Originally published in
South American Explorer
March 1994

Technology has advanced a whole bunch in the 4 years since this was written, but the way the system works, how well it works, and the commentary on desirable features is the same. The big change is that you can really buy a decent GPS receiver for $150 now.

The Global Positioning System

Technological Magic for Explorers of the New Millenium

Jim Lux & Sally Lux

About five centuries ago, some brave explorers, using what was then secret military technology, ventured forth across the Atlantic Ocean to discover, among other things, South America. Some of the original users of this high technology were probably accused of witchcraft, burned at the stake, or at least required to answer to some distinctly unfriendly inquisitors. This technology, the map and magnetic compass, in a relatively short time, resulted in a revolution of exploration. As the "secret" of the map and compass spread, so did explorers of all kinds. For the first time, you could venture forth and have a reasonably good chance of arriving close to where you set out for.

Today, all of us explorers of South America are in a similar situation. A technology, originally developed for military applications, is becoming available to everyone, at a reasonable price. Fortunately, you probably don't risk being burned as witch for using it, either, even though it does seem like magic when you use it. For a few hundred dollars, you can go out and buy a handheld unit that will tell you where you are (including altitude), anywhere in the world, any time of day or night, in any weather, with an accuracy of about 30 feet. It's called the Global Positioning System (GPS), and it will revolutionize life for those of us who have a need to know where we are, where we were, and where we are going. This article explains a little bit about what GPS is and how it works, what kind of receivers are currently available, and the results of some of our field testing.

You may ask, "Why would I want to know where I am within 30 feet?" We provide the following "true to life" scenarios where having a GPS receiver proves useful, if not positively essential:

You have been hacking your way through almost impenetrable forest in the Andean foothills for two weeks. Suddenly, your partner falls into a hole and cries out in pain. When you peer over the edge, you see that what you have been walking on is the roof of a storeroom containing the Inca gold which was hidden away from the Spanish conquistadores, and that your partner is mortally wounded, having broken his leg on the legendary gold chain which once graced the House of the Sun. Heartless though it may seem, you decide that since it will take two weeks to get to medical attention, it is pointless to try and save your partner. Besides, his life insurance beneficiary is the South American Explorers Club, so many will benefit from his misfortune, if only you can find your way back to this location. You whip out your GPS receiver, wait a few seconds until it calculates and displays your location, and carefully note it. You cover over the hole so that your partner's moans and cries don't disturb the indigenous wildlife. When you return next year to retrieve the treasure, you can use the receiver to guide you to the exact spot. No more Lost Dutchman Mine or the Seven Cities of Cibola.

You are wandering, lost and confused, in the streets of Lima, searching for the succor of the South American Explorer's Club, however none of the street signs seem to be readable, even if you did know Spanish, and the map you have must have been drawn 50 years ago. Fortunately, you know the exact latitude and longitude of the clubhouse, so you get out the trusty GPS receiver, enter the coordinates of the clubhouse as your destination, and let it guide you onward. A few minutes later, you arrive, soon to be rested, informed, and ready for further exploration.

You get off the bus in the small, dusty town of San Cristobal, however, nothing looks like you expected. Asking the local residents, you discover that you may be at the "wrong" San Cristobal, having gotten on the wrong bus. The clouds of dust during the trip prevented you from seeing anything outside. You check your map, and find that there are 3 possible San Cristobals that you might be in. No problem: you take out the GPS receiver, get a fix, and instantly, you know that you are at San Cristobal de la Cruz, when you wanted to be at San Cristobal del Oro. Oh well, it's probably only another 20 hour bus ride back.

You are an anthropologist studying primitive indian settlements in the Amazon forest. You want to be able to return to each settlement after you find it, but all the rivers look the same from your boat. You use a handheld GPS unit to store the location of each settlement as you find it, and then use it to guide you back to it later. You can also use the locations stored in your GPS unit for identifying features on aerial photos taken with horizontal control from another GPS receiver, or standard satellite images.

What is the Global Positioning System?

