both AIS channels from it simultaneously — at a fraction of the cost of dedicated equipment, on hardware that can also receive many other signals.

Not for navigation

SDR-based AIS reception is intended for hobby and research use only. It must not be used for navigation, safety-of-life applications or commercial vessel operations — those require type-approved equipment meeting international maritime standards. Read the full disclaimer.

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