The Global Positioning System (GPS) was developed by and is maintained by the U.S. Air Force. It's primary purpose is to provide accurate locating of friendly and hostile units. The system consists of 24 satellites in orbit around the Earth. Each satellite transmits an ID and time pulse. A ground based receiver picks up the pulse and uses the information to compute its location. Now, one satellite is not enough to provide a position fix, so the receiver listens for other pulses. Two satellites are needed for a fix, three or more provide a better, more accurate position.

It didn't take very long for GPS receivers to show up in the civilian market. The first units were slow, large and very expensive. They could only receive information from one satellite at a time, the information received was stored and the receiver listened for another satellites pulse. That information was also stored and the process repeated itself until the receiver had enough data to compute the position. Anything blocking a clear view of the sky, trees, mountains, inside a building, prevented the signal from reaching the receiver.

Todays units are much better. They are smaller and can receive up to twelve signals at the same time, so obstructions don't affect them as much. They have faster processors so they can obtain a position fix in only a couple of minutes. The price has also dropped dramatically, a small, simple GPS unit costs around $100. If all you need is a unit that will lead you back to your vehicle after a days hike these units are perfect. If you want more, and you have deep pockets, units are available with build-in maps or you can connect them to your computer and download maps from the Internet.

All GPS units give you a choice of which coordinate system, called grids, you want to use. The most popular systems are;

Universal Polar Stereographic (UPS)- A special grid for the North and South Poles.

Thomas Brother's Page and Grid- A grid used on road maps printed by Thomas Brothers.

Don't buy a receiver unless it supports UTM and Latitude/Longitude since these are universal grids. At this point you may be wondering what these other systems are, after all, the only grid you ever heard of growing up in grade school is Latitude/Longitude(L/L). The truth is that Latitude/Longitude works very well on the high seas but not so well for land navigation. L/L was developed to use celestial sighting as the primary means to determine a position. The navigator uses a sextant to take a sighting on the Sun, Moon or a star, and using a chronometer for time, does a mathematical computation to determine where he is. All positions are measured in Degrees, Minutes and Seconds. It is difficult to achieve an accurate fix in forests or mountainous areas where the star you need may not be visable.

A better, more accurate system is UTM. You are probably thinking "Oh no! I've got to learn another system?" RELAX. UTM is really easy. A UTM coordinate reads like this:

The UTM Grid is not used above lattitude 84 deg. N or blow lattitude 80 deg. S. Those areas are under the Universal Polar Stereographic (UPS) grid. The UTM grid divides the Earth into 60 Zones. Each Zone is numbered and is 6 degrees wide. Zone 1 is located at 180 degrees West Longitude. As you move east the Zone number increases, for instance Wyoming is located in Zone 13. That is, we are 13 zones east of the starting position. Zones have vertical and horizontal grids. Each Zone is divided in half vertically by the Zone Meridian which is ALWAYS labeled 500000 m.E. Coordinates greater than 500000 are east of the meridian, coordinates less than 500000 are west of the meridian.

The Northing coordinate indicates how far north or south of the equator you are. In the Northern Hemisphere the number reads exactly how many meters north of the equator you are, the equator has a value of 0000000 m.N. For the Southern Hemisphere you have to do some calculating, the equator has a value of 10000000 m.N.

An example: Northern Hemisphere 4572500 m.N. is 4,572,500 meters north of the equator.

Southern Hemisphere 4572500 m.N. is 10000000-45500=5,427,500 meters south of the equator.

The UTM grid is build onto all USGS maps. The maps are marked with the Zone number, and in the margins you will notice blue tick marks with numbers just below them. Those are UTM coordinates. Use a long ruler to draw a line between corresponding tick marks and you have a UTM grid on your map. Each square is 1 kilometer or 1,000 meters on each side. You can buy special rulers to use with maps in the 1:24,000, 1:25,000, 1:63,360 scales. These rulers make an already simple grid even easier to use. They allow you to determine coordinates from the map for entry into a GPS receiver as waypoints.

For more information on UTM and using your GPS receiver I recommend the book "GPS Made Easy" by Lawrence Letham, it is an excellent resource and is available from Amazon.com, REI, Eastern Mountain Sports, and many other places for about $15.00

Major manufacturers are Garmin, Magellan Systems, and Brunton. There are others but these are the big three.

GARMIN produces some very fine instruments. Their entry level unit is the eTrex GPS which sells for about $100, through I have seen it on sale at Target stores for as low as $79.99. The receiver stores up to 50 waypoints and can be connected to a PC, through you have to purchase the cable separately. They also make the eTrex Legend($170), the eTrex Vista($270), and the eTrex Vista C ($400). Each has more advanced features including internal maps and greater memory. Garmin is the sole producer of a unique GPS, the RINO series. These combine a GPS and a Family Radio Service (FRS) transceiver into one small unit. Using two or more units, they have the ability to display on the map screen the positions of other members of your party. The disadvantage is that for this feature to work you have to purchase more units.

MAGELLAN Their entry level unit is the SporTrak($150). I have this unit and have found it to be reliable, fast, and easy to use. It has a 1 megabyte memory and can be connected to a PC to download/upload information. The SporTrak also has a function to compute the best times to hunt or fish.

Magellan has just come out with a new receiver series called the eXplorist which is available in three models. These units are small and can display information in ten languages. I have not yet seen a price on these units.

BRUNTON This company only produces two receivers, the Atlas at $200 and the Multi-Navigator(MNS)at $360.

The Atlas has built-in maps and can be updated using Atlas Info MMC cards. You can connect an external antenna for use in vehicles.

The MNS has a built-in digital compass which operates separately from the GPS function. Using just the compass the batteries last for up to 60 days. The unit also has an Altimeter, and StormWatch Barometer.

The key to using all these units is PRACTICE! Take them on short hikes in areas that you are familiar with and use them. Get familiar with the different screens and how to navigate between them. Learn how to enter waypoints and routes and, more important, how to recall the information. ALWAYS carry extra batteries and a compass as a back-up.

Zone	Easting	Northing
13	404460 m.E	4572500 m.N