GLOBAL POSITIONING SYSTEM (GPS)

ABSTRACT :
Modern technology offers us an affordable mechanism called “GPS” which can help us find our way in situations like lost in a completely unknown place, hiking, snowmobiling, snowshoeing, mountain -biking, fishing, cross-country skiing, ORV expeditions, hunting 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 40-foot range. These variations dep end on the model and type of the GPS receiver you have. GPS is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these satellites as reference points to calculate positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter! In a sense it's like giving every square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated circui ts and so are becoming very economical. And that makes the technology accessible to virtually everyone. These days GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop computers.

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1.0 BACKGROUND AND HISTO RY :
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 1 978 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. Basically, GPS is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. As shown in figure, linking 24 satellites makes GPS work.

Fig.1 Constellation of 24 Satellites 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.

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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 . 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. Other common features are Sunrise / Moonrise time tables, Altimeters, and Ma p 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 have been, GPS is your tool. A typical handheld GPS receiver is slightly larger than a standard cellular phone. Like this model (below), GPS receivers are designed to fit in your hand as well as a pocket or purse. They can be taken anywhere.

Fig.2 GPS receivers CITC (CE DEPT.) 3

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2.0 FUNCTIONALITY :
The entire GPS system actually consists of 3 “segments”. The first and most important is the Space Segment. That segment consists o f the 24 earth-orbiting satellites. 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 th e 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).

2.1 Here’s how GPS works in five logical steps :
1) The basis of GPS is triangulation from satellites. 2) To “triangulate,” a GPS receiver measures distance using the travel time of radio signals. 3) To measure travel time, GPS needs very accurate timing which it achieves with some tricks. 4) Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. 5) Finally you must correct for any delays the signal experiences as it t ravels through the atmosphere. Details of all 5 steps are as follows :

2.1.1 Triangulating :
Position is calculated from distance measurements (ranges) to satellites. Mathematically we need four satellite ranges to determine exact position. Three ranges are enough if we reject ridiculous answers or use other tricks. CITC (CE DEPT.) 4

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Actual GPS triangulation occurs in 3 dimensional space. As we 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 our house (for example) because the GPS receiver must be able to “see” the satellites .

Fig 3.Triangulation of Satellites CITC (CE DEPT.) 5

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2.1.2 Measuring Distance :
Velocity (60 mph) x Time (2 hours) = Distance (120 miles) In the case of GPS we're measuring a radio signal so the velocity is going to be the speed of light or roughly 186,000 miles per second. Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite. To make the measurement we assume that both the satellite and our receiver are generating the same pseudo -random codes at exactly the same time. By comparing how late the satellite's pseudo -random code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light and you've got distance. If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be extremely good, because if their timing is off by j ust a thousandth of a second, at the speed of light, that translates into almost 200 miles of error! The secret to perfect timing is to make an extra satellite measurement. That's right, if three perfect measurements can locate a point in 3 -dimensional space, then four imperfect measurements can do the same thing.

2.1.3 Getting Perfect Timing

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Accurate timing is the key to measuring distance to satellites. Satellites are accurate because they have atomic clocks on board. Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove errors. We have been assuming that we know where the GPS satellites are so we can use them as reference points. But how do we know exactly where they are? After all they're floating around 11,000 miles up in space. CITC (CE DEPT.) 6

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2.1.4 Satellite Positions :
To use the satellites as references for range measurements we need to know exactly where they are. GPS satellites are so high up their orbits are very predictable. Minor variations in their orbits are measured by the Department of Defense. The error information is sent to the satellites, to be transmitted along with the timing signals. To get the most out of the system, a good GPS receiver needs to take a wide variety of possible errors into account. Here's what they've got to deal with.

2.1.5 Correcting Errors

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One of the basic assumptions is not exactly true. We've been saying that you calculate distance to a satellite by multiplying a signal's travel time by the speed of light. But the speed of light is only constant in a vacuum. The earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors. Some errors can be factored out using mathematics and modeling. The configuration of the satellites in the sky can magnify other errors. Differential GPS can eliminate almost all error.

2.2 The Pseudo Random Code :
It is a fundamental part of GPS. Physically it's just a very complicated digital code, or in other words, a complicated sequence of "on" and "off" pulses. The signal is so complicated that it almost looks like random electrical noise. Hence the name "Pseudo - random". CITC (CE DEPT.) 7

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There are several good reasons for that complexity: First, the complex pattern helps make sure that the receiver doesn't accidentally sync up to some other signal. The patterns are so complex that it's highly unlikely that a stray signal will have exactly the same shape. Since each satellite has its own unique Pseudo -Random Code this complexity also guarantees that the receiver won't accidentally pick up another satellite's signal. So all the satellites can use the same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system. In fact the Pseudo Random Code gives the DOD a way to control access to the system. But there's another reason for the complexity of the Pseudo Random Code, a reason that's crucial to making GPS economical. The codes make it possible to use "information theory" to amplify the GPS signal. And that's why GPS receivers don't need big satellite dishes to receive the GPS signals.

