Jesse's Hunting > GPS

GPS

The Global Positioning System , GPS, is a satellite-based navigation system consisting of a network of 24 orbiting satellites, eleven thousand nautical miles in space, in six different orbital paths. The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours. If you do the math, that's about 1.8 miles per second. Now that's really moving! The GPS satellites are referred to as NAVSTAR satellites.

Russia now has a satellite navigation system of their own called the Global Navigation Satellite System (GLONASS), in place. Both the United States NAVSTAR GPS and GLONASS transmit spread spectrum signals at two frequencies in the L-band (1.2 GHz and 1.6 GHz), and have pledged to make a partial set of signals available for civil use without any user fees for the next 10 years, or more. On its own, GLONASS provides greater accuracy than NAVSTAR (if selective availability is turned on), with its superior spread of satellites transmitting "unfudged" signals, although you might have to scout around a bit to find shops selling GLONASS receivers at the moment. The logical progression from there has happened very recently, with the California based company Ashtec producing a combined GPS / GLONASS receiver which can lock onto so many satellites it can provide a position accurate within 7 to 15 meters without any differential correction - and an unbelievable 40 cms with DGPS!

Wikipedia GPS info

• The first GPS satellite was launched way back in February, 1978.
• Each 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!
• Each satellite transmits on three frequencies. Civilian GPS uses the 'L1' frequency of 1575.42 MHz.
• Each satellite is expected to last approximately 10 years. Replacements are constantly being built and launched into orbit. The GPS program is currently funded with replacements through 2006.

The orbital paths of these satellites take them between roughly 60 degrees North and 60 degrees South latitudes. What this means is you can receive satellite signals anywhere in the world, at any time. As you move closer to the poles (on your next North Pole expedition!), you will still pick up the GPS satellites. They just won't be directly overhead anymore. This may affect the satellite geometry and accuracy, but only slightly.

One of the biggest benefits over previous land-based navigation systems is GPS works in all weather conditions. No matter what your application is, when you need it the most, when you're most likely to get lost, your GPS receiver will keep right on working, showing right where you are! So what information does a GPS satellite transmit? The GPS signal contains a 'pseudo-random code', ephemeris (pronounced: ee-fem-er-is) and almanac data.

The pseudo-random code identifies which satellite is transmitting, in other words, an I.D. code. We refer to satellites by their PRN (pseudo-random number), from 1 through 32, and this is the number displayed on a GPS receiver to indicate which satellite(s) we are receiving. So why there are more than 24 PRN numbers? This simplifies maintenance of the GPS network. A replacement satellite can be launched, turned on, and used before the satellite it was intended to replace actually fails! They simply use a different number (again from 1 through 32) to identify the new satellite.

Ephemeris data is constantly transmitted by each satellite and contains important information such as status of the satellite (healthy or unhealthy), current date, and time. Without this part of the message, your GPS receiver would have no idea what the current time and date are. This part of the signal is essential to determining a position, as we'll see in a moment.

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. By now the overall picture of how GPS works should be getting much clearer. (Clear as mud, right?) Each satellite transmits a message which essentially says, "I'm satellite #X, my position is currently Y, and this message was sent at time Z." Of course, this is a gross oversimplification, but you get the idea. Your GPS receiver reads the message and saves the ephemeris and almanac data for continual use. This information can also be used to set (or correct) the clock within the GPS receiver.

Now, to determine your position the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is. If we add distance measurements from a few more satellites, we can find our position. GPS is based on knowing the distances from a receiver to satellites, and the process is properly called trilateration (for three satellites) or multilateration (for an indefinite number of satellites).  Similarly, celestial navigation is a multilateration process because it is based on knowing the distance from the observer to the celestial subpoints of the stars used for the navigation calculation.  Since GPS usually uses at least four satellites, multilateration is a good overall term to use.

This is exactly what a GPS receiver does. With a minimum of three or more satellites, your GPS receiver can determine a latitude/longitude position, what's called a 2D position fix. With four or more satellites, a GPS receiver can determine a 3D position fix which includes latitude, longitude, and altitude. By continuously updating your position, a GPS receiver can also accurately provide speed and direction of travel (referred to as 'ground speed' and 'ground track').

