How It Works
The Global Positioning System (GPS) consists of 24 earth-orbiting satellites. These satellites allow any person who owns a GPS receiver to determine his or her precise longitude, latitude and altitude anywhere on the planet. For as little as $100, you can know exactly where you are and where you have been. For anyone who has ever been lost -- while hiking in the woods, boating in the ocean, driving in a unfamiliar city or flying a small airplane at night -- a GPS receiver is a miracle. When you use GPS receiver, you're never lost!

How is this possible?We'll look at the details of how the GPS satellites and GPS receivers work together to pinpoint a location. You'll find that the GPS system is an amazing technological tour de force!

Trilateration
In order to understand how the GPS satellite system works, it is very helpful to understand the concept of trilateration. Let's look at an example to see how trilateration works.

Let's say that you are somewhere in the United States and you are TOTALLY lost -- you don't have a clue where you are. You find a friendly-looking person and ask, "Where am I?" and the person says to you, "You are 625 miles from Boise, Idaho." This is a piece of information, but it is not really that useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles, like this:

If you know you are 625 miles from Boise,
you could be anywhere on this circle.

So you ask another person, and he says, "You are 690 miles away from Minneapolis, Minnesota." This is helpful -- if you combine this information with the Boise information, you have two circles that intersect. You now know that you are at one of two points, but you don't know which one, like this:

If you know you are 625 miles from Boise
and 690 miles from Minneapolis, then you
know you must be at one of two points.

If a third person tells you that you are 615 miles from Tucson, Arizona, you can figure out which of the two points you are at:

With three known points, you can determine
that your exact location is somewhere near
Denver, Colorado!

With three known points, you can see that you are near Denver, Colorado!

Trilateration is a basic geometric principle that allows you to find one location if you know its distance from other, already known locations. The geometry behind this is very easy to understand in two dimensional space.

This same concept works in three dimensional space as well, but you're dealing with spheres instead of circles. You also need 4 spheres instead of three circles to find your exact location. The heart of a GPS receiver is the ability to find the receiver's distance from 4 (or more) GPS satellites. Once it determines its distance from the four satellites, the receiver can calculate its exact location and altitude on Earth! If the receiver can only find three satellites, then it can use an imaginary sphere to represent the earth and can give you location information but no altitude information.

For a GPS receiver to find your location, it has to determine two things:

• The location of at least three satellites above you
• The distance between you and each of those satellites

Measuring Distance

Do You Own a GPS??? Do you own a GPS? If so, millions of How Stuff Works readers would love to know how you like it, and what you think are the best and worst features of your GPS. !!

GPS satellites send out radio signals that your GPS receiver can detect. But how does the signal let the receiver know how far away the satellite is? The simple answer is: a GPS receiver measures the amount of time it takes for the signal to travel from the satellite to the receiver. Since we know how fast radio signals travel -- they are electromagnetic waves and so (in a vacuum) travel at the speed of light, about 186,000 miles per second -- we can figure out how far they've traveled by figuring out how long it took for them to arrive.

Measuring the time would be easy if you knew exactly what time the signal left the satellite and exactly what time it arrived at your receiver, and solving this problem is key to the Global Positioning System. One way to solve the problem would be to put extremely accurate and synchronized clocks in the satellites and the receivers. The satellite begins transmitting a long digital pattern, called a Pseudo Random Code, as part of its signal at a certain time, let's say midnight. The receiver begins running the same digital pattern, also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern. The length of the delay is equal to the time of the signal's travel. The receiver multiplies this time by the speed of light to determine how far the signal traveled. If the signal traveled in a straight line, this distance would be the distance to the satellite.

The only way to implement a system like this would require a level of accuracy only found in atomic clocks. This is because the time measured in these calculations amounts to nanoseconds. To make a GPS using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. Atomic clocks usually cost somewhere between $50,000 and $100,000, which makes them a little too expensive for everyday consumer use!

The Global Positioning System has a very effective solution to this problem -- a GPS receiver contains no atomic clock at all. It has a normal quartz clock. The receiver looks at all the signals it is receiving and uses calculations to find both the exact time and the exact location simultaneously. When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point, as illustrated above. Four spheres of this sort will not intersect at one point if you've measured incorrectly. Since the receiver makes all of its time measurements, and therefore its distance measurements, using the clock it is equipped with, the distances will all be proportionally incorrect. The receiver can therefore easily calculate exactly what distance adjustment will cause the four spheres to intersect at one point. This allows it to adjust its clock to adjust its measure of distance. For this reason, a GPS receiver actually keeps extremely accurate time, on the order of the actual atomic clocks in the satellites!