A GPS receiver works by using radio signals transmitted from more than 20 satellites orbiting about 11,000 miles above the earth.The information in the signals is used to calculate your position in a seconds or two, potentially to an accuracy of centimeters, although the inevitable noise and errors mean that positions are typically accurate to only(!) 30 feet. The system works anywhere in the world in any kind of weather as long as you can "see" enough of the sky so that you can receive the satellite signals (GPS signals go through clouds, so you don't need blue sky). It tells you not only your latitude and longitude, but also your elevation above (or below) sea level. Each satellite orbits the earth twice a day, so there are always new satellites "rising" and old satellites "setting", with any one satellite "in view" about 3 or 4 hours per day. There are at least 4 satellites visible from anywhere on earth, any time of day, and in middle latitudes, you can often "see" seven or eight.

The system was originally designed by the United States Department of Defense for military purposes like guiding tanks and missiles. GPS receivers designed for the military originally cost $50,000 and up, putting them out of reach of mere civilian consumers. However, technology marches ever forward, so now, for less than a thousand dollars, you can buy a unit which works better than the original $50,000 units, and also fits in a (big) shirt pocket. Another nice feature is that the signals are available to anyone, anywhere in the world, without having to pay a user fee.

See the sidebar for more information on how GPS works, how accurate it is, and more.

How well does it work.

The first thing you should know is that it works great! GPS is such an awesome advance over other ways of finding where you are that it seems like some sort of magic. Unfortunately, you rapidly get spoiled by being able to punch a button and find your location within a few feet. So, when you have bad satellite geometry, or you are somewhere without a good view of the sky, and you can only get a fix to within 500 feet, or not at all, you feel somehow cheated. In only a week of casual walking, driving, and flying around with a handheld GPS receiver, I got hooked on always knowing where I was, watching the position display change, and so forth.

In an open field or parking lot, it works great. You typically get your first fix less than a minute after turning on the receiver, and then, it tracks your position to an amazing degree of accuracy. Of course, did I really want to know that I walked to the mailbox 583 feet away at a heading of 47 degrees with an average walking speed of 2.4 miles per hour? Sometimes it is possible to have too much information.

When you go inside, your receiver will probably lose most of the satellites it is tracking, until you put the receiver next to a window, where it can see the sky. If it can see enough satellites it will recalculate your position, although there is a good chance that you will have bad geometry (see Sidebar), because you can only see one direction from the window. Another thing which may prevent you from using the window is if it is covered with a thin metal film. Most office buildings use a reflective film on the windows to reduce solar heat loads (and for that "mirror building look"). The same film that blocks the heat of the sun also blocks the radio signals used by your GPS receiver. I tried to get a GPS fix in a hotel atrium, and it never did work, because of the film on the skylights.

It works great strapped to the handlebars of a bicycle with a bungee cord. In a car, it works great if you set it on the dashboard (some velcro or duct tape is handy to keep it from sliding off). The satellites are visible through the windshield. However, if you have some sort of metallic film over the windows, the film will probably block the GPS signals. All of the receivers have some sort of external antenna option to allow you to put the antenna outside, while the receiver is inside.

Trees don't seem to have much of an effect, although if you were in a torrential downpour, the water on the trees might have a shielding effect, particularly if the vegetation was very dense. GPS does work inside a tent in the rain.

If you are standing at the base of a cliff (or tall building), GPS may or may not work, depending on whether you can "see" the satellites. If you are very close to the cliff (a few feet), the receiver might pick up the signals reflecting from the cliff, rather than the direct signal, (just like "ghosts" on a fringe TV station) and give you a false position.

Kinds of GPS receivers

There are three basic kinds of GPS receivers available for non-military users. All of the receivers work with the same satellite system. The more expensive units are designed for a particular application, and often have features which allows the positions calculated by more than one receiver to be compared, giving much more accuracy.

