1. What TankerMap is receiving
TankerMap's canonical fleet database contains 7,286 tanker profiles; 6,341 have a latest observation within the past day and 6,550 within the current live freshness window. A position timestamp describes when the vessel was observed, not a promise of continuous coverage.
Coverage quality varies enormously by region: European coastal waters, the Singapore Strait and the US Gulf produce dense, frequent updates, while parts of the South Atlantic, the Indian Ocean and high latitudes can go hours between usable messages. When you compare two ships on a map, you are also comparing two reception environments.
2. What an AIS message contains
Automatic Identification System transponders broadcast dynamic fields such as position, course over ground, speed over ground, heading, rate of turn and navigation status, plus static and voyage fields such as MMSI, IMO number, ship name, call sign, vessel type, declared draft and destination. Update frequency scales with maneuvering: anchored ships may report every few minutes while a turning, fast vessel reports every few seconds.
Two properties of AIS shape everything downstream. First, it is self-reported: the ship's own equipment decides what to transmit. Second, much of it is typed in by humans: destination and ETA are notorious for stale entries like "FOR ORDERS" or last month's port. IMO numbers are the durable identifier - they stay with the hull for life - while MMSI codes and even names can change with flag or ownership.
3. Terrestrial and satellite reception
Coastal receivers generally provide frequent updates within radio range, but geography, antenna coverage and message collisions create gaps. AIS was designed for line-of-sight VHF: hills, curvature of the earth and interference limit each receiver's footprint to tens of miles.
Satellites extend coverage offshore, yet the physics cuts both ways: a satellite passing over crowded seas - the Singapore Strait, the North Sea - receives thousands of overlapping messages in a single pass and must discard collisions. Add processing latency and store-and-forward delays, and two aggregators can legitimately show different last-seen times for the same ship without either inventing a position.
Practical consequence: always compare a vessel's track against the freshness indicators shown with it, and distrust any analysis that treats missing points as missing ships without checking reception conditions in that area.
4. Why a tanker appears to go dark
A gap in a tanker's track can result from weak reception, satellite scheduling, congestion, equipment failure, maintenance, power management or an intentionally disabled or manipulated transponder. Crews legally switch AIS off in piracy-risk areas; states can spoof broadcasts; and ordinary hardware dies at sea.
A missing message alone therefore proves nothing. Strong analysis asks structured questions: How long was the gap? Was the last reliable position consistent with the first position after it, given feasible speed? Did the ship resurface near an STS area or a sanctioned-export anchorage? Do port-call records, thermal imagery or official designations corroborate either reading?
That discipline - testing the physically feasible explanation against the suspicious one - separates maritime analytics from pattern-matching. See shadow-fleet indicators for how risk signals combine.
5. How to interpret a live map responsibly
Check the observation time before interpreting a marker as current - a marker is a measurement, not a live feed. Compare identity fields across IMO, MMSI and name because MMSI codes and names can change. Treat destination as a crew-entered signal, not a verified contract.
For movement evidence at scale, use track history and chokepoint analytics: counting detected crossings through Hormuz or Suez sidesteps the weaknesses of single-position snapshots. And when you need the workflow end-to-end - from raw broadcast to interpreted voyage - read how TankerMap tracks tankers.
Finally, respect the boundary between observation and accusation. AIS data supports descriptions of what happened; conclusions about intent require corroborating evidence from beyond the transponder.
6. Frequently asked questions
What does AIS stand for in shipping?
Automatic Identification System. It is a VHF transponder mandated on most commercial vessels that broadcasts the ship's position, speed, course, identity and voyage fields to other ships, shore receivers and satellites.
How often does AIS update a ship's position?
It depends on speed and maneuvering: every 2–10 seconds for a fast turning ship, every few minutes for an anchored one, with Class B equipment reporting less often than Class A. Reception conditions, not just the transponder, determine how often you actually receive an update.
Why do ships turn off AIS?
Legitimate reasons include piracy-risk transits, equipment maintenance and military-security practice. Deliberate shutdowns can also serve sanctions evasion or illegal fishing concealment. A gap alone does not distinguish motive - analysts test whether movement during the gap was physically feasible and look for independent corroboration.
Can AIS be faked or spoofed?
Yes. AIS is unauthenticated and unencrypted: fields can be edited from onboard equipment, and shore-based spoofing campaigns have broadcast phantom positions for real ships. Cross-checking IMO numbers, comparing multiple reception sources and looking for kinematically impossible jumps are the standard defenses.
Do all tankers have AIS?
Virtually all commercial ocean-going tankers carry Class A AIS under international treaty (SOLAS). Some smaller inland and coastal craft carry only Class B or none at all, which is why small-bunker movements can be invisible even close to shore.