GPS guide & receivers

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GPS guide & receivers

People and Boaters are constantly on the move and are always looking for a better, faster, easier way to get to where they are going. A traveler without a destination is a wanderer, but even those with a map can have trouble getting to where they want to be. Especially in areas with few landmarks, travelers need a navigation system, or a means for determining their exact position, course and distance.

New advancements in GPS technology have made traveling easier than ever! Youll never get lost again with step by step directions and path tracking. Handheld GPS receivers now even allow you to find your exact location anytime.

Throughout history we can see incremental improvements in mapping and navigation. With these improvements, people have progressively traveled farther, faster and with greater safety and reliability. During the last century, the pace of improvement in navigation and transportation accelerated exponentially. We even sent a man to the moon and robotic explorers to mars. We have regular missions into space, and satellites orbiting around Earth have become crucial to our everyday reality.

With our ability to send satellites into orbit came a new development in navigation. We can now map places with precision that was unimaginable prior to this advancement. This new navigational ability is due to the Global Positioning System (GPS), a system developed by the United States Department of Defense.

GPS was initially designed to assist ships and planes by providing specific navigational information, but advances in miniaturization and integrated circuits have allowed for GPS receivers to become more widespread. GPS technology now comes in many cars, boats, and even on construction and farm equipment.

Hand-held, Portable GPS receivers are also becoming easily accessible and are making many industries safer and more efficient.

GPS Signals

All GPS signals are in the L band of the frequency spectrum. Since L-band waves penetrate clouds, rain, fog, storms and vegetation, GPS units can collect accurate data in turbulent weather or beneath a forest canopy. Some situations where GPS units cannot get accurate readings are inside concrete buildings, in some mountainous regions or under especially heavy and wet forest canopies. Precision may also be affected by factors of both the satellite and the receiver. Eliminating all odds of inaccuracies is unfeasible, although corrections can be made after the fact. Despite the chance of inaccuracy, current GPS technology is outstandingly reliable.

The three types of GPS signals are:

-Coarse Acquisition Code
-Ephemeris Data
-Almanac Data.

Ephemeris Data is constantly transmitted by each satellite to indicate the time, date and the practical condition of the satellite.

Coarse Acquisition Code (C/A-Code) , is the most frequently used data received by a GPS unit, identifies the time it takes for the unit to get a reply from a satellite.

Almanac Data is an indication of the precise orbital location of the satellite and is transmitted to the five earth-based tracking stations that constantly monitor the GPS satellite signals.

GPS Works

GPS consists of a network of 24 satellites and their corresponding ground stations.

There are 21 primary satellites and 3 back-ups which orbit the earth in a predictable pattern at an altitude of roughly 10,900 miles. The satellites are used as points of reference to calculate an exact location within a few meters or better. Five tracking stations, located in various places around the Earth's equator, constantly monitor satellite signals.

A GPS receiver determines its location by solving a set of equations calculating the distance between it and three or more satellites. This calculation is an ancient surveying technique known as "triangulation." The travel time of radio signals between the satellites is what determines the location. At least three satellites must be available to get an accurate horizontal location, and at least four satellites are required to get an accurate horizontal and vertical location. The more satellites that are available to use for the calculation, the more precise the location can be identified.


The Wide Area Augmentation System (WAAS) provides the most accurate determination of position. The Federal Aviation Administration's has now made this system the objective standard for all civil aviation.

Common Terms

Waypoint - Synonymous with landmark or coordinate, a waypoint is a location that can be entered and stored in the GPS unit at any time. Waypoints can be a final destination or be placed along the way to organize a route. They mark your position along a route and can be added or deleted at any time throughout a trip. You can begin with a start and end point, entering waypoints along the journey that deviate from the original plan. This gives you the liberty to roam and explore other interesting places without getting lost. Some GPS units allow you to give your waypoints names or symbols.

Route a recorded path that illustrates how to get from one waypoint to the next. It is formed by connecting the waypoints that are entered into the GPS unit. The waypoints split the route into "legs."

Track back, the way in which the GPS unit records and stores your path or journey.

WAAS (Wide Area Augmentation System) the technology that allows us to pinpoint a specific location within three meters or less. If you are looking for a high-level of precision, make sure that your GPS unit has WAAS.

Types of GPS Receiver Units

GPS units are basically compasses with multidimensional accuracy. They are programmed with both magnetic north and true north settings; the user has a choice of one or the other. When using a GPS unit in conjunction with a compass, the magnetic north setting should be used.

The three types of GPS units that are typically used the most are: non-mapping, base-mapping and mapping.

Non-mapping GPS units are the simplest. They usually have no map detail. The plotter screen shows your path by tracking waypoints, routes or track logs. They may also display latitude and longitude and time of day, as a compass would.

Base-mapping GPS units are similar to non-mapping units in their plotting, tracking and route features, but they also display a map to help the user visually verify their location. If a map is not accessible but essential to find the final location, base-mapping units are helpful sources. GPS Base-mapping units usually show state, interstate and US coastlines, highways, major thoroughfares, airports, rivers, lakes and exit locations. They can calculate your current location, speed and direction of travel.

