by Richard L. Carey, minor revisions July 3, 2012.
GPS is a commonly heard acronym these days. It refers to the Global Positioning System and the navigation receivers that make use of it. This article will attempt to describe, in laymen's terms, how the system works and what advantages it offers out on the trail. I have been using a GPS navigator extensively for several years now and will relate my experiences and also offer opinions on current units now on the market, should you be considering a purchase.
Background and History of GPS
The concept of the Global Positioning System was proposed more than 20 years ago as a worldwide means of navigation for the US military. An early satellite navigation system known as TRANSIT, now shut down, proved that navigation from satellites with an unlimited number of users was a workable concept and offered advantages of higher accuracy and world-wide coverage unavailable in existing systems. After years of testing, equipment refinement, and launch delays due to the Challenger disaster, the number of satellites, in late 1993, finally reached the minimum number necessary for 24-hour coverage around the globe. At that time there were 24 satellites in orbit and, after further testing, the Air Force declared the system to "have full operational capability" on April 27, 1995. Total cost, from concept to completion, has been about 12 billion dollars.
Although it was developed for the military, the advantages of such a system for civilian uses was seen early on. Also, advances in electronics have brought down the price of GPS receivers from the over $10,000 range to a few hundred dollars today. For many years only the military could afford them and now they are affordable for commercial and recreational users. What has come about is a system run by the U.S. military, but available to anyone. At present the commercial market has grown to where the number of commercial units sold outnumbers the military units by 3 to 1. GPS is far better than other systems and it is on its way to becoming the primary means of navigation in most applications.
Satellites - The Heart of the GPS System
The GPS system has 24 satellites in orbit at an altitude of about 11,000 miles above the earth's surface. Each make a full revolution every 12 hours. They are spread out in six orbital planes so that from any point on earth at least five satellites are in view at all times, meaning that they are above the horizon. This high orbit, well above the earth's atmosphere, yields a very precise and stable orbit that can be measured very accurately by ground stations. The Air Force, at its master control station at Schriever AFB near Colorado Springs, Colorado, and at four other monitor stations, can check each satellite twice a day. A coded message is continuously transmitted by each satellite on two channels that contain timing information and data about the orbit of the satellite. To maintain a very precise timing signal which the user on earth needs to calculate his position, several atomic clocks are used as a reference in the satellite. As with most satellites there are solar cells and backup batteries to maintain power for an expected 7 1/2 year lifetime.
Of the two channels transmitted, called L1 and L2, the civilian user can only make use of the L1 channel that has unencrypted information. The L2 channel is for the military with special receivers. The signal consists of a coded acquisition signal for the timing process and a message signal that gives orbital data for the satellite and also for the other 23 satellites.
The Navigation Process
Every GPS receiver has a small antenna to receive the satellite signals. To function properly the unit must be outdoors and have good visibility to most of the sky. Units will not work in caves, underwater, in buildings, or in deep canyons where sky visibility is severely restricted. Some units work better than others in jungle or dense forest; this will be discussed later. Signals are not generally affected by clouds or bad weather.
After activation the receiver will look for satellite signals and try to receive as many as its design can handle. Most receivers give an indication of the number of satellites being received or "tracked" and some have a graphic display showing the number and position of each satellite. To determine a position the receiver does a lot of calculations that are basically trigonometry problems. By using the coded timing signal the receiver measures the range to each of four satellites and calculates four equations with four unknowns. The most accurate position or "fix" is obtained from satellites widely distributed about the sky and more than 20 degrees above the horizon. If the satellites are bunched up in one direction or your view of any is blocked then the "fix" may not be as accurate. Most receivers will give an error number indicating the positional accuracy.
This whole process may sound complicated, but is done automatically by the receiver during the few minutes before a position is displayed. There is usually an error "beep" and display if the receiver cannot find enough satellites. If you have just established a fix then your next one will be fairly fast. If you have not used the unit in some time then your first fix may take up to 13 minutes as the receiver has to download all the message data about the orbits. Once it has current data subsequent fixes are much faster.
I mentioned that the receiver needs data from four satellites. This is true for it to provide a complete solution which consists of your location horizontally and your elevation. This is called "3D" or three-dimensional solution. If only three satellites are visible then the unit can provide a "2D" or two dimensional solution which gives your horizontal location but no elevation. Most receivers indicate whether a 2D or 3D answer is being provided. The receiver can also provide accurate time of day and velocity if your are travelling.
The question that often comes up is "How accurate does it compute my position?" The GPS position for the civilian user was deliberately degraded for many years in a policy called "Selective Availability", but this was stopped on May 1, 2000. Thus the civilian user now has essentially the same accuracy as the military user. For horizontal position the error is plus or minus 5 meters for 95% of the time and elevation is generally within plus or minus 10 meters.