  1. Industrial units for surveying and data collection. These run from $5000 and up, and have a lot of features, lots of interfaces, and (with appropriate accessories) can provide accuracies to millimeters. Archaeologists mapping a site in hilly or forested terrain take note! They may be expensive, but these boxes are almost miraculous in their function, and can save an immense amount of time in mapping a complex site.
  2. Medium cost units intended for the marine and aviation market. They have functions aimed at steering your boat or plane, man overboard return, and things like that. These are typically in the few thousand dollar range, although the moving map display models cost substantially more. The marine versions have NMEA interfaces to hook up to your autopilot, depth sounder, etc. The aviation versions have the appropriate interfaces to connect to your avionics. Usually, they work primarily in latitude/longitude, and display speeds in knots and distances in nautical miles. Often, they include a "canned library" of locations, like airports, harbors, and so forth.
  3. The new breed of handheld units aimed at outdoorsmen, explorers, and so forth. They are relatively inexpensive ($700-$1500), provide outputs in a variety of units, and generally don't have external interfaces. These are the units reviewed in this article.

With anything as technology oriented as a GPS receiver, there is a steady stream of "new and improved" models from the manufacturers. Newer models get a fix faster, use less battery power, weigh less, and (hopefully) are easier to use. By the time you read this, the models we describe may have been superseded by newer ones. The sophistication of the technology in a GPS receiver is similar to that in a cellular phone, so we can expect the prices of GPS receivers will probably drop as their popularity increases, just like the phones. It will probably be a couple of years before the handheld units get down to $300, though.

It is important to remember that almost all GPS receivers have the same basic accuracy, which is primarily determined by the where the satellites are when you get a fix (the geometry) and the inherent accuracy of the satellite signals. What differs is how long it takes to figure out your position, battery life, and what sort of user features are provided. Just like with any other type of electronic equipment, each manufacturer has added what they think is an essential set of features and capabilities. The way in which the makers distinguish themselves is by these "bells and whistles". Whether a particular feature is important depends on if you are going to use it. For instance, some of the receivers feature a NMEA (National Marine Electronics Association) interface, which is really useful if you are hooking your GPS receiver up to your boat, but not much use in a backpack or airplane.

Here are some features that most receivers have, over and above the basic telling you where you are and what time it is:

Satellite data

All the receivers can tell you which satellites are being received, how good the signals are, and where the satellites are. Some are fancier than others, with graphic maps, and others with just numbers. In reality, the fancy display isn't necessary, since as long as the receiver tells you where you are, you don't care how it figured it out. Knowing what the direction is to each satellite is handy if you are having trouble receiving enough satellites, and want to figure out if something is blocking the signals.

Storing fixes or locations:

Typical GPS receivers can store 100-250 locations, which you can use later to plot positions on a map, or to use to retrace your steps (like the lost Inca gold in the example given earlier). You can store your current position (and time) into the unit's memory for later retrieval with a single button (often labelled Man Overboard, with obvious marine applications). You can also enter locations manually, say from a map or gazetteer, and later use the GPS unit to guide you to them.

Navigate to stored location

The receivers all provide a function in which the receiver gives you instructions on how to go to a previously entered location: turn left or right, and an indication of how far you have to go, as well as how fast you are moving right now. Some do this with a little picture that looks like a road you're supposed to follow, the others do it by giving you a steering indicator, like a needle on a compass.

Trips and Routes

The receivers allow you to enter a route or trip, defined as a series of locations (called waypoints) along the path. The receiver tracks your progress along the path and tells you how far off path you are, and whether to turn right or left to get back on the path. Usually, the receiver will tell you when you are expected to get to the end of the trip, assuming your speed remains constant.

External antennas

As we discussed before, GPS works by receiving signals from satellites in the sky. If you can't see the sky, you can't figure out where you are. This presents a real problem in a vehicle. Therefore, all of these units either have a detachable antenna and extension cable or an add-on external antenna, which you can put outside (using a magnet, suction cup, or the ubiquitous baling wire and duct tape), connected by a cable to the receiver which is inside.

External interfaces

The more expensive units have interfaces to external equipment, like a computer or autopilot. Depending on your needs, this may not be particularly useful (NMEA interfaces, the most common, are not directly compatible with most PC's).