One problem with this method is the measure of speed. As we saw earlier, electromagnetic signals travel through a vacuum at the speed of light. The earth, of course, is not a vacuum, and its atmosphere slows the transmission of the signal according to the particular conditions at that atmospheric location, the angle at which the signal enters it, and so on. A GPS receiver guesses the actual speed of the signal using complex mathematical models of a wide range of atmospheric conditions. The satellites can also transmit additional information to the receiver.

From this discussion you have learned several important facts about the Global Positioning System:

• The Global Positioning System needs 24 satellites so it can guarantee that there are at least 4 of them above the horizon for any point on earth at any time. In general there are normally 8 or so satellites "visible" to a GPS receiver at any given moment.
• Each satellite contains an atomic clock.
• The satellites send radio signals to GPS receivers so that the receivers can find out how far away each satellite is. Because the satellites are orbiting at a distance of 11,000 miles overhead, the signals are fairly weak by the time they reach your receiver. That means you have to be outside in a fairly open area for your GPS receiver to work.

Finding the Satellites
The other crucial component of GPS calculations is the knowledge of where the satellites are. This isn't difficult because the satellites travel in a very high, and predictable orbits. The satellites are far enough from the earth (11,000 miles) that they are not affected by our atmosphere. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

Finding Location

The most essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, so that it can mathematically determine the receiver's position on Earth. The basic information a receiver provides, then, is the latitude, longitude and altitude (or some similar measurement) of its current position. Most receivers then combine this data with other information, such as maps, to make the receiver more user friendly. You can use maps stored in the receiver's memory, connect the receiver up to a computer that can hold more detailed maps in its memory or simply buy a detailed map of your area and find your way using the receiver's latitude and longitude readouts.

Geographers have mapped every corner of the Earth, so you can certainly find maps with the level of detail you desire. You can look at a GPS receiver as an extremely accurate way to get raw positional data, which can then can be applied to geographic information that has been accumulated over the years. GPS receivers are an excellent navigation tool, with seemingly endless applications!

What It Can Do
The Global Positioning System, a collection of 24 earth-orbiting satellites, has a number of possible applications, spanning across several areas of society. For most of us, the way we can take advantage of GPS is to purchase a handheld GPS receiver, or have one installed in our car. Handheld receivers are compact, and the most basic ones are fairly affordable. You can pick one up for as low as $100!

This could certainly be a sensible purchase, when you consider all of the things a GPS receiver can do for you. The basic function of a GPS receiver is to figure out its location on Earth. To everyone who's ever lost their way in the woods, driven off course on a cross-country trip or gotten turned around while piloting a boat or airplane, the advantages of this simple function are obvious. But most GPS receivers go far beyond providing this simple navigational data. They can act as an interactive map, and they have a number of recreational applications. We'll look at many of the things you can do with a GPS receiver!

The Basics
At its heart, a GPS receiver is simply a device that can locate itself on Earth. It does this by communicating with at least four satellites overhead (see this page for details). For this reason, a GPS receiver is limited as to where it can function. It has to be able to "see" the satellites to calculate latitude and longitude, which means it usually won't work inside. So, one of the basic characteristics of GPS receivers is that they find your location only when you are outside.

The simplest GPS receiver would give you just the coordinates of your location on Earth in latitude, longitude and altitude. Latitude and longitude are basically X and Y axes of a big imaginary grid wrapped around the planet, and altitude is a measure of your distance above sea level. If you had a GPS receiver that gave you these simple coordinates, and you had a map of your area that used this same coordinate system, you could find your location simply by reading the map. In this regard alone, a GPS receiver is a amazing device. Without a GPS receiver, you would have to find your position based on the position of the stars in the sky, using complicated tools and calculations. And you wouldn't have near the same level of accuracy!

But today's handheld GPS receivers give you much more than this raw data. Even low-end receivers have some sort of electronic map stored in memory, so you don't have to carry around a bunch of paper maps. The receiver takes the coordinate information and applies it to its electronic map, graphically pointing out to you where you are in relation to roads, bodies of water, etc. Maps vary a great deal in the level of detail they offer; but the basic idea behind this function is to give you a map that automatically marks your location, without you having to consider your coordinates. This is a great convenience any time you need to use a map, and is extremely helpful at times when you can't take the time to find your location on a map, such as when you're driving down the highway.

