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GPS (Global positioning system) means a space-based satellite navigation system provides location and time information in all weather. Maintained by the United States government US government and is freely accessible by anyone with GPS receiver overview official name navigational satellite timing and ranging global positioning system. It consists of 30 GPS satellites in medium Earth orbit made up of two dozen satellites working in harmony are known as a satellite constellation mainly used for navigation map making and surveying.
History of GPS System:
In early times navigating my Stars requires clear night and clear measurements most widely used for centuries. Secondly the GPS project was developed in 1973 to overcome the limitations of previous navigation systems GPS was created and realized by the US Department of Defense and was originally done with 24 satellites. It became fully operational in 1995 “Bradford Parkinson” “Roger L Easton” and “Ivan A. Getting” are credited with inventing it.
GPS elements are divided into three segments
- space segment
- control segments
- user segment
GPS satellites fly in circular orbits at an altitude of twenty thousand and two hundred kilometer and with a period of 12 hours powered by solar cells. The satellites continuously orient themselves to point the solar panels toward the Sun and the antenna towards the earth. Orbital planes are centered on the earth orbits are designed so that at least six satellites are always within line of sight from any location on the planet.
The control segment consists of three entities the control segment
- master control system
- monitor stations
- ground antennas
Master control station:
The master control station located at Falcon Air Force Base in Colorado Springs. It is responsible for overall management of the remote monitoring and transmission sites. Checkup is performed twice a day by each of six stations as the satellites complete their journeys around the earth can reposition satellites to maintain an optimal GPS constellation.
Monitor stations checks the exact altitude position speed and overall health of orbiting satellites. The control segment ensures that the GPS satellite orbits and clock remains within acceptable limits. A station can track up to 11 satellites at a time. This checkup is performed twice a day by each station.
Ground antennas monitor and track the satellite from horizon– to horizon. They also transmit correction information. They also transmit correct information to individual satellites and communicate with the GPS satellites for command and control purposes.
GPS receivers are generally composed of
- an antenna
- receiver processors
- high stable clock ( commonly a crystal oscillator )
They can also include a display for showing location and speed information to the user. A receiver is often described by its number of channels as of recent receivers usually have between 12 and 20 channels you can handle.
You can see this diagram on your screen yeah you can see the ground segment at the right bottom and just left next to that you can see monitor station and next to that user segment you can see satellites which is on the eleven thousand miles the altitude is eleven thousand miles you can see that .
Working Principle of the GPS:
You can find once location if you know its distance from other already known locations. Things which need which need to be determined will speak current locations of GPS satellites. The distance between receivers position and the GPS satellites current location of GPS satellites.
Current location of GPS satellites:
GPS satellites are orbiting the Earth at an altitude of 11,000 miles and I believe that you know that one mile is equal to 1.6 kilometers aprox. The orbits and the locations of satellites are known in advance. GPS receivers store this orbit information for all of the GPS satellites in ALMANAC. An ALMANAC is a file which contains positional information for all of the GPS satellites.
Distance between receivers position and GPS satellites:
A GPS receiver can tell its own position by using the position data of itself and compares the data with three or more GPS satellites to get the distance to each satellite by measuring the amount of time taken by radio signals to travel from the satellite to the receiver radio waves travel at the speed of light that is about 1 lakh 86,000 miles per second the distance from the satellite to the receiver can be determined by the formula distance equals to speed multiplied by time hence receivers position find out using trial literation trial iteration now you can see the diagram of two satellites and the orbit distance measurement from two satellites limits our location to the intersection of two spheres which is a circle.
a third measurement narrows our location to just two points you can see you can see the difference between these two you have a third orange circle now which is narrowing this part with two points you can see them you can see those two points yellow one 1/4 measurement determines which point is our true location you can see that pink one has intersected the circumference of pink one has intersected the point so that is your exact location so we require at least four satellites to get our exact location.
you can see in the right lab Tygra in the right diagram.
The position calculated by GPS receiver relies on three accurate measurements firstly current time secondly position of the satellites thirdly the time delay for the signal the GPS signal in space will provide a worst-case accuracy of seven point eight meters at a 95% confidence level. GPS time is accurate to about 40 nanoseconds. Higher accuracy available today by using GPS in combination with augmentation systems. These enable real-time positioning to within a few centimeters.
- Coarse / acquisition code
- Precision encode
- Navigation message
- Data updates
- L1 (1575.42 MHz)
- L2 (1227.60 MHz)
- L3 (1381.05 MHz)
- L4 (1379.913 MHz)
- L5 (1176.45 MHz)
The C/A code which stands for the course acquisition is transmitted on the L1 frequency. L1 and L2 frequencies transmit both the precision code. L3 is used by the different supposed program to signal direction the signal detection of missile launches nuclear detonations and other applications. The solar radiation ionized the atmosphere for which L4 is used for additional correction. The civilian safety of life signal used L5.