Adjustable receiver parameters

Some receivers give the user the opportunity to adjust some of the receiver's operational parameters. These are the high tech equivalent of all the knobs on an expensive stereo. The problem is, unless you know a lot about GPS, and how the receiver works, you wouldn't have any idea how to set these values. However, if you are a GPS expert, you might want a receiver that lets you twiddle the knobs.

Field Tests

All well and good, from a theoretical standpoint. But what's really important is how well these little boxes work in the field. We asked the leading manufacturers to provide evaluation units for us to, well, evaluate. We didn't have a budget to go to the Amazon, or even Hawaii, for jungle testing, nor to the Andes for testing in the mountains, so we tested them in Southern California, where there are mountains, deserts, and jungles (of the urban sort).

Normally, reviews of high technology products have lots of comments about how the insides of the box work, and lots of esoteric detail, often designed to show the erudition of the review author. We feel that the user really doesn't care how it works, they only care if it works well, and does something useful. Therefore, we won't talk about how many channels the receiver has, or any other technical specs, except as they pertain to the overall usefulness of the unit. Call us up if you want to talk about esoteric details like early/late gate vs. tau dither loops, software channel multiplexing, and geodetic datums. The manufacturer's literature (copies on file at the club) also gives you all of this information, and more.

Accordingly, we focused on three basic areas of evaluation. The first is the issue of how long does it take the unit to get the first fix, and how well does it keep up with you after that. The second is battery life. The third is how well will it work in the field, actually being used to navigate or measure positions.

We have tried to give you a feel for what these GPS receivers can do, and what kinds of receivers are currently available. It is very important to go out and try the receiver you are thinking of buying. Hold it in your hand, press the buttons, walk around the parking lot with it, see if the display is readable, and so forth. Call the manufacturer to find out who stocks their receivers in your area (local marine and aviation supply places are a good bet).

Environmental Ruggedness

The units we reviewed are all designed for field use, and are relatively rugged and durable. They are all splashproof or waterproof, which is essential for field use. Not only might you need to use it in the rain, you would hate to have your moderately expensive receiver ruined by an unexpected dunking when the canoe overturns. Sure, there are lots of places you might want to go, like the Atacama desert where waterproofness is superfluous. Bear in mind that waterproof also means dustproof. You are more likely to encounter dust than water, and that fine, omnipresent grit is great at grinding away delicate electronic components.

Most of these units will work at any temperature where the user can work. They all use Liquid Crystal Displays (LCDs), which tend to fade and temporarily stop working in cold temperatures. If you get them cold enough, the liquid in the LCD may freeze, damaging the display (although not the rest of the receiver), so if you plan on taking one of these units to the top of Aconcagua, you will want to carry it inside your parka.

How long does it take to get the first fix

This is probably the primary difference between the receivers. When you turn your GPS receiver on, it has to scan for and lock onto the signals from the satellites that are currently in view. If the receiver can guess where it is and what time it is, it can use an internal almanac to figure out which satellites (of the 20 or more) are good candidates (i.e. above the horizon). If you are starting from scratch (i.e. you don't know where you are or what time it is), the receiver doesn't know which of the satellites might be in view, so it has to look for all 24 of them, one at a time, until it has enough to get a fix. This process can take 10 minutes or more. If the receiver knows which satellites are probably in view, it only has to scan for the signals from those satellites, which generally takes about 30 seconds to a minute.

Most of the time, you only have to do the long satellite search if you have travelled a long way (like Miami to Lima on a plane), or if the batteries have been removed for a while (even "dead" batteries have a little juice left, enough to keep the receiver's memory intact). Most receivers have a provision where you can tell it approximately where you are and what time it is, which speeds up the long search process.

Once the receiver is locked onto a set of satellites and has calculated your position, it can calculate a new position almost instantly (typically less than a second). The receiver continuously monitors the signals from the satellites, and when they get weak or disappear (like if the satellite has gone below the horizon), it scans for a new satellite(s) to lock on to. This is especially important when you are in a valley or an urban environment, where cliffs or buildings block the signals from the satellites as you move around.

