The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). The GPS uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, that enable GPS receivers to determine their location, speed, direction, and time. GPS was developed by the United States Department of Defense. Its official name is NAVSTAR-GPS. Similar satellite navigation systems include the Russian GLONASS, the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India.
Following the shooting down of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available free for civilian use as a common good. Since then, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce and scientific uses.
GPS satelite system The first GPS satellite was launched in 1978. A full constellation of 24 satellites was achieved in 1994. 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, transmitter power is only 50 watts or less.
In 1997, U.S. Secretary of Transportation Federico Pena stated, "Most people don't know what GPS is. Five years from now, Americans won't know how we lived without it."
Since the beginning of time, mankind has been trying to figure out a dependable way to know where they were, and to guide them to where they wanted to go and get back again. Major developments in early navigation were the compass and the sextant . Considering the less accuracy of this methods, Britain formed a group of well-known scientists called the Board of Longitude. In 1761, a man named John Harrison developed a timepiece called a ‘chronometer’.
Radio-based navigation systems were developed in the early twentieth century, and were used in World War II. When Sputnik was launched into space by Russia on October 4th, 1957 it became known that "artificial stars" could be used for navigation. The U.S. Navy experimented with satellite navigation. In the mid-sixties there was the ‘Transit System’ that was developed for submarines carrying Polaris nuclear missiles. In measuring the Doppler shift of the radio signals the submarines could locate its position within fifteen minutes.
The Global Positioning System, now commonly known GPS was designed and built and is operated and maintained by the U.S. Department of Defense. It used to be known as the “Navstar Global Positioning System” and was first brainstormed at the Pentagon in 1973 as they were looking for a satellite system that was error-proof. In 1978 the first operational GPS satellite was launched. By the mid-1990s the system was fully operational with 24 satellites.
The current GPS consists of three major segments. These are the space segment (SS), control segment (CS), and user segment (US). Space segment The space segment (SS) comprises the orbiting ‘GPS satellites’, or ‘Space Vehicles’ (SV) in GPS architecture. The GPS design originally called for 24 SVs, eight each in three circular orbital planes, but this was modified to six planes with four satellites each.
Control segment The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA) . The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). So to change the orbit of a satellite, the satellite must be marked ' unhealthy ', so receivers will not use it in their calculation. Then the message can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked ‘ healthy’ again.
User segment GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right). In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock. Trimble Leica Garmin A typical OEM GPS receiver module measuring 15×17 mm
GPS broadcast signal Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-week, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The messages are sent in frames, each taking 30 seconds to transmit 1500 bits. Navigation signals
The almanac consists of coarse orbit and status information for each satellite in the constellation and information to relate GPS derived time to Coordinated Universal Time (UTC). Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public , and the Precise (P) code, which is usually encrypted and reserved for military applications. Each satellite has its own C/A code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency. The P code is encrypted by the Y-code to produce the P(Y) code, which can only be decrypted by units with a valid decryption key.
GPS can aid in the recovery of a stolen car. Integrated with the automobile security system, GPS can notify the car owner by phone or e-mail when the car alarm is triggered, and indicate the location of the car. GPS also monitors the car condition, and issues warnings when the battery is low or when it is time for an oil change. The Ford automobile company has developed a new ‘ telematic system’ through GPS on the Ford Focus, that will alert emergency services when an airbag deploys.
The development of GPS technology opens the door to the design of an unmanned vehicle. Commercial fishermen can use GPS receivers to keep track of the best spots where they have caught fish. GPS technology is also a valuable aid to targeting locations for military personnel, firefighters, and construction workers.
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.