First the good news; now the bad news! What makes a GPS receiver perform below its best capability or accuracy? There are several items which add error to your GPS position, preventing you from achieving the best possible accuracy. The first of these items, and the largest source of position error, is Selective Availability (or SA). SA is an intentionally imposed degradation in the accuracy of civilian GPS by the U.S. Department of Defense. Under SA, GPS accuracy can be degraded to a maximum of 100 meters (328 feet). Of course, they don't typically degrade GPS accuracy to that level, but errors of 30 meters or more are not unusual.

Here is a link to see the latest accuracy for your GPS. GPS error

Why does SA exist? GPS was originally designed and built for military applications. As the system evolved, many folks realized that it has numerous civilian applications as well. By presidential proclamation, Ronald Reagan declared in the early 1980s that GPS would be made available to everyone with the exception that the best accuracy would still be reserved for the military. Since that time, satellites capable of being degraded with SA have been launched regularly. Today, all GPS satellites are capable of and subject to SA degradation. The rationale behind SA is to deny hostile military or terrorist organizations the maximum accuracy benefits of GPS. The Cold War may well be over, but the US is still wary about turning off Selective Availability, although the pressure is on for President Clinton to do something. With civilian use of GPS now dominant, the US tax payers are demanding for themselves the capabilities the system was designed to deliver. President Clinton has said the situation will be reviewed annually from the year 2000, but the bottom line is that SA is already becoming largely defunct.

Another factor affecting GPS accuracy is satellite geometry. In simple terms, satellite geometry refers to where the satellites are located relative to each other (from the perspective of the GPS receiver). If a GPS receiver is locked onto four satellites and all four of these satellites are in the sky to the north and west of the receiver, satellite geometry is rather poor. In fact, the GPS receiver may be unable to provide a position reading! Why? Because all the distance measurements are from the same general direction. This means triangulation is poor and the common area where these distance measurements intersect is fairly large (i.e., the area where the GPS receiver thinks our position is covers a large space, so pinpoint positioning is not possible). In this scenario, even if the GPS receiver does report a position, accuracy will not be very good (maybe off as much as 300-500 feet).

With those same four satellites, if we spread them out in all directions, our position accuracy improves dramatically. Suppose these four satellites are separated equally at approximately 90 degree intervals (north, east, south, west). Now satellite geometry is very good since distance measurements are from all directions. The common area where all four distance measurements intersect is much smaller, meaning we're much more certain where our exact position is. In this scenario, even with SA, our accuracy may be within 100 feet, or better.

Satellite geometry also becomes an issue when using a GPS receiver in a vehicle, near tall buildings, or in mountainous or canyon areas. When the GPS signals are blocked from several satellites, the relative position of the remaining satellites will determine how accurate the GPS position will be (and the number of remaining satellites will determine if a position can even be determined). As more and more of the sky is obstructed by buildings or terrain, it becomes increasingly difficult to determine a position. A quality GPS receiver indicates not only which satellites are available for use, but where they are in the sky (azimuth and elevation) so you may determine if the signal of a given satellite is being obstructed.

Another source of error is multipath. Simply put, multipath is the result of a radio signal being reflected off an object. Multipath is what causes 'ghost' images on a television set. We don't see this on a television much nowadays since it's most likely to occur with those old style 'rabbit ears' antennas, not on cable. With GPS, multipath occurs when the signal bounces off a building or terrain before reaching the GPS receiver's antenna. The signal takes longer to reach the receiver than if it traveled a direct path. This added time makes the GPS receiver think the satellite is farther away than it really is, which adds error to the overall position determination. When they occur, multipath errors typically add well under 15 feet of error to your overall position. Are there any other sources of error? Sure. Propagation delay due to atmospheric effects can affect accuracy. So can internal clock errors. In both cases, the GPS receiver is designed to compensate for these effects and will do so quite efficiently. But, very small errors due to these items can still occur. If you're wondering, propagation delay is the 'slowing down' of the GPS signal as it passes through Earth's ionosphere and troposphere. In space, radio signals travel at the speed of light, but they are significantly slower once they enter our atmosphere.

Keep the batteries fresh.
A lot of the electronic gizmos I've used had the problem with the "ON" button getting accidentally pushed and the batteries drained. You want to keep batteries in the unit because you'll forget to install them before you head out like I always do. A simple trick to keep the batteries from being drained is to turn one battery around in the battery compartment. This won't hurt the unit and it breaks the electronic circuit so that the batteries cannot be drained if the "On" button is pushed. On a 9 vdc battery, just reverse the battery. When you go to use the unit, reverse the battery to it's correct contact position and Shazam!!, fresh batteries. No uncontrolled weeping because you only have 1 spare set of batteries and they are for your penlight.