Satellite acquisition and tracking is where clever design on the part of the manufacturer can make a big difference. There are lots of ways to go about searching for good satellites, and also how to anticipate when to switch off to a new one. Everybody has their own technique and design, manifested as different performance under the same satellite configuration. It gets particularly tricky if you want to make the whole system low powered.

Battery life.

If you are going to be trusting your navigation to a device which runs off of electricity, you had better make sure you have electricity when you need it. All of these portable receivers use batteries for power. The burning question becomes, how long do those batteries last? Am I going to have to carry a hundred pounds of AA batteries, just to find out where I am?

We used a mountain of batteries and drained them flat when testing these receivers. Better that we should run out of batteries across the street from Kmart than you should in the jungle outback. Do the manufacturers tell the truth when giving battery life specs?

It varies. They will all run for well over the minimum time given, but it's probably pretty hard to get the absolute maximum. It turns out that a lot of battery life is determined by the user's habits. Do you always use the backlight for the display (it typically draws a lot of power)? Do you use the fast fix mode? Just how fresh were those batteries, anyway? Is it really cold out (below freezing, alkaline batteries lose their power)?

In expedition slogging, where you are trekking towards a goal and you just want to find where you are occasionally, you don't need to leave the receiver on. When you need a location fix, turn the receiver on, wait the minute for it to find the satellites, and record the position. Then turn it off. If you do this once an hour, a receiver with a paltry 4 hour battery life (they are all better than that) will work for 10 days. If you use this mode of operation the "time to first fix" is important, since once you have that first fix, you will probably turn the unit off.

In an continuous navigation mode, where you are holding the receiver in front of you like a compass, the battery life issue changes to how much power does the unit draw while it is running continously. Some receivers, like the Trimble Scout, have a mode where most of the electronics (except the display) is turned off, except for every 5 seconds when it takes a fix. There is also usually an option that just turns the receiver off automatically after a preset interval (like 15 minutes) with no button pushes, if you forget to do it yourself.

If you are going to be doing a lot of travelling in a vehicle, an external power adapter would be a good investment. That way, you can run off the car battery forever, without running down those AA batteries. For extended field use you can get portable rechargeable 12 Volt batteries. You could even use NiCd rechargeables, and recharge them with a solar panel.


Trimble Scout (list price $795)

This is a new model from Trimble Navigation, who was an early pioneer in low cost GPS receivers. Based on their earlier Ensign models, it weighs about 14 oz (with batteries), and is a slim curved handheld unit with a very nice feel. The antenna is built in to the top of the case. The Scout uses a eight buttons to cycle through a series of menus on its 4 line LCD display.

Unlike the Ensign, which was a marine unit, the Scout is designed for use on land. The slickest feature is the the ability to display and enter positions in a whole host of formats, as opposed to just Lat/Lon or UTM. For instance, if you tell it the map scale, you can display and enter positions as inches or centimeters on the map relative to a known location (like the corner of the map). For users who are in the United States, it comes with a complete road atlas, and will give you positions as page and grid in the atlas, or as detailed page and grid for the Thomas Brothers series of city street atlases (in Los Angeles, this feature alone would pay for a Scout if you were always getting lost). It can also tell you the phase of the moon (and where it will be) for any date, time, or position, which you might be able to use for predicting eclipses, should it prove useful in encounters with primitive, but aggressive, natives.

Trimble has chosen an overall philosophy of emphasizing user needs and functions, as opposed to the technology. You don't get a lot of options to set filter parameters, manually select satellites, etc. That is all buried inside the unit. What you do get is dozens of ways to display your position, in convenient units. Overall, most users will find this a boon, since they really don't care about the esoterica of GPS, but don't want to have to interpolate between grid lines on a map.

