The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by
the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions,anywhere in the world, 24 hours a day. There in no subscription fee or setup charge to use GPS.

How it works

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and
track movement. With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude).

Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track,trip distance, distance to destination, sunrise and sunset time and more.

How accurate is GPS?

Today’s GPS receivers are extremely accurate, thanks to their parallel multi-channel design. GARMIN’s 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks,even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy
of GPS receivers. GARMIN GPS receivers are accurate to within 15 meters on average.

Newer GARMIN GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters.The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

The GPS satellite system

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there’s no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for

  • The first GPS satellite was launched in 1978.
  • A full constellation of 24 satellites was achieved in 1994.
  • Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
  • A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
  • Transmitter power is only 50 watts or less.

What’s the signal?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF
band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid
objects such as buildings and mountains.

A GPS signal contains three different bits of information ? a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your GARMIN GPS unit’s
satellite page, as it identifies which satellites it’s receiving.

Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

  • Ionosphere and troposphere delays? The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
  • Signal multipath? This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
  • Receiver clock errors ? A receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
  • Orbital errors ? Also known as ephemeris errors, these are inaccuracies of the satellite’s reported location.
  • Number of satellites visible ? The more satellites a GPS receiver can “see,” the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
  • Satellite geometry/shading ? This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites arelocated in a line or in a tight grouping.
  • Intentional degradation of the satellite signal ? Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals.
    The U.S. government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Mapping: Where am I

Have you ever looked at a map and wished you could pinpoint your exact location? Are youor is someone you know “directionally challenged”? Ever find a great hunting or fishing spot and want to get back to it easily? A GPS unit may be just what you need to know where you are and where you are going. GARMIN units are available with different types of map data. Models vary from having no map, to a basemap,to a highly detailed map./p>

  • Nonmapping Units
    GPS units with no map detail have a plotter screen that can show an overhead view of your location to any waypoints, routes, or track logs (see Glossary) you have created. The plotter screen will aid in determining your position in relation to these items. Most GARMIN GPS receivers will have the ability to show this basic information. Some models have an additional city point database that displays city locations.
  • Basemap Units
    A GARMIN unit with a basemap will typically show interstates, US and state highways, major thoroughfares in metro areas, lakes, rivers, railroads, coastlines, cities, airport locations, and exit information for the federal interstate highway system.
  • Mapping Units
    By stepping up to a unit with either built-in detailed map data, or the ability to download detailed map data from CDROMs or DVDs, on-screen information really takes a lead forward. Map data may include business and residential streets, restaurants, banks, gas stations, tourist attractions, marine navigational data, boat ramps, topographic detail, off-road trails and much, much more. Imagine being able to look up and navigate to any street address contained in a huge database using an electronic map that shows street-level detail! Map data can be incorporated into the unit either by using a data cartridge or by downloading the information directly from a CD or DVD to the GPS unit. Some units utilize GARMIN’s preprogrammed cards for specific areas or regions. Others use a blank cartridge, combined with a PC and a MapSource CD or DVD, allowing you to select an area of detail to program into the data cartridge. Yet other units can have data loaded directly into internal memory without the need for data cartridges, and some recent models have detailed mapping preloaded.


* Reprinted with permission of Garmin International.

Central is an Authorized Garmin Dealer.