Using the Position Information - Coordinate Systems
The most common coordinate system is latitude and longitude, which divide the earth into lines of latitude parallel to the equator and lines (meridians) of longitude perpendicular to the equator. Zero degrees latitude is on the equator and zero degrees longitude is at the Prime Meridian in Greenwich, England. Your position is an angular measurement north or south of the equator and east or west of the Prime Meridian in Greenwich, England. This system has been in use for hundreds of years and all GPS units will give a position in these units. The display can be in degrees, minutes, and seconds or in degrees and fractional minutes. This system is fine for nautical charts, but for land navigation it is awkward to think of distance in terms of degrees and minutes when we are more used to miles and feet or meters. The degree is divided into 60 minutes which is not as easy to do compared to the division by 10 in the metric system.
Although awkward to use, we find that all topo maps are arranged with edges based on latitude and longitude. As we go north on the map the distance of one degree latitude is consistent from the equator to the pole, but not so for degrees longitude. The longitude lines all merge at the poles and so the width of one degree longitude gets progressively smaller as we travel north. This can be readily seen in maps of California where the width of a map in Del Norte county is smaller than one in San Diego county.
A better system for the land navigator is the UTM or Universal Transverse Mercator coordinate system. At the risk of oversimplification I'll say that it is a projection of the spherical surface of the earth onto a horizontal cylinder. To keep the error small this is done in zones six degrees wide and covers territory from 80 degrees south to 84 degrees north latitude. A map on a cylinder can be split and laid out flat and thus the problem of representing a round globe on a flat piece of paper is solved. There are 60 numbered zones starting with Zone 1 from 180 to 174 degrees west longitude and increasing toward the east. In California we are in two zones: zone 10, from 126 to 120 degrees, and zone 11, from 120 to 114 degrees.
All 7.5 minute topo maps (1:24,000 scale) have the UTM grid which is a metric grid of one kilometer spacing. On a lot of maps the grid is drawn in with very fine lines and on others it is shown on the margin with blue tick marks. The zone the map is in is identified in the text in the lower left corner of the map. On those maps with just the tick marks I recommend drawing in the grid if you plan to read UTM values from the map. Note that the UTM grid is usually not in alignment with the latitude and longitude lines on the map and will be slanted left or right depending on which side of the center of the zone the map is located.
The nice thing about the UTM grid is that it is a consistent 1 Km grid which does not vary with the location of the map! The vertical units are the distance in meters north from the equator and will vary from 3,602,000 along the Mexican border to 4,649,000 at the California-Oregon border. For horizontal measurements the distance is also in meters and to keep the numbers positive a value of 500,000 meters is assigned to the center of the zone. On the typical California map, depending on your location, the horizontal UTM numbers will vary from about 200,000 to about 800,000 increasing from left to right. Using a grid like the UTM allows you or your GPS receiver to calculate bearings and distances between points using simple trigonometeric formulas.
Most all GPS units now on the market will display a position in the UTM system. The display will show the zone and two numbers - a six-digit number will be meters east and a seven-digit number will be meters north. It is easy to visually divide the 1 Km grid into 10 units and place your position reading on the map to within 100 meters.
Coordinate Systems for Other Countries
Many countries have adopted their own grid systems that are similar to the UTM, but have a different origin to optimize the grid for their territory. For example, in Great Britain it is called the British National Grid and many receivers will have the option of selecting this. If you plan on using the GPS unit in a foreign country having the option to select the proper coordinate system is important. Some of the lower cost units are very limited and assume that use will be confined to the U.S.
Datums - Defining the Shape of the Earth
This is a subject the average user doesn't need to know a lot about other than the fact that using the right datum is important, just as is knowledge of the coordinate system. The receiver, when it calculates where you are, uses an elliptical model for the shape of the earth. This is called the ellipsoid. There are eight numbers that define its shape and origin and make up the datum. There are datums that have evolved from satellite measurements that are optimized for the entire earth and there are those set up for just a region or country. Our topo maps are based on the North American Datum of 1927 (abbreviated NAD27) and this is the one that your GPS receiver should be set to for use in the US. As it comes, the receiver will probably be set to WGS84, so you should change it to NAD27 to be compatible with existing topo maps. There are well over a hundred datums defined for various areas of the world and the more the GPS receiver has in its memory the better.
Other GPS Receiver Features
We have talked about taking a reading from the GPS receiver and placing yourself on the map and we can also do the reverse. This is useful for taking a point on the map such as a summit or trail junction and storing it in memory and then finding your way to that point. Saved positions are called "waypoints", shorthand for "points along the way", and most GPS units will allow at least 100 waypoints to be saved. Usually a five or six digit name can be assigned to them to aid in remembering where they are. It is quite handy if you are going to a new area to spend some time beforehand with the topo map and store waypoints for important places on your route. There are several special plastic rulers available to allow you to read UTM positions on your map to within 10 meters.
