global positioning system

The aim of this presentation is to give brief idea about Global Positioning System.
The Global Positioning System (GPS) is a space-based Global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and 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.
GPS was created and realized by the U.S Department Of Defense (DOD) and was originally run with 24 satellites. It was established in 1973 to overcome the limitations of previous navigation systems

SEGMENTS OF GPS

SPACE SEGMENT

The space segment consists of a constellation of 24 satellites in six orbital planes. The satellites have been put into 55° inclined circular orbits at an altitude of 20,200km. The six orbits are spaced 60 degrees apart with four satellites in each orbital plane

CONTROL SEGMENT

The control or the ground segment consists of the Master Control Station (MCS) at Colorado Springs in the United States and five monitor stations sited at;
Ascension Island
Diego Garcia
Kwajalein
Hawaii
Colorado Springs
Ground upload antennae are also located at the monitoring stations.

User segment

The user segment consists of the users with their GPS receivers. 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.

COMMON USE OF GPS

Four Primary Functions of GPS

Position and coordinates.
The distance and direction between any two way points, or a position and a way point. (Way points are locations or landmarks that can be stored in your GPS.)
Travel progress reports.
Accurate time measurement.

GPS TIME

Time transfer along with positional information is one of the primary designed aims of the GPS system.
Accurate timekeeping is also essential for correct positional information – to know where you are, you must know when you are! The satellites have very stable atomic clocks for time keeping and the Master Control Station monitors and corrects errors from the reference or GPS time.

CONCEPT OF OPERATION

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellite high above the Earth. Each satellite continually transmits messages that include
The time the message was transmitted
Precise orbital information
The general system health and rough orbits of all GPS satellites.
The receiver utilizes the messages it receives to determine the transit time of each message and computes the distances to each satellite.
Three satellites might seem enough to solve for position, since space has three dimensions and a position near the Earth's surface.
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.

To get accurate position?

Need signal from at least four SVs for 3D position
One SV provides a time reference
Distance to three remaining SVs is determined by observing the GPS signal travel time from SV to the receiver
With three known points, and distances to each, we can determine the GPS receiver’s position (transliteration)

GPS Signals

GPS signals are broadcast on two UHF band carrier frequencies. Two pseudo random codes are used to phase modulate these frequencies: a precision (P) code and a coarse acquisition (C/A) code.
Separate frequencies are earmarked for the control segment to up link and down link data. The carrier frequencies are:-
L1 frequency : modulated by both the C/A-code and the P-code.
L2 frequency : modulated by the P-code only.
L3 frequency : used for nuclear explosion detection and has no significance for navigation.

COARSE/ACQUISITION (C/A) CODE

The Coarse/Acquisition (C/A) Code is the code used by all civilian GPS users to determine range. As the name suggests, it has a dual purpose in that it is also used to help a US military receiver gain access to the accurate P code.
Characteristics of the C/A code:-
(a)It modulates the L1 frequency only and provides the data for Standard Positioning Service (SPS), which is the usual position, generated for most civilian receivers.
(b)It is modulated by the Navigation/Timing Message code.

PRECISION (P) CODE

P code was designed for military use and to provide a higher level of positioning accuracy and a greater immunity to jamming than the C/A code.
Though the P code was initially quite secure the details of the code are now well known and documented. The P code has now been changed into an encrypted code called the Y code. A special code key enables the use of encrypted code in the PPS (Precise positioning service) available to US Military and selected allied and civilian users.

SELECTIVE AVAILABILITY (S/A)

The Defense Department dithered the satellite time message, reducing position accuracy to some GPS users.
S/A was designed to prevent America’s enemies from using GPS against us and our allies. In May 2000 the Pentagon reduced S/A to zero meters error.
S/A could be reactivated at any time by the Pentagon.

GPS Integrity

Along with the accuracy of the system another factor that needs consideration is system integrity, by which is meant the ability of the system to warn users in case of failures. The GPS system inherently has poor integrity with no provision of warning the users in case of system malfunction or erroneous positions due to other causes. The GPS system may provide erroneous information for relatively long periods without having the capability to warn users.
It must also be borne in mind that the GPS ground stations have a slow reaction time and that the GPS control segment stations do not provide 100% tracking coverage hence leading to gaps in the monitoring network.

DIFFERENTIAL GPS

Differential GPS or DGPS is a technique by which a reference station at an accurately known position is used to compare the position obtained by GPS with the known position and hence calculate errors.
This gives a higher level of accuracy and integrity and is increasingly being used to improve the system performance.