GLOBAL POSITIONING

SYSTEMBy

S.Mudassir

IT IS A SATELLITE BASED NAVIGATION SYSTEM PROVIDED BY

U.S. DEPARTMENT OF DEFENSE.

GPS(GLOBAL POSITIONING SYSTEM)

USED TO LOCATE POSITIONS ON

EARTH.

ITS FULL NAME IS NAVSTAR GPS (NAVIGATION OF SPACE WITH TIME

AND RANGING GLOBAL POSITIONING SYSTEM).

HOW GPS WORKS?

GPS USES A CONSTELLATION OF 24 SATELLITES CONTINOUSLY ORBITING THE EARTH. Speed of each satellite= 14000km/hAltitude of each satellite from the Earth= 20000km

THE SATELLITE ORBITS ARE DISTRIBUTED SO THAT AT LEAST 4 SATELLITES ARE ALWAYS VISIBLE FROM ANY POINT ON THE EARTH AT ANY GIVEN INSTANT

STRUCTURE OF GPS

Space Segment Control Segment

User Segment

IT CONSISTS OF SATELLITES ORBITING AROUND THE

EARTH

Space Segment

Each satellite contains several on board atomic clocks.

The satellites transmit low radio signals with a unique code on different frequencies, allowing GPS to identify the signals.

The main purpose of this signal is to allow the gps receiver to calculate travel time of the radio signal from the satellite to the receiver.

Travel time multiplied by speed of light is equal to distance of a particular point on Earth.

Velocity x time = distance.

CONTROL SEGMENT

CONSISTING OF CONTROL AND MONITORING STATIONS Control segment tracks the satellites

and then provides them with corrected orbital and time information.

It consists of one master control station and five base stations and ground antennas.

The main function of base control stations is to keep the record of the satellite and send the corrected information to the master control station.

The corrected data is known as “EPHEMERIS” data.

This corrected data from Master Control stations is sent to satellites through ground antennas.

Master control station is located at Colorado springs.

Five Base Control Stations are located at five various positons on Earth.

USER SEGMENT

GPS RECEIVERS OWNED BY CIVILIANS AND MILITARY This segment consists of the users using

GPS receivers. There are many applications in this

segment.

HOW GPS DETERMINES A POSITION

A GPS receiver should receive information from at least four satellites to determine the position accurately.

It proceeds in following three steps: Precise location of satellites Distance from each satellite Triangulation to determine the position

PRECISE LOCATION OF SATELLITES When gps receiver is first turned on, it

downloads orbit information from all satellites called an almanac. This process , the first time , can take as long as 12 minutes; but once this information is downloaded, it is stored in the receiver’s memory for future use.

DISTANCE FROM EACH SATELLITE The gps receiver calculates the distance

from each satellite to the receiver by using the distance formula as stated earlier.

The receiver already knows the velocity, which is the speed of the radiowave.

TRIANGULATION When the receiver receives the signals

from at least three satellites it should be able to calculate its approximate posotion.

The receiver need at least four or more satellites to calculate a more accuate 3d position.

APPLICATIONS So far gps has limited uses in fields of

surveying, navigation, millitary etc. Recreational uses of gps are increasing

day by day. Gps receiver chips are now a days used

in mobile phones.