The Basics of GPS Courtesy of:
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Table of Contents Page Introduction So what is GPS? The Satellite Network Background and History What are GPS used for? GeoCaching Winter Sports Warm Weather Sports Marine Fishing On the Road Paintball Child Locator and Protection Vehicle Locator How GPS Works The GPS System Basic Triangulation What to look for in a GPS Take a Tour of a GPS End Note Where to get more information
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Introduction GPS to the Rescue! January 27, 2004: Cell phones, night-vision goggles and GPS satellite tracking were credited with helping save the lives of 14 fishermen who were stranded on ice in Lake Erie. The Coast Guard was able to more easily find the men because they had a global positioning system (GPS) receiver, which allowed rescuers to track a satellite signal to their location. Everyone was back on land within two hours and no one was injured. (Source: ABCNews) February 24, 2004: Golfers at The Den at Fox Creek (Bloomington, IL) can now use technology to back up their game. Global positioning system (GPS) technology will provide golfers with information about how far they are from the pins, water hazards and sand traps. (Source: PantaGraph.Com) March 20, 2004: When Matthew Martin lost his foothold on a steep and rugged area of southern Utah's Kaiparowits Plateau, his last coherent thought was that he was about to die. "My feet were in a precarious position on just some sage roots. When I slipped, I thought, 'this is it. This is the end of my life'". He slid down a snowy shale slope for about 150 feet before plummeting from a 30-foot drop-off. But the hiker supplied his parents with his GPS coordinates and then called the guide service that had taxied him to the trailhead. When the helicopter flew in Thursday at about 8:30 a.m., Martin understandably felt tremendous relief. (Source: Salt Lake Tribune)
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Have you ever had the unfortunate experience of being completely lost? Did you wish there were some simple way to find out where you are and how to get back to familiar ground? Or maybe you have found that perfect fishing, camping, or picnic spot and wished there were an easy way to record that exact location for future reference. Perhaps you enjoy backcountry activities such as hiking, snowmobiling, snowshoeing, mountain-biking, fishing, crosscountry skiing, kayaking, ORV expeditions, hunting, etc., and wish that you didn’t have to rely on trail markers, landmarks, or intuition to find your way. Fortunately, modern technology offers us an affordable mechanism called “GPS” which can help us find our way in these situations and many others. GPS tells us exactly where we are on the planet at any time. It can do this with precisions ranging from exact precision to points within a 40foot range. These variations depend on the model and type of GPS receiver you have. Many automobiles manufacturers install GPS systems as part of the basic package. Boats, both recreational and maritime vessels, make heavy use of GPS to guide them in open water. In surveying and geological applications, it is important to know exactly where you are digging or exactly what point you are marking. Again, GPS to the rescue! So, if you want to know exactly where you are at any time (within an acceptable range of a few feet), a GPS receiver is your tool. GPS can do other things as well. Most GPS systems have a built-in Trip Odometer for instance.
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Other common features are Sunrise / Moonrise time tables, Altimeters, and Map features. Combined with a transmitting device, GPS can also perform locator services. So you can see that GPS is more than a toy; if it matters at all where you are, where you are going, or even where you’ve been, a GPS is your tool.
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So what is GPS? The term GPS means “Global Positioning System.” This is a system of 24 Earth-orbiting satellites established by the United States Department of Defense. The Satellite Network The satellites in the GPS network are arranged in such a way that you can see at least 4 of them at a time from any given point on Earth. Actually, you can’t really “see” them; it is your GPS device that is doing the seeing. A GPS device can lock onto 4 or more available satellites in the GPS satellite network quickly and thus provide a point of reference. The following is a graphical representation of the GPS satellite network as it would be viewed from outer space.