Keep the juice flowing.
Every few months you should inspect your battery compartment of your GPS. Make sure the battery contacts or springs still have enough tension to keep the battery pushed into the other battery contact. Look for any green/blue corrosion from leaking batteries and scrub it out with electronic cleaner if you find any. You can clean the springs or contacts with a pencil eraser.

Keep current on software upgrades.
Every few months you should check the website of the company that made your GPS to see if there are any software upgrades. When new software bugs are found or sometimes just when needed features are added, the company will have a downloadable software upgrade that will just take a few minutes to get.

Here is a link to see the latest accuracy for your GPS. GPS error

How accurate is GPS, really? A typical civilian GPS receiver provides 10 to 60 feet accuracy with selective availability off and 60 to 225 feet accuracy with selective availability on. The accuracy of your GPS unit also depends on the number of satellites available, and the geometry of those satellites.

WASS. There is a new upgrade to the GPS system we have used in the past called WAAS (Wide Area Augmentation System). The Garmin eTrex Legend, GPSmap 76 and Vista are currently the only civilian GPS models that are capable of using WAAS. The only drawback to using WAAS right now is that your geographic location and the terrain you're in will affect how well it works. From a review by Joe Mehaffey and Jack Yeazel, "There are only 2 WAAS satellites as of July 2001, one of these is low on the SE horizon from the Eastern USA and the other low on the SW horizon in the western USA. If you cannot see at least ONE of these WAAS satellites, WAAS will not operate."

WHAT IS WAAS? The basic GPS service fails to meet the accuracy (the difference between the measured position at any given time to the actual or true position), availability (the ability of a system to be used for navigation whenever it is needed by the users, and its ability to provide that service throughout an air flight operation), and integrity (the ability of a system to provide timely warnings to users or to shut itself down when it should not be used for navigation) requirements critical to safety of flight. In order to meet these requirements the FAA is developing the Wide Area Augmentation System or WAAS. WAAS is a safety-critical navigation system that will provide a quality of positioning information never before available to the aviation community. It is what the name implies, a geographically expansive augmentation to the basic GPS service. The WAAS improves the accuracy, integrity, and availability of the basic GPS signals. This system will allow GPS to be used as a primary means of navigation for enroute travel and non-precision approaches in the U.S., as well as for Category I approaches to selected airports throughout the nation. The wide area of coverage for this system includes the entire United States and some outlying areas such as Canada and Mexico.

The WAAS is based on a network of approximately 35 ground reference stations that covers a very large service area. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist. The WAAS will improve basic GPS accuracy to approximately 7 meters vertically and horizontally. Click here and here for more WASS info.

More sophisticated and expensive GPS receivers, costing several thousand dollars or more, can provide accuracies within a centimeter by using more than one GPS frequency. However, a typical civilian GPS receiver's accuracy can be improved to fifteen feet or better (in some cases under three feet!) through a process known as Differential GPS (DGPS). DGPS employs a second receiver to compute corrections to the GPS satellite measurements.

How are these corrections provided to your GPS receiver? There are a number of free and subscription services available to provide DGPS corrections. The U.S. Coast Guard and U.S. Army Corps of Engineers (and many foreign government departments as well) transmit DGPS corrections through marine beacon stations. These beacons operate in the 283.5 - 325.0 kHz frequency range and are free of charge. Your only cost to use this service is the purchase of a DGPS Beacon Receiver. This receiver is then coupled to your GPS receiver via a three wire connection, which relays the corrections in a standard serial data format called 'RTCM SC-104.'

Subscription DGPS services are available on FM radio station frequencies or via satellite. Of course, in either case you need a separate receiver to pick up these transmissions and then send them to your GPS receiver. In some cases, the prices vary according to the level of accuracy desired.

Do-it-Yourself Accuracy Test from Sam Wormley

Measure the accuracy of your GPS receiver by following these four steps. A set of measurements is worth a thousand expert opinions! Forget about EPEs! Trust your own plot.

1. Find any convenient unobstructed place.
2. Record the UTM coordinates for that place. Don't throw away any data points!
3. Make a graphic plot of Eastings and Northings (pencil and paper works really well for this).
4. Repeat steps 2 and 3 (at different time of day and night) for at least a week.

Sam Wormley's GPS Accuracy Webpage

If you would like to look at some testing on the accuracy of a GPS receiver click here to go to David Wilson's GPS accuracy webpage.

