By Paul Titus, Contributing writer
Even though the Global Positioning System (GPS) navigation is one of the best man-made navigation systems available, it requires the user to have a basic understanding as to how the system works in order to get the most out of his/her GPS receiver.
Did you ever have your GPS unit indicate a direction or course that you should take in order to reach a "GO TO" or a saved waypoint that you know was not correct? Did the receiver indicate that the desired course should obviously be in a different direction and away from your intended destination? If so, I bet you were stationary for a period of time when the inaccurate heading was displayed.
GPS uses a triangulation method for determining positions. It uses signals originating from geosynchronized satellites in an orbit about 12,000 miles above the earth. The GPS nominal constellation of 24 operating satellites transmits a one-way signal that gives the current GPS satellites positions and time. These satellites orbit the earth in less than 24 hours in very specific orbits. Modern GPS receivers can usually receive up to 12 of these satellite transmissions simultaneously and after receiving four or more of these signals can determine its position to within 15 meters.
Notice that I said position, not direction, that the GPS unit or your boat is pointing. Although some hand-held units have an optional electronic compass that coordinates the GPS unit to a position and a direction that it is facing. Most fixed-mounted consumer GPS units do not have this built in compass option.
In order to determine a direction, the GPS receiver must move. The system allows the receiver to determine its position, and once it is moved, it can determine the new position. A move of 20 or 30 feet should be more than enough for the unit to establish the change of position and give you a correct course. By comparing the stationary position to the second position after moving, the internal software in the unit can indicate a direction of travel and the track. This direction is usually displayed on the "compass" or "map" screen showing the direction of movement relative to the North setting that you choose. The GPS unit will then display the correct course or heading values.
Remember: change your position before setting out on a course to get a correct direction displayed in the GPS unit.
About these directions that are displayed for your courses and headings:
The tracks and courses in a GPS receiver are usually expressed in degrees of a circle, the same as a standard magnetic compass (0-360°).
This degree value is expressed as compared to a North direction that is in either magnetic or true values. You, as the GPS user, can configure your unit to indicate either value. But careful considerations should be given when making these settings.
If you use the True North setting the unit will display all of the track, course and "Go To" directions based on North being the Geographic North Pole or Terrestrial North Pole. It describes a direction to that pole location. If you use the Magnetic North setting, the GPS unit will display all courses, tracks, routes and directions using compass readings that are based on the Magnetic North Pole. The Magnetic North Pole is at this time generally considered to be near Ellesmere Island in northern Canada and is slowly moving westward toward Russia about 35 miles per year. This means that the difference between the two settings is constantly changing as time progresses and as you move on the surface of the earth. It is the direction that is indicated by a magnetic compass as North.
The difference of the angle between the True North and Magnetic North is called Variation on marine charts and Declination on land maps such as U.S.G.S. Topographical maps. These differences can be substantial, depending on your position.
All marine navigation schools teach to plot your courses using True North, and then apply Variation and Deviation (compass interference) factors. This is because the true north values will not change as could the deviation and variance factors. However, with GPS you may want to consider using the Magnetic North option. This is now practicable with GPS receivers as the software in the unit, using the satellite data, can determine the amount of variation that should be applied for any position at any time. This empirical data is stored in the GPS unit and applied to the heading, course, track and route displays in the unit by converting the True North values to the present required variation settings which is based on the position of the unit and time and dates.
This means that the GPS unit displays will indicate the same headings and courses as presented by a properly installed magnetic compass. The user can apply course, "GO TO" headings and that are the same in the GPS and the compass.
I use and recommend the magnetic setting in my seminars, so that the GPS and the compass will display the same degree values. I let the unit do the variation converting math for me. It allows me to use the compass and GPS together, each complimenting the other.
Of course, if I plot a course for archiving or for a customer, I plot the values in the time proven method of using True North values.
- Captain Paul Titus