04-03-2013, 10:48 AM
The Global Positioning System (GPS)
The Global Positioning.docx (Size: 166.48 KB / Downloads: 151)
Introduction
Trying to figure out where you are is probably one of humankind's oldest problems. Navigation and positioning are crucial to so many activities and yet the process has always been quite cumbersome and inexact. In the earliest days mankind used the stars to navigate. Early instruments also sited the stars to determine position. The science of horology began in part because navigation depended on precise timing the movement of the stars.
Over the years all kinds of technologies have tried to simplify the task but every one has had some disadvantage. Finally, the U.S. Department of Defense decided that the military had to have a precise form of worldwide positioning. Fortunately they had the deep pockets it took to build something really good. The result is the Global Positioning System, a system that's changed navigation forever.
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.
GPS uses these "man-made stars" 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 circuits 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.
Soon GPS will become almost as basic as the cell telephone. In fact, GPS technology is being used now in cell phones to implement the emergency ('911') location system.
How GPS Works
Here's how GPS works in six 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. You must correct for any delays the signal experiences as it travels through the atmosphere.
6. Finally (for us), you can now obtain the precise time from the GPS satellites.
We'll explain each of these points in subsequent sections.
Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth.
That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth.
Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space.
Triangulation
• 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.
• Another range is required for technical reasons to be discussed later.
Random Code
The Pseudo Random Code (PRC) 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."
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 Department of Defense 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.
We glossed over one point in our silly 'Stairway to Heaven' analogy. It assumes that we can guarantee that both the satellite and the receiver start generating their codes at exactly the same time. But how do we make sure everybody is perfectly synced? Stay tuned and see.
How GPS Works
The Global Positioning System (GPS) is a technical marvel made possible by a group of satellites in earth orbit that transmit precise signals, allowing GPS receivers to calculate and display accurate location, speed, and time information to the user.
By capturing the signals from three or more satellites (among a constellation of 31 satellites available), GPS receivers are able to use the mathematical principle of trilateration to pinpoint your location.
With the addition of computing power, and data stored in memory such as road maps, points of interest, topographic information, and much more, GPS receivers are able to convert location, speed, and time information into a useful display format.
GPS was originally created by the United States Department of Defense (DOD) as a military application. The system has been active since the early 1980s, but began to become useful to civilians in the late 1990s. Consumer GPS has since become a multi-billion dollar industry with a wide array of products, services, and Internet-based utilities.
GPS works accurately in all weather conditions, day or night, around the clock, and around the globe. There is no subscription fee for use of GPS signals. GPS signals may be blocked by dense forest, canyon walls, or skyscrapers, and they don’t penetrate indoor spaces well, so some locations may not permit accurate GPS navigation.
GPS receivers are generally accurate within 15 meters, and newer models that use Wide Area Augmentation System (WAAS) signals are accurate within three meters.
While the U.S. owned and operated GPS is currently the only active system, five other satellite-based global navigation systems are being developed by individual nations and by multi-nation consortiums.
Global Positioning System (GPS) Technology and Cars
Over the years, the technology involved in manufacturing an automobile has become more advanced, as automakers shift their focus from basic transportation to the design of features that make a vehicle safer, more comfortable, and more easily operated. One such feature is the global positioning system (GPS).
A GPS unit consists of a space segment, a control segment, and a user segment. The space segment is a constellation of two dozen satellites orbiting the earth twice every 24 hours, at approximately 10,900 nautical miles above the earth's surface (1). These satellites are funded and controlled by the U.S. Department of Defense. The control segment is a series of monitoring stations located at different sites on earth. These stations update and correct errors in the navigational message of the satellites. The user segment is a receiver that receives radio waves from the satellites in orbit. It can determine how far away it is from each satellite by keeping track of the time it takes for a radio wave to travel from the satellite to the receiver (2).
Four satellites are used simultaneously to pinpoint the precise position of the receiver on the earth. Information from the first three satellites narrows down the range of possible locations to two points; one of these is usually illogical and indicates a point not on the earth. A fourth satellite is used to confirm the target location (3).
The accuracy of a typical GPS receiver is about 10-15 meters. This may not be practical for locating a small object such as an automobile, which is about three meters long. Differential GPS (DGPS) is a system that improves the accuracy of the GPS receiver to about one to two meters (4). Several reference GPS receivers are placed at stationary locations, whose coordinates are known. These receivers compare their known locations to the location information they receive from satellites, and broadcast the range errors they detect from each other and from every satellite. A DGPS receiver can pick up this range error information and correlate it with the satellite signals it is receiving, to find out its true position (5). The accuracy is dependent on how fast the reference receivers broadcast their signals.