Image courtesy of the US Department of Defense
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As mentioned, the satellites rotate in a very specific pattern so that the network gets maximum exposure from anywhere on the Earth. Background and History So why and when was this network built? Interestingly, the GPS network has been around longer than the Personal Computer. The US Department of Defense designed this system back in the 1970’s. It was (and still is) called NAVSTAR by the Department. The first satellite went into orbit in 1978 and the network reached its current “full constellation” in 1994 with 24 satellites. Originally, the Department of Defense designed this system with a strategic military application in mind. However, the widespread usefulness of the GPS network was quickly recognized and it was made available for civilian use during the 1980’s. Today anyone can use it, and use it you should. If you’re a US citizen, you are paying for this system with your tax money! So how does someone “buy” a GPS? Usually when someone refers to their “GPS” what they are really talking about is a GPS receiver. GPS receivers are small devices that, as the name implies, receive signals from the GPS network of satellites and provide location and other data. Through a process call Triangulation, they are able to determine the exact location by locking onto the positions of 4 (or more) satellites. The satellites broadcast their positions and the GPS receivers pick their signal and then make the necessary calculations. Sometimes, you will hear the term GPSr (GPS Receiver), but in most cases, it’s just GPS.
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When someone says, “check out my cool GPS”, they are probably talking about a GPS receiver. A typical handheld GPS receiver is slightly larger than a standard cellular phone. Like this model, the Meridian Marine by Magellan, GPS receivers are designed to fit in your hand as well as a pocket or purse. They can be taken anywhere.
Image courtesy of Magellan
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What are GPS used for? As mentioned, the original intent for GPS was military. However, GPS has emerged as a recreational tool as well. A wide variety of recreational activities can benefit from the use of a GPS receiver. GeoCaching
Basically, GeoCaching is a “high-tech treasure hunt”. You download coordinates and perhaps some hints from a GeoCaching website and with the assistance of your GPS, go hunt for them. This is a relatively new sport and is still evolving. But in its basic form, the rules are quite simple: 1) Find a GeoCache 2) Claim the Cache goodies 3) Replace the Cache goodies with something else 4) Sign the log book Yes, you can’t just take things; you have to give back. Don’t worry, the types of items in a cache are generally quite basic. Examples of items you might find at GeoCache sites are paperback books, lottery tickets, books of stamps, CD’s, batteries, school supplies, scented candles.. well the list The Basics of GPS (v 1.8) © 2004 The GPS Place
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goes on. You can usually find good GeoCache items in your closets, basement or at your local dollar store. GeoCaching is a terrific family activity and it’s great for people of all ages. You can even bring the dog (they can often assist in finding the cache!). All you need a GPS unit. One word of warning; it is so much fun that it’s addicting! It gets you out of the house and gets everyone into a spirit of adventure. To learn more about GeoCaching, please visit the links page at The GPS Place.
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Winter Sports
If you’re a skier who sticks to the established runs, you’ll probably never need to worry about getting lost. But what if you enjoy "fringe slope" activities such as Snowmobiling, Snowshoeing, or Cross-Country Skiing? It's pretty easy to leave it all behind with these sports and just hit the open snow. What if you take a wrong turn and get lost? What if you and your group get a long ways from the trailhead and bad weather sets in? Worse, what if you and your group were buried in a sudden avalanche? In all these cases, a GPS (along with a cell phone) could help save lives. Emerging in the electronics market are Cell Phones with GPS built in so you can have both if you want. Now, we are not suggesting that you become reckless winter sports enthusiasts and abandon all safety rules, but a GPS can give you and your family peace of mind so that you really can enjoy these wonderful winter activities with fewer worries.
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Warm Weather Sports
Do you enjoy hiking or backpacking in the wilderness? How about horseback riding or off-road-vehicle activities? Perhaps you like to take to the air by way of hang-gliding or hot air ballooning. Maybe you like those out-of-the way camping or picnic sites. But the last thing you want is for you or one of your family members to get lost. If the activity you enjoy takes you to remote areas, take along a GPS. You’ll always know where you are, which direction you came from, and where you are headed. When our kids were young, we all loved to go camping. But we always worried about one of them wandering off. You read about such stories every year, and the thought of this becomes a parent’s worst nightmare. Well once again, GPS technology to the rescue! Fortunately, kids love to use GPS devices. It’s both fun for them and educational. And it’s satisfying to teach them something which they’ll both enjoy and learn from.