GPS in Motion
A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing. If you've read the article on How a GPS Receiver Works, you know that a receiver must know the exact time to find its location. If it combines these two pieces of information -- your changing location and the exact time -- it can also calculate how fast you are going. A receiver can use all of this basic data to give you several pieces of valuable information:

• How far you've traveled (odometer)
• How long you've been traveling
• Your current speed (speedometer)
• Your average speed
• A "breadcrumb" trail showing you exactly where you have traveled on the map
• The estimated time of arrival at your destination if you maintained your current speed

User Input
Most receivers have a certain amount of memory available for you to store your own navigation data. This greatly expands the functionality of the receiver, because it essentially lets you make a record of specific points on Earth. The basic unit of user input is the waypoint. A waypoint is simply the coordinates for a particular location. You can save this in your receiver's memory in two ways:

• You can tell the receiver to record its coordinates when you are at that location.
• You can find the location on a map (the internal map or another one) and enter its coordinates as a waypoint.

• Good camp sites
• Favorite road-side shops
• Excellent fishing spots
• Scenic overlooks
• Where you left your car

Computer Connections
Receivers with route capabilities will let you save a certain number of waypoints to memory so that you can use them again and again. If the receiver has a data port, you can also download your routes to a computer, which has much more storage memory, and then upload them again when you plan to follow those routes.

Because they have so much more storage capability, computers can do a lot more with GPS location data than your average receiver. A receiver with a data port can feed the raw coordinates of your location into a computer running more complicated software. There are a number of available software applications that can place you on detailed maps of particular areas. If you want to use your receiver for complicated navigation, down backroads for example, this capability will help you out tremendously. You can also update your computer maps, so that they include any surveying adjustments or changes in an area, whereas a receiver's onboard map usually can't be changed. When you use your receiver in conjunction with your computer, you increase the receiver's capabilities considerably. Also, your receiver won't be outdated as quickly, because in conjunction with a computer, all it needs to do is provide coordinates -- your computer does the rest.

Some recent receivers let you download detailed maps of an area into the GPS, or supply detailed maps with plug-in map cartridges. These maps can give you street-level detail in cities and the receiver may even provide driving directions as you drive!

GPS receivers have been a favorite of hikers, boaters and pilots for years, and are now becoming commonplace as prices fall. Check out the handy feature chart to help you decide which features you need!

Features
If you are thinking about buying a GPS receiver, it is helpful to know all of the features that GPS receivers offer today. That way you can pick the receiver that is perfect for you. The chart below highlights the most important features available today:

Essential
Feature	Option	Description
Receiver	Multiplex	Multiplex receivers have only one channel. They pick up one satellite signal at a time, cycling through a few satellites. They work much better in open environments, as their connection can easily be disrupted by buildings or other obstacles. The most affordable models use multiplex receivers.
	Parallel-channel	Parallel-channel receivers have several channels, and lock onto many satellites at the same time. They don't lose satellite connections very easily and they can pinpoint the location more exactly. These receivers were once fairly expensive, but there are several affordable models now on the market. If you plan to use your receiver in a big city or mountainous area, you should probably get one with parallel channels.
Antenna	Quadrifilar	Quadrifilar antennas are a length of coiled wire in a plastic housing that protrudes from the receiver. You may want to look for a model with a removable quadrifilar antenna, so you can place the antenna on your dashboard for a better "view" of the satellites. Quadrifilar antennas are best at receiving transmissions from satellites near the horizon, and not so adept at receiving signals from satellites overhead.
	Patch	Patch antennas are flat, and they are usually built in to the receiver. They have the reverse strengths and weaknesses of a quadrifilar antenna: They are better at detecting satellites that are directly overhead and not as good at detecting satellites near the horizon.
Power	Battery	Hand-held receivers will use batteries as a power source. This means portability. Be sure to find out what kind of batteries a hand-held unit uses, and how long they typically last.
	External Source	Some handheld receivers can accept external power, which is handy if you plan to be driving all day with your GPS on and don't want to drain the batteries. Car, boat or airplane in-dash GPS receivers will run on an external power source provided by the larger unit it's hooked up to. These devices are not mobile.
Display	LCD Panel	All GPS receivers display information on an LCD panel.
	Color LCD Panel	These displays make it easier to read maps on the receiver and help you distinguish between different routes you have created in the same area. Color panels often use more power than B&W panels, so they drain batteries faster.
Map Datum	WGS 84	WGS 84 is the default map datum for any GPS receiver. It is a system developed around the emergence of GPS technology and is standardized for universal use.
	Additional	Eventually maps of the whole world will be converted to WGS84, a GPS standard datum. In the meantime, check to see that the GPS receiver recognizes the map datums used in your area, or areas you plan to travel to.

