Use & limitations of GPS/Chart plotters

KNOWLEDGE OF: 

  • Use & limitations of GPS / Chart Plotters

Make candidates aware of the pros and cons and differences between GPS and Chart Plotters. 

Items to cover: 

  1. What is a waypoint? (Lat / Long)
  2. Mark two waypoints on a chart
  3. Join the two waypoints, measure the bearing (T) and distance
  4. Briefly explain where a GPS / Chart Plotter gets its position information from (satellites) 
  5. Turn on your GPS / Chart Plotter & show your students the basics of what they see on the screen/s i.e. Lat / Long, Speed, Track, Heading
  6. Input the two waypoints you have taken from your chart (check for clarity) 
  7. Show your students how to set and start a route between the two waypoints
  8. Wrap up the session by discussing the limitations of GPS / Chart Plotters

Limitations of GPS / Chart Plotters:

Human error - inputting the wrong position

No accounting for hazards, unlike satnav, a GPS/Chart plotter does not know where hazards are located. 

Technology error - satellite geometry, aerial damage or positioning

Using a fixed object (buoy) as a waypoint - you may well hit it!

Pre-set Variation & Deviation

True or Magnetic bearings?

Datum offset i.e. OS36 or WGS84?


Additional useful info for you (the instructor) background knowledge base:

  • GPS = Global Positioning System
  • ECS = Electronic Chart System
  • GNSS (Global Navigation Satellite System)
  • ATON = Aids TO Navigation
  • SOG = Speed Over Ground
  • COG = Course Over Ground
  • BTW = Bearing To Waypoint
  • DTW = Distance To Waypoint
  • DR = Dead Reckoning

E.G. 24-satellite GPS constellation in motion with the Earth rotating

By Paulsava - Own work, CC BY-SA 4.0, Link

COG Vector: 

  • Use SOG to project motion of the vessel over the earths surface.
  • Both COG and SOG are derived from a GNSS receiver & show the vessel movement after any effect of current and/or wind. 

Heading Vector: 

  • Shows the bearing of the vessels head / bow relative to North. 
  • Input comes from a heading sensor fitted to the vessel. 
  • Length of heading vector is derived from vessel speed through the water (using input from water speed log)
  • Only takes into account heading & speed, does not consider leeway or current.