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Marine
Marine usage is one of the primary applications of GPS technology. What would you do if you were lost at sea? There are usually no landmarks or buoy markers on the open ocean. But a GPS can always tell you where you are. You can see that GPS is a natural fit for Maritime travel but they are also valuable take-alongs on canoe or kayak trips. It’s easy to loose track of where you are and how far you’ve paddled. Many of the GPS models are waterproof and some of them even float if you happen to drop them in the water! So, don’t worry about taking them along on your water adventures.
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Fishing
Do you have that favorite fishing spot? One that is hard to find? You can make it easier to find for return trips by “marking the spot” with a GPS. Most GPS receivers have the capability to store and remember locations (called “Waypoints”). The push of a button can tell you where you are in relation to any given Waypoint. And there are special GPS-type devices called FishFinders that are made just for fishing. With the help of a device called a Transducer, they can provide a picture of what lies under your boat. There is nowhere the fish can hide! Technically this is not a GPS, but it does incorporate GPS technology. These types of systems provide a highdefinition display that gives a clear picture of fish, thermoclines, structure and bottom surface. What an unfair advantage you get over the fish!
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On the Road
Have you ever been driving in a strange place and found yourself completely lost? GPS to the rescue again! Mercedes Benz was one of the first auto manufacturers to install GPS systems as standard equipment in their automobiles. Now a lot of other auto makers have followed suit with in-dash GPS displays that prevent you from getting lost. Even if you don’t have a vehicle with built in GPS, you can buy dashboard mounts for many handheld GPS models and use your GPS in your vehicle as needed. Mounts are available for bicycles and motorcycles as well. Since many GPS models have built in mapping capability, you can see right where you are on a map.
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Paintball Games
Do you like Paintball Games? With GPS receivers in hand, your team can have an instant tactical advantage. Communication before and during a game is essential to good Paintball play. A team may get flanked, a player might get shot, or you might run out of paint. These things need to be communicated! Meetings at intervals can help your team regroup and change plans as necessary. You can plan exact locations at which to meet as well as what intervals to meet at because GPS receivers keep very accurate time!
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Other consumer uses for GPS technology - As well as having a lot of recreational uses, GPS technology offers more than a few practical uses as well! Child Locator and Protection Devices
It seems that from the moment children learn to walk, they’re wandering off somewhere. They have a natural desire to discover the world around them. And of course, parents have the natural instinct to keep them from danger. GPS technology again to the rescue! Specialized GPS devices that can transmit location data have become small and affordable and can give parents of small children that much needed peace of mind. Wherify Wireless makes a small wearable wrist device that can be worn by children and transmits their location. If the child wanders out of sight, the parent can log into the internet with her or his computer, PDA, or web-enabled cell phone and see the exact location of the child in real-time. To learn more about this product, please visit the links page at The GPS Place.
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Vehicle Protection
Image courtesy of CNN
Thankfully vehicle security has gotten better; in the years since 1995, incidents of auto theft have declined significantly. This is largely due to new technology that makes it more difficult for car thieves to operate. In addition, GPS technology has made it easy for law enforcement and vehicle owners to know the exact whereabouts of their vehicle at all times. A vehicle tracking device made by Aertrax, is a small affordable device that can be placed covertly in your vehicle and afford you the peace of mind of knowing where your vehicle is. You can also set up alerts to be notified if your vehicle moves. To learn more about this product, please visit the links page at The GPS Place. So you can see that while GPS devices go along with fun, they provide very useful services as well. And new uses for GPS are emerging all the time!
