Global Positioning System (GPS) is the only system today able to show
ones own position on the earth any time in any weather, anywhere. This
paper addresses this satellite based navigation system at length. The
different segments of GPS viz. space segment, control segment, user
segment are discussed. In addition, how this amazing system GPS works,
is clearly described. The various errors that degrade the performance of
GPS are also included. DIFFERENTIAL GPS, which is used to improve the
accuracy of measurements, is also studied. The need, working and
implementation of DGPS are discussed at length. Finally, the paper ends
with advanced application of GPS.
INTRODUCTION: The Global Positioning System (GPS) is a satellite-based Navigation system
developed and operated by the US Department of Defense. GPS Permits
land, sea and airborne users to determine their three-dimensional
position, velocity and time. This service is available to military and
civilian users around the clock, in all weather, anywhere in the world.
HISTORY:   Since
prehistoric times, people have been trying to figure out a reliable way
to tell where they are, to help guide them to where they are going, and
to get they back home again. The earliest mariners followed the coast
closely to keep from getting lost. When navigators first sailed into the
open ocean, they discovered they could chart their course by following
the stars. Unfortunately for Odysseus and all the other mariners, the
stars are only visible at night – and only on clear nights.  The next
major developments in the quest for the perfect method of navigation
were the magnetic compass and the sextant. The needle of a compass
always points north, so it is always possible to know in what direction
you are going. The sextant uses adjustable mirrors to measure the exact
angle of the stars, moon, and sun above the horizon.
In the early 20th century several radio-based navigation systems were
developed. A few ground-based radio-navigation systems are still in use
today. One drawback of using radio waves generated on the ground is that
you must choose between a system that is very accurate but doesn’t
cover a wide area, or one that covers a wide area but is not very
accurate. High-frequency radio waves (like UHF TV) can provide accurate
position location but can only be picked up in a small, localized area.
Lower frequency radio waves (like AM radio) can cover a larger area, but
are not a good yardstick to tell you exactly where you are. A
transmitter high above the Earth sending a high-frequency radio wave
with a special coded signal can cover a large area and still overcome
much of the “noise” encountered on the way to the ground. This is the
main principle behind the GPS system.
GPS has 3 parts: the space segment, the user segment, and the control
segment. The space segment consists of 24 satellites, each in its own
orbit 11,000 nautical miles above the Earth. The user segment consists
of receivers, which you can hold in your hand or mount in your car. The
control segment consists of ground stations (five of them, located
around the world) that make sure the satellites are working properly.
Space segment:
The complete GPS space system includes 24 satellites, 11,000 nautical
miles above the Earth, which take 12 hours each to go around the Earth
once (one orbit). They are positioned so that we can receive signals
from six of them nearly 100 percent of the time at any point on Earth.
There are six orbital planes (with nominally four Space Vehicles in
each), equally spaced (60 degrees apart), and inclined at about
fifty-five degrees with respect to the equatorial plane.
Satellites are equipped with very precise clocks that keep accurate time
to within three nanoseconds. This precision timing is important because
the receiver must determine exactly how long it takes for signals to
travel from each GPS satellite. The receiver uses this information to
calculate its position.
The first GPS satellite was launched in 1978. The first 10 satellites
were developmental satellites, called Block I. From 1989 to 1993, 23
production satellites, called Block II, were launched. The launch of the
24th satellite in 1994 completed the system.

Control Segment:
The control segment consists of a worldwide system of tracking and
monitoring stations.The ‘Master Control Facility’ is located at Falcon
AFB in Colorado Springs, CO.
The monitor stations measure signals from the GPS satellites and relay
the information they collect to the Master Control Station. The Master
Control Station uses this data to compute precise orbital models for the
entire GPS constellation. This information is then formatted into
updated navigation messages for each satellite.

User Segment:
The user segment consists of the GPS receivers, processors and antennas
utilized for positioning and timing by the community and military. The
GPS concept of operation is based on satellite ranging. Users figure
their position on the earth by measuring their distance to a group of
satellites in space. Each GPS satellite transmits an accurate position
and time signal. The user’s receiver measures the time delay for the
signal to reach the receiver. By knowing the distance to four points in
space, the GPS receiver is able to triangulate a three-dimensional


