Captain’s Log: Navigating with GPS

Eurodam’s bridge looks a lot closer to ready than it did the first time we saw it.

Today Captain Jeroen van Donselaar explains some of the finer points of nautical navigation:

GPS is short for “global positioning system.” It is a satellite-based navigation system that was commercially introduced in the early 1990s. It is based on the principle of measuring time differences between signals sent from three or more satellites that are “visible” at the location of the receiver. This receiver can be a built-in navigation system in a car, a ship or an airplane. These days there are even hand-held receivers.

Before GPS, other, much less accurate navigation systems were used such as NNSS (Navy Navigation Satellite System, the forerunner of GPS, only gave a “fix” every 90 minutes), Loran C, Omega, Decca and Consol. Other means of “position finding” are RDF (radio direction finding), visual/radar bearings in combination with radar distances.

The major difference between the older systems and GPS is that GPS provides a continuous and very accurate real-time position versus a somewhat accurate position every now and again. GPS works regardless of weather conditions, visibility and at any latitude, and is extremely reliable. I say latitude because most of the other systems were not available in the very northern or southern regions of the globe.

GPS has taken the guesswork out of navigation, so to speak. This is somewhat unappreciated sometimes by our younger deck officers who always have had the luxury of never having any doubt about the ship’s location. Myself and the chief officer are probably the only ones on board who started our careers before the advent of GPS, so we certainly appreciate the difference it has made!

In the early days GPS was just a box that would display latitude and longitude and course and speed over the ground. We would have to take this information and plot it on a paper chart in the same way we did with the other systems, only with much greater accuracy. During the first Gulf War the accuracy of GPS was further increased when the deliberate inaccuracy effect called “dithering” was turned off, supposedly for the benefit of guided missiles. These days I would say GPS is accurate to within 10 feet.

Nowadays our GPS receivers are more advanced and are at the heart of our integrated bridge systems. Although we still use paper charts as a backup, our primary navigation is now done via ECDIS, which means Electronic Chart Display Information System. The STN Atlas integrated bridge system on the Vista- and Signature-class ships takes it a step further and also superimposes a radar image including the AIS, or Automatic Identification System labels of other ships on the same screen.

In my office on the ship, which is located between the bridge and my quarters, I have a large flat-screen monitor that is basically a repeater of the main navigation display on the bridge. In this way while I am working at my desk I can keep an eye on things.