When using the GPS unit you can ask for a range and bearing to the closest waypoint you have saved or to a particular one. Bearing will be in degrees true or you can have the display be a magnetic reading by subtracting the local compass declination. (Or add the declination if in an area where the compass points west of true north). Range to the waypoint can generally be displayed in miles rather than kilometers even though the grid is metric. You will find that you need your trusty compass when using a GPS unit. GPS gives a bearing to a point, but which direction is that? To head in the right direction a compass is still essential!
Various GPS units have a variety of other features such as a small graphic display of your route and a more elaborate bearing indicator display that would be useful in a boat for navigation. This will show graphically how to steer to a waypoint without being concerned with numeric values or obstacles enroute and also your velocity will be indicated with good accuracy for judging time of arrival. There are other features and manufacturers are sure to dream up new ones to "enhance" their product. Many of these may not be needed and just add unwanted complexity.
Using GPS in the Forest on in a Car
Most all GPS units work fine out on the ocean or in the desert where there are few obstructions, but in a dense forest there are problems due to signal attenuation. Tree trunks and leaves weaken the satellite signal considerably and certain receivers work better than others under these conditions. We mentioned earlier that the receiver must lock onto the signal from at least four satellites to determine a 3D position. To save power some receivers use a sharing technique called "multiplexing" where the unit switches channels among the satellites. The satellite signal thus is not continuously tracked. Most new receivers have dedicated channels for each satellite and maintain a better track in marginal conditions such as in a forest. If operation in the forest is essential then be sure to take this into account when buying a unit. The best arrangement is 12 parallel channels which means the receiver can track all satellites in view.
Using a GPS unit in your car can be very useful when trying to find road junctions and landmarks particularly on unmarked desert or mountain roads. The signal will penetrate through the windshield and some units work OK when placed up on the dash. A better solution is to attach an external antenna. Most manufacturers offer a magnetic mount antenna that can be plugged into the receiver and put on the roof with the cable run through a window. On some lower cost units the provision for attaching an external antenna has been deleted. In my experience performance is much improved with the outside antenna. For car use a cable with plug to connect to the cigarette lighter is a good idea to save on battery power.
GPS navigation has progressed significantly in recent years allowing businesses to use GPS technology . wirelessmatrix.com provides vehicle tracking systems to improve shipping routes and decrease logistics costs.
Summary and Trends
Navigation using GPS is proven and reliable at this time and is here to stay. Receiver prices have come down a lot in the last few years and are still trending lower. I think their use in the backcountry will increase and a GPS unit will be regarded as another tool for navigation, along with the traditional map, compass, and altimeter. It does not replace these methods but complements them. Hiking guides and trip descriptions will start to include UTM coordinate points as standard practice to identify important trailheads, junctions, passes, road washouts and so on. GPS can help you plan a safer, more enjoyable and successful trip.
Opinions on Several GPS Receivers
These are my own opinions based on my use of several receivers, personal research and from postings on an Internet news group. These are all hand-held units, suitable for hiking or backpacking. I do not represent or sell any of these products.
Garmin 12, 12XL, GPS II plus, and GPS III
These are the latest 12 channel parallel receivers that Garmin makes and they perform well and are considerably improved over earlier units. These units acquire satellites under marginal conditions much better and getting a fix is faster too. The 12XL at a cost of about $199 is very popular and now offers storage of up to 500 waypoints. The model 12 is quite similar to the 12XL, but lacks the rear connector that allows use of an external antenna. This is not a serious drawback for most people and the model 12 is cheaper at typically $149. The GPS II plus uses the same receiver, but has a different case design with a fold-up antenna and display that allows it to work horizontally on the dash or vertically in the hand. Cost is about $250. The GPS III is a slightly larger unit with a big map database built-in and a nice map display. The maps are fine for road travel, but are not good enough for hiking. This unit is about $360.
Garmin eTrex series
The eTrex is Garmin's latest product that operates with only two AA batteries and is even smaller in size. The computer interface connection has been changed so that a different cable is needed. The cable from the older model 12 or 12XL will not work. I had a chance to observe the operation of a friend's eTrex and I was disappointed with the way they present or don't present the information. They seem to have gone overboard with cute graphics that take up valuable display space and don't say much. For example: after the unit finds the satellites it comes up with a screen saying, "Ready to navigate with XX feet error". There is a little man shown and no coordinates! I thought this page is darn near useless since I usually want to see coordinates. To get them you have to press Page three times to get to the Menu then press Enter. The UTM coordinates are in a very small font at the bottom with another cute graphic taking up most of the page. Those of us over 45 are going to need a magnifier to see the numbers! They had enough space to make the text much larger and easy to read.