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How GPS Works How exactly does a GPS receiver determine its precise location? What kind of magic does it use? The GPS System The entire GPS system actually consists of 3 “segments”. The first and most important is the Space Segment. That segment consists of the 24 earth-orbiting satellites discussed earlier. The second segment, the Control Segment, consists of 5 ground based stations. As the name implies, they control and monitor the satellite network. The Control Segment is in continuous contact with the satellites and can send them corrective signals when they stray off course. They also keep the satellite’s almanac data updated. In using the GPS system, you will probably never come in contact with a control station, but they are an integral part of the whole system. The third segment is the User Segment. This consists of me, you, and anyone else who uses the system (along with our GPS units of course). Triangulation How does a GPS get its information and pass it along to you, the user? The process that allows GPS receivers to “read” a position from a group of satellites in the GPS network is called Triangulation. In a nutshell, Triangulation is used by receivers to determine the distance from each satellite. By The Basics of GPS (v 1.8) © 2004 The GPS Place
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using this information, they can deduce their location. This is best explained by simple example. Suppose you were somewhere on land and you had absolutely no idea where (you need to use your imagination here). Then suppose in front of you, there are some small notes stacked. On the top note, it says “Chicago – 181 miles”. Now, instead of having no idea where you are, you know that you’re 181 miles from Chicago. That means you could be at any point on an imaginary circle around the city of Chicago:
But that information by itself is not much help. So you look at the 2nd note and it says “Pittsburg – 360 Miles”. Now you know a little more; you know that you are at some point on a circle that is 181 miles away from Chicago AND ALSO some point on a circle that is 360 miles away from Pittsburg. With that information, you can now narrow the location down to two possible points:
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You now know that you are at one of the points where these two imaginary circles intersect. But which one? So you look at the 3rd note and it says “St. Louis – 243 Miles”. Aha! Now you know exactly where you are:
There is one and only one point that is at some point on a circle 181 miles from Chicago AND at some point on a circle 360 miles from Pittsburg AND ALSO at some point on a circle 243 miles from St. Louis. Only one place fits all three criteria: Indianapolis.
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Note the name “Triangulation” as in “Triangle” or 3 points. From three distances, we were able to determine our location in this (admittedly) oversimplified example. This is Triangulation on 2 dimensions, but actual GPS triangulation occurs in 3 dimensional space. As you might imagine, this suddenly becomes more complex and a 4th point of reference is required to determine position. This is why a GPS receiver must have access to at least 4 satellites in order to determine position. So Triangulation is simply the process of using relative distances from satellites to determine a position. This is why GPS devices are used mostly in outdoor activities. GPS devices will not work in your house (for example) because the GPS receiver must be able to “see” the satellites. Trimble is a leading innovator of GPS technology for scientific and engineering applications. They have a very good Flash-Media explanation of Triangulation on their site if you care to learn more about it. http://www.trimble.com/gps/triangulating1.html
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What to look for in a GPS Like many electronic gadgets, GPS receivers are packed with interesting options. Depending on your needs (and your budget), it helps to know some of the features generally found in GPS receivers. Grayscale versus Color One of the first choices that many people think about is whether to get a GPS with a grayscale (black and white) or color display. As you might expect, a color GPS is generally more expensive than a grayscale GPS with similar features. To give an example, the Magellan Meridian Color retails for around $400 while the Garmin eTrex Legend retails for around $215 – almost one-half! Yet, each unit is similar with respect to its feature set. When we think of personal computers, there really is no other choice than a color display. Monochrome displays for computers went out of existence years ago. However, when we talk about small handheld consumer electronics, it’s a different story. In small battery-powered devices such as handheld GPS, the power consumption is significantly greater when you have a color display. Using the same two GPS models above as examples, the Magellan Meridian Color GPS will get about 13 hours from 2 AA batteries, while the Garmin eTrex Legend will get about 18 hours from 2 AA batteries. The obvious advantage to a color GPS is the stunning visual effects. So if your budget allows it, they are very nice to look at. But when we take a tour of a GPS in the next chapter, you’ll see that a grayscale GPS can provide a full set of features without using color. The Basics of GPS (v 1.8) © 2004 The GPS Place
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Waterproof GPS is basically an outdoor activity. Handheld units tend to be taken along on many of the rugged adventures mentioned earlier, so they are subject to getting wet when it rains or snows. Or they may even get dropped in the water! That’s OK because many GPS units are built to be waterproof. And this is not necessarily a high-end GPS feature. For example, the Garmin Geko is completely waterproof and retails for well less than $150. WAAS Enabled The WAAS (Wide Area Augmentation System) is a supplementary set of ground GPS stations situated in various locations throughout the United States established for the purpose of enhancing the GPS network. A GPS that is compatible with this system (WAAS Enabled), will get far greater accuracy – within 3 meters of target about 95 percent of the time – than it would otherwise. The WAAS system is not yet fully operational in all parts of the U.S. so at this time, this is somewhat of an optional feature. It is also worth mentioning that power consumption increases when WAAS is used on a GPS. BaseMap or Mapping A BaseMap or mapping capability refers to the type of builtin maps a GPS unit has. Depending on how you might plan to use the GPS, it can have city maps, waterway maps, or topographical maps. Maps can also be purchased as software and installed on mapping GPS units. Some of the entry level GPS models do not have built-in mapping capability. So if this feature is important to you, make sure the model you select includes built-in mapping.
