What is GPS?
Global Positioning System (GPS)
It is a satellite-based navigation system.
Made up of a network of 24 satellites placed into orbit
by the u.s. department of defense.
Gps was originally intended for military applications, but
in the 1980s, the government made the system
available for civilian use.
Gps permits land, sea, and airborne users to determine
their three-dimensional position, velocity, and time.
It can be used by anyone with a receiver anywhere on
the planet, at any time of day or night, in any type of
GPS uses radio transmissions. The satellites transmit
timing information and satellite location information.
The system can be separated into three parts:
Figure illustrates how the three segments
The space segment consists of the satellites themselves.
According to the United state Naval observatory there
are currently 27 operational GPS satellites about 11,000
miles up in space.
This constellation provides between five and eight GPS
satellites visible from any point on the earth.
It takes each satellite about twelve hours to orbit the
earth. There are six orbital planes with at least four
satellites in each plane.
The control segment is a group of ground stations that
monitor and operate the GPS satellites. There are
monitoring stations spaced around the globe and one
Master Control Station located in Colorado Springs
There are actually five major monitoring
systems, the figure below does not include
the Hawaiian station
The user requires a GPS receiver in order to receive the
transmissions from the satellites. The GPS receiver
calculates the location based on signals from the
The users consist of both the military and civilians.
Table of Content
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.
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.
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 and track
four or more satellites in view, the receiver can
determine the user's 3D position
How accurate is GPS?
GPS receivers are extremely accurate due to parallel
Garmin's 12 parallel channel receivers are quick to lock
onto satellites when first turned on and they maintain
strong locks, even in dense foliage or urban settings
with tall buildings.
Certain atmospheric factors and other sources of error
can affect the accuracy of GPS receivers.
GPS receivers are accurate to within 15 meters on
GPS receivers with WAAS capability can improve
accuracy to less than three meters on average.
Users can also get better accuracy with Differential GPS
(DGPS), which corrects GPS signals to within an average
of three to five meters.
What's the signal?
GPS satellites transmit two low power radio signals,
designated L1 and L2.
Civilian GPS uses the L1 frequency of 1575.42 MHz in
the UHF band.
The signals travel by line of sight, meaning they will pass
through clouds, glass and plastic but will not go through
most solid objects such as buildings and mountains.
A GPS signal contains three different bits of
Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect
accuracy include the following:
• Ionosphere and troposphere delays
• Signal multipath
• Receiver clock errors
• Orbital errors
• Number of satellites visible
• Satellite geometry/shading
• Intentional degradation of the satellite signal
Advantages of GPS:
GPS is extremely easy to navigate as it tells you to the direction for
each turns you take or you have to take to reach to your
GPS works in all weather so you need not to worry of the climate as
in other navigating devices.
The GPS costs you very low in comparison other navigation
The most attractive feature of this system is its100% coverage on
It also helps you to search the nearby restaurants, hotels and gas
stations and is very useful for a new place.
Due to its low cost, it is very easy to integrate into other
technologies like cell phone.
The system is updated regularly by the US government and hence is
This is the best navigating system in water as in larger water bodies
we are often misled due to lack of proper directions.
Disadvantages of Global Positioning System
Sometimes the GPS may fail due to certain reasons and in that case
you need to carry a backup map and directions.
These systems rely on batteries. Once the battery is drained out,
the system does not work.
Sometimes the GPS signals are not accurate due to some obstacles
to the signals such as buildings, trees and sometimes by extreme
atmospheric conditions such as geomagnetic storms.
Price of the system is a major concern. GPS systems cost a high
amount if they have lots of features.
Accuracy is again an issue. The systems are not always updated, so
we do not know when a road has been closed or when a new road
has been opened.