Geo-Informatics
Lecture 7-8 What is GPS?
GPS Stands for Global Position System
A very precise positioning system
Introduction to GPS Developed and maintained by the US Department of
Defense (DOD)
Feasibility studies begun in 1960’s.
Pentagon appropriates funding in 1973.
First satellite launched in 1978.
Nabeel Ahmed System declared fully operational in April, 1995.
dr.nabeelahmedgul@gmail.com Satellite Based
0344-5485232 24 satellites
20,200 km high orbit
Characteristics of GPS
Free
Precise
Reliable
All weather
How GPS Works
Anytime & anywhere
Unlimited user capacity
Segments of GPS Three Segments of the GPS
Space Segment
Space Segment
Control Segment
Control Segment User Segment
User and Equipment
Ground
Master Station Monitor Stations Antennas
Control Segment
Space Segment
US Space Command
Cape Canaveral GPS satellites (24) fly in
Hawaii circular orbits
Kwajalein Atoll at an altitude of 20,200 km and
Diego Garcia
with a period of 12 hours.
Ascension Powered by solar cells, the
satellites continuously orient
Is.
themselves to point their solar
panels toward the sun and
their antenna toward the
earth.
Orbital planes are centered on
the Earth
Master Control Station Monitor Station Ground Antenna
User Segment Four Basic Functions of GPS
Military Position and coordinates
Search and rescue
Disaster relief
The distance and direction
Surveying between any two waypoints, or a
Marine, aeronautical and terrestrial navigation position and a waypoint
Remote controlled vehicle and robot guidance Travel progress reports
Satellite positioning and tracking
Accurate time measurement
Shipping
Geographic Information Systems (GIS)
Determining GPS Position Position is Based on Time
Suppose the distance Signal leaves satellite
from Satellite A to our at time “T” T
position is 11,000 miles +
Satellite A
At this point we could
be located anywhere on
the specified sphere T+3
+
Next, let us take Satellite B
another measurement Signal is picked up by the
from a second satellite, receiver at time “T + 3”
Satellite B
Signal From One Satellite Signal From Two Satellite
The receiver is
somewhere on
this sphere.
Three Dimensional (3D)
Three Satellites Position
Selective Availability (S/A) Sources of GPS Error
The Defense Department dithered the Standard Positioning Service (SPS ):
satellite time message, reducing position
accuracy to some GPS users. Civilian Users
S/A was designed to prevent America’s Source of Error Amount
enemies from using GPS against us and our Satellite clocks 1.5 to 3.6 m
allies.
In May 2000 the Pentagon reduced S/A to Orbital errors <1m
zero meters error. Ionosphere 5.0 to 7.0 m
S/A could be reactivated at any time by the Troposphere 0.5 to 0.7 m
Pentagon. Receiver noise 0.3 to 1.5 m
Multipath 0.6 to 1.2 m
Receivers Errors are Cumulative Sources of Signal Interference
Earth’s Atmosphere
System and other flaws = < 9
meters
Solid Structures
User error =
+- 1 km
Metal
Electro-magnetic Fields
Planning a Navigation Route
Waypoint
A waypoint is based on coordinates entered into a
GPS receiver’s memory
It can be either a saved position fix, or user
entered coordinates.
= Waypoint
Start
How A Receiver Sees Your
Route
GPS Satellite Geometry
Satellite geometry can affect the quality of GPS
signals and accuracy of receiver trilateration.
Dilution of Precision (DOP) reflects each satellite’s
position relative to the other satellites being
accessed by a receiver.
Position Dilution of Precision (PDOP) is the DOP
value used most commonly in GPS to determine
the quality of a receiver’s position.
It’s usually up to the GPS receiver to pick satellites
which provide the best position triangulation.
Ideal Satellite Geometry Good Satellite Geometry
N
W E
Good Satellite Geometry Poor Satellite Geometry
N
W E
Poor Satellite Geometry Poor Satellite Geometry
Real Time Differential GPS
x+5, y-3
x+30, y+60
Differential GPS x-5, y+3
Receiver DGPS Receiver
DGPS Site
DGPS correction = x+(30-5) and True coordinates =
y+(60+3) x+0, y+0
True coordinates = x+25, y+63 Correction = x-5, y+3
Wide Area Augmentation System How good is WAAS?
Geostationary GPS Constellation
With Selective Availability set
WAAS satellites to zero, and under ideal
conditions, a GPS receiver
without WAAS can achieve +-15 meters
fifteen meter accuracy most
of the time.*
+-
3 meters
Under ideal conditions a
WAAS equipped GPS
receiver can achieve three
meter accuracy 95% of the
time.*
WAAS Control
Local Area Station (East
* Precision depends on good satellite geometry, open sky view, and no user
WAAS Control System (LAAS) Coast)
Station (West Coast) induced errors.
Future GPS
(a) GPS in USA
(b) GLONASS Program for Russia
(c) GALILEO from European countries