2.3 Differential GPS (DGPS) :
A horizontal accuracy of approx. 20 m is probably not sufficient for every situation. In order to determine the movement of concr ete dams down to the nearest millimeters, for example, a greater degree of accuracy is required. In principle, a reference receiver is always used in addition to the user receiver. This is located at an accurately measured reference point (i.e. the co -ordinates are known). By continually comparing the user receiver with the reference receiver, many can be eliminated. This is because a difference in measurement arises, which is known as Differential GPS (DGPS). In the case of differential processes in use today, a general distinction is drawn between the following:

1. Local area differential GPS 2. Regional area differential GPS 3. Wide area differential GPS

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2.4 What to look for in a GPS receiver :
Like many electronic gadgets, GPS receivers are packed with in teresting options. Depending on our needs (and our budget), it helps to know some of the features generally found in GPS receivers.

1. 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 we might expect, a color GPS is generally more expensive (double price approx.) than a grayscale GPS with similar features. Yet, each unit is similar with respect to its feature set.

2. 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.

3. 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 purpo se 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. CITC (CE DEPT.) 9

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4. Base map or Mapping :
A Base Map or mapping capability refers to the type of built -in 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 bu ilt-in mapping capability. So if this feature is important to us, make sure the model selected includes built-in mapping.

5. 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.

6. 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 use rechargeable Lithium batteries.

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7. Waypoint Storage :
Waypoints are a way of marking an exact location and saving it for future reference. They make points easier to refer to. For example, “Waypoint 1” is easier than “the point at North 39° 42.662 West 104° 56.861”. Most all GPS units have the ability to store Waypoints. If we use your GPS to permanently keep track of interesting points, you may need lots of Waypoint storage over time.

8. 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.

9. 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).

10. Computer Interface :
A GPS receiver device is designed to work indepen dently of a computer. But sooner or later, we may want to install software (such as additional maps), updates, or downloaded Waypoints to our GPS receiver. This will require that our GPS have some way to communicate with our computer. Most commonly this i s a serial cable, although some GPS units have USB interfaces.

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11. 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.

12. 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.

13. 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 GPS are specifically designed for the growing sport of GeoCaching. They have screens and modes that are very specific to that sport. Some are 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). Some advanced PDA (Personal Digital Assistant) contains GPS functionality. It combines GPS with PDA technology so you get both technologies in one unit.

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2.5 GEOCACHING :
It is the widely popular, high -tech game of treasure hunting. With an appetite for adventure, you're sure to find hidden treasure and see the world. Several lightweight, rugged, waterproof GPS handhelds are perfect for geocaching. These units have long battery lives and lots of waypoint storage — some even come with built-in altimeters, compasses, mapping capabilities and more. GeoCaching is a terrific family activity and it’s great for people of all ages. A pet dog can also be brought. All it needs is a GPS receiver.

2.5.1 Introduction
Basically, geocaching is a high -tech version of treasure hunting. Geocachers seek out hidden treasures utilizing GPS coordinates posted on the Internet by those hiding the cache. Using a GPS unit, they then trek out into the backwoods or urban jungles to find the hiding spot of the cache. Once found, a cache may provide the visitor with a wide variety of rewards. If the visitor takes something out of the cache, they are asked to leave something in return. For some, the biggest reward is the thrill of the search and the discovery of a place that they have never been. This is a relatively new sport and is still evolving. 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, etc. In a few short years, geocachi ng has grown incredibly popular. Relatively cheap and accurate GPS receivers and wid espread access to the Internet have helped the sport flourish throughout the world. One of the nice things about geocaching is that just about anyone can do it; your gender, age, or economic status do esn’t matter. The main requirements are a spirit of adventure, a love of puzzles and mysteries, and a good sen se of fun. CITC (CE DEPT.) 13

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Geocaching is played by comp uter geeks, families & friends, outdoor recreationists etc. 1. Computer geeks : Because geocaching involves gadgets (GPS receivers and the Internet), in the early days of the sport, a number of co mputer geeks were initially drawn to the activity GeoCaching has become so popular that the average nongeeks currently outnumber the technology geeks. 2. Families and friends : Geocaching is a very family-oriented sport. Although you can certainly geocache by yourself, the social aspects of the activity and having more than one set of eyeballs to look for a well-hidden geocache are well suited to multiple-person outings. 3. Outdoor recreationists : A fair number of hikers, hunters, fishers, rock hounds, and other types of recreationists have been using the outdoors long before GPS came into being. Because they typically already own a GPS receiver, many of these outdoorsmen and women have added geocaching to their primary outside interests, getting in a little geocaching while they’re biking, hiking, fishing, four-wheeling, or engaging in some other sport.

