Electronic navigation tools

Electronic navigation tools for mariners have become more sophisticated and more powerful as they follow the aircraft model of integrated electronic devices that talk to each other via a system bus. It’s not an exaggeration to say that a thoroughly equipped modern voyaging boat has better navigation electronics than some of the older 727 airliners flying today.

For the ocean voyager, the mainstay of electronic navigation is GPS. It has worldwide coverage and continuous availability, and it has been around long enough so that the price of receivers has become affordable for most voyagers. While there is a variety of important issues to be aware of when using a GPS receiver, the fact is that GPS is a highly effective tool for the ocean voyager.

Standard accuracy for civilian GPS receivers is about 100 meters. Considerable media attention has been given to the higher accuracy possible from differential GPS (DGPS) systems. And while they do provide more accurate positions(down to 10 meters in certain cases), the fact is that for the bulk of the time, 100-meter position accuracy is great for the ocean sailor. When you are 250 miles from Hawaii inbound for Honolulu, for example, knowing your position within a mile is accurate enough, let alone 100 meters. (The GPS signal specifications call for 100-meter accuracy over the whole earth and averaged over 24 hours. At any one point on the earth and at any moment in time, the inaccuracy may actually be more than 300 meters.)

The key to using GPS prudently is to make it yet another arrow in your navigational quiver. Don’t rely on it slavishly; mix GPS positions with celestial lines of position, dead reckoning, radar fixes (if within range of the coast), loran positions, soundings, visual bearings, etc. The prudent navigator is well-rounded and can gather information from a variety of sources. The reason for this is so that you don’t put all your eggs in one basket. GPS satellites have been known to malfunction, and the 12-volt electrical systems of voyaging boats have been known to fail. GPS is a great system, but you have to avoid relying too heavily on it. Why all this stress on prudence? You don’t want your prized voyaging boat ending its life prematurely as a wreck on a reef.

Datum differences

All the GPS satellites may be working perfectly and your electrical system may be fully charged and healthy, yet you may still run into difficulties navigating solely with GPS. The problem stems not from GPS but rather from the mismatch between the lat/long grid used by GPS and the lat/long grid used by a particular chart maker. The model for registering the lat/long grid on a chart to the actual surface of the earth is called a chart datum. Different areas of the world use different chart datums. In order to reduce datum effects to a minimum, you should set your GPS to the datum of the chart you’re using. Most GPS receivers allow you to pick a suitable datum from a list. (For more on datums see “The roots of chart accuracy,” Issue No. 87.)

For most areas of the world, voyagers will have the 100-meter accuracy of the uncorrected civilian GPS signal. And when in U.S. waters, voyagers with DGPS-equipped receivers can take advantage of the 10-meter positioning possible from DGPS corrections broadcast by the Coast Guard’s DGPS radiobeacon system.

And GPS receivers may be among the most widely available of marine electronics equipment. GPS capability is being built into everything from fish-finders to VHF radios. Given the “low dynamics” of the average ocean voyaging boat, which typically operates at six to 10 knots, most GPS receivers on the market will provide excellent position-fixing capability.

Loran, an electronic navigation system that many voyagers may think has disappeared in the age of GPS, is still alive and well in the U.S. and in other areas of the world. There are working loran chains in Europe, Saudi Arabia, Japan, South Korea, and China. In fact, there is even an effort underway in Europe to use loran signals for sending DGPS corrections (see “Loran expected to stay alive,” Issue No. 88). This technique, along with loran’s advantages as a back-up system to GPS, should help loran continue to be used well into the next century.

For those voyagers who would like to use loran, one of the biggest problems may be finding a loran unit. With the ascendancy of GPS, many manufacturers have dropped loran from their product lines.

Another growing area of electronic navigation is electronic charts. There are two approaches to electronic charts. One involves displaying electronic charts on a dedicated unit. These often use cartridges for chart storage and they often have monochromatic display screens (although color screens are also available). These types of units, sometimes called chart plotters, often have a built-in GPS receiver.

A second approach to using electronic charts involves displaying the charts on a laptop or desktop computer, rather than via a single-purpose box. Sometimes, electronic charts are also divided by the type of imaging technique they use: vector charts vs. raster charts. These imaging methods each have their advantages and disadvantages.

Electronic chart differences

Vector charts are easy to update and can be separated into layers that can be turned on and off (e.g., turning off soundings deeper than 50 feet). However, they don’t generally look like paper charts (some newer versions, like C-Map’s NT series, more closely approximate paper charts).

Raster charts are digital pictures of paper charts and so they look exactly like the paper charts that most mariners are familiar with. This is an advantage because there is a tremendous amount of detail on a paper chart. On the downside, updating a raster chart involves putting a “patch” over the changed section of the chart, rather than updating the basic chart. And displaying raster charts requires more processing horsepower than displaying vector charts.

While most dedicated electronic devices can only display vector charts, a computer-based system is fully capable of showing both types of charts, raster or vector. All that’s required is for chart manufacturers to put their charts in a vector format that can be read by a PC.

In the vector chart world, there are two major suppliers of charts: C-Map and Navionics. Both of these electronic charting companies have extensive libraries of worldwide charts, and both of these companies have adapted their charts to be used on a laptop computer.

On the raster side, there are several charting formats. For charts of U.S. waters, NOAA issues raster charts via its private sector partner BSB electronic charts. Another purveyor of raster charts is the Resolution Mapping Company under its Maptech brand. Recently, Resolution Mapping purchased BSB. Both libraries of charts will be sold under the brand name of Maptech.

The British Admiralty also offers raster charts of their exhaustive chart library. Called Admiralty Raster Chart Service (ARCS), these charts are copyrighted by the British Admiralty. They tend to be more expensive than NOAA raster charts. For non-U.S. waters, the National Imaging and Mapping Agency (NIMA) produces paper charts. But it does not produce raster charts for foreign waters as NOAA does for U.S. waters. For international raster charts, Maptech charts are the way to go. Maptech’s catalog includes raster charts for some or all of Europe, New Zealand, Australia, Central America, the Caribbean, Hawaii, and southeast Brazil.

The future of electronic charts seems to be some sort of hybrid display system that involves the use of both raster and vector techniques. These hybrid charts are already in development and they may point the way toward a worldwide electronic chart catalog that uses the best of both raster and vector electronic chart display methods.

By Ocean Navigator