The Global Positioning.docx (Size: 166.48 KB / Downloads: 151)
Introduction
Trying to figure out where you are is probably one of humankind's oldest problems. Navigation and positioning are crucial to so many activities and yet the process has always been quite cumbersome and inexact. In the earliest days mankind used the stars to navigate. Early instruments also sited the stars to determine position. The science of horology began in part because navigation depended on precise timing the movement of the stars.
Over the years all kinds of technologies have tried to simplify the task but every one has had some disadvantage. Finally, the U.S. Department of Defense decided that the military had to have a precise form of worldwide positioning. Fortunately they had the deep pockets it took to build something really good. The result is the Global Positioning System, a system that's changed navigation forever.
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.
GPS uses these "man-made stars" 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 circuits 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.
Soon GPS will become almost as basic as the cell telephone. In fact, GPS technology is being used now in cell phones to implement the emergency ('911') location system.
How GPS Works
Here's how GPS works in six 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. You must correct for any delays the signal experiences as it travels through the atmosphere.
6. Finally (for us), you can now obtain the precise time from the GPS satellites.
We'll explain each of these points in subsequent sections.
Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth.
That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth.
Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space.
Triangulation
• 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.
• Another range is required for technical reasons to be discussed later.
Random Code
The Pseudo Random Code (PRC) 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."
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 Department of Defense 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.
We glossed over one point in our silly 'Stairway to Heaven' analogy. It assumes that we can guarantee that both the satellite and the receiver start generating their codes at exactly the same time. But how do we make sure everybody is perfectly synced? Stay tuned and see.
How GPS Works
The Global Positioning System (GPS) is a technical marvel made possible by a group of satellites in earth orbit that transmit precise signals, allowing GPS receivers to calculate and display accurate location, speed, and time information to the user.
By capturing the signals from three or more satellites (among a constellation of 31 satellites available), GPS receivers are able to use the mathematical principle of trilateration to pinpoint your location.
With the addition of computing power, and data stored in memory such as road maps, points of interest, topographic information, and much more, GPS receivers are able to convert location, speed, and time information into a useful display format.
GPS was originally created by the United States Department of Defense (DOD) as a military application. The system has been active since the early 1980s, but began to become useful to civilians in the late 1990s. Consumer GPS has since become a multi-billion dollar industry with a wide array of products, services, and Internet-based utilities.
GPS works accurately in all weather conditions, day or night, around the clock, and around the globe. There is no subscription fee for use of GPS signals. GPS signals may be blocked by dense forest, canyon walls, or skyscrapers, and they don’t penetrate indoor spaces well, so some locations may not permit accurate GPS navigation.
GPS receivers are generally accurate within 15 meters, and newer models that use Wide Area Augmentation System (WAAS) signals are accurate within three meters.
While the U.S. owned and operated GPS is currently the only active system, five other satellite-based global navigation systems are being developed by individual nations and by multi-nation consortiums.
Global Positioning System (GPS) Technology and Cars
Over the years, the technology involved in manufacturing an automobile has become more advanced, as automakers shift their focus from basic transportation to the design of features that make a vehicle safer, more comfortable, and more easily operated. One such feature is the global positioning system (GPS).
A GPS unit consists of a space segment, a control segment, and a user segment. The space segment is a constellation of two dozen satellites orbiting the earth twice every 24 hours, at approximately 10,900 nautical miles above the earth's surface (1). These satellites are funded and controlled by the U.S. Department of Defense. The control segment is a series of monitoring stations located at different sites on earth. These stations update and correct errors in the navigational message of the satellites. The user segment is a receiver that receives radio waves from the satellites in orbit. It can determine how far away it is from each satellite by keeping track of the time it takes for a radio wave to travel from the satellite to the receiver (2).
Four satellites are used simultaneously to pinpoint the precise position of the receiver on the earth. Information from the first three satellites narrows down the range of possible locations to two points; one of these is usually illogical and indicates a point not on the earth. A fourth satellite is used to confirm the target location (3).
The accuracy of a typical GPS receiver is about 10-15 meters. This may not be practical for locating a small object such as an automobile, which is about three meters long. Differential GPS (DGPS) is a system that improves the accuracy of the GPS receiver to about one to two meters (4). Several reference GPS receivers are placed at stationary locations, whose coordinates are known. These receivers compare their known locations to the location information they receive from satellites, and broadcast the range errors they detect from each other and from every satellite. A DGPS receiver can pick up this range error information and correlate it with the satellite signals it is receiving, to find out its true position (5). The accuracy is dependent on how fast the reference receivers broadcast their signals.