The manual is clear and well organized. It doesn't go into too much detail about how GPS works, but concentrates on using the Scout. If you need to know how it works, Trimble publishes a very good 80 page booklet about how GPS works. When you first pick up the unit, you need to spend a few minutes getting used to how the menus are arranged, and how you use the "cursor keys" to select menu items, but once you have done that, it is quite easy to use. Entering numbers and letters is a little bit of a pain, since you have to use the cursor keys to step through A-Z,0-9, and punctuation. A little practice makes this fairly fast though.



Garmin GPS-75 Personal Navigator (list price $1419)

This receiver looks like a slighly overgrown Walkman in size and weight (about 19 oz with batteries) , and has a graphic LCD display, as well as a 20 key keypad. It has a cylindrical antenna (which you are supposed to aim vertically) attached to the side of the case by a pivot. The GPS-75 is a newer, more sophisticated version of Garmin's GPS-50 ($1195), which was primarily a marine product, and which does not have the graphic display.

The graphic display is really the Garmin's best feature. It is clear and sharp, and has a really neat feature which displays a map of your selected route, as well as your actual path along the route. The Garmin, just like all the other receivers, uses a series of menus to control operation, enter waypoints, and so forth. It has a numeric keypad, which simplifies entering numeric coordinates. The GPS-75 can display positions in lat/lon coordinates, UTM coordinates, as well as British and Irish Grid.

A unique feature of the Garmin receiver is that you can enter up to 9 "proximity waypoints" which cause an alarm to sound if you are within a user specified distance of them. In marine applications, this is used to warn of rocks or reefs, but there are probably similar land based applications.

The Garmin has a whole slew of features to display charts and graphs of where the satellites are, how strong the signals are, and so forth. However, in practice, most users don't really care about this, only wanting to know where they are or where their destination is.

The GPS-75 has an external interface built in which can talk to NMEA compatible marine electronics, as well as receiving differential corrections, which can improve the accuracy of the position it calculates to within a few feet. The external interface can take any DC power from 5 to 40 Volts (like 12V from a car battery), and can also sound an external alarm when certain conditions are met (loss of satellite lock, closeness to preset locations, etc.)

The GPS-75 acquires within 2 minutes for a 2D fix, although a special "warmstart" mode gets a fix in 15 seconds. The 4 AA batteries should last 5 hours in normal mode, and 7 hours in power saver mode. As always, your mileage may vary.

The manual is fairly complete, providing a lot of information on how to use the GPS-75, with diagrams and pictures for the most common operations. However, it seems that for many of the more sophisticated features, the description of how to use it is somewhat sketchy. Also, there isn't a whole lot of information about what to do when things go wrong. Sure, it's nice to know that you have "Degraded Accuracy", but some helpful hints about what to do might be useful.



Magellan GPS NAV 5000D (List price $1299)

The Magellan receiver is the largest and heaviest of the lot we reviewed (a little less than 2 lbs, with batteries). It has a group of function keys above a numeric keypad, and a 4 line LCD display. The antenna is a sort of rectangular box attached with a pivot to the side of the unit. This receiver is representative of the previous generation of handheld GPS units. Magellan is coming out with a new model, the Trailblazer, in the fall of 1993, which I'll talk about at the end of this section.

In all, the 5000D has basically the same accuracy as other GPS units, it has the usual features to remember waypoints, plan trips, and guide you along a stored route. It has an external NMEA interface to connect to various marine electronics. One feature of the 5000D, as opposed to the ordinary 5000, is that it has the capability to receive "differential corrections" from another special GPS receiver, which results in corrected positions accurate to about 3-10 feet.

A useful feature of the 5000D is that it can calculate a satellite availability schedule for a specified location and date, telling you if there are any times that you might not be able to get a full 3D, or even a 2D fix. It takes 4 satellites in view to get a 3D fix, and due to satellite failures or your location, it is possible (although unlikely) that not enough satellites will be visible.

The manual for the 5000D is very well written, and has a helpful section with troubleshooting and operating tips. It shows a picture of the message on the screen, and tells you what to do to fix it. The manual shows the NAV 5000's marine orientation, as most, if not all, of the examples are related to sailing a boat.

