Introduction
Out of sight of land, with no landmarks or roads, mariners historically navigated by stars, compass, and dead reckoning. Today, GNSS guides ships across oceans and into ports with precision.
The Marine Environment
Unique challenges include the absence of landmarks, salt water exposure, vessel motion, and multipath from calm water surfaces. Unique advantages: usually excellent sky view, no buildings or trees, and stable platforms on large vessels.
Phases of Marine Navigation
| Phase | Accuracy Needed | Technology |
|---|---|---|
| Ocean passage | 100–500 metres | Standard GPS sufficient |
| Coastal navigation | 10–50 metres | DGPS or SBAS recommended |
| Harbour approach | 1–10 metres | DGPS or RTK |
| Docking | <1 metre | RTK, laser, or vision systems |
IMO Requirements
The International Maritime Organization sets carriage requirements through GMDSS (Global Maritime Distress and Safety System) and SOLAS (Safety of Life at Sea). Performance standards require 10-metre accuracy (95%), integrity warnings within 10 seconds, and 99.8% continuity over 3 hours.
Marine DGPS
The IALA Beacon System broadcasts DGPS corrections on 283.5–325 kHz marine beacon frequencies from reference stations at lighthouses and coast guard sites. Ships with beacon receivers apply these corrections for 1–5 metre accuracy up to 200–500 km offshore, free to all mariners worldwide.
ECDIS: Electronic Chart Display
Electronic charts combined with GPS position revolutionized marine navigation. Ships show their position on electronic charts with automated route monitoring and danger warnings. SOLAS vessels are now required to carry ECDIS, though paper charts remain as backup.
AIS: Automatic Identification System
Vessels broadcast their identity, position (from GPS), course, speed, and destination. This is received by other ships for collision avoidance, by vessel traffic services, and tracked on websites like MarineTraffic. AIS is useless without accurate GPS, and spoofed AIS positions from fake GNSS signals are a growing concern.
Special Marine Applications
- Hydrographic surveying: Mapping the seafloor; RTK/PPK for vertical accuracy; tidal corrections essential
- Dredging: Maintaining channel depth; RTK on dredge; real-time cut/fill monitoring
- Offshore construction: Platform installation, pipeline laying; RTK/PPP with heave compensation
- Fishing: Finding grounds, returning to pots/nets, boundary compliance monitoring
Future: e-Navigation
The IMO's e-Navigation initiative aims for integrated systems, better information exchange, and improved safety. This includes high-integrity PNT, resilient multi-system positioning, and cyber-secure design by default.
Vital Points
- GNSS is the primary means of navigation at sea
- Marine DGPS provides free corrections in coastal areas worldwide
- ECDIS + GPS revolutionized marine navigation
- AIS depends on accurate GNSS position
- Special applications (dredging, surveying) use RTK/PPK
- Future: resilient PNT through e-Navigation