Satellites, Signals, And You

In The Market For A Handheld GPS?

June 5, 2001 By Elizabeth A Tennyson

Do you know where you are, where you're going, and how you're going to get there? For pilots, those aren't existential questions - they're at the heart of every cross-country flight. Every student learns to find his or her way above the world with the help of charts, landmarks, and radio navigation aids such as VORs and NDBs. While it's vital that pilots master and practice these tried-and-true navigation techniques, there's another alternative in the modern cockpit - the global positioning system (GPS).

Not all aircraft are equipped with GPS, but that doesn't mean you can't take advantage of this technology. Purchasing your own portable, handheld GPS receiver gives you the opportunity to carry a unit with which you are intimately familiar in any airplane you fly. Pilots who rent aircraft with different panel-mounted GPS units soon learn that every model works a little differently, and every manufacturer has its own set of operating conventions. That's just one reason why it's a good idea to fly with your own receiver. Another advantage of a handheld unit is that it can run on battery power - a great navigation backup if your aircraft's electrical system quits. Add to all that the fact that GPS is widely regarded as the single easiest way to navigate, and you've got plenty of reasons to go shopping for a unit of your own.

While some people have begun using GPS for ground-bound navigation, it is still far more common in aviation and marine applications. If you're like the majority of pilots, you probably had little or no exposure to GPS before you joined the flying community. So exactly what is GPS, and what do you need to know before making a purchase?

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 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 at the same time 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 satellites 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 five channels - 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 one or more 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. But the military does have the authority to resume its signal degradation at any time - so it's important to be aware of the possibility. Still, for most aviation applications - particularly VFR navigation - the signal is accurate enough to get you to your destination, even when it is being intentionally degraded.

Types Of Receivers

Now that you know how GPS functions and some of its potential problems, what should you look for in a receiver?

While GPS receivers may be designed for a variety of functions - aviation, marine navigation, land navigation, etc. - how effectively they function in each of these environments depends on their design. There are three primary types of receiver: sequential, multiplex, and continuous. Sequential receivers are the least expensive, but they're no good for aviation use because they have only one or two channels, meaning they are relatively slow to pinpoint your position and cannot track satellites while moving at high speeds. Like sequential receivers, multiplex receivers switch back and forth between satellites being tracked. While they can follow more satellites than sequential units, they are still slower and less accurate than continuous receivers. As the name suggests, the latter can track several satellites at the same time on a continuous basis. Continuous receivers require a minimum of four channels to read the data from three satellites. Five channels allow the receiver to follow four satellites simultaneously and so on. Continuous receivers known as all-in-view receivers have as many as 12 channels to track all satellites within their range simultaneously, and these are fast becoming the most common aviation receivers.

In addition to these different receiver designs, receivers generally operate in two different ways: code correlation and carrier phase. Code correlation receivers determine position by processing information from the code that is transmitted by the satellites. These receivers, which are the most common and the most affordable, are accurate to within about five meters. Carrier phase receivers determine position by processing the satellite signals' carrier phase. Such receivers may be accurate to within a few centimeters, but they are very costly and impractical for most uses.

Features To Look For

Your best choice for flying is a continuous, code correlation GPS. But that's only the tip of the iceberg when it comes to available features. Most GPS units have a moving map, zoom function, a variety of views, a "nearest" function to help you find the closest airport in an emergency, a "direct" function that will show you the shortest route from where you are to the location of your choice (called a waypoint), and backlighting to make it possible to see the display at night or in bright sunlight. Some even include communication transceivers, melding two useful handheld items into one package.

Beyond that, the options are many and varied. GPS units may have full color or monochrome displays. They also come in a variety of screen sizes and resolutions. To maximize safety and minimize head-down time in the cockpit, it's important that you be able to easily see and quickly interpret the information on your GPS display. So be sure to look at a variety of units and choose one that you can see clearly.