The 5000D is rated to take 55 seconds to first fix from a cold start and 35 seconds from a warm start (last fix was less than an hour ago, and still using same satellites), however, it always seemed to take longer than that for us when we were testing. The 6 AA batteries should last 10 hours of continuous use. The 5000D comes with an external interface/power cable to connect to NMEA devices.



Just as this article was being finished up, I found that Magellan is releasing their version of a lightweight handheld GPS receiver called the Trailblazer. The Trailblazer weighs less than a pound, and runs for 4 hours minimum on 3 AA batteries. It has a very handy feature which turns most of the receiver off, except every 10 minutes when it wakes up, takes a fix, and stores it in memory. You can then select a "backtrack" mode which will guide you back along those saved positions. It displays positions in lat/lon or UTM, and comes with an external interface. Best of all, Magellan expects it to sell for about $500. Look for this unit when it comes out.

Other receivers we didn't have a chance to review

Of course, there are other manufacturers of handheld GPS receivers. We haven't reviewed them in detail, either because we couldn't find anything out about them or because we couldn't get an evaluation unit from the manufacturer before the deadline for this article. GPS is a new technology, and many of the manufacturers are having trouble figuring out where to position it and which division should handle their GPS products. This makes tracking down information a little bit difficult.

Motorola has a unit called the Traxar which is priced in the $700-800 range, which has been out for about a year. It weighs 19 ounces and is about 4 x 8 x 1 1/2 inches. The Traxar uses 6 AA batteries, which should last 6 hours. It is primarily a marine unit with the low end unit displaying in lat/lon only (the Traxar MG+ can also display in UTM/UPS coordinates). Motorola sells a "Smart Bracket" which provides a NMEA interface and external power. It is sold through marine distributors, as well as some catalog outlets (e.g. L.L. Bean). The Motorola unit has a 3 year warranty and toll-free technical support.

Sony has a relatively sophisticated unit, the Pyxis IPS-720, which has a graphic display, and appears to be aimed at the marine and aviation markets, since they will have little memory cards with chart data preprogrammed in it available. You will probably see Sony's original GPS offering, the Pyxis IPS-360 in the discounter catalogs for about $500. It may seem like a good deal, but it is reputed to have very short battery life, and take forever to get a first fix. The Trimble Scout or Magellans Trailblazer are much better units at the same or slightly more money.

ICOM, better known for handheld radios and amateur gear, also has a GPS receiver, however, after seeing it once at a marine supply store, we haven't been able to find out anything about it.

Which one do I like?

My personal opinion, based primarily on gut feel, favors the Trimble Scout. It's cheap, it looks real cool, it's lightweight, and it displays positions in a wide variety of formats, including inches on a map. It would be nice if it had an external interface, but you can't ask for everything, and there will probably be a model that has that option soon. The other units provide more information about where the satellites are (with neat little graphs in the case of the Garmin unit), however, I would rather have the plethora of ways to display my position, like the Scout does. The "moving map" on the Garmin GPS-75 is pretty neat, but isn't much use on land, where you are usually constrained to a road or trail.

The Magellan Trailblazer, due out in fall 1993, looks very interesting, and apparently competes directly against the Trimble Scout. I'll be waiting to see what it looks like and how it works.


Thanks to all the people at the manufacturers and users of GPS gear that provided information. Thanks particularly to Dave Sprague at Trimble, Jim White at Magellan, and Mike Keener at Garmin, for supplying GPS receivers for us to test so that we could give you some real hands-on feedback.

Author's Bio:

Jim and Sally Lux, when not exploring, run a computer consulting firm in Westlake Village, California (about 50 miles from Los Angles) called Innovation Consulting and Design. Jim has been working with high tech stuff like GPS since he was a very little boy, dismantling the family vacuum cleaner, although now he builds things more than he takes them apart. Sally is really an actress, but since that doesn't pay very well, she has been training users and evaluating hardware and software for over 10 years. Their next expedition is to see the total eclipse in 1994.

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