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Global Positioning System (GPS):
The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver with some technical limitations which are only removed for military users. The GPS program provides critical capabilities to military, civil and commercial users around the world. It is an engine of economic growth and jobs, and has generated billions of dollars of economic activity. It maintains future warfighter advantage over opponents and is one of the four core military capabilities. In addition, GPS is the backbone for modernizing the global air traffic system. The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. GPS was created and realized by the U.S. Department of Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1994. In addition to GPS, other systems are in use or under development. The Russian Global Navigation Satellite System (GLONASS) was in use by only the Russian military, until it was made fully available to civilians in 2007. There are also the planned European Union Galileo positioning system, Chinese Compass navigation system, and Indian Regional Navigational Satellite System.

How it works
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the

user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

How accurate is GPS?
Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Garmin's 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin® GPS receivers are accurate to within 15 meters on average.

Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

Uses of GPS:
Military Uses for GPS Although the GPS system was completed only in 1994, it has already proved to be a valuable aid to U.S. military forces. Picture the desert, with its wide, featureless expanses of sand. The terrain looks much the same for miles. Without a reliable navigation system, U.S. forces could not have performed the maneuvers of Operation Desert Storm. With GPS the soldiers were able to go places and maneuver in sandstorms or at night when even the Iraqi troops who lived there couldn’t. More than 1,000 portable commercial receivers were initially purchased for their use. The demand was so great that before the end of the conflict, more than

9,000 commercial receivers were in use in the Gulf region. They were carried by soldiers on the ground and were attached to vehicles, helicopters, and aircraft instrument panels. GPS receivers were used in several aircraft, including F-16 fighters, KC-135 aerial tankers, and B-52 bombers. Navy ships used them for rendezvous, minesweeping, and aircraft operations. GPS has become important for nearly all military operations and weapons systems. It is also used on satellites to obtain highly accurate orbit data and to control spacecraft orientation.

GPS in Everyday Life
The GPS system was developed to meet military needs, but new ways to use its capabilities in everyday life are continually being found. As you have read, the system has been used in aircraft and ships, but there are many other ways to benefit from GPS. We’ll mention just a few to give you an idea of its many uses.

GPS is helping to save lives and property across the nation. In 2002, it enabled rescuers to drill a shaft to free trapped miners in Somerset PA. Many police, fire, and emergency medical-service units use GPS receivers to determine the police car, fire truck, or ambulance nearest to an emergency, enabling the quickest possible response in life-or-death situations. GPS-equipped aircraft can quickly plot the perimeter of a forest fire so fire supervisors can produce updated maps in the field and send firefighters safely to key hot spots. Mapping, construction, and surveying companies use GPS extensively. During construction of the tunnel under the English Channel, British and French crews started digging from opposite ends: one from Dover, England, and one from Calais, France. They relied on GPS receivers outside the tunnel to check their positions along the way and to make sure they met exactly in the middle. Otherwise, the tunnel might have been crooked. GPS allows mine operators to navigate mining equipment safely, even when visibility is obscured.

Remember the example of the car with a video display in the dashboard? Vehicle tracking is one of the fastest-growing GPS applications today. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations at all times for both efficiency and driver safety. Automobile manufacturers are offering moving-map displays guided by GPS receivers as an option on new vehicles. The

displays can be removed and taken into a home to plan a trip. Several major rental car companies have GPS-equipped vehicles that give directions to drivers on display screens and through synthesized voice instructions. Imagine never again getting lost on vacation, no matter where you are. GPS-equipped balloons monitor holes in the ozone layer over the polar regions as well as air quality across the nation. Buoys tracking major oil spills transmit data using GPS to guide cleanup operations. Archaeologists, biologists, and explorers are using the system to locate ancient ruins, migrating animal herds, and endangered species such as manatees, snow leopards, and giant pandas. The future of GPS is as unlimited as your imagination. New applications will continue to be created as technology evolves. GPS satellites, like stars in the sky, will be guiding us well into the 21st century.

Uses of GPS in India
In India, GPS services are used to control and manage vehicular flow in many cities. Automatic Vehicle Locater System is a hardware device that uses GPS technology to monitor vehicle movements. Using this, you can well track a fleet of vehicles. Vehicle tracking system is made up of two units. The mobile vehicle unit and the fixed base station units. GPS technology is also used for harvesting. Tractors provided with GPS guidance systems are used for accurate ploughing, harvesting, spraying insecticides and fertilizers on specified areas, location and tagging of soil samples etc. GPS can also be used for tourism and adventure sports purposes. With GPS enabled hardware technologies, you can, not only receive route signals on mountain terrains, you can also get information about terrain altitudes and future weather conditions. GPS together with GLONASS satellite technology has been

working together in combination since 1990’s. Integration of both these satellite constellation network resulted in the production of a receiver that monitors satellite signaling more perfectly.