Mapping GPS units have the ability to record the information entered and transmit it to a computer through an RS-232 cable. Mapping units can be personalized, depending on the amount of memory space, by uploading area maps or road maps from another digital source. Mapping units can be used for real-time applications. The amount of memory available may vary between mapping GPS units, so it is important to choose software carefully.

Getting Started With GPS

GPS technology has developed far beyond the simple navigational assistance it was originally intended to provide, and improvements keep coming. New uses for GPS in work or recreation are just waiting for necessity or someone's imagination to discover them.

Before it can work, every GPS unit needs input data. The first piece of information that it needs to be aware of is its own location. When it is initially turned on, the unit will obtain a satellite signal to determine its starting point. After identifying the beginning location, you can track your waypoints as you travel. Tracking waypoints can build a guide for future travel along the same route. Some GPS units can record up to 700 waypoints.

Official U.S. Government web site about the Global Positioning System Navigation Charts, Selection, Plotting and Stowage

What features...

Each boater must determine what features are important for their particular needs prior to purchasing a unit. For instance, offshore cruisers may be more interested in the COB function while a boater who always uses the Intracoastal Waterway may want a moving map feature. Both may want a graphic course indicator that shows cross-track error and course-to-steer to assist them in maintaining the center of their route.

GPS - Accuracy


Selective Availability (SA) is the ability of the Department of Defense to degrade the accuracy of civilian use GPS to a maximum of 328 feet, reserving the most accuracy for our military and deny hostile forces that same accuracy. SA was lifted in May of 2000, increasing accuracy to at least 50 feet.


Differential GPS (DGPS) was created because of the need for more accurate GPS positioning in response to SA. The USCG used existing low frequency radio beacon stations to transmit error correction messages. DGPS operates on lower frequencies (200-500 kHz) than GPS, requiring a different antenna and receiver. This system is limited to the range of the transmissions which is 50 to 100 miles from any US coastal radio beacon. To take advantage of this additional accuracy all you need is a differential-ready GPS receiver and antenna.


Wide Area Augmentation System (WAAS) is an aviation sister to the marine DGPS. It is a newer technology and incorporated by Raytheon into their GPS receivers. WAAS uses ground reference stations (currently located only in North America) similar to DGPS, but Inmarsat geo-stationary satellites to relay the corrections to the GPS receiver instead of differential beacons. Since GPS is satellite based, WAAS eliminates the need for a separate receiver for the differential beacons of the DGPS.


Alarms can alert you to being off course, passing waypoints or when at anchor, in the event that the boat moves more that a preset distance from where you anchored.

Displays are either graphic LCD or alphanumeric, color or monochrome. Resolution, contrast, screen size, and character size all play a part in screen visibility. A good screen, especially color, should be viewable in many lighting conditions.

The Raymarine Company , also award winning CHIRP DownVision Electronic Technology gives boat owners underwater vision (designed in 50-channel GPS sensor)so close to reality, that they call it "Visionality".

Backlit Display enhances the viewing ability of the LCD screen in most light conditions.
Data Sharing with an autopilot, chart plotter, radar and other navigational instruments allows you the freedom to keep one hand for the boat and one for yourself.

Memory is available in some units in the form of a lithium battery in addition to the operational batteries. When the operational batteries die or the unit is not receiving enough voltage from a house bank to operate, this battery keeps the current waypoints and data in memory.
The greater the number of receiver channels, the greater the number of satellites the GPS will search for and the greater the number of satellites the unit can “lock” into for greater positioning. Most units offer 12 channel receivers.

The Spec Functions

These are not included in all GPS models:

Course to Steer indicates what course you need to steer to your next waypoint.


Cross Track Error (XTE) gives the distance and direction to the rhumb line between two waypoints—an off-course measurement.
Route storage allows the user to create a path using two or more waypoints. Some GPSs are capable of only one route while others allow many routes to be created. The number of waypoints in a route varies widely by model. The ability to reverse the route may be available.


Speed Over the Ground (SOG) gives the actual speed on the surface of the Earth.
Turn or Steer indicates the number of degrees to turn, port or starboard, to your destination.


Velocity Made Good (VMG) is the rate of speed at which you're closing in on your destination. This function is used in conjunction with any GPS that features time functions such as ETA .

Waypoint storage enables the user to plug in and store many locations or waypoints and some receivers will allow you to name the waypoint or location using 4 to 16 alphanumeric characters. The number of waypoints available varies widely among models. Some units have the capability of downloading waypoints from a computer.

Weather&water resistance does not mean the same thing as water proof. GPS units are not waterproof, unless the manufacturer specifically states that they are, and if the GPS is subjected to more than spray, rain or wet hands, should be kept below decks on in a waterproof bag.

To Buy a gps

Typically, for small open boats, the advantage to a handheld unit is the ability to remove it or securely stow it in a locker when you leave the boat. Determine what gps-functions are important to you for your type of boating.

Ease of use is the next important consideration in purchasing a GPS:

Is the keyboard easy to use?
Are the function keys easy to use and understand?
Is the LCD screen large and clear enough to read in the conditions in which you'll be using the unit?

If you're using displaying charts or graphics, would a color screen be better suited to see the overlays more clearly? If you operate an open boat, an internal antenna is sufficient, but boats with enclosed steering stations may require an external antenna for better satellite reception.