GLOBAL POSITIONING
SYSTEM
What is GPS?
GPS, which stands for Global
Positioning System, is the only
system today that provides an
answer to the question,
Where on Earth am I ?
GPS is able to show you your exact
position on the Earth anytime, in
any weather, anywhere.
• Global Positioning System was
originally established by the
U.S.Department of Defence (DOD)for
Military purpose at a cost of
US$13billion.
• It was made free to all users from May
2000.
• GPS is a short form of NAVSTAR GPS –
(Navigation System with Time And
Ranging).
GPS Segments
The three parts of GPS are:
• The Space segment - Satellites
• The Control segment- Ground
stations
• The User segment - Receivers
Space Segment
• There are quite a
number of satellites in
space.
• They are used for a
wide range of purposes:
• satellite TV, cellular
phones, military
purposes Remote
Sensing etc.
• Satellites can also be
used by GPS receivers
GPS Constellation
• The GPS Operational
Constellation consists of
24 satellites that orbit
the Earth at 20200km
altitude in very precise
orbits twice a day.
• GPS satellites emit
continuous navigation
signals.
• There are 6 orbital paths
with 4 satellites in each.
Control Segment
• It is a system of tracking stations located
around the world near the equator.
• One Master control station at Colarado
Springs.
• Four Monitor stations - Hawaii, Kwajalein,
Ascension Island and Diego Gracia.
• The path of the satellites are tracked by
the monitoring stations.
• The control stations•
•
•
•
Tracks the GPS satellites
Updates their orbiting position
Calibrates and synchronizes their clocks.
Predict its path for the next 24 hours.
User Segment
• GPS receivers come in a variety of formats,
from devices integrated into cars, phones, and
watches, to hand held devices.
• The GPS receiver is the user segment (US) of
the GPS.
• In general, GPS receivers are composed of an
antenna, tuned to the frequencies transmitted
by the satellites, receiver-processors, and a
highly-stable clock).
• They may also include a display for providing
location and speed information to the user.
Receivers and Satellites
• GPS units are
made to
communicate with
GPS satellites
(which have a
much better view
of the Earth) to find
out exactly where
they are on the
global scale of
things
Signal Refraction
Signals from
satellites can be
like light. When
they hit some
interference (air
patterns in the
atmosphere,
uneven
geography, etc.)
they sometimes
bend a little.
Light Refraction
Refraction is the bending of light as it
travels through one media to
another.
Sometimes the GPS signal from the
satellite doesn’t follow a straight
line.
Line of Sight Transmissions
• Line of sight is the
ability to draw a straight
line between two
objects without any
other objects getting in
the way. GPS
transmission are lineof-sight transmissions.
• Obstructions such as
trees, buildings, or
natural formations may
prevent clear line of
sight.
Signal Interference
• Sometimes the
signals bounce
off things
before they hit
the receivers
Atomic clocks
• Each GPS Satellite has several accurate
Atomic Clocks.
• The clocks operate at a frequency of 10.23
MHz.
• They generate signals that are broadcast
from the satellites.
• They broadcast two Carrier waves in Lband which travel at the speed of light.
L 1 Carrier Wave
• Transmits two codes:
• The C/A Code or Coarse/Acquisition Code
and
• The P-Code or the Precision Code.
• The L-2 Carrier transmits one P- Code
GPS Signals
•
Each GPS satellite transmits data that
indicates its location and the current
time.
• All GPS satellites synchronize
operations so that these repeating
signals are transmitted at the same
instant.
• The signal is just a complicated digital
code, i.e. a sequence of “on” and “off”
pulses.
GPS Signals
• A GPS signal contains three different bits of
information :
• A pseudorandom code-or an I.D. code that
identifies which satellite is transmitting information.
• Ephemeris data which tells the GPS receiver
where each GPS satellite should be at any time
throughout the day.
• Almanac data, contains important information
about the status of the satellite (healthy or
unhealthy), current date and time..
Time Difference
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.
• A GPS receiver must be locked on to the
signal of at least three satellites to calculate
a 2D position (latitude and longitude).
• 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.
Signal Errors
• Ionosphere and troposphere delays — The
satellite signal slows as it passes through the
atmosphere. The GPS system uses a built-in model
that calculates an average amount of delay to
partially correct this type of error.
