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Feeling Lost? An Overview of Global Positioning Systems

Until about five years ago, the Global Positioning System (GPS) existed in the realm of high-tech military thrillers. Fictional spies would tote hand-held units that precisely displayed their locations (or that of their objectives) anywhere on earth – with street maps and 3D topographic representations to boot!

Reality imitates art. In the past ten years, three amazing things have happened. First, the U.S. military opened up the Global Positioning System for civilian use. Then, the price of receivers plummeted from the $1,000-$10,000 range to $100-$200, making them widely affordable. And last, advanced computer-controllable units have appeared, making integration with personal computers a reality. With your Macintosh and about $300 in additional hardware and software, you can do things that were science fiction just a little while ago.

In this article, I explain the technology behind the Global Positioning System and discuss some receiver units currently available. Later in this issue, TidBITS Managing Editor Jeff Carlson talks about GPS technology from a user’s perspective, and reviews GPSy, a GPS communications software package I developed for the Macintosh.

Your Tax Dollars at Work — The Global Positioning System is truly amazing. Developed by the U.S. military at a cost of several billion dollars, GPS is based on 24 orbiting satellites (space-heads call them SVs, which is short for "Space Vehicles"). These satellites broadcast a precise data signal that allow GPS receivers to locate themselves anywhere on the planet. A receiver can calculate its position (latitude and longitude), altitude, velocity, heading, and precise time of day. Most units also have a built-in mapping feature that displays their positions relative to waypoints you’ve pre-programmed into them and a plot trail that shows where you’ve travelled. Advanced models have built-in street or waterway maps, plus serial ports for computer connections.

Military and high-end survey-grade models are accurate to the millimeter level (less than one-sixteenth of an inch). However, standard over-the-counter civilian models are nominally accurate to "only" about 100 meters (roughly a city block). This is due to military-induced Selective Availability – a euphemism for scrambling the GPS signal just enough to reduce the accuracy to sub-military levels. Such scrambling leaves the signal accurate enough to find your favorite fishing hole, but prevents you from accurately dropping a cruise missile into the government’s classified Area 51 base in Nevada. One amusing consequence is that many car navigation systems that use GPS will put you slightly off the road – making it seem as though you’re driving into a river or building!

If you need better accuracy than 100 meters, an FM radio receiver called a Differential GPS unit (DGPS) used in conjunction with your GPS receiver can provide three to ten meter accuracy. The U.S. Coast Guard broadcasts DGPS signals for free along the entire coastline of the United States, and inland for a small subscription cost from various DGPS broadcast companies. The inland cost should go away soon because the Federal Aviation Administration (FAA) wants to use GPS for all aircraft and plans to begin wide-scale broadcasting of free DGPS signals around the year 2000. DGPS receivers currently go for about $500, but once the FAA plan goes into action GPS units should start to have built-in DGPS receivers.

Behind the Scenes — The 24 satellites have a staggered orbit designed so four satellites will be visible from any location on earth 95 percent of the time. This number four is important, as we will see.

Each satellite broadcasts a repeating message, indicating the position and orbital parameters of itself and the other satellites (almanac), a bill of health for the satellites (health bit), and the precise atomic time. The information is encrypted into a signal with strict timing characteristics.

In order to understand how the GPS system works, we’re going to jump into a bit of simple algebra. Remember echolocation from high school physics? If we send out a pulse of sound or radio waves and wait for them to bounce off something and come back, we can determine the distance to the object by dividing the time it took for the reply by the speed of sound (or light).

Distance = Speed * Time

Time = Distance / Speed

GPS works on much the same principle, except that unlike RADAR/SONAR, where the transmitter is also the receiver of the signal, GPS satellites only transmit the timing data pulses; GPS receiver units only receive.

So how does the system work? Imagine you and a friend had precision-synchronized watches and were standing in a football field. If she shouted, "I’m at the far right cornerpost and it’s now 5:00 and 0.0000 seconds!" and you heard this message at 5:00 and 0.333 seconds, you could determine how far away she was by the timing delay of 0.333 seconds. Estimating the speed of sound at around 300 meters per second, you can guess she’s about 100 meters away from you (or that you’re 100 meters away from the far right cornerpost).

Suppose you had another friend at the far left cornerpost and he shouted the same message at the same time and you calculated him to be 150 meters away. Could you tell where you were? Pretty much. You know that you’re 100 meters away from your first friend, so you could take a diagram of the field and draw a circle with a 100 meter radius around her known position. Then you could draw a circle with a 150 meter radius around your second friend’s known position. The two circles should intersect at two points – one of which should be your real position. With three friends, you’d have no ambiguity.

Draw this on a piece of paper if it doesn’t make sense as a written example.

Shouting from the Stars — The Global Positioning System works on this principle, although it uses much more precise clocks and the speed of light. There’s a hitch, though. The above example required that each person had precision-synchronized clocks. If each GPS unit had to have an atomic clock, it would be outrageously expensive. With three friends (or three satellites) we can solve three of these four variables:

X, Y = horizontal position

Z = altitude

t = time

With only three satellites and an imprecise clock, we have to assume altitude to be a known constant (e.g., sea level), since we can only solve for three variables using three satellites: X, Y, and time. But if we have four visible satellites, we can solve for all four variables: X (longitude), Y (latitude), Z (altitude), and t (precision time). The pleasant side effect is that not only do we have our precision location, but we also have precision time – which makes GPS valuable technology not only for geophiles, but chronophiles as well. Many people are now synchronizing their systems or network clocks to GPS signals, since it’s a cheap and highly accurate source.