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. HOW ACCURATE IS GPS
<ul><li>Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. </li></ul><ul><li>2. Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. </li></ul><ul><li> </li></ul><ul><li> cntd…. </li></ul>Sources of GPS signal errors
<ul><li>3. Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. </li></ul><ul><li>4. Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location. </li></ul><ul><li>Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground. </li></ul><ul><li> cntd…. </li></ul>
The receiver can utilize multidimensional Newton-Raphson calculations. Linearize around an approximate solution, say from iteration k, then solve four linear equations derived from the quadratic equations above to obtain Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted signals can be generated using readily available signal generators. Position Derermination P(Y)code
The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry. The military applications of GPS span many purposes: Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies. Target tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile. MILITARY
Missile and projectile guidance: GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions. Search and Rescue: Downed pilots can be located faster if they have a GPS receiver. Map Creation: The military use GPS extensively to aid mapping. The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an Electro-Magnetic Pulse (EMP) sensor (W-sensor) which form a major portion of the United States Nuclear Detonation Detection System.
This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing. The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation. The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. CIVILIAN
GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes. GPS tours are also an example of civilian use. The GPS is used to determine which content to display. For instance, when approaching a monument it would tell you about the monument. GPS functionality has now started to move into mobile phones.
With a GPS Data Logger you can place a device on the car it will record locations as the car drives around. You can then retrieve the device later and connect it to your PC to read the data. The advantage of a data logger is very detailed report at a low cost with no monthly fees. The disadvantage is that you can not find our where a car is or has been until it returns and you retrieve the device. This is a good solution to monitor the driving behavior of a teen or employee over the past few days or weeks but will not help you find a missing car. GPS data loggers
A Real-Time GPS Tracking System has a built-in connection to the internet so you can log-on and see where the device is and check most current location as well as the history from anywhere in the world. Real time GPS tracking system
The Seek is essentially a portable GPS device that's "meant to help novice and experienced looking for local theaters, restaurants, clubs etc." Also, it has touch screen based functions with other functions like a portable music player to keep you entertained. So if you are in a new town, just seek the guidance of Seek and you will see the whole city. Portable GPS
GPS features will soon be added to this list, and will offer the capability of tracking any cell phone user. The capability of tracking a cell phone user becomes critical in emergency situations, like a car accident, through the use of 911 emergency phone call, where every minute may be a matter of life or death. In situation like this, the GPS system may pinpoint the precise location of the road accident, enabling emergency services to be quickly dispatched. GPS in cell phones
G-Guard is a new generation of high-tech satellite security system. G-Guard not only provides you with safety protection, emergency rescue and special services, it also offers you and your vehicle an all-time protection and full function services. Security guard? Cntd….
Unmanned Control Center is specially designed for the G-Guard users to have vehicle location Search, Tracking and Emergency reporting services. …….
When you are using a Bluetooth GPS receiver paired with a PDA in a vehicle, the receiver should be placed in a position that has a clear view of the sky. However, the PDA can be positioned and mounted on the car dashboard or window so that you have the best possible screen visibility. This positioning makes it easy to see and access the PDAs screen and menu options. Bluetooth GPS reciever Cntd….
Every Bluetooth GPS receiver has software that is included with it when purchased. The software generally includes United States maps, a point of interest (known as POI) database, and special features, which usually include route recalculation. ……..
Natural sources Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver. Solar flares are one such naturally occurring emission with the potential to degrade GPS reception. GPS signals can also be interfered with by naturally occurring geomagnetic storms . Disadvantages
Artificial sources Man-made EMI can also disrupt, or jam, GPS signals. The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military claimed to destroy six GPS jammers during the Iraq War (including one that was destroyed ironically with a GPS-guided bomb).
Accuracy enhancement Augmentation : Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Precise monitoring The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways. Advantages
The most certain aspect of the future of GPS is its increased usage and its expansion into new areas of application. Bradford Parkinson , from the University of Minnesota Center for Transportation Studies, predicts that by 2010 there will be more than 50 million GPS users that perform applications relating to the following fields: - automobiles - ships - farm vehicles - aircraft - military systems
If you jump into the future, 2012, we’ll see that Pocket PC's will probably be small enough to include them into a larger sized watch. The GPS satellites would have probably been updated to punch out more power or better and quicker signal fixes would be achieved by ground sites. Watches would not have a problem receiving GPS based signals, but privacy will be a big issue.
Technology Additional advances in GPS technology will also include increased positional accuracy and more reliable calculations. The addition of civilian codes and civilian frequencies will be developed to solely meet the needs of civilian users with little to no military application.
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