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Thinking Like Your GPS System

Even the best manuals can be confusing. Here are some tips for getting the most out of your GPS.

October 4, 2007

Today’s electronic navigation systems let you accomplish just about any navigation task in three or four ways. And therein lies the problem. Some of these systems offer so many options we can easily lose track of the fundamental task at hand: getting from point A to point B safely and relatively gracefully.

It helps to think of an electronic GPS or chart plotter as an assembly of four separate units: position finder, navigation computer, waypoint library and control display unit.

The position finder is the heart of the electronic navigation system. The important thing to understand is that the position-finder portion of GPS does nothing more than determine the current position of the vessel over and over again-usually at one-second intervals. These fixes are then sent to the navigation computer and the display unit of the GPS or the chart plotter.

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As most of us know, the vessel’s instantaneous (current) position is displayed alphanumerically as latitude/longitude. If the system has plotting or charting capability, the current position will be displayed with an icon on the plotter/chart view. Most systems offer a “track mode that will display subsequent fixes. (Some users like to think of this feature as the “bread crumb mode.) You can turn it on and off and select the sampling rate.

To this point, our system is simply establishing positions sequentially and displaying them. Now we can really put the navigation computer to work and get the most out of these sophisticated units.

The first task of the navigation computer is to derive and display course over ground (COG) and speed over ground (SOG). Because the system knows where the vessel was a few seconds ago and where it is now, the navigation computer can approximate SOG and COG. “Approximate, of course, is the operative word. If the vessel is running at a constant speed and along a straight path, SOG and COG will be quite accurate. Alternatively, if the vessel is maneuvering and changing speed, the accuracy of SOG and COG calculations will deteriorate.

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Your nav-system setup page will enable you to choose the COG display in true or magnetic degrees. For most navigators, true is the practical choice. The system will ask you to input the local variation or select an automatic mode that will provide variation based on your position. If you have a gyroscopic compass or flux gate on board, you will want to keep the displays in degrees true, to match your chart plots.

Though it seems obvious, it is important to understand that SOG is not necessarily speed through the water. SOG is a resultant of speed through the water, current and leeway. In a simple example, if your vessel is making 10 knots into the face of a 3-knot current, your SOG will be 7 knots. Similarly, COG is not necessarily the same as true or magnetic heading. Consider the situation in which you wish to track due west across a swiftly moving north-to-south current. While the COG will be 270T, your boat’s heading will be greater than 270T, because you must crab into the current to make good the desired course.

A sophisticated navigation computer can take speed through the water from a transducer and compare it with ship’s heading (from a compass system), SOG and COG to come up with current set and drift, a task that used to be hashed out with pencil, paper and calculator.

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WAYPOINTS

To this point, we’ve dealt only with the “where we are half of the navigation equation. The “where we are going half-obviously just as vital-involves waypoints.

Most systems allow you to define waypoints in several ways. Typically, you can enter lat/long of the desired waypoint or its range/bearing from your current position. Systems with plotting devices usually let you define a remote waypoint by moving a cursor to the appropriate place and pushing a button to capture and store the position. Most modern GPS systems have huge waypoint-storage capacities-1,000 or more is common.

Once you have defined a waypoint for your system, you can ask it to guide you there. This is one of the most common and easily understood functions of a GPS. As soon as you hit GoTo, the system begins computing a new collection of navigation parameters beyond SOG and COG. You may be able to see (or select) displays of ETE (estimated time en route), ETA (estimated time of arrival), TTG (time to go) and XTE (cross-track error). You’ll also have the opportunity to select new display types: CDI (course-deviation indicator) and BRI (bearing/range indicator). These are all functions that will help improve the system beyond getting from point A to point B, creating a more complete navigation tool.

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Displays of ETE, ETA and TTG are instantaneous-that is, they are accurate if course and speed have been stable for a few moments and you continue to maintain your velocity. If you change speed, the system will recompute those parameters.

XTE is a measure of how far your vessel has moved left or right of the course that was established when you pushed GoTo. The lazy navigator’s method to remedy XTE is to push GoTo again. This generates a new track from the boat’s present position to the selected waypoint, eliminating the error. It is important to keep track of this information, ensuring you don’t wander over a navigational hazard.

The correct way to deal with XTE is to get back on the original course, then add a heading correction to keep your vessel on track. Your correction angle will be proportional to your rate of drift off course. Here’s a method some pros use to correct cross-track error:

1. The first step is to keep XTE from increasing. Make a 5- or 10-degree turn toward the desired track and hold the new heading while you monitor XTE. If XTE continues to grow, make another correction toward the original track, this time about half the magnitude of the first correction.

2. Once you have momentarily stabilized the XTE, your steering compass will indicate the corrected heading necessary to maintain the desired track when you get back on it. Now, turn toward the track, and once you have returned to it, turn back to the corrected heading.

This technique sounds a lot more complicated than it is in practice. Soon, you’ll be doing it automatically. In short, stop the drift, return to course and resume with the corrected heading. Use the CDI to visualize the rate of departure or closure to or from the desired track. Your autopilot can also be set up to interface with the GPS, monitoring the XTE. As with any piece of modern navigation equipment, however, it is important to understand the fundamentals. That way, you’ll know why your boat is changing course.

Finally, one waypoint you should know how to generate blindfolded is the MOB (man overboard) waypoint. Typically, the MOB waypoint is created when the user pushes MOB and holds it for a number of seconds, or pushes it a set number of times. The MOB waypoint automatically becomes the active waypoint; the displays direct you back to the place where you activated it, presumably where you lost somebody or something overboard.

ROUTES

One of the most beneficial elements of the modern GPS is the “route function. You can hook up a string of waypoints into a named route and tell the system to execute (i.e. follow) the route. In this situation, the system guides you to each waypoint in the route and automatically transitions to the next waypoint as the voyage continues. When running a route, your nav system will compute, at your request, the familiar nav parameters-SOG, COG, ETA, ETE and TTG-to the next waypoint or to the final destination. Make sure you know which waypoint (active or final) the system is using, and be sure you are still monitoring your course and changes in waypoint. It is often handy to have a waypoint list and check off each as you progress.

At any time, you can tell the system to skip a waypoint and proceed directly to another waypoint down route. You can also tell the nav computer to reverse the route and guide you back to the origin. Look at your owner’s manual for specifics.

Each nav system stores a finite number of routes, each with a finite number of waypoints. The specification will usually read something like “a maximum of 50 routes with 20 waypoints each.

USING YOUR SYSTEM

All electronic navigation systems do the things we’ve discussed. Where they differ is in the nuances of control and display. The best way to get to know your GPS and chart plotter is to use them at every opportunity. Have them up and running even when you’re cruising locally. Compare what you see on the displays with what you see and sense in the real world. Explore the menu pages and try different display configurations. Customize your display units to work with your charts.

The more time you spend with your GPS and chart plotter, the better they will serve you.

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