So what's the best GPS receiver for me? Now, that's the ultimate question, isn't it? And certainly the hardest one to answer. A number of issues come into play here:

1. First and foremost, make sure you buy a 12 channel parallel RECEIVER!!! 8 channel, 12 channel - what's the difference? In terms of performance, about the same as the difference between a 386 PC and a 233 MMX Pentium. The 8 channel receiver is "multi-plexing", which means that although it can track up to 8 satellites, it can only receive data from one at a time. So it does a sort of Round Robin, getting data from one satellite, then switching to the next. A 12 channel parallel receiver can lock on to more satellites, devoting a channel to each one, so once locked on, the only break in that link will be if the satellite is physically obscured from view. 12 channel parallel receivers will always work better in those fringe reception areas where a multi-plexing receiver might throw in the towel - walking through woods, for example. They're much quicker at getting a fix when you switch on, and they're also likely to offer greater accuracy, because they take position information from different groups amongst the satellites visible, then overlay them and average out the differences. Given the price of 12 channel receivers, 8 channel models simply aren't worth buying any more.

2. In some cases, you may still have a lot of choices from which to choose. For example, if your intended use is hiking or hunting, a GPS for outdoor recreation is suitable but so is a handheld GPS designed for boating or flying. In this case you may have to examine specific features more closely. Unless you plan on flying too, all the extra information of airports contained in the aviation handheld GPS probably isn't worth the extra price. A marine GPS which uses cartridges to show navigation markers and depth contours won't help out much on the trail either (unless you also want to use the GPS on your yacht!).

3. What is the price range? Once you've narrowed the field, you'll most likely still have several models over a range of prices from which to choose. Examine each model closely. What do the high-priced models have that the lower priced models do not? Do you need the extra features or accessories that come with the higher priced model or is the lower priced model sufficient to do the job?

4. Which model do you like the best? Choosing the right GPS receiver for you is two parts rational planning and one part simple preference. If rational planning still leaves you with two or three models to choose from, find a dealer for these models and try operating each one. Sometimes the differences in operation are dramatic. You may find one real easy to use and understand, while another seems much more complicated and difficult to use. Choose the GPS receiver that you LIKE best! You're more likely to still be happy with the decision you made after one month or one year.

If you want additional info on how to determine what model GPS you need, click on the links below to go to:
 

Buyer's guide to GPS receiver features
Comparison guide for entry level GPS receivers, ($100 to $200)
Comparison guide for mid-level GPS receivers, ($200 to $300)
Comparison guide for handheld GPS receivers with map databases, ($375 to $600)

Different maps have different coordinate systems. The two most common systems, used by the US Geological Service TOPO maps, are:
Latitude (LAT) and longitude (LONG), with units of degrees, minutes and seconds. The UTM (Universal Transverse Mercator) Grid system, with units of meters.

Lat/Long is a system of units most of us are familiar with. The distance of a location from the equator to the poles is identified by latitude 0 to 90 degrees north or south (horizontal lines on the globe). Going around the earth east or west, the earth is divided into 360 degrees (vertical lines on the globe). The line that passes through Greenwich, England is called the prime meridian, and longitudes are measured to 180 degrees east and west of this line. It is difficult to manually measure distance between two points using this system because the size of a degree varies with ones position on the round earth.

The UTM system is one that is preferred by map users who need precise distance information. This system selects a meridian and identifies how far east or west of that meridian you are and how far north or south of the equator you are (in meters). If you are unfamiliar with this system, but have a map marked with UTM coordinates, or need the accuracy that this system provides, please check my topo webpage which has more in-depth info on the UTM coordinate system.

When you first setup your GPS receiver, make sure you set the GPS to the coordinate system you will be using. This can be done in the 'menu mode'. There should be a choice of LAT/LONG or UTM. Also, while your in the 'menu mode', select the 'map datum' you will be using. Most of the hunters I've met use 7 1/2 minute topo maps which use the North American Datum of 1927, (NAD 27). When you have the coordinates of your position, you can look it up on a map and know just about where you are (depending on the accuracy of your unit). When you look up your destination on a map, you can input these coordinates and save the position in your GPS unit for route planning. Transparent plastic plotting rulers, (also known as roamers), make it easier to estimate a coordinate from a map. I have links to buy these plotters on my topo webpage also. Some GPS units have a feature that allow you to identify coordinates of a position on a map by just using the map name, and scale, and by measuring how many inches the position is from the lower right hand corner.

