GPS Navigation and Its Use on Board Ships

GPS- the Global Positioning System, has revolutionised position fixing at sea. Here is a simple explanation on how the system works and why it is close to indispensible to mariners today.

• About GPS & Marine Navigation

  You would have to search hard to find a ship that does not carry a GPS receiver – sometimes more than one- today. When I first started sailing in the seventies, navigation in the open sea was done in the old-fashioned way; with laborious calculations based on Sextant measurement of the altitudes of planets and stars. The accuracy of these in terms of the ship’s position (Latitude, Longitude) was measured in miles. Then came the SatNav, a satellite navigating system that was soon superseded by the vastly superior GPS Navigation system that you see today. It’s accuracy is measured in metres.

• Development of GPS

  First developed for the US Department of Defense almost forty years ago, today this global navigational satellite system uses between 24 and 32 satellites orbiting 11000 nautical miles in space, normally making two orbits around the earth every day. Each satellites continually transmits, through radio waves, precise data on its identification, position and time- essentially saying, I am satellite A, my position right now is B and the time now is C. The satellite also transmits some almanac data on all the satellites in the system, plus a signal which lets the GPS receiver on board set (or correct) its time.

• Use on Board Ships

  On board, the GPS receiver stores the almanac data for continuous use. It also calculates, (based on the A, B and C data above) exactly how far the satellite is from the ship at any given instant. Three such satellites and the calculations give us an exact 2D fix on board- Latitude and Longitude. If there are four satellites used, one can calculate altitude as well. By keeping a record of the ship’s positions, another simpler calculation determines the ship’s speed, and the course it has ‘made good’ in the time between positions.
  
  

  
  

  
  
  What is even remarkable about the GPS system is its accuracy; normally 10-15 meters for civilian systems, and a huge improvement on the older sextant centered methods. Another advantage of the system is that it is an ‘all weather’ one. Even when one cannot see a landmark 50 metres away in dense fog, the GPS will give you your position without any decrease in accuracy.
  
  
  Installation is fairly simple. Fix the receiver to any convenient place on the navigating bridge, run the cable and fix the antenna to a suitable point outside, power the GPS receiver, feed in some ship-specific data (e.g., height of antenna above sea level) and that is about it. Some minor corrections may be necessary to plot GPS positions on a navigational chart at sea, though. This is because there is a difference in the datum and the projection of the chart being used. This correction is so small that it is often ignored.
  
  
  GPS information at sea is often replicated on other navigational equipment on the bridge like radars, electronic navigational systems and communication systems resulting in convenient, easy and seamless navigation- essential in these days of minimum manning on ships.
  
  
  Don’t throw away that sextant, though. If for nothing else, you still need it to pass any navigational exam.
  
  By Manu