     The principle behind GPS is the measurement of distance (or
“range”) between the receiver and the satellites. The satellites also
tell us exactly where they are in their orbits above the Earth. Four
satellites are required to compute the four dimensions of X, Y, Z
(position) and Time. GPS receivers are used for navigation, positioning,
time dissemination, and other research.
One trip around the Earth in space equals one orbit. The GPS satellites
each take 12 hours to orbit the Earth. Each satellite is equipped with
an accurate clock to let it broadcast signals coupled with a precise
time message. The ground unit receives the satellite signal, which
travels at the speed of light. Even at this speed, the signal takes a
measurable amount of time to reach the receiver. The difference between
the time the signal is sent and the time it is received, multiplied by
the speed of light, enables the receiver to calculate the distance to
the satellite. To measure precise latitude, longitude, and altitude, the
receiver measures the time it took for the signals from four separate
satellites to get to the receiver.
        It works something like this: If we know our exact
distance from a satellite in space, we know we are somewhere on the
surface of an imaginary sphere with radius equal to the distance to the
satellite radius. If we know our exact distance from two satellites, we
know that we are located somewhere on the line where the two spheres
intersect. And, if we take a third measurement, there are only two
possible points where we could be located. By taking the measurement
from the fourth satellite we can exactly point out our location.
Need for DGPS:  As
the GPS receivers use timing signals from at least four satellites to
establish a position, each of those timing signals is going to have some
error or delay, depending on what sort of perils have befallen it on
its trip down to receiver. Since each of the timing signals that go into
a position calculation has some error, that calculation is going to be a
compounding of those errors.
      The sheer scale of the GPS system solves the problem. The
satellites are so far out in space that the little distances we travel
here on earth are insignificant. So if two receivers are fairly close to
each other, say within a few hundred kilometers, the signals that reach
both of them will have traveled through virtually the same slice of
atmosphere, and so will have virtually the same errors.
       The underlying premise of differential GPS (DGPS) is that any two
receivers that are relatively close together will experience similar
atmospheric errors. Differential GPS involves the cooperation of two
receivers, one that’s stationary and another that’s roving around making
position measurements. Since  the reference receiver has no way of
knowing which of the many available satellites a roving receiver might
be using to calculate its position, the reference receiver quickly runs
through all the visible satellites and computes each of their errors.
Then it encodes this information into a standard format and transmits to
the roving receivers. It’s as if the reference receiver is saying: “OK
everybody, right now the signal from satellite #1 is ten nanoseconds
delayed, satellite #2 is three nanoseconds delayed, and satellite #3 is
sixteen nanoseconds delayed…” and so on. The roving receivers get the
complete list of errors and apply the corrections for the particular
satellites they’re using.

Implementing DGPS:
The three main methods currently used for ensuring data accuracy are
real-time differential correction, reprocessing real-time data, and post

1. Real-Time DGPS

       Real-time DGPS occurs when the base station calculates and
broadcasts corrections for each satellite as it receives the data. The
correction is received by the roving receiver via a radio signal, if the
source is land based or via a satellite signal, if it is satellite
based and applied to the position it is calculating. As a result, the
position displayed and logged to the data file of the roving GPS
receiver is a differential corrected procedure.

2. Reprocessing Real-Time Data

      Some GPS manufacturers provide software that can correct GPS data
that was collected in real time. This is important for GIS data
integrity. When collecting real-time data, the line of sight to the
satellites can be blocked or a satellite can be so low on the horizon
that it provides only a weak signal, which causes spikes in the data.
Reprocessing real-time data removes these spikes and allows real-time
data that has been used in the field for navigation or viewing purposes
to be made more reliable before it is added to a GIS.


3. Post processing Correction

      Differentially correcting GPS data by post processing uses a base
GPS receiver that logs positions at a known location and a rover GPS
receiver that collects positions in the field. The files from the base
and rover are transferred to the office processing software, which
computes corrected positions for the rover’s file. This resulting
corrected file can be viewed in or exported to a GIS.
     Thus, Differential GPS or “DGPS” can yield measurements good to a
couple of meters in moving applications and even better in stationary
situations. That improved accuracy has a profound effect on the
importance of GPS as a resource. With it, GPS becomes more than just a
system for navigating boats and planes around the world. It becomes a
universal measurement system capable of positioning things on a very
precise scale.
      GPS can provide worldwide, three-dimensional positions, 24 hours a
day, in any type of weather. However, the system does have some
limitations. There must be a relatively clear “line of sight” between
the GPS antenna and four or more satellites. Objects, such as buildings,
overpasses, and other obstructions, that shield the antenna from a
satellite can potentially weaken a satellite’s signal such that it
becomes too difficult to ensure reliable positioning. These difficulties
are particularly prevalent in urban areas. The GPS signal may bounce
off nearby objects causing another problem called multipath


   GPS receivers were used in several aircraft, including F-16 fighters,
KC-135 aerial refuel, and B-2 bombers; Navy ships used them for
rendezvous, minesweeping, and aircraft operations.  
   GPS has become important for nearly all military operations and
weapons systems .In addition, it is used on satellites to obtain highly
accurate orbit data and to control spacecraft orientation.
GPS is based on a system of coordinates called the World Geodetic System
1984 (WGS 84). The WGS 84 system provides a built-in frame of reference
for all military activities, so units can synchronize their maneuvers.
GPS is also helping to save lives. Many police, fire, and emergency
medical service units are using GPS receivers to determine the police
car, fire truck, or ambulance nearest to an emergency, enabling the
quickest possible response in life-or-death situations.

GPS, a satellite based navigation system, thus can be used to determine
the position of an object on earth. As discussed above, its application
field is vast and new applications will continue to be created as the
technology evolves. GPS can also interface with other similar projects
such EU’s GALILEO to account for unpredictable applications. Thus, the
GPS constellation, like manmade stars in the sky, can be used for
guiding and navigation.