Standard
Feature	Description
Internal Maps	All receivers will give you your latitude, longitude and altitude, but they don't all show you your location on a detailed map. When you're shopping for a receiver, decide what kind of map you'll need and make sure the receiver you purchase offers that type of map. Many receivers contain a general map for the world in memory, but this map may only show you major roads and bodies of water. Some receivers have a wide array of other maps stored in memory or can download detail maps.
Map Cartridges	Some receivers accept special map cartridges with more detailed maps of specific areas.
Download Maps	Some of the newer GPS receivers have download capability that allows you to download maps stored in your computer into your receiver.
Way Point Capability	With this feature, you can record certain way points -- locations along your path or on a map -- and arrange them in a route. Your receiver will then guide you from way point to way point along your route. This route mapping is handy because you can record the way you got somewhere so you can easily backtrack. You can also plan routes on detailed maps before you leave for a trip, and record all the information you need on your hand-held receiver.
Track Logging	Receivers with a track logging feature can record your path as you move. This is useful if you want to backtrack or document your exact route for future use. It's also helpful to view your progress this way while you are traveling.
Storage Memory	If you plan to use route-mapping and track logging extensively, you'll want to find a receiver that has enough memory. Consider how many way points you would want to store and find out what a receiver's maximum storage capability is. Also, look for a receiver with a backup system that will hold onto your information while you change the receiver's batteries.
Data Port	One way to place yourself on a detailed map is to hook the receiver up to a computer (desktop, laptop or PDA). A data port provides such a connection so that you can use GPS data in conjunction with a number of software applications. Receivers with computer connection capability may also be able to download information to the computer. This is a good feature if you want to keep a collection of route maps (favorite hiking paths, tricky driving directions, good fishing spots). A receiver has limited memory, but you can store an entire catalog of route maps on your computer.
Sunrise/Sunset Times	Some receivers can give you the times for sunrise and sunset at any particular location. This helps you plan your trip so you don't have to travel in the dark, which can be very useful to hikers, sailors, and pilots alike.
Odometer	In most modern receivers, you can track how far you have traveled. Just like the odometer in your car, this feature can be useful in any number of ways.
Speedometer	Most GPS receivers these days can track how fast you are moving. This is extremely helpful for estimating how long it will take you to get to your destination. Most receivers with speedometers will also give you an ETA.
Measurement Units	Make sure a receiver can display the measurement units you will be using. If you will use the receiver in sailing navigation, for example, you will probably want a receiver that can give you measurements in nautical miles. Another feature to look for is the ability to display multiple measurement systems at a time, so that you could have elevation in feet, say, and geographical distance in kilometers.
Accuracy Warning	Most receivers have some sort of system that tells you when something may be causing inaccurate positioning. This could either be due to poor satellite reception or to a receiver malfunction. In a lot of GPS applications, accurate positioning is critical, so be sure to find a receiver that will tell you when there is an accuracy problem.

Extras
Feature	Description
Differential GPS	Differential GPS is a technique that utilizes a second GPS receiver at a known location to correct for satellite signal inaccuracies. If a receiver already knows its exact location, it can check the accuracy of the signals it is receiving. This second, stationary receiver then broadcasts any accuracy adjustments to your receiver.
Built-in Database	GPS receivers designed specifically for airplanes or boats may have way points, or landmarks, already programmed into them. These might include airports and ports.
Rotatable Screen	Some GPS receivers have a display that rotates from a vertical position to a horizontal position. This feature might be useful if you plan to mount your receiver horizontally in your car some of the time and carry it vertically in front of you at other times.
User-changeable Fields	Receivers with this feature give you some extra control over how you look at information. Basically, you can customize different fields so they show you only the information you need for a particular activity.
Waterproofing	If you will be using GPS on a boat or while hiking, you should look for a receiver with good waterproofing. Some receivers are sealed so that they are completely waterproof while others are merely constructed so they resist water. Consider the conditions in which you will be using your receiver, and look for an adequate amount of weatherproofing.

Some GPS receivers have speed limits.
GPS receiver manufacturers sometimes program speed limits into the devices, so that if the device is moving above a certain speed, it will not work properly. A receiver meant to be used in a car may not work on an airplane, which travels much faster than a automobile. This is more often the case in car, airplane or boat-mounted receivers than in the hand-held models.

GPS receivers have temperature limits.
Like most electronic devices, especially those with LCD screens, GPS receivers may not function properly above or below certain temperatures. If you plan to use your receiver in any extreme temperature situations, such as mountain climbing or hiking in the desert, you should check to make sure the receiver model can function in those conditions.