2.5.2 History :
The whole geocaching concept isn’t that new. Over 100 years ago, something similar developed in England: letterboxing. Letterboxing comprises placing a blank logbook and a custom-made rubber stamp in a waterproof container and then hiding it. Clues are distributed with the container’s location, and searchers armed with inkpads and notebooks try to f ind the hidden box. If they’re successful, they stamp the logbook in the box with their own personal rubber stamp and also stamp their logbook with the box’s stamp . CITC (CE DEPT.) 14

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2.5.3 Rules
This growing outdoor sport has a simple set of rules, mak ing it easy for anyone to play. The challenging part is finding the cache. Some caches have even been planted on mountainsides and underwater! Once geocachers find a cache, they follow these basic rules: a) Fill out the logbook. b) Take something out of the cache. c) Put something in the cache. d) Return the cache to the exact position and condition in which it was found. Caches typically consist of a waterproof container discreetly placed within the local terrain. The container will include a logbook and any number of more or less valuable items such as toys, books, money, trinkets, etc.

2.5.4 Why should someone geocache? :
a) b) c) d) e) f) g) To master your GPS receiver To see new places To get some exercises To challenge yourselves To hangout with family and friends To educate kids To build teams

2.5.5 Geodashing :
This is a contest in which random points are selected and players need to get within 100 meters of the location. There are no caches, hints, or terrain difficulty ratings, and the points can be anywhere on Earth. In fact, some locations can be impossible to reach. A new cont est takes place roughly every month. The goal of the game is for teams to collect all the points first or to get as many as they can before the contest ends.

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2.5.6 Ideal features of GPS unit for GEOCACHING :
Lightweight and compact — makes it easier to carry. Waterproof — caches can be hidden under or near water . Long battery life — a must for those hard-to-find, remote caches. Waypoint storage — should hold more cache locations . Screen readability — should be easy to read in daylight an d at night. : WAAS-enabled — means your GPS unit will give you better position accuracy within 3 mt(10 ft.) 95% of the time. Built-in Compass — convenient and comes in handy when you get within 100 feet of your cache. Altimeter — shows how high up you are for those vertically hidden caches. Mapping Capabilities — makes it easier to navigate when you can download maps to your unit and see the terrain. External antenna — for use under heavy tree cover or difficult terrains.

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2.6 WAAS
It stands for Wide Area Augmentation System. Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. Try an average of up to five times b etter. A WAAS-capable receiver can give you a position accuracy of better than 3 meters 95 % of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.

2.6.1 The origins of WAAS

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The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

2.6.2 How it Works

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WAAS consists of approximately 25 ground ref erence stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbi t and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

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2.6.3 Who benefits from WAAS?

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Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in Sou th America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land -based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.

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2.7 Applications of GPS :
GPS has a variety of applications on land, at sea and in the air. Basically, GPS is usable everywhere except where it's im possible to receive the signal such as inside most buildings, in caves and other subterranean locations, and underwater. The most common airborne applications are for navigation by general aviation and commercial aircraft. At sea, GPS is also typically us ed for navigation by recreational boaters, commercial fishermen, and professional mariners. Land -based applications are more diverse. The scientific community uses GPS for its precision timing capability and position information. Surveyors use GPS for an increasing portion of their work. GPS offers cost savings by drastically reducing setup time at the survey site and providing incredible accuracy. Basic survey units, costing thousands of dollars, can offer accuracies down to one meter. More expensive syste ms are available that can provide accuracies to within a centimeter. Recreational uses of GPS are almost as varied as the number of recreational sports available. GPS is popular among hikers, hunters, snowmobilers, mountain bikers, and cross-country skiers, just to name a few. Anyone who needs to keep track of where he or she is, to find his or her way to a specified location, or know what direction and how fast he or she is going can utilize the benefits of the global positioning system. GPS is now commonplace in automobiles as well. Some basic systems are in place and provide emergency roadside assistance at the push of a button (by transmitting your current position to a dispatch center). More sophisticated systems that show your position on a street map are also available. Currently these systems allow a driver to keep track of where he or she is and suggest the best route to follow to reach a designated location.

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3.0 CONCLUSION :
GPS has already revolutionized the observational techniques and basic

philosophies of land, hydrographic and geodetic surveying. Within the next few years navigators on land, in the air and at sea will be the next group to fully adopt this new positioning system. The number of civilian users of GPS already well far exceeds the number of potential military users. Consequently, the future of GPS can no longer be solely in the hands of the US DOD (Department Of Defense).The beginning of the next phase of the historical development of GPS is already started. The number and diversity of the applications of GPS continues to increase on an almost daily basis. As an inevitable result it is almost impossible to forecast the future prospects for the Global Positioning System. However, it is certain to affect one of the most revolutionary changes in our concept of positional information for many years.

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4.0 REFERENCES : http://www.garmin.com Garmin Home Page http://www.magellangps.com Magellan Home Page http://www.the-gps-place.com Great prices and selection on GPS units! http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html University of Colorado explanation of GPS http://www.trimble.com Trimble home page

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