07-07-2014, 02:42 PM
GLOBAL POSITIONING SYSTEM
GLOBAL POSITIONING SYSTEM.ppt (Size: 1.68 MB / Downloads: 8)
Introduction
The Global Positioning System (GPS) is a space-based satellite navigation system 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 to anyone with a GPS receiver.
The GPS program provides critical capabilities to military, civil and commercial users around the world. 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.
Basics 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
Satellite position at time of message transmission
The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light
GPS ELEMENTS
GPS elements consist of three basic parts:
Space segment
User segment
Control segment
Space segment
Space segment of the system consist of the GPS satellites. These space vehicles (vs.) send radio signal from space. The nominal GPS operation constellation consist of 24 satellites that orbit the Earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to repeat older satellites. The satellites orbit repeats all most the same ground track once each day the orbit altitude is such that satellite repeat the same track.
There are six orbital plane equally spaced (60 degree apart) and inclined at about 55 degree with respect to the equatorial plane
User segment
User segment consist of receivers, which you can hold in your hand or mount in your path. The GPS receiver converts SV signals into position, velocity and time.
Four satellites are required to compute the 4 dimensions:
X, Y, Z and Time.
GPS receivers are used for navigation, position, time and other research.
Control segment
Control segment consist of a system of tracking stations located around the world. These stations measure signals from the sves which are incorporated into orbital model for each satellites. The models compute precise orbital data and sv clock correction for each satellites.
The master clock station uploads ephemeris and clock data to sves.
The master control facility determines the orbit and clock parameters using measurements from five monitor stations distributed around the world.
GPS Error sources
There are various types of error in GPS . These are:
Satellite clock
Receiver clock
Satellite orbit error
Atmospheric error: Ionosphere and troposphere
Multi-path errors
These errors in the satellites produce error in distance measurements
Satellite clock error
One nano second of inaccuracy in a satellite clock results in about 30 cm (1 foot) of error in measuring the distance to that satellite
For this reason satellites are equipped with very accurate cesium atomic clock.
Even these very accurate clock accumulates an error of one nano second every 3 hours
Receiver clock error
Similar to satellite clock error, any error in the receiver clock causes inaccuracy in distance measurement. However it is not practical to equip receiver with very accurate atomic clocks. Atomic clocks weigh more than 20 kgs, cost about US$ 50,000.
Assume that at a given time our receiver clock has an error of 1 ms causing a distance error of about 300,000 meters. If the distance to all satellites are measured exactly at the same time. We can therefore include the receiver clock error as one of the known that we must solve for
Tropospheric Error
The lower level of the atmosphere which contains water vapors is called the troposphere. It has the effect of slowing down both code and carrier.
The effect of the troposphere cannot be removed using dual frequency system. The only way to remove the effect of the troposphere is by measuring its water vapour content, temperature and pressure, and applied mathematical model that can compute the delay of troposphere.