1.1Abstract

For ask above questions we must know Using the Global Positioning System (GPS)),

s i m p l y , i t i s a process used to establish a position at any point on the globe) the

following two values can be determined anywhere on Earth:

1. Ones exact location (longitude, latitude and Alti tude (height co-ordinates)

accurate to within a range of 20 m to approx. 1 mm.

2. The precise time (Universal Time Coordinated, UTC) accurate to within a range of

60ns to approx. 5ns. Speed and direction of travel (course) can be derived from these

co-ordinates as well as the time. The coordinates and time values are determined by

28 satellites orbiting the Earth .

Fig. 1.1 : The basic function of GPS.

1.2 Introduction (GPS) is a Satellite-Based navigation (i.e Satellites that estimate the position of a point by using

a group of satellites.) System made up of a network of 24 Satellites placed into orbit by the U.S.

Department of Defense (DOD).

GPS was originally intended for military applications, but in the 1980s, the government made the

system available for civilian use .

Comparison of Some Satellite-based Navigation systems :

1.3 History of GPS

Prior to the development of the GPS system, the first satellite system was called Transit and

was operational beginning in 1964. Transit had no timing devices aboard the satellites and

the time it took a receiver to calculate its position was about 15 minutes. Yet, much was

learned from this system. GPS is a great improvement over the Transit system. The original

use of GPS was as a military positioning, navigation, and weapons aiming system to replace

not only Transit, but other navigation systems as well. It has higher accuracy and stable

atomic clocks on board to achieve precise time transfer.

The first GPS satellite was launched in 1978 and the first products for civilian consumers

appeared in the mid 1980's. It was in 1984 that President Reagan announced that a portion

of the capabilities of GPS would be made available to the civil community. The system is

still being improved and new, better satellites are still being launched to replace older ones.

1.4 What is GPS?

The Global Positioning System (GPS) is a location system based on a constellation of about

24 satellites orbiting the earth at altitudes of approximately 11,000 miles. GPS was

developed by the United States Department of Defense (DOD), for its tremendous

application as a military locating utility. The DOD's investment in GPS is immense. Billions

and billions of dollars have been invested in creating this technology for military uses.

However, over the past several years, GPS has proven to be a useful tool in non- military

mapping applications as well.

GPS satellites are orbited high enough to avoid the problems associated with land based

systems, yet can provide accurate positioning 24 hours a day, anywhere in the world.

Un-corrected positions determined from GPS satellite signals produce accuracies in the

range of 50 to 100 meters. When using a technique called differential correction, users can

get positions accurate to within 5 meters or less.

Fig. 1.2 : GPS constellation

Today, many industries are leveraging off the DOD's massive undertaking. As GPS units

are becoming smaller and less expensive, there are an expanding number of applications for

GPS. In transportation applications, GPS assists pilots and drivers in pinpointing their

locations and avoiding collisions. Farmers can use GPS to guide equipment and control

accurate distribution of fertilizers and other chemicals. Recreationally, GPS is used for

providing accurate locations and as a navigation tool for hikers, hunters and boaters Many

would argue that GPS has found its greatest utility in the field of Geographic Information

Systems(GIS).

With some consideration for error, GPS can provide any point on earth with a unique

address (its precise location). A GIS is basically a descriptive database of the earth (or a

specific part of the earth). GPS tells you that you are at point X,Y,Z while GIS tells you that

X,Y,Z is an oak tree, or a spot in a stream with a pH level of 5.4. GPS tells us the "where".

GIS tells us the "what". GPS/GIS is reshaping the way we locate, organize, analyze and

map our resources

1.5 How it Works

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. Now, with distance

measurements from a few more satellites, the receiver can determine the user's position and

display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D

position (latitude and longitude) and track movement. With four or more satellites in view,

the receiver can determine the user's 3D position (latitude, longitude and altitude). Once

the user's position has been determined, the GPS unit can calculate other information, such

as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and

more.

Comparison of Some Satellite-based Navigation systems :