Buyer's Guide to Handheld GPS

More than direct-to

Garmin GPSMap 396

Garmin GPSMap 196

Garmin GPSMap 296

Lowrance Airmap 500

Lowrance Airmap 1000

hether you consider GPS (global positioning system) to be the best thing since sliced bread or you see it as a too-rich-for-my-blood alternative to your trusty sectional chart, you have to admit that this form of navigation has made getting from point A to point B easier for an entire generation of pilots. Because of GPS's popularity and the advancement of computer technology, several companies have entered the market for handheld GPS units, joining familiar faces. Our guide helps you to understand GPS and the product features out there so that you can better determine what unit suits your style of flying best.

How it works

The global positioning system is made up of 24 military-owned satellites and the ground stations that keep them working effectively. The United States Navy and Air Force first proposed what was then called a Defense Navigation Satellite System in 1973, and the first of the system's 2,000-pound satellites was launched in 1978. So GPS, in one form or another, has a fairly long history. But the FAA didn't approve use of the system for civil aviation until 1993, and GPS wasn't officially integrated into the national air traffic control system until the following year, so it's still pretty new to the world of general aviation.

The theory behind GPS — triangulation — may take you back to high school geometry classes. The idea is that by measuring the distance from your position to each of three satellites, you can pinpoint your location in space. Of course, it's not quite that simple. Actually, measuring the distance between your location and three satellites narrows down your position to two possible points. More often than not, one of those points is not a reasonable answer — it may be moving very quickly or show you hundreds of miles above the Earth's surface. That means your receiver can then reject that improbable answer and put your location at the other possible point with reasonable certainty.

Most of the time, your receiver is in view of more satellites than it needs to find its location. As a result, it must choose which satellites to use as measurement points. Most receivers will select satellites that are as widely separated as possible in order to provide more accurate position information. If necessary, a fourth measurement can identify your actual location from the two possibilities and provide you with altitude information. Aviation GPS receivers do take a fourth measurement, but that measurement is used primarily for timing rather than identifying location. More about that later.

Potential errors

To accurately measure the distance from the satellite to your receiver, you must have an extremely precise way of measuring how long it takes a radio signal — which each GPS satellite broadcasts to civilian users on 1575.42 MHz — to travel from the satellite to the receiver. Because it takes only a few hundredths of a second for a signal from a satellite directly overhead to reach the Earth, it's vitally important that the time measurement be precise — even a small inaccuracy could make navigation impossible. (A measurement that is off by just one one-thousandth of a second would put you approximately 200 miles off course.)

There's no really good direct way to make sure that your GPS receiver keeps time accurately. That would take an atomic clock. And while every GPS satellite is equipped with such a clock, they are much too heavy and expensive to put into every GPS receiver. Instead, there's an indirect way to make sure that your receiver measures time as precisely as an atomic clock. Every GPS satellite broadcasts a pseudo random code or PRC. (The PRC isn't really random; it's just such a complex digital code that it looks very much like random electrical noise.) Each satellite has its own PRC, allowing your receiver to identify exactly which satellite it is receiving. The difference between the time any given portion of a satellite's PRC is transmitted and the time it reaches your receiver can be translated into the distance between the two — but only if your receiver is in sync with the atomic clock on the satellite. To make that work, your receiver must take a measurement from a fourth satellite.

As you already know, your receiver identifies its location by triangulating its position relative to three satellites. The fourth satellite measurement will not intersect with the other three. When that happens, the receiver tries to find a single correction factor that will make all four measurements intersect at a single point — your location. That correction factor is a time measurement. Once the receiver has determined what that measurement is, it applies the correction to all of its timing measurements, bringing them into perfect sync with the satellites' atomic clocks. What that really means to you as a GPS buyer is that you want a unit with at least four channels (which all aviation-specific handhelds have) — that way it can make all four measurements simultaneously, improving the speed and accuracy with which the unit finds itself, and you.

The timing problem isn't the only one that GPS designers have had to resolve to make the technology both accurate and affordable. In order for a GPS receiver to identify its location relative to satellites, it must know exactly where those satellites are at any given moment. GPS satellites orbit at high altitude — some 11,000 miles above the Earth's surface — to keep clear of atmospheric interference. Outside of the atmosphere, they orbit according to relatively straightforward mathematical formulas. Even so, over time those orbits may wobble a little bit as the satellites come under the gravitational influence of the moon and sun.