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Parallel Channels A “channel” on a GPS refers to its satellite tracking. For example, a 12-channel GPS can track 12 satellites at any one time. “Parallel” refers to the tracking method. A GPS using parallel channels is generally superior to one using multiplexing channels because it can hold on to a signal in dense woods or among tall buildings. Handheld GPS models typically are 12-channel Parallel units. Battery Life One important question to ask is how long will the GPS unit operate on its batteries? Since GPS units are generally used outdoors and in sometimes inclement weather, you may want to be prepared by carrying spares. The battery life range is usually about 10 continuous hours up to 20 hours. Most handheld models use AA batteries and some higher end models, such as the Garmin Street Pilot III, use rechargeable Lithium batteries. Waypoint Storage Waypoints are a way of marking an exact location and saving it for future reference. They make points easier to refer to. Which is easier: “Waypoint 1” or “the point at North 39° 42.662 West 104° 56.861”? Most all GPS units have the ability to store Waypoints. The question to ask is how many will it store? If you use your GPS to permanently keep track of interesting points, you may need lots of Waypoint storage over time.
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Route Assistance GPS units offer a wide variety of features that remember route details for you. One such feature is called “Bread Crumb Trails” which, as the name implies, shows the trail you have taken from a starting point to your current position. Some GPS units have the ability to remember complete routes should you need them in the future. Navigational Assistance Most GPS units have a built-in compass and a built-in altimeter (to tell you what your elevation is). Other options that are commonly available are readouts of heading, bearing, speed, direction, and ETA (Estimated Time of Arrival). Computer Interface A GPS receiver device is designed to work independently of a computer. But sooner or later, you may want to install software (such as additional maps), updates, or downloaded Waypoints to your GPS. This will require that your GPS have some way to communicate with your computer. Most commonly this is a serial cable, although some GPS units have USB interfaces. Check to see if the GPS comes with a cable; if not you may want to plan on purchasing a data cable for it. External Power GPS receivers operate primarily on batteries. But there are times when you can save your batteries and benefit from using other power sources. Most commonly this is done with an AC adapter that plugs into a cigarette lighter in a car, truck, or RV. If you plan to use your GPS in a vehicle or a boat, make sure to purchase a power adapter for it. The Basics of GPS (v 1.8) © 2004 The GPS Place
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Size and Weight It may seem that GPS receivers are about the same size; particularly the handheld models. However, a few ounces and variations in shape can make a difference. Some have slimmer designs than others, making them more “handheld” than others. Make sure to choose the correct fitting unit. Specific Functionality GPS units are evolving to suit a variety of uses and as a result, many GPS receivers are designed to serve a specific purpose. Some examples: the Garmin GPS MAP 60C / 60CS units are specifically designed for the growing sport of GeoCaching. They have screens and modes that are very specific to that sport. The Garmin ForeTrex 201 is designed to be worn on the wrist rather than placed in a pack, purse, or pocket. This is for people who want a GPS suitable for hands-free activity (such as jogging or rowing). And the Garmin iQue 3600 is actually a PDA (Personal Digital Assistant) which contains GPS functionality. It combines GPS with PDA technology so you get both technologies in one unit. So GPS receivers are becoming “specialized”. If you have a specific purpose in mind for your GPS, make sure and pick one that is best suited for that usage. On the other hand, if you just want a general purpose GPS, there are several of those available. Examples of GPS receivers that can be used in a variety of uses are: Garmin eTrex Series Lowrance iFinder Series Magellan SporTrak Series Magellan Meridian Series
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Taking a Tour of a GPS Let’s take one of the most popular GPS receivers ever - the Garmin eTrex Legend - and take a closer look. The Legend works by way of a series of buttons. On the left side of the unit are three buttons (Zoom in, Zoom out, and Find) and a “Click Stick” on the face of the unit. The Power button and Page button are on the right side of the unit.