It was launched in 2005 L2C is civilian GPS signal designed specifically to mean commercial needs. L2C enables ionospheric corrections a technique that boots accuracy. It delivers fast a signal acquisition, enhanced reliability and greater operating range. L2C broadcast at a higher effective power making it easier to receive under trees and even indoors.
Sources of GPS signal errors:
Different errors can cause a deviation of plus minus 50 to 100 meters from the actual GPS receiver positions which are firstly:
- Satellite clock
- Receiver clock
- GPS jamming
One nanosecond of in an accuracy in a satellite clock result in about 30 centimeter of error in measuring the distance to the satellite.
We have error in the distance measurement when the receiver clock causes in accuracy. Accurate atomic clock is not practical to equip receiver. Atomic clock weigh more than 20 kg kilograms cost about a U.S dollar fifty thousand.
Thirdly GPS jamming it limits the effectiveness of the GPS signal. GPS jammer is low-cost device to temporarily disable the reception of the civilian course acquisition code.
Speed of GPS signal is affected by ionosphere in troposphere which caused a deviation of 0 to 30 meter from the actual position of receiver.
Multi path error:
Bouncing of GPS signal due to a reflecting surface be called before reaching to the receiver antenna which caused a deviation of 0 to 1 meter from the actual position of receiver.
Methods of improving accuracy:
- Dual frequency monitoring
- Carrier phase enhancement
- Relative kinematic position
Dual frequency monitoring:
- It refers to the systems that can compare two or more signals.
- These two frequencies are affected in two different ways
- After monitoring these signals it is possible to calculate what the error is and eliminate it
- Receivers that have the correct disk decryption key can decode the p/y code transmitted on signals to measure the error
Carrier phase enhancements (CGPS):
CPGPS uses the L1 carrier wave which has a period 1,000 times smaller than that of the sea a bit period to act as an additional clock signal and resolve as uncertainity. The phase difference error in the normal GPS amounts to between two and three meters of ambiguity. CPGPS works to within one percent of perfect transition to reduce error to three centimeters of ambiguity by eliminating this source of error. DGPS coupled with CPGPS normally realizes between twenty and thirty centimeters (8 – 12 inches) of absolute accuracy.
Relative kinematic positioning:
Determination of range signal can be resolved to an accuracy of less than 10 centimeters resolves the number of cycles in which the signal is transmitted and received by those receiver accomplished by using a combination of DGPS correction data transmitting gps signal phase information and ambiguity resolution techniques via statistical tests possibly with processing in real-time.
It relies on the external information being integrated into the calculation process. Some augmentation system transmits additional information about the sources of error. Some provide direct measurement to how much the signal was off in the past. Another group could provide additional navigation or vehicle information to be integrated in the calculation process.
Nationwide differential GPS system (NDGPS):
Ground based augmentation system that provides increased accuracy and the integrity of GPS information to users on U.S land and waterways. The system consists of the maritime differential GPS system operated by the US Coast Guard and an inland component funded by the Department of Transportation.
Wide area augmentation system:
Satellite based augmentation system operated by the Federal Aviation Administration (FAA) supports aircraft navigation across North America.
Global differential GPS (GDGPS):
NASA Jet Propulsion Laboratory JPL high accuracy GPS augmentation system developed to support the real-time positioning timing and determination requirements of NASA science missions. Future NASA plans include using the tracking and the data relay satellite system (TDRSS) to transmit via satellite a real-time differential correction message.
GPS can provide worldwide three-dimensional positions 24 hours a day in any type of weather but there must be a relative clear light line of sight between the GPS antenna and four or more satellites. Hence it becomes too difficult to ensure reliable positioning these deceitful these difficulties are particularly deeply prevalent in urban areas. Multipath interference occur when the GPS signal may bounce off nearby objects.
Surveyors use absolute location to make maps and determine property boundaries.
GPS integrated with computer and mobile communications technology in automotive navigation system.
Vehicle tracking located we can anytime anywhere in this world deepest car tracking system is more effective way to locate car. It consists of boxes which contain batteries and tracking receiver and antenna.
Monitor track spy:
- Exact location on the map
- speed at which the vehicle is traveling
- total distance travelled by a vehicle
- fuel refill theft
- Ignition control of the vehicle
Applications in military:
GPS indicated in two fighters tankers helicopters ships submarines tanks jeeps and the soldiers equipment target taking tracking search in the rescue.
GPS satellite based navigation system thus can be used to determine the position of an object on earth this application field is vast and new application will continue to be created as the technology evolves the GPS can also interfaced with other similar projects such as use GALILEO to account for unpredictable applications. Thus the GPS constellation like man-made stars in the sky can be used for guiding and navigation.