However, thinking back to our example, there are some important caveats. Our friends, or the satellites, must be spaced well apart. If they’re too close together, the timing difference between their signals isn’t enough to calculate our location precisely. In GPS parlance, this is your "dilution of precision," and it greatly affects your accuracy. Also, there must be a clear path between us and them – we can’t have anything blocking our signals, or a large reflective object causing unwanted echoes ("multipath" signals). These errors can further degrade the accuracy of our location.

GPS signals work in the microwave band. They can pass through glass, but are absorbed by water molecules (wood, heavy foliage) and reflect off concrete, steel, and rock. This means that GPS units have trouble operating in rain forests, urban jungles, deep canyons, inside automobiles and boats, and in heavy snowfall – among other things. These environmental obstacles degrade positional accuracy or make it impossible to get a fix on your location.

GPS Receiver Technology — The core of GPS receivers come in two major variations: sequential single-channel and parallel multi-channel. Single-channel GPS units have only one radio receiver unit, and they must step sequentially through all possible satellites. This takes time and degrades their accuracy, since they may lose a "lock" each time they switch channels. Parallel units have from between four and twelve receivers, each dedicated to one particular satellite signal, so strong locks can be maintained on all the satellites.

There are some two-channel units out there, but in practice these are only slightly better than single-channel units. Parallel-channel units are up to 15 times faster in satellite acquisition times and they are unparalleled (sorry for the pun) in their ability to lock onto the satellite signals even in difficult situations like heavy foliage or urban skyscraper canyons.

Boaters, however, may be content with single or dual channel sequential units, since there are few environmental obstacles on the open ocean. These models are now considered outmoded technology, so you may be able to pick one up cheaply. But for others, don’t settle for anything less than a full 12-channel parallel system, especially since the price differential has closed greatly in the past six months.

Reviewing Some Receivers — Let’s take a quick peek at a few low-end consumer GPS units in the $150 to $300 range. All these units have parallel 12-channel receivers. They also all have the same 100 meter accuracy, because the U.S. government reduces all civilian GPS units in a similar fashion. If the government were to remove the SA interference, the units would be accurate to about 15 meters.

If you’re shopping for a unit, pay attention to features like form factor (handheld versus mounted); external antennas; mapping, and computer-controllability.

I’ve included on my Web site a list of resellers where you can buy these GPS units. For local shopping, try your neighborhood boating stores or outdoor sports stores, they often carry the lower-end units.

< index.html#resellers>

Garmin — My favorite GPS receivers are from Garmin, Inc. Their main handheld unit, the Garmin GPS 12XL, is about $250. The 12XL is designed for handheld use and fits in your palm. It runs on 4 AA batteries for about 12 hours and has a backlight for night use. Garmin also sells the lower-end GPS 12, a 12XL without an external antenna connection or audible beeper for less than $200, but otherwise sporting the same handheld form factor.

The GPS II+, on the other hand, is designed for vehicular use and sits on a dashboard or console. It has dedicated zoom buttons which make it easier for one-handed use while piloting (though not recommended while driving). Garmin also sells a nifty handlebar mount. I have an older GPS II mounted on my Honda CX500 Custom motorcycle and it’s my constant companion when I tour New England. The II+ has a battery life of around twenty hours on four AAs.

The greatest thing about Garmin units is that they have a bidirectional serial port that allows them to hook up to your computer. Though many GPS units can only transmit their current positional information, the Garmin units also allow you to transfer their waypoint databases, route tables, and other useful information. The wide availability of programs supporting the Garmin transfer protocol makes the Garmin units good choices for computer-based use.


Eagle — Eagle/Lowrance sells an inexpensive twelve-channel unit called the Eagle Explorer (approximately $200). The Explorer has a strong 12-channel GPS receiver, but unfortunately the user interface is harder to use than the Garmins. The unit also lacks an external antenna connection, which makes it more difficult to use inside vehicles. Eagle/Lowrance units have a proprietary data transfer protocol that’s not currently supported in any Macintosh products. I personally wouldn’t recommend that you buy a Eagle Explorer. It’s too difficult to use either by itself or with a computer.

If you’re into boating, Eagle also has a four-channel unit called the Accunav Sport, which has the ability to support plug-in map modules for coastal waterways. It’s a nice unit, but the receiver technology is two years old and a bit dated. Shop around for better deals.


DeLorme — DeLorme Mapping publishes CD-ROM and paper maps of the United States. Their most famous product is Street Atlas, a street level map of the entire U.S. on CD-ROM. But they also manufacture a neat little 12-channel unit called the DeLorme Tripmate. The Tripmate has no display or controls of its own and is designed strictly to be used with a computer. That said, it’s a remarkable unit with good performance – and a reasonable $150 price tag.


Data Cables — The biggest problem facing Macintosh GPS users is the unavailability of data cables. Most data cables come in PC-style DB-9 connectors, not the Macintosh-style DIN-8. Currently, the only commercial source for GPS cables for the Macintosh appears to be my company. However, you can make your own cables if you’re handy with a soldering iron or wire-crimps. My GPS Cable Page provides details.


Finding Your Way — GPS receivers aren’t yet standard equipment in cars, but with price drops and improvements, it’s only a matter of time. Ubiquitous availability of GPS devices will help eliminate the angst of finding yourself, in at least one sense.

For more information about GPS, check out resources from the U.S. Coast Guard, NOAA (National Oceanic and Atmospheric Administration), Iowa State University, and my own Web site.





[Karen Nakamura founded Global Mapping Systems, a Mac-centric mapping and GPS/GIS software development company. Her "other" day job is as a sociocultural anthropologist studying deaf social movements in Japan and United States.]

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