• Signal multipath — This occurs when the GPS
signal is reflected off objects such as tall buildings or
large rock surfaces before it reaches the receiver.
This increases the travel time of the signal, thereby
causing errors.
• Receiver clock errors — A receiver's built-in clock
is not as accurate as the atomic clocks onboard the
GPS satellites. Therefore, it may have very slight
timing errors.
• Orbital errors —these are inaccuracies of the
satellite's reported location.
• Number of satellites visible — The more satellites
a GPS receiver can "see," the better the accuracy.
Buildings, terrain, electronic interference, or
sometimes even dense foliage can block signal
reception, causing position errors or possibly no
position reading at all. GPS units typically will not
work indoors, underwater or underground.
• Satellite geometry/shading — This refers to the
relative position of the satellites at any given time.
Ideal satellite geometry exists when the satellites are
located at wide angles relative to each other. Poor
geometry results when the satellites are located in a
line or in a tight grouping.
• Selective Availability (SA): is an
intentional degradation of the signal once
imposed by the U.S. Department of
Defense. Users of Differential GPS are not
much affected.
• Anti Spoofing (A-S): Denies civilians and
hostile powers the access to the P-code
Geometric Principle of Triangulation
Geometric Principle:
You can find one
location if you
know its distance
from other,
already-known
locations
Triangulation
Triangulation
GPS satellites use Atomic
Clocks for accuracy
1.The dashed lines show the actual intersection point, and the gray bands
indicate the area of uncertainty.
2.The solid lines indicate where the GPS receiver "thinks" the spheres are
located. Because of errors in the
receiver's internal clock, these spheres do not intersect at one point.
3.The GPS receiver must change the size of the spheres until the
intersection point is determined. The relative size of each sphere has
already been calculated, so if the size of one sphere is changed, the
other spheres must be adjusted by exactly the same amount.
Latitude and Longitude
GPS uses Latitudes and
Longitudes to determine
locations.
GPS Receiver
• There are number of pages on the GPS
Receiver :
• Satellite Page
• Trip Computer Page
• Map Page
• Compass Page
• Main Menu Page
• The Find Page
• Waypoint Information Page
Recievers
What You can do with a Receiver?
• Mark Waypoints
• Create and view Tracks
• Create a Route
Waypoints
• Waypoints are
locations or landmarks
that can be stored in
your GPS.
• waypoints may be
entered directly by
taking a reading with
the unit at the location
itself, giving it a name,
and then saving the
point
Direction o
waypoint
Your location
Waypoint
Date
and
Time
Latitude and
Longitude
Tracks
• Tracks are like footprints.
• They show you exactly where you have
moved.
• Tracks appear as small dotted lines on
you receivers Map page.
• The GPS receiver automatically creates
tracks as you move and displays the
dotted lines called as Track log.
Routes
• A Route gives you straight line direction
from one point to another or many other
points.
How accurate is GPS? today
• GPS receivers are extremely accurate.
• Certain atmospheric factors, dense foliage or tall
buildings can affect the accuracy of GPS
receivers.
• Garmin GPS receivers are accurate to within 15
meters on average.
• GPS receivers with WAAS (Wide Area
Augmentation System) capability can improve
accuracy to less than three meters on average.
Differential Correction
• Differential correction
is a technique that
greatly increases the
accuracy of the
collected GPS data. It
involves using a
receiver at a known
location - the "base
station“- and
comparing that data
with GPS positions
collected from
unknown locations
with "roving
receivers."
Advantages of GPS
• Intervisibility between points is not
required.
• Can be used at any time of day or in any
weather.
• Has high geodetic accuracy.
• More work can be accomplished in less
time with fewer people.
Uses of GPS
• There are five main uses of GPS :
1. Location : determining a basic position.
2. Navigation : getting from one location to
another.
3. Tracking :monitoring the movement of
people and things.
4. Mapping :creating Maps.
5. Timing :providing precise timing.
Applications of GPS
•
•
•
•
•
•
•
•
•
•
Agriculture
Aviation
Environment
Road rail and ocean Transport
Military
Public safety
Recreation
Space
Surveying
Timing
Competitors of NAVSTAR GPS
• GLONASS – Russia
• GALILEO - European Union