Since the object of using a handheld GPS is to help you get where you're going, the type of database it holds is critical. The price of the database, and therefore the GPS, depends on what you need. Most aviation databases include airports, VORs, NDBs, airway intersections, ILS markers (even though no handheld units are currently approved for IFR operations), and other features. In addition, some databases include communication and weather frequencies, airport and runway information and diagrams, and even information about restaurants and other ground features. Databases are available in many sizes and may contain information for a region including a few states, for a single country such as the United States, for a continent such as Europe, or for the entire world. How much information you want dramatically affects the price you can expect to pay, so don't buy more than you need unless you just can't resist having the world at your fingertips.

Once you have the database information in your unit, it's good for only a limited period of time. To make sure that you have up-to-date navigation information, you will need to update your database frequently. Some companies offer subscription services that include updates at regular intervals, such as 56 days or six months. This can be a convenient option, particularly if you rely heavily on the navigation information in your GPS. You should also consider whether or not a specific unit is field upgradeable. In some cases, you can replace the data card in your machine yourself or download new data from a Web site. In other instances you must physically return your GPS to the manufacturer for a data update - a major inconvenience if you do a lot of flying, since you can expect to be without your unit for at least a couple of weeks.

As GPS applications become more varied, some units can be converted for use in the airplane, on the ground, or on the water. These units typically have interchangeable data cards that contain information appropriate to the application. So, by carrying one GPS and a couple of small data cards, you can find your way to the airport of your choice and then navigate to your hotel without ever having to stop and ask for directions.

In addition to the data that is included in the database you select, you can program your own waypoints into most units. The number of self-created waypoints and flight plans that you can store varies a great deal among GPS receivers. You probably want a unit that will hold at least a handful of reversible flight plans and waypoints so that you don't have to keep reprogramming your unit with routes and locations that you fly to frequently.

The fact that handheld GPS units are battery-operated is an important part of their value. Different models use different amounts of power so it's important to know how long any unit you purchase will run on batteries. If you can, test this out for yourself or ask others who own the same unit; manufacturers are notorious for overestimating battery life. Most units will last at least three hours on new or fully charged batteries, but expect that time to decrease dramatically if you are using the backlighting feature. Some units use standard batteries - most often AA or AAA size - while others use specially designed rechargable battery packs. Both work well, provided that you remember to recharge or replace the batteries before each flight. Also remember that the number and type of batteries can make a huge difference in a unit's weight.

In addition to batteries, many units can be run from an external power source, such as the airplane's cigarette lighter. Be sure to find out whether any unit you buy can be operated from an external power source and if a power cable is included with the unit or is available for purchase as an option. Having a power cable for your GPS is a good idea because it allows you to fly with the GPS or program it on the ground and save the batteries for when you really need them.

Another feature available on some models is a remote antenna. This is typically a small, removable antenna, often with a suction cup, that can be placed in a window to reduce the time it takes the unit to acquire satellites. How important a remote antenna is depends a great deal on what kind of aircraft you fly - the more metal between you and the sky, the more likely you are to need one. Even if you don't really need a remote antenna, you may find one useful - anything that improves acquisition time can be a bonus.

Other nice-to-have items include yoke mounts, wrist or knee straps, and carrying cases. These items are standard equipment for some units and available as options on most others.

Price

GPS units have come down dramatically in price in recent years, putting this technology in reach of most pilots. GPS units, without a database, are available for as little as $160. Add the database and you can be out flying for under $300. That, of course, is a bare-bones model, but bare bones may be exactly what you need, depending on what type of flying you do. Fancier models, including those with color, can cost upwards of $1,500. Again, whether or not this is a worthwhile investment depends on your preferences and the type of flying you do.

Because the type and number of features available vary so much, it's important to know what comes with your unit. For instance, one unit may sell for $500, but by the time you add a power cable, battery pack, case, and yoke mount, your price climbs to $700. Another unit may sell for $650 but include all those "extras." So be sure you know exactly what you're getting.

Once you've selected a unit, check pilot supply stores, the manufacturer, and catalogs for the best deals, because prices vary. When you have purchased your GPS, spend time with it on the ground practicing programming and activating flight plans, changing views, and scrolling through data. If you must experiment with your new toy in the cockpit, be sure to bring a safety pilot. Moving maps can be mesmerizing, and you are sure to find yourself head-down more than is really safe until you have mastered your model.