Global Positioning System

The Global Positioning System (GPS) operated by the United States Department of Defense is based upon 24 Navstar satellites which each circle the Earth twice per day. Their orbits are calculated so that no matter where you are located, there will always be seven or eight satellites overhead. These satellites constantly broadcast their position and the time which ground based receivers use to automatically calculate their own position. GPS satellite signals are relatively weak and can be obstructed by terrain and buildings, so GPS receivers need a clear view of the sky. A handheld portable GPS receiver can be a useful navigational tool for any outdoor activity and can be especially useful for the Militia member participating in disaster relief or search and rescue operations. However, you should remember that the system relies upon the U.S. government for continued operation and the satellite signals can be degraded or shut off at any time. Having a GPS receiver doesn't mean you can throw away your compass. A GPS unit can tell you which way to go, but unless you are moving along a particular track, it won't tell you which way is which. Also, while your compass will continue to work without batteries, a GPS receiver is dependent upon them. Alkaline batteries provide the longest operational times (typically from 6-24 hours of continuous operation). Rayovac Renewal reusable alkaline batteries are very cheap to use so long as you have access to 110 volt AC power for the special charger they require. Nickel cadmium rechargeables (which weigh less than alkaline batteries) and a solar powered charger can keep your GPS receiver operating in the field.

Initialization and Finding a Position. New GPS receivers must be initialized to acquire almanac data telling them which satellites to look for. This procedure takes anywhere from 3 to 45 minutes depending upon the particular model and satellite locations. Although some GPS receivers have a built-in backup battery to retain the unit's memory, units that have had their batteries totally discharged or removed for an extended period or receivers that have been moved over 500 miles with the power off must be re-initialized. Once almanac data has been stored a position fix can usually be determined within two minutes of power-up; each satellite has a 30 second data transmission that must be collected before it can be used for navigation. GPS receivers tracking three satellites can provide a two-dimensional position fix and altitude can also be determined with input from a fourth satellite. Once a first position has been found, updates are much quicker and are typically calculated every second. Some units let you specify less frequent updates to conserve battery strength. For example, in battery saver mode the Garmin GPS 38 automatically reduces the update rate when you travel a steady course without constant speed or heading variations.

Accuracy. GPS satellites provide two signals; Standard Positioning Service (SPS) capable of accuracy of 15-25 meters for civilian use and an encrypted Precise Positioning Service (PPS) capable of accuracy of 10-15 meters for military use. Civilian GPS receivers are subject to the U.S. DOD imposed Selective Availability program, which inserts random errors into the broadcast signals and degrades accuracy to 100 meters. Differential GPS (DGPS) is a system which uses ground based radio beacons to supplement the satellite signals and eliminate Selective Availability errors, providing accuracy within 5-10 meters. DGPS requires a separate radio beacon receiver and is not much use for land navigation. DGPS in the United States is operated by the U.S. Coast Guard as a maritime navigational aid and is only available along coastal waterways, the Great Lakes and the Mississippi river. Altitude information provided by civilian GPS receivers is not noted for its accuracy, with errors of up to 500 feet being typical.