Of course, the products to enhance GPS receiver capabilities are becoming more available, and less expensive. Keep an eye out for new software products with exciting features, that seem to come out every month. There are atlas type software, with streets, highways, restaurants and hotels, that let you find almost any address in the continental US. There are topographic-type map programs for several regions with real USGS Topo maps or hiking guides for popular mountainous areas (with more coming soon). There are new PC cable + software packages being offered by GPS manufacturers to upload/download GPS information. Some GPS units can be connected to a portable computer, with mapping software, and will actually track REAL-TIME, your location on the PC screen, also called "Moving Map". GPS application will continue to evolve and improve like computers, but DON'T WAIT, if you need to use it NOW. I have links and reviews for this CD-ROM software below in the "Links" section.

If you have a GPS for sale or are looking for a used GPS we have a JHO Classifieds section at Classifieds

Also you can check out used GPS units at ebay and Aamzon.com Auctions

Adventure GPS Products .... Retailers of name brand GPS units. Also cool place for learning about GPS and how to use one.
Bass Pro Shop .... They sell the Eagle, Garmin and Magellan models.
Peter Bennet's GPS Webpage .... Excellent site about GPS and NMEA-0183 data.
Blue Hills Innovations  ....  Excellent site to get Garmin GPS cables for a good price.
Brigade Quartermaster's .... They sell GPS and holsters.
Cabela's ....They sell the Eagle, Garmin and Magellan models.
CommSystems Intermountain .... GPS cables, antennas and software.
DeLorme  .... Makers of map atlases (gazetteers), Street Atlas software and  TOPO USA, a software program of USA topo maps that you can interface with your GPS. I have a review of this software on my topo map software webpage here  Topo map software reviews.
Eagle Electronics .... Makers of GPS receivers, sonar, and software for GPS.
Fugawi .... Makers of GPS and topo map software and hardware.
Garmin .... Makers of GPS handheld units and topo software.
GPS City .... Garmin dealer with good prices.
GPS Nuts .... Ron Wilson and Andrew Kalinowski are heavy Lowrance/Eagle GPS users. Make sure you visit here if you own or are looking at one of these GPS receivers.
GPS Store .... Online retailers of GPS receivers.
GPSUnitsAndAccessories.com .... Handheld, Tracking and Fitness GPS Units and Accessories. Name Brands, Superior Service, Great Prices!!
James Associates .... A Garmin dealer with good prices.
JHO GPS Talk Forum .... Great place to talk GPS and navigation.
JHO Classifieds .... Sell or buy a GPS.

Lat26 .... Lat 26 has introduced a synthetic inkjet printer paper for topo maps that is tear proof and renders the ink waterproof. $26.95 for 25 sheets. For you peeps using Maptech, DeLorme or TopoZone to print out maps, this is a godsend. Laminating the maps runs about the same cost ($1.00 per map), but if you put the maps back to back to laminate, lamination would be about half the cost ($.50) of this new printer paper. The paper is not listed on their website yet, so call 1 800-305-0036 to order. They also have the GPS/Topo Coordinator which is a plastic cartographic overlay using the Universal Transverse Mercator Projection (UTM).
Louisiana GPS Fishing Waypoint Page .... Hundreds of waypoints for Louisiana fisherman by Mike Guerin
Lowrance  .... Makers of GPS receivers.
Magellan .... Makers of GPS handheld units and topo software.
Terrain Navigator .... Maptech's topo map software on CD-ROM with GPS interface.  I have a review of this software on my topo map software webpage here  Topo map software reviews .
Mark's GPS Links  .... LtCol Mark Schmidt's vast page of GPS links. No matter what you're looking for on GPS, you'll find a link here. Bookmark this one folks.
Joe Mehaffey and Jack Yeazell's GPS website .... All kinds of GPS reviews and info links. Great website! This is a must visit site for anything related to GPS.
National Geographic .... Topo software on CD-ROM which allows you to upload/download info between  your computer and your GPS. We have a review of this software on our topo map software webpage here  Topo map software reviews . Available here in the
NavtechGPS.com .... Dealer for Garmin, Lowrance, Magellan and Canadian Marconi.
Outfitter Satellite .... GPS vehicle tracking, stolen vehicle locators, and satellite phones.
Ozie Explorer .... OziExplorer Software which will work with Magellan, Garmin, Lowrance and Eagle GPS receivers for the upload/download of data and most brand of GPS receivers for on screen tracking of position (Moving Map). You can also import DRGs (Digital Rastor Graphics or digital computer topo maps), for viewing and editing with your GPS. I use this program and it is the best one I have found to plot your tracks, waypoints and other map info.
REI .... Online retailers of GPS and other outdoor gear.
Silva .... Makers of the first and only handheld satellite navigator with a built-in electronic compass.
TVNAV.COM .... A Garmin and Delorme dealer with outstanding prices.
Tom's Garmin GPS Pages  .... If you own a Garmin GPS or are thinking about getting one, you need to check out Thomas Born's excellent website.
TOPOGUIDE .... Hanta Yo's topo map software on CD-ROM with GPS upload/download interface.
Trimble .... Makers of the 'ScoutMaster GPS'.
TRM Enterprises .... Makers of 'Topo Companion' and the 'Coordinator'. Plastic plotter/rulers for topo maps with UTM and LAT/LONG scales from 1:24000 to 1:250,000.. You can get the 'Topo Companion' at REI for $6.00.
Sam Wormley's GPS Info Website .... Great place to read up on GPS. Tons of info here.

