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Selecting a GPS Receiver

By Tom Burden

What they do

The Global Positioning System is a satellite-based navigation system that provides accurate position fixes 24 hours a day, on land, sea and air, in any weather, with no subscription or fee. Originally built between 1978 and 1994 for the U.S. Department of Defense with 24 satellites in orbit 12,000 miles above the Earth (three are spares in case one fails), this multi-billion dollar system is available for all users.

GPS units receive high frequency, low power signals (50 watts or less) from these satellites orbiting the Earth every 12 hours. By timing the signals sent by the satellites (which each carry a highly accurate atomic clock) and by knowing the exact orbital locations of the satellites, a GPS receiver can determine your location to an accuracy within 6 meters 95 percent of the time, and your altitude to within approximately 10 meters. Navigators using paper charts and traditional tools can use these position fixes to plot courses. But GPS receivers can perform these operations automatically, and most will show your location on an incredibly detailed bright electronic color map. They can aid in creating routes, identify interesting landmarks and points of interest, and some can even talk to you, providing simple, clear directions to your destination.

How accurate is the GPS system?

GPS receivers need to locate three satellites in order to calculate a two-dimensional position, and can develop a three-dimensional fix when they receive signals from four satellites. Since, in theory, four satellites are “in view” from every location on Earth, problems with reception are caused by buildings, mountains and forests blocking the line-of-sight signals. The GPS system transmits signals on several bands (with some reserved for the U.S. Military). The civilian signals were intentionally degraded in accuracy until May 2000, when the program called Selective Availability was discontinued.

Now you can expect accuracy in the range of 6 to 12 meters (about 20 to 40 ft.) from any GPS receiver. The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) have developed the WAAS (Wide Area Augmentation System) program, which corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

WAAS, with about 25 ground reference stations across the United States that monitor GPS satellite data and two master stations on either coast, creates a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, with a fixed position over the equator. Any GPS receiver equipped to receive WAAS (all except the least expensive units) has its accuracy improved to three meters (just less than 10 ft.).

WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications.

Degree of portability: handheld, mountable, or fixed-mount?

For small boat outings, dinghy sailing or hiking, one of the many pocket-sized, handheld GPS units is ideal. In addition to being conveniently portable, these battery-operated receivers provide an additional measure of safety should a larger vessel's electrical system fail.

GPS Recievers

GPS 73 Handheld GPS Navigator is as basic as you can get. No mapping, but you get your position, and it outputs NMEA 0183 data for your autopilot or VHF radio with DSC.

The next step up in size are the mountables (or portables, as they are sometimes called), combining the properties of both handheld and fixed-mount units. These receivers have larger displays and large keypads or touchscreen interfaces for easier use on a bouncing boat or moving car, and can be powered by batteries or external 12V power. They have well designed mounts for dash or nav station installation, and can use either a built-in antenna or connect to an external antenna for better reception. Both mountable and handheld units are often available in Land and Sea versions, like the Montana Marine Value Pack, which include all the mounts for automotive and marine use, as well as BlueChart g2 mapping software boating and City Navigator NT for street-level land navigation.

Larger vessels with a navigation station will use one of the fixed-mount receivers with the larger, high–resolution displays and larger keypads that are the most convenient to use.  Fixed-mount units will connect to the vessel's 12V DC power, eliminating the need to change batteries, and will frequently use an external antenna for a better view of the sky.

Maps in a variety of formats

All GPS receivers other than the most basic handhelds feature electronic maps, so you can see your position on a detailed chart. If you only want to get position fixes and navigate using just traditional paper charts, a receiver like the Garmin eTrex 10 will provide that capability, and will let you plan routes with a collection of waypoints.

Most GPS units are preloaded with a “basemap”, with varying levels of detail depending on the individual receiver. To really achieve the full level of mapping detail, with charts showing information down to the level of a few feet, nearly all receivers use electronic moving maps designed for marine, automotive, off-road or backcountry use that show your position superimposed directly on the chart.

There are several ways this digital map data is delivered:

CD-ROM downloads: some receivers, especially handheld units, require that you connect them to a computer and download sections of a CD, either directly to the GPS or to data card. In some cases buying the CD allows you to download one region, and you need to buy unlock codes for every additional region you choose.

