Reference layer
What the data actually means
Anyone can show you a dot on a map. These explain what the dot is, where it came from, how often it lies, and what it cannot tell you. Written from the standards, with the sources listed at the bottom of every page.
Photo: cottonbro studio / Pexels · source · Pexels License
Trying to understand a gap on the map? Start with where our shore receivers can hear ships.
How AIS actually works
AIS is a VHF radio broadcast, not a tracking service. Here is what a transponder sends, on which channels, how often, and why the signal stops at the horizon.
Takeaway Every dot on a live map is a ship voluntarily broadcasting its own position over VHF radio, and a receiver close enough to hear it.
Why AIS coverage is uneven — and what it means when a ship disappears from the map
A vessel that vanishes mid-Atlantic has not gone dark. It has gone out of radio range. Here is how shore-based AIS coverage works, where it is dense, and where it fails entirely.
Takeaway Coverage follows the receiver network, not the ocean. A missing ship is almost always a missing receiver.
Reading an MMSI: what the nine digits mean
An MMSI is not a serial number. Its first digits tell you the flag state, and some prefixes mean the transmitter is not a ship at all. Here is how to read one.
Takeaway The first three digits of a ship's MMSI are its flag state — but only when the identity is a ship station in the first place.
AIS receiver range and coverage limits
VHF is line-of-sight and shore receivers are fixed to land. Here are the actual range figures, what Class A and Class B transponders reach, and where the open feeds this site uses fall short.
Takeaway Coverage depends on transmit power, antenna height, and where receivers happen to be installed. Most vessels more than 40 to 60 NM from a coast are outside range.
AIS Navigational Status: All 16 Codes Explained
What do AIS navigational status codes 0–15 mean? See all 16 values, why a ship's reported status can be wrong, and how to check it against speed.
Takeaway Navigational status is a human-entered field, not a sensor reading — useful context, never evidence.
AIS ship type codes explained
Every AIS ship type code explained — cargo (70–79), tankers (80–89), high speed craft (40–49) and more. Decode the two-digit decade structure in one read.
Takeaway The first digit of an AIS ship type is the family; for most families the second digit says whether the ship is carrying dangerous goods.
Why ships "go dark" on an AIS map
A ship disappearing from an AIS map usually means its radio signal is no longer reaching a receiver. Learn how range, coverage, equipment and AIS rules explain a missing position.
Takeaway A missing AIS position tells you what this site's receivers have not heard; it does not tell you that a ship stopped transmitting.
AIS vs radar vs satellite tracking: what each can and cannot see
Three ways to find a ship, with three completely different blind spots. AIS asks, radar looks, satellites do both badly and brilliantly at once.
Takeaway AIS sees identity but only for cooperating ships; radar sees hulls but not names; satellites trade resolution and revisit rate for reach.
AIS transponders explained: Class A, Class B and receive-only
What a marine AIS transponder actually transmits, how Class A, Class B/SO and Class B/CS differ in power, reporting rate and data, and the questions that decide which one a boat needs.
Takeaway A receiver lets you see AIS traffic; only a transponder makes you visible, and its class decides how often and how loudly you are heard.
AIS antennas and splitters: height, cable and sharing a VHF antenna
Why antenna height sets AIS range more than transmit power, when one VHF antenna can serve both radio and AIS, what an active splitter does, and how much range cheap coax quietly throws away.
Takeaway Put the antenna as high as practical, feed it with low-loss cable, and never share it between a radio and a transponder without an active splitter.
Chartplotters explained: charts, AIS targets and what the screen cannot tell you
What a marine chartplotter displays, how an onboard AIS receiver or transponder supplies targets, which connections to check, and why the picture is never complete.
Takeaway A chartplotter puts electronic charts, your boat's position and received AIS targets on one screen; the AIS picture is only as complete as the broadcasts your equipment can hear.
Marine radar basics: range, target tracking and choosing a set
How an onboard marine radar makes its picture, why range depends on antenna height and target size, what ARPA does, and what to check before choosing a scanner.
Takeaway Radar detects echoes from nearby objects whether or not they transmit AIS, but its useful range and tracking depend on the installation, conditions and target.
AIS MOB, AIS-SART, EPIRB and PLB: which beacon alerts whom
Distress beacons split into two systems — local AIS and DSC alerts heard by boats nearby, and 406 MHz satellite alerts that reach rescue coordination centres. What each device does, who it alerts and how it appears on AIS.
Takeaway An AIS MOB beacon alerts the boat you fell from; a PLB or EPIRB alerts the rescue services. They solve different problems, and many crews carry both.
Marine VHF radio: fixed, handheld and DSC explained
How to choose a fixed or handheld marine VHF radio, what Channel 16 and DSC do, why an MMSI and position source matter, and where AIS fits.
Takeaway Start with the antenna and where the radio will be used; add DSC with a registered MMSI and a reliable position source if you want digital calling.
DSC on marine VHF: distress alerts, MMSI and AIS
How a VHF radio's digital selective calling identifies a station, what a distress alert can carry, and why DSC is different from AIS.
Takeaway DSC is digital calling on marine VHF. An assigned MMSI identifies the radio, while a current position source lets a distress alert include a location.