Usenet newsgroups where you can find current discussions on GPS: news:sci.geo.satellite-nav , news:comp.infosystems.gis , news:sci.engr.surveying , news:rec.aviation.products , news:rec.aviation.marketplace ,  news:rec.aviation.misc , news:rec.boats ,  news:rec.boats.electronics ,  news:rec.boats.marketplace

  If you know of any other good links about GPS units please email us at

'The Aviator's Guide to GPS' .... Paperback by Bill Clarke, 3rd ed., 304pp.  ISBN: 0070094934, Publisher: McGraw-Hill Companies, Pub. Date: March  1999,  Edition Desc: 3RD, $23.96.

'Comprehensive Guide to Land Navigation With GPS' .... Good paperback on learning how to use a GPS to navigate by Noel J. Hotchkiss. Format: Paperback, 3rd ed., 315pp., ISBN: 189268800X,  Publisher: Alexis Publishing, Pub. Date: March  1999,  Edition Desc: REV, $24.95.

'Global Positioning System: Theory and Practice' .... This is some deep reading for the surveyors and professional users of GPS receivers by B. Homan-Wellenhof. Format: Paperback, 1st ed., 370pp.,  ISBN: 3211828397, Publisher: Springer-Verlag New York, Incorporated,  Pub. Date: January  1992,  Edition Desc: 4TH REV, $54.95.

'GPS Instant Navigation: A Practical Guide from Basics to Advanced Techniques' .... Kevin Monahan, Don Douglass, Format: Paperback, 320pp.,  ISBN: 0938665480,  Publisher: Fine Edge,  Pub. Date: April  1998,   $23.96.

'GPS Land Navigation: A Complete Guide For Backcountry Users of NAVSTAR Satellite Systems' .... This is the Bible of GPS terms and info by Michael Tucker. This is my favorite GPS book. Excellent sections on how GPS works, how to use your GPS, limitations of GPS, GPS accessories, coordinate systems and map datums. If you only buy one map or GPS book, this should be it. Format: Paperback, 240pp., ISBN: 0965220257,  Publisher: Glassford,  Pub. Date: September  1997,  $15.96.

'GPS Made Easy, Using Global Positioning Systems in the Outdoors' .... Excellent book by Laurence Lethan covering all aspects of GPS receiver use. A must read by Garmin owners. Format: Paperback, 2nd ed., 224pp., ISBN: 0898865921,  Publisher: Mountaineers Books, The,  Pub. Date: September  1998, Edition Desc: 2ND, $11.96.

GPS Outfitters .... Demo videos on different models of GPS, and video on how to use topo maps with GPS, $24.95 each with shipping. ph# 800-477-4868. Take a test drive before you buy.

'Outdoorsman's Guide to GPS' .... VHS video by Tim Ezelle covering GPS navigation topics. 32 minutes, NTSC format for U.S. and Canada, $39.95, ISBN #6304376413.

'Yachtsman's GPS Handbook: A Guide to the Global Positioning System of Satellite Navigation' .... By Colin Jones, .Format: Hardcover, 128pp., ISBN: 1853104701, Publisher: Voyageur Press, Incorporated,  Pub. Date: August  1996, $17.46.

If you know of any other good books about GPS units please email us at