In spite of these complaints the eTrex works well with accuracy as good as the old units and has the advantage of smaller size and less weight. The current price for the eTrex is about $129.00.
Garmin GPS 38, 40 and 45
These older units are still being sold in some stores, but are being phased out and I do not recommend them unless priced very cheap (under $50). The model 40 has a built-in antenna and is a little smaller than the model 45. The 45, with its fold-up external antenna performs better in marginal situations. The model 38 is similar to a model 40, but with some newer software features. All three of these units track up to eight satellites using a multiplexing method and do not work well in a forest. Each can store up to 250 waypoints and an optional cable and software allow transfer of data to and from a computer. Garmin will send data on its products by calling the factory at (800) 800-1020.
Eagle and Lowrance
These products are apparently made by the same company with the Lowrance brand and are not sold by mail order whereas the Eagle can be. The older GlobalNav Sport and AccuNav Sport units were good 5 channel receivers that were better than the Garmin 38, 40 and 45 series in forested and marginal situations. They used six AA cells and had rather short battery life. The newer 12 channel units from Eagle are much better performers. The Eagle Expedition II is a well-regarded unit that can store 750 waypoints and has numerous software improvements.
The Eagle AccuNav Sport has a larger screen than the Garmin 12XL, but is also about twice the size and weight. It has a numeric, backlighted keypad that makes entering waypoint data easier. This unit requires six AA batteries. A big drawback is that it does not support the UTM grid. Cost is about $300. Eagle has a web page at: http://www.eaglegps.com
Magellan Trailblazer, Meridian XL, GPS 2000, GPS 3000
The Trailblazer and Meridian XL are older units in a larger case. There have been numerous reports of trouble with these units such as failing to find satellites or giving wild and erroneous readings. Some users have had poor response from the factory getting them fixed. I do not recommend these units even if low-priced on the used market.
The 2000XL and 3000XL are very compact units about the same size as the Garmin 12XL. The 2000XL stores 100 waypoints and has no provision for an external antenna. The 3000XL can store 200 waypoints and connect to an optional external antenna. They both display in UTM grid as well as latitude and longitude. Some of Magellans units have had a two-channel multiplexed receiver which is advertised as 12 channel tracking. This is somewhat deceptive as it is not a true 12 channel parallel receiver. Some of the latest units are true 12 channel parallel, but the model number has not been changed! Watch out for this if considering one of these units since the 12 channel parallel version is very much preferred. Magellans web site is at: http://www.magellangps.com/
More Sources of Information on GPS
If you have a computer with access to the World Wide Web then I suggest the Web page on GPS at the University of Texas Geography Department. It is at: http://www.automechanicschools.com/docs/gps.html
Navtech is a company that offers training on GPS and has a very complete book list. They also sell receivers from their Arlington, VA offices and are one of the few places that can answer technical questions about their products. They have a web site at: http://www.navtechgps.com that has an extensive listing of links to other GPS related web sites.
Trimble Navigation publishes a good primer on GPS that at one time was available free from the company. This covers the technical aspects of how GPS works rather than actual use in the field. The title is "GPS - A Guide to the Next Utility" by Jeff Hurn and costs $9.95 from Navtech. Trimble's number is (800) 827-8000 in Sunnyvale.
GPS enthusiasts Joe Mehaffey and Jack Yeazel have an informative web page which gives good reviews of equipment and software. Find it at: http://joe.mehaffey.com/
Peter Bennett in Canada has a web page with a wealth of GPS information on it. It is well worth looking at. Find it at: ftp://sundae.triumf.ca/pub/peter/index.html.
The February, 1996 issue of Scientific American has a good article on GPS and some of the design decisions that went into it. A new book that is one of the best is: "GPS Land Navigation" by Michael Ferguson. Available for $19.95 from Glassford Publishing, Box 2895, Boise, ID. 83701-2895
A book which I can recommend is: "GPS Made Easy: Using Global Positioning Systems in the Outdoors" by Lawrence Letham (The Mountaineers, Seattle, 1995). It costs $12.95 at REI and Navtech. This book covers the use of GPS very well and shows the display of several receivers in its examples.
Another book "A Comprehensive Guide to Land Navigation with GPS" by Noel J. Hotchkiss, 2nd edition, 1996, gives a very good explanation of the UTM grid. It is a bit pricey at $29.95 and unfortunately only uses the Magellan Trailblazer in its examples, one of the poorest performing units on the market. Available from Navtech.
About the Author - Richard L. Carey is a retired Electronics Engineer who worked at the Spawar Systems Center in San Diego. He has been an active hiker/mountaineer since 1970. He admits to being a "peakbagger" and enjoys exploring remote mountains usually with his GPS receiver. He would like to thank Gail Hanna for editing this article.