Pre-loaded: a convenient feature provides the data pre-loaded on the receiver, using either flash memory or a hard drive.  One version of the Garmin echoMAP™ 70dv is pre-loaded with LakeVü HD inland lake maps, and another echoMAP™ 70dv model is pre-loaded with U.S. coastal marine charts, for example. Worldwide basemaps that are part of the firmware of many receivers also include a reasonable level of detail, so rough outlines of coastlines are still shown when you move out of an area covered by your electronic cartography.

Cartridges: many charts are sold on small cartridges containing a regional data file that are inserted into ports on the receiver. To travel to new locations you purchase additional chips for each region with C-Map, Navionics, BlueChart g2 or other charts compatible with the receiver. Newer Garmin handheld units use tiny MicroSD Cards, which are about 1/4 the size of a postage stamp. These miniscule cartridges, with up to 32GB of storage, have the same BlueChart g2 marine regions as Garmin’s larger Preprogrammed Data Cards. More information to help select cartographic charts is available in the West Advisor, Selecting Electronic Cartography.

GPS Recievers

741xs Fishfinder/GPS Combo is a chartplotter, a fishfinder, and the hub of an NMEA 2000 network. It uses a touchscreen with pinch-to-zoom, like your smartphone.

Stand-alone, network and combo receivers

Fixed mount receivers are available as stand-alone units known as chartplotters. Many of these have the capability to display underwater information from a “black box” fishfinder module and fishfinder transducer using a split screen or superimposed display format. Manufacturers find new combinations all the time, mixing GPS with other electronic gadgets in an expanding variety of options.

Fishfinder/chartplotter combos (shown in the fishfinder section) give all the performance of both devices with the footprint of only one, so you can place one larger display where two smaller ones would be a tight fit. Switching between functions is easy.

Networked GPS receivers displays allow the connection of a wide variety of sources of information; including radar scanners, fishfinders, video cameras, weather data receivers, inverters and SiriusXM satellite music services. Networks feature plug-and-play high-speed connections, which simplify wiring and installation. Plugging in a new device, like a radar dome, is simple, as the network recognizes the new peripheral and assigns it an IP address. The most elaborate networks use “Network” or “Multi-function Displays”. Multiple steering stations are perhaps the best use of multifunction network navigation displays. Garmin, Raymarine, Simrad and other manufacturers offer excellent systems that allow you to share information between displays; say between the pilothouse and the flybridge.

Several of these can be connected at different locations on a larger boat, and each display is capable of showing any or all of the data independently. Garmin’s GPSMAP 8500 Series, the Raymarine e-Series and the Simrad NSO evo2 all provide the ultimate in networked systems. Using network hubs, Ethernet connections and NMEA 2000 networking, these systems can be customized with just about any desired combination of displays, receivers and sensors. Raymarine, Garmin, Simrad, Lowrance and B & G systems also allow networking of their instruments and autopilots. Our Technical Sales staff at 1-800-BOATING can help you customize a networked system to meet your requirements, order all the cables, mounts and other components at once, and possibly save some money on package deals.

GPS Recievers

The B&G Zeus² 12 Chartplotter includes many sailing-specific functions. It can tell you when you need to tack to lay your weather mark. Multi-touch touchscreen with keypad and rotary knob.

Internal or external antenna

Many receivers are sold in two versions, with either an internal antenna for use in open-cockpit boats (where the receiver has a good view of the sky), or an external antenna (with a cable between 20 and 30ft. long) so the GPS can be mounted below deck. A boater with a center-console or a bass boat, for example, might select a combination chartplotter/fishfinder with an internal antenna, which would save space on a small instrument console. Nowadays, more units are sold with internal antennas, which work fine even below deck on many boats, thanks to the new high-sensitivity GPS receivers.


The U.S. military invested billions of dollars building NAVSTAR (the original official name for the GPS system) for military purposes and created the world’s greatest navigation system for civilian use as an afterthought. Okay, it did cost $14 billion to create the system and launch the satellites, but now we get to use the satellites essentially for free.