GPS Applications:
Tracking Devices One of the easiest applications to consider is the simple GPS tracking device; which combines the possibility to locate itself with associated technologies such as radio transmission and telephony. Tracking is useful because it enables a central point to monitor the position of several vehicles or people, in real time, without them needing to relay that information explicitly. This can include children, criminals, police and emergency vehicles or military applications. The tracing devices themselves come in various different flavors. They will always contain a GPS receiver, and some GPS software, along with some way of transmitting the resulting coordinates. GPS watches, for example, tend to use radio waves to transmit their location to a tracking center, while GPS phones use existing cell phone technology. The tracking center can then use that information for coordination or alert services. One application in the field is to allow anxious parents to locate their children by calling the tracking station – mainly for their peace of mind. GPS vehicle tracking is also used to locate stolen cars, or provide services to the driver such as locating the nearest gas station. Police can also benefit from using GPS tracing devices to ensure that parolees do not violate curfew, and to locate them if they do. Navigation Systems

Once we know our location, we can, of course, find out where we are on a map, and GPS mapping and navigation is perhaps the most well-known of all the applications of GPS. Using the GPS coordinates, appropriate software can perform all manner of tasks, from locating the unit, to finding a route from A to B, or dynamically selecting the best route in real time. These systems need to work with map data, which does not form part of the GPS system, but is one of the associated technologies that we spoke of in the introduction to this article. The availability of high powered computers in small, portable packages has lead to a variety of solutions which combines maps with location information to enable the user to navigate. The first such application was the car navigation system, which allows drivers to receive navigation instructions without taking their eyes off the road, via voice commands. Usually, these systems take their map data from a CD which can be replaced when the driver moves from one geographical location to another. Then there are handheld GPS units, such as those from Garmin, which are commonly used by those involved in outdoor pursuits, and only relay very limited information such as the location, and possibly store GPS waypoints. A waypoint being a location that is kept in memory so that the unit can retrace the path at a later time. More advanced versions include aviation GPS systems, which offer specific features for those flying aircraft, and marine GPS systems which offer information pertaining to marine channels, and tide times. These last two require maps and mapping software which differ vastly from traditional GPS solutions, and as such can often be augmented with other packages designed to allow the user to import paper maps or charts. The map source

software is one such industry standard package. There are even GPS solutions for use on the golf course. Golf GPS systems help the player to calculate the distance from the tee to the pin, or to know exactly where they are with relation to features such as hidden bunkers, water hazards or greens.

The Elements of GPS:
The GPS system is made up of three segments—space, control, and user—all of which contribute to overall accuracy, reliability, and functionality. Space Segment The baseline GPS constellation consists of at least 24 satellites in six planes inclined at 55 degrees relative to the equatorial plane. The operational constellation includes additional satellites to ensure that maintenance and anomalies will have minimal impact on service. The satellites are positioned about 20,000 kilometers above Earth in approximately 12-hour orbits. With this configuration, almost every point on Earth can see at least five GPS satellites, and often many more.

The GPS satellites have solar panels to generate power and use shaped-beam antennas to provide nearly constant signal strength over Earth. Satellite lifetimes typically exceed ten years, thanks to a high degree of system redundancy. Navigation performance is highly dependent on the stability of the cesium and rubidium atomic clocks. These highquality space-qualified atomic clocks have stabilities of better than 1 part in 1013 over a period of one day, which translates to an error buildup of less than 10 nanoseconds (3 meters) per day. To keep accuracy high, Air Force Space Command computes and uploads clock corrections to the satellites, which in turn broadcast this information to the user as part of the data messages. The more stable the atomic clocks, the less frequent the satellite uploads need to be to maintain a desired ranging accuracy. The first GPS satellite was launched in 1978. Initial operational capability was established in December 1993 when the full constellation of 24 satellites was completed. Final operational capability was announced the following year.