To compensate for those wobbles, the Department of Defense regularly monitors each satellite, using radar to check its precise position and speed from a ground station. Once the Defense Department has determined the satellite's position, it sends that information to the satellite itself. The satellite then broadcasts this updated position information, known as ephemeris information, with its signal. Your GPS receiver decodes that signal, making it possible to determine exactly where each satellite really is at any time.

A variety of other factors can also bring inaccuracies into GPS. As the signal passes through the atmosphere, it can be slowed down a little — something known as propagation delay. Modeling the likely conditions at a given moment in time is one way to predict and compensate for these errors. A more precise method is to use a dual-frequency measurement to compare the relative speeds of two satellite signals. Most GPS receivers do not have dual-frequency capabilities, but the errors are so small as to be insignificant in most applications. Multipath error is another potential problem for GPS. It occurs when the satellite signal bounces off obstructions in the area of the receiver before the signal reaches the receiver itself. Signal rejection techniques employed by some receivers help to minimize this type of error.

Finally, there are intentional errors. The military recently stopped degrading GPS signals for civilian users — a practice that intentionally decreased the reliability of the signal called selective availability. But the military has the authority to resume its signal degradation at any time — so it's important to be aware of the possibility. Still, for VFR navigation the signal is accurate enough to get you to your destination, even when it is being intentionally degraded.

WAAS

More recently, the accuracy of GPS navigation was ratcheted up a notch when the FAA turned on the WAAS (wide area augmentation system) signal for use by general aviation on July 10, 2003. WAAS is a system of satellites and ground stations that make corrections to errors in the GPS signal, enhancing the accuracy of the signal — and the position information delivered by a WAAS-enabled GPS receiver — by up to five times its norm.

WAAS was developed beginning in 1995 by the FAA in an effort to move GPS navigation toward a point at which it could provide ILS-type approaches (precision lateral and vertical navigation) to aviation users. While at the time of this writing there were no ILS-type approaches (called LPV or localizer performance with vertical guidance approaches) activated for general aviation use, the first of the panel-mount WAAS-capable IFR GPS navigators, the Garmin AT CNX80, was expected to be certified for that use sometime during summer 2004. LPV approaches were expected to follow shortly thereafter.

While handheld receivers are not approved for IFR use, several handhelds available are marketed as WAAS-capable. What does this mean for you as a VFR user? The handheld is simply that much more accurate — instead of being accurate to 20 to 50 meters, the WAAS-corrected signal is accurate to 2 to 3 meters vertically and 1 to 2 meters horizontally.

Types of receivers

There are two basic varieties of handheld GPS receivers: those specifically designed for aviation use, and those which involve aviation-specific programs that run on general-use personal digital assistants (PDAs) or laptop or tablet PCs.

Dedicated aviation units consist of a hard case with several buttons, an attached antenna or port for a remote antenna, an internal battery pack or outlet for a power supply, and a color or black-and-white screen. A mounting mechanism attaches the unit to either your aircraft panel or control yoke.

GPS receivers for use with PDAs and portable PCs typically involve the software program (either on CD-ROM, DVD, or downloaded from the Internet), a GPS antenna to plug into the device, a stylus or mouse to navigate onscreen buttons, and a power cord to plug into the aircraft power supply. Yoke or panel mounts are also available for PDAs and some tablet PCs.

Features to look for

Screen size, color, and resolution. Some obvious differences between units lie within the screen. Units themselves vary in size from shirt-pocket receivers to tablet PCs that cover your lap. To a certain extent, screen size is directly related to the size of the unit, but how information is displayed on the screen also plays a role in how easily you can read the map and data.

A screen's layout is divided into a map or, in some models, simulated panel portion, with a number of data blocks arranged on the sides of the screen. The user often has some choice of layouts, with the further option to determine the data displayed in the blocks. Depending on the size of the blocks, they can take up a good portion of the screen — so the option of a full-screen display in which you can see more of the map as you progress along your route (and more detail as you zoom in) is nice to have.

Screen resolution determines how clearly you can see the detail on the map, and in a limited fashion how well you can read the map in strong sunlight. The refresh rate defines how quickly screen information is updated, and how quickly the map and data is redrawn. A slow refresh rate shows up most obviously in turns and as you move from page to page within the unit.

Some manufacturers offer color screens in addition to black and white screens. While color adds a modicum of detail and snap to a display, it also adds cost to the unit. Black and white screens currently use gray-scale to show detail, which somewhat mitigates the impact of color. But for higher-end features such as terrain and weather depiction, color is important.