Using a GPS Receiver. While all GPS receivers have their own unique method of operation and special features, they perform the same two basic functions; locating your present position and providing navigational guidance to a destination. Most GPS receivers can provide position information in latitude and longitude (degrees, minutes and seconds) or Universal Transverse Mercator (UTM) grid coordinates. While various GPS receivers support a wide variety of other grid coordinate systems (such as the Military Grid Reference System, OSGB, British, German, Irish, Maidenhead, Swedish, Swiss and Taiwan grid formats), UTM grid coordinates are generally the most useful for the civilian backpacker or Militia member since they can be used for finding a position on U.S. Geological Survey topographic maps. Some models like the Trimble Navigation ScoutMaster can tell you how many inches over and up from a corner of your map you will find a position. Depending on the model, from 100 to 250 landmarks or waypoints can be recorded in internal memory either by manually entering map coordinates or by pressing a button to mark your present position. A few GPS units can provide directional information in mils (6400 to a circle), but all can display an azimuth (direction or bearing) in degrees. Some GPS receivers can automatically correct azimuths for magnetic declination, which makes them easy to use with even a simple compass. Most GPS receivers allow you to specify a desired course over a route with many intermediate waypoints. The number of routes and the number of legs per route varies with each different model. Most GPS units can tell you the azimuth to any distant position and the distance, which can be expressed in miles, kilometers or nautical miles. Most GPS receivers can also tell you how far off a desired route you are and in which direction you should head to get back on course. When following a route, GPS receivers typically can tell you the estimated time of arrival, estimated time enroute for each leg, the distance traveled, the distance to your destination and the distance and azimuth to any waypoint from your current position. Some GPS receivers have extra features like sunrise and sunset times, moon phases or a trip odometer. Many GPS receivers can display a map plot of your desired route and the actual track you are following. The various Scout models by Trimble Navigation, while being among the most expensive, have small screens and no map display.

Selecting a GPS Receiver. You can find civilian handheld GPS receivers that sell anywhere from $190 to about $1,400. Various models from different manufacturers offer a variety of special features which you may or may not need. The Magellan Trailblazer and Patriot models ($460-$699) feature a detachable Quadrifilar Helix antenna that can be used with an optional external cable for vehicle mounting. This type of antenna has a theoretical advantage of better satellite reception than the built-in flat antennas used with most other receivers, but a field test reported in the July 1995 issue of "American Survival Guide" could not determine any practical advantage. Magellan GPS receivers can track up to twelve satellites, while most others can track only eight, but this also has little practical advantage; seldom will there ever be more than eight satellites flying over your position. Some Magellan and Trimble Navigation GPS receivers can provide Military Grid Reference System map coordinates, which would be useful for an active duty service member, reservist or member of the National Guard. The Trimble Navigation Scout models ($700-$1395) have support for seven languages (English, Spanish, French, German, Italian, Japanese and Norwegian). This might be useful for coordinating operations with coalition peacekeeping forces or United Nations occupation troops, but (with the possible exception of Spanish) extra language support offers little appeal or advantage to Militia members in Washington State. While new models are constantly being introduced and prices continue to fall, as of the Summer of 1996, the absolute best buy has to be the Garmin GPS 38 (about $200 at Wal-Mart). It has the capability of storing 250 waypoints and 20 reversible routes with 30 legs each and also features a map plot screen and an automatic track log of up to 768 points with a backtrack function that works without manually recording waypoints. The GPS 38 compares very favorably with and even exceeds the capabilities of some GPS receivers selling for two to five times as much. Compared with most other GPS receivers, the GPS 38 is smaller (6.15"x2"x1.23"), weighs less (9.5 ounces with batteries) and has a longer battery life (12-20 hours with 4 AA alkaline batteries). Accessories for the GPS 38 are also generally less expensive than those from other manufacturers. For example a PC kit with data interface cable and software, which allows you to quickly and easily enter routes, waypoint coordinates and waypoint names with a home or laptop computer costs about $100. An equivalent PC kit for the Trimble Navigation Scout costs about $400. While the GPS 38 can't use an external antenna for mounting the receiver inside a vehicle, the Garmin GPS 40 (about $270 at Wal-Mart) can. The more expensive GPS 40 also comes with a carrying case and an instructional videotape, but has older software with less options and features than the cheaper GPS 38.

The June 1992 issue of "American Survival Guide" reported on the Sony IPS-360, one of the first consumer oriented handheld GPS receivers. The article concluded with the statement, "With a retail price of $1,500 not many of us will have the extra change to afford a GPS receiver, but it's amazing what technology can offer." Now that the price of a full featured GPS receiver like the Garmin GPS 38 is around $200, you have to ask yourself if you can afford not to add one to your land navigation tool chest.