Control Segment GPS employs a worldwide ground network to monitor the health of the satellites, keep them in their intended orbits, and update their clock and position data. Five globally distributed monitor stations track the GPS satellites and send ranging data to a master control station in Colorado Springs. The master control station processes the ranging measurements in a Kalman filter every 15 minutes to determine satellite orbit and clock corrections. Periodically, roughly once per day for each satellite, the

master control station predicts the orbits and clocks and forms a navigation message. The navigation message is sent to a ground antenna for upload to the satellite on an S-band data link and transmitted to the user on the GPS signal. The navigation message is transmitted on both the L1 and L2 channels at a rate of 50 bits per second. The message has a 1500-bit frame (30-second duration) consisting of five 300-bit subframes (6 seconds each). Subframe 1 contains clock parameters. Subframes 2 and 3 contain orbit parameters. Subframes 4 and 5 contain almanac data (less accurate orbit data that is used only for signal acquisition), single-frequency ionosphere model parameters, and GPSUTC (Coordinated Universal Time) offset data. User Segment A GPS receiver tracks selected satellites and computes user position. A receiver consists of an antenna (typically omnidirectional), filtering and amplification circuits, and signal-tracking components. Satellite positions are computed from navigation message data. The pseudorange measurements are corrected for satellite clock errors, Earth rotation, ionospheric delay, tropospheric delay, and relativistic effects. The corrected pseudorange data and satellite positions are used to compute user position, velocity, and time. The computation may be done using GPS alone or integrating data from other sensors such as altimeters, compasses, and inertial measurement units. Depending on the application, user position may be superimposed on a map, used to make corrections to a weapon in flight, or transmitted to a central processing facility.

The Future of GPS:
GPS is playing an increasingly important role in all aspects of military operations—from ground troop maneuvers to

precision weapon delivery. But the role of GPS in civilian applications is expanding even faster. As navigation technology matures, the trend will continue toward embedded GPS applications integrated with communication systems and large databases. For example, integrated systems could provide immediate traffic information and route alternatives to rush-hour drivers or advertise a particular restaurant to potential customers in its vicinity as the dinner hour approached. In fact, given the emphasis on complete system integration, future users may not even be aware that satellite navigation technology will be at work in their daily lives. Promising applications are abundant in the transportation arena: real-time traffic information, route guidance, fleet control, collision avoidance, automated accident reporting, and automated toll charges, to name just a few. Other uses —such as auto insurance pricing based on when, where, and how fast a car is driven—might not be so popular with the general public. GPS has become an essential element in the global infrastructure and has exceeded the expectations of even its early developers. Aerospace played a prominent role in the development of this dual-use space system, and will continue to guide and support its future evolution.

Basic concept of GPS:
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include ◦

the time the message was transmitted precise orbital information (the ephemeris) the general system health and rough orbits of all GPS satellites (the almanac). The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites' locations are used with the possible aid of trilateration,

depending on which algorithm is used, to compute the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes. Three satellites might seem enough to solve for position since space has three dimensions and a position near the Earth's surface can be assumed. However, even a very small clock error multiplied by the very large speed of light — the speed at which satellite signals propagate — results in a large positional error. Therefore receivers use four or more satellites to solve for the receiver's location and time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.

The GPS satellite implementation system:
The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour. GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket

boosters on each satellite keep them flying in the correct path. Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS): ◦ ◦ The first GPS satellite was launched in 1978. A full constellation of 24 satellites was achieved in 1994. ◦ Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit. ◦ A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.

At least 24 GPS satellites in operation at any given time with a number of on-orbit spares in case one fails. Each one is in a 12 hour orbit (meaning it takes 12 hours to orbit the

earth). They are in a variety of six different orbits and are not just locked into a geosynchronous orbit (meaning they stay over roughly the same place on earth at all times, like your satellite TV and communications satellites) like some satellites. All GPS satellites are owned and operated by the US Air Force and are controlled specifically by the 2d Space Operations Squadron at Schriever AFB in Colorado Springs, CO. There is not an easy way to deny the GPS capability to our enemies without also denying our own capability, so it is a free system open to anyone that has the technology to utilize it. GPS satellites carry not only positional data but also extremely precise timing signals, which help the GPS receivers on the ground to triangulate their position and are even used to validate and secure financial transactions, etc. When the system was first created, artificial timing errors were put into the signal to try to reduce the effectiveness of the system for non-military users, but it was removed in 2000. The GPS satellites also have NUDET (Nuclear Detonation) sensors on them to detect nuclear detonations almost anywhere on earth. To use GPS you need to be in clear view of at the very least 3 satellites but you should be in view of 6 satellites at any given time unless some are blocked by objects, mountains, etc. So, GPS usually doesn't work well in-doors or even in a forest or valley at times.


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