Mandatory worldwide ship-position-reporting system at 6-hour intervals via Inmarsat-C (or Iridium), separate from AIS.
Regulatory detail & full definition
Required by SOLAS Ch. V Reg. 19-1 for passenger ships, cargo ships >= 300 GT on international voyages, and offshore drilling units. Position is reported to the flag state's LRIT data centre, which may share with port states (within 1,000 NM of coast) and the IMSO (International Mobile Satellite Organization). Distinct from AIS — LRIT is encrypted and not publicly viewable.