Image Courtesy of Garmin
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The Legend uses a series of 5 screens.
Image Courtesy of Garmin
When initially powered up, the Legend starts with the Satellite Page. There is a period of a few minutes when the GPS is powered up where you must wait for it to synch with the available satellites. This is called Satellite Acquisition.
(Image Courtesy of Garmin)
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Once Satellite Acquisition is complete, the Legend will display your current location information and your current elevation. You can also see details about available satellites, such as their relative position and their signal strength. You will also see what your accuracy rating is. In the screen shot above, the accuracy is within 10 feet. That means if you relay that position to anyone else, they can come within 10 feet of you by using their own GPS. You can move between pages by pressing the Page button. On the Map Page, the Position Icon will indicate your current location in relation to a point on a map. You can also see the map’s scale as well as highway numbers and city names.
(Image Courtesy of Garmin)
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The third screen is the Navigation Screen. It takes the form of a compass and will show you your bearing (which direction you are headed), a destination waypoint if you have one selected, the straight-line distance to your waypoint, and your ETA (Estimated Time of Arrival). It will also show you data at the bottom of the screen. In this screenshot it shows elevation and time, however those are programmable and you can select from a variety of data. Examples of the data that can be shown are sunrise, sunset, speed, trip odometer, and average speed.
(Image Courtesy of Garmin)
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The fourth screen is the Trip Computer Screen. This displays any data relating to your current journey. The trip odometer at the top will show the distance you have traveled since last initializing your satellite signals. How far is it to school? This is one way you can tell. The data shown are the most commonly used data fields, but you can select and program data from a variety of choices.
(Image Courtesy of Garmin)
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The fifth screen is the setup screen and it is here that you can access various configuration setting options. Each option shown has many selections listed under it when selected.
(Images Courtesy of Garmin)
At the top of all screens you will find a menu bar, similar to the one you might see on your computer. The “X” closes the current screen. The box to the left of the X is the options menu. When selected, it will show options relating to that function of the GPS. Moving between screens and making selections are performed by using a combination of the buttons and the click stick. The click stick operates in a manner very similar to a stationary pointing device on a computer. It moves in all directions and it also responds when you press down on it. That’s a whirlwind tour of the Legend. But please note that the Garmin eTrex Legend is a feature-packed GPS and to cover it completely would take many more pages. The Basics of GPS (v 1.8) © 2004 The GPS Place
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End Note This has been a very basic introduction to GPS. If you want to learn more or dig deeper, see the links on the next page. And there is no better way to get to know a GPS than to actually try one out for yourself. That’s why The GPS Place offers a “no questions asked” 30 day return policy. See our store for details: http://www.the-gps-place.com
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Where to get more information http://www.cobra.com
Cobra Home Page http://www.garmin.com
Garmin Home Page http://www.lowrance.com
Lowrance Home Page http://www.magellangps.com
Magellan Home Page http://www.navman.com
Navman Home Page http://www.the-gps-place.com
Great prices and selection on GPS units! http://vancouver-webpages.com/peter/idx_gps.html
GPS Information Files, Equipment, and Software Reviews http://www.gps-practice-and-fun.com/
More in-depth information on GPS http://www.aero.org/publications/GPSPRIMER/EvryDyUse.html
More about uses for GPS in everyday life http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
University of Colorado explanation of GPS and related systems
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