Know Your GPS
What is GPS?
GPS, or Global Positioning System, is a satellite-based navigation system developed by the U.S. Department of Defense to provide a consistent, accurate method of simplifying navigation. It was originally designed for the military, however, it provides both commercial and recreational users 24 hour, worldwide navigation coverage with a possible accuracy to 15 meters (49 feet).
Like any other method of navigation, you should not rely solely on your GPS. You should use every method of navigation available and compare the results to make sure that you are where you think you are.
What are the Advantages of GPS Navigation?
For centuries, mariners have been searching for an accurate method of traveling the world’s waterways. From celestial navigation to lorans and SatNav, each system has had its problems with weather, range and reliability.
GPS takes navigation to a higher level by providing accurate position and course information, anywhere in the world, regardless of the weather or your proximity to land. The accuracy and coverage of GPS navigation can help make your boating safer, smarter and more efficient wherever you may travel.
The GPS is a powerful tool. To better understand its operation and capabilities, it may be helpful to review the basic terms and concepts explained below.
Navigation is the process of traveling from one place to another and knowing where you are in relation to your desired course.
Position is an exact, unique location based on a geographic coordinate system. Marine navigation is based on the latitude/longitude coordinate system.
Meridians of longitude are a set of imaginary circles around the earth that pass through the north and south poles. Longitude describes position in terms of how many degrees it is east or west of the Prime Meridian (0° Longitude which runs through Greenwich England).
Parallels of latitude are another set of imaginary circles that are perpendicular to the earth’s polar axis. Latitude describes position in terms of how many degrees it is north or south of the equator (0° Latitude).
A waypoint marks an exact position fix so it can be recalled for future use. The GPS lets you mark waypoints electronically, without physical landmarks.
Bearing is a compass direction to a particular destination (waypoint) from your present position.
Track is a compass direction representing your course over ground or course made good.
How Does GPS Work?
GPS Navigation uses orbiting satellite signals to determine your position. These satellites continually send out radio signals containing precise position and time information back to earth. By knowing the position of 3, 4 or more of these satellites and calculating various time differences between the transmitted signals, your GPS receiver can determine its present position anywhere on earth. Once underway, your GPS continually updates your position and provides speed and track information.
Various manufacturers make GPS’s and some have more features than others but all give basically the same information. Almost all of them have graphic displays and some even have a cartographic feature which allows you to see your location on an electronic chart. If you are going for a fixed mounted GPS rather than a handheld portable, I would vote for a model that does support electronic charts. This provides yet another way of checking your position, especially if you are in site of land or an object on the chart.
You should expect your GPS to accept waypoints (where you want to go) and routes (a series of waypoints leading to your final destination). Once you have input a waypoint the GPS will calculate your current position and give you, at minimum, the following information:
- A course to steer to the waypoint (continually updated)
- The distance to the waypoint (continually updated)
- Once underway your speed (continually updated)
- The time it will take to get to the waypoint at your current speed (continually updated)
- Turn, Steer or Off-Course Error — the GPS should tell you when you are off course and what direction to turn to get back on course
Various alarms should be available such as:
- An arrival alarm which sounds when approaching a waypoint
- A proximity alarm which sounds when you come within a preset distance of any of several waypoints, regardless of whether they are your destination
- An anchor alarm which sounds when you travel more than a preset distance from a waypoint
an off-course alarm which sounds whenever you are exceeding a preset distance from your intended course