There are also GPS systems that are handheld. These can be used not only when driving, but also when hiking, biking, hunting and fishing, boating and during any other outdoor activity. Some of the more familiar names in global positioning system equipment include Garmin and Datus. GPS is a wildly handy tool when you need to locate a specific position, and/or to help you stay on track to avoid getting lost.

Many GPS products, used by both businesses and government, have been integrated into mobile radios, cell phones and mobile data terminals. Tracking vehicle locations and meeting the needs of vehicle fleet managers, GPS is finding more and more uses.

What is GPS?

The Global Positioning System (GPS) is a collection of satellites owned by the U.S. Government that provides highly accurate, worldwide positioning and navigation information, 24 hours a day. The system consists of 24 Navistar GPS satellites that orbit 12,000 miles above the earth. They are constantly transmitting the precise time and position in space. GPS receivers on or near the earth's surface listen to the information that is received from three to 12 satellites, and from that, determines the location of the receiver, as well as how fast an din what direction it's moving.

GPS uses the triangulation of signals from the satellites to determine locations on earth. GPS satellites know their location in space and the receivers determine their distance from a satellite by using the travel time of a radio message coming from the satellite to the receiver. After its relative position to three or four satellites is calculated, a GPS receiver can calculate its position using triangulation. There are four highly accurate atomic clocks on board GPS satellites and they have a database of the current and expected positions for all satellites. This is updated frequently from earth, so that when a GPS receiver locates a satellite, it can download all satellite location information. This way it can find the remaining needed satellites more quickly.

The GPS system was initially set up for use by the military to provide precise position information. The military uses GPS products to coordinate and track the movement of soldiers and equipment in the field, to guide military ships, and to provide position and navigation information to military aircraft. The accuracy of these systems has improved because Selective Availability (a program implemented by the U.S. Department of Defense to make GPS less accurate for non-military users) has been removed. Therefore, the accuracy of the systems today may be within 10-15 feet.

There are, of course, errors that can occur, even with highly accurate atomic clocks. The most significant errors are due to variations in the earth's ionosphere, which effects the speed of GPS radio signals. Another source of error is from water vapor in the troposhere (portion of the atmosphere below the stratosphere).

Consumer Use

In the last several years, an increasing number of affordable GPS receivers have been manufactured for consumer use. As the technology has improved, additional features have been added, while the price and size have decreased. What was "state-of-the-art" GPS in 1996, is now among the least expensive units. Today's "latest-and-greatest" systems have features that were only dreamed about 4-5 years ago.

What is available today are handheld GPS receivers that vary in price from $100 to $600. Almost all GPS receivers available to the average consumer are 12 parallel channel and data capable. Most can be connected to a laptop computer and used with street-level mapping software for "real-time" automobile navigation. Different kinds of available GPS receivers have background moving highway maps for auto navigation; handheld and mounted GPS receivers for boating and fishing; aviation GPS receivers with built-in Jeppensen airport information, and GPS receivers that combine with radios or e-mail.

What to Look For When Purchasing GPS for Automobile Navigation

As with most tools, GPS receivers come with lots of features. Which ones are important to you depend on what and how you will use the system. The features needed for automobile navigation, however, are a little different from those used when hiking. According to some reviewers of GPS, the following are essential features for automotive navigation:

Some of the features that aren't essential but nice to have include address to address routing, and an external antenna. Address-to-address routing isn't available in low-end units (i.e., Emap/StreetPilot ColorMap), but a "Find Address" feature is a substitute. For top-of-the-line GPS units, you can expect voice instructions that instruct you on distance, street names and directional turning, and fast destination address entry. These units are most often $1000-$3000 automobile navigation units built into the vehicle.

Among the lower-cost units available are Garmin G-III Plus, G-12 Map, StreetPilot, ColorMap SP, Lowrance GM1200 and Magellan 410. Introduced in Spring of 2000, Garmin's eMap is Garmin's entry into the low cost mapping GPS receiver. Garmin's other entries in the mapping GPS receiver market have been GPS-175/G-III+/G-12Map.

eMap is a new kind of GPS receiver system with a high definition residential street level mapping GPS receiver. It's simple to use, provides excellent maps and has a highly readable map screen. However, if you want a mapping GPS with features for hiking, biking, boating or flying use, you'll need to consider other models. emap comes with a built-in BASE MAP covering 100 percent of the Federal highways and all major state highways. There is an optional 8 meg or 16 meg cartridge memory that permits the uploading of additional maps from Garmin's MapSource and MetroGuide CDROM map product line.

eMap is a pocket size unit with simple controls and features; fewer than in ordinary GPS receivers designed for hiking, boating or aviation. It is designed to be an electronic map with a mid-sized high resolution viewing screen. Replacing the traditional paper map, eMap can load the MapSource MetroGuide, WorldMap, USA Topo and Roads and Recreation CD maps into its 8 or 16 megabyte optional memory. One of the features missing in eMap is address-to-address routing. However, it is possible to find a specific address if MetroGuide maps are loaded. eMap will point it out on the map. Although somewhat limited, eMap provides a high resolution display which is easy to view. At this time, only Garmin map collections can be used. Keep in mind eMap is a new generation of simplified GPS receiver and is designed to be an electronic map, as simple to use as a paper map.

Datus also manufactures a Personal Navigation Assistant (PNA) that is quite good, the Datus Routefinder. Also a handheld GPS navigator, it is more like the Magellan PathMaster or the Alpine automobile navigation system than it is a standard GPS receiver.Designed specifically for automobile navigation, the Datus Routefinder does have address-to-address route calculation, as well as turn-by-turn voice instructions and a built-in GPS receiver. PNA uses an external amplified antenna module rather than a built-in antenna.

The Datus Routefinder serves as an excellent co-pilot to any driver. You simply enter a destination city and street address, enter "GO" and in less than a minute, the Datus PNA will begin reciting its voice guidance instructions.

For more information on handheld GPS receivers, refer to the manufacturers' web sites.