Basic functions. Most GPS units offer some variation on several core functions: the ability to fly direct from your present position to a waypoint, the ability to enter a flight plan route, and the ability to call up the nearest airport, navaid, service, or waypoint from your present position.

Because the Direct-to feature is the one most used by pilots in VFR navigation, most units have a dedicated button either on the case (in the case of a dedicated aviation unit) or on the screen (in the case of a PDA- or PC-based program). After pressing the button, the unit calls up a menu of waypoint types, or allows you to enter a waypoint name. This feature should feel easy to use and logical to you, since you'll probably use it more than any other one function.

Many units offer you the ability to enter and store flight plans, and recall flight plans from memory. The number of flight plans you can store varies from unit to unit, as does the way you enter and name those flight plans. You can also use database information to look up airport layouts, frequencies, and airspace.

Another critical feature is the Nearest function. Nearest points you to the airport, navaid, or fix closest to your present position, and many units offer the ability to find the nearest flight service station or air traffic control frequency. While useful for everyday flying, Nearest helps guide you to safety when an emergency or urgency situation arises. If you encounter headwinds stronger than forecast, you may need to find the nearest airport with fuel service, and many units offer databases with this information. If you become disoriented or encounter deteriorating weather, you may need to quickly locate an airport, navaid, or ATC frequency for a clearance or assistance. And if you suffer a mechanical problem, Nearest can help you quickly determine if you can make it to the nearest suitable airport. Like the direct-to feature, most units offer either a dedicated button or on-screen button to activate the Nearest function.

Advanced functions. Because computer technology continues to advance, more features have been added to both dedicated units and PDA- and PC-based programs. Chief among these are vertical navigation (VNAV), simulated panels, trip planning functions, runway extensions, datalink weather, and automobile navigation and databases.

VNAV has been available in some dedicated aviation units for several years. You can enter a desired rate of descent and distance from an airport or waypoint, and the GPS tells you when you reach the top of your descent or VNAV profile.

Simulated panels take GPS-derived altitude, heading, groundspeed, and vertical speed and use these values to mock up an instrument panel, interpreting the values into displays on standard instruments, such as altimeter, horizontal situation indicator, vertical speed indicator, turn coordinator, and airspeed indicator. While the panel page cannot be substituted for required instruments in the airplane, and while there are several caveats to the use of this information for aircraft control, the panel page can provide some degree of position information in the event of an emergency. And some pilots find the analog displays easier to interpret than digital readouts.

Trip planning functions allow you to look ahead to the next leg of your trip and help you determine fuel stops, time en route, and possible diversion airports. By entering the fuel flow and amount of fuel on board your airplane, you can use the unit for advanced flight planning.

Runway extensions project the extended runway centerline for, typically, the primary hard-surface or instrument runway at an airport. This line may help you navigate to the airport, visualize the airport layout, or set up for a traffic pattern more easily.

Datalink weather, primarily available to PDA- and PC-based devices, uses either satellite or ground-based installations to deliver data to the cockpit. Weather data available often includes Nexrad radar graphics, Metars, TAFs, convective sigmets, sigmets, airmets, echo tops, and/or lightning strike information. Information is either downloaded in a continuous stream or offered on a request-reply basis.

Automobile navigation and databases provide you with direction after you land and once you've started the crew car. Some units offer databases with local services for travelers as well as highway exit and street-level information. Marine navigation is also available on some units.

Price. As with many portable electronics, you can spend within a range and get many of the options listed here.

New dedicated aviation units start around $500 and go on up to $1,800, often excluding accessories such as additional databases, yoke mounts, and adapters. Cost to upgrade the unit's database varies. As a VFR pilot it's up to you how often you update your database, but it's wise to do so every six months to one year.

PDA- and PC-based software typically runs from $100 to $300, with additional cost for GPS receivers, remote antennas, yoke mounts, power adapters, and additional databases. If you don't already own a compatible PDA or laptop or tablet PC, you need to factor in that purchase as well. PDAs cost from $100 to $500 and up, and tablet PCs from $1,800 to $3,000 and up. You face the same expense of upgrading the program's database as with dedicated handhelds, though some programs offer subscriptions included with the price.

For more information on GPS receivers and programs, please visit the following links.

• AOPA Subject Report: GPSs and Multifunction Displays
• "GPS: New pilots guide to satellite navigation", June 1998 Flight Training