Geo-Informatics
Lecture 9-10 Selective Availability (S/A)
The Defense Department dithered the
satellite time message, reducing position
GPS, DGPS and WAAS accuracy to some GPS users.
S/A was designed to prevent America’s
enemies from using GPS against us and our
allies.
In May 2000 the Pentagon reduced S/A to
zero meters error.
Nabeel Ahmed
dr.nabeelahmedgul@gmail.com
S/A could be reactivated at any time by the
0344-5485232
Pentagon.
Sources of GPS Error Receivers Errors are Cumulative
Standard Positioning Service (SPS ): System and other flaws = < 9
Civilian Users
meters
Source of Error Amount
Satellite clocks 1.5 to 3.6 m
Orbital errors <1m User error =
Ionosphere 5.0 to 7.0 m +- 1 km
Troposphere 0.5 to 0.7 m
Receiver noise 0.3 to 1.5 m
Multipath 0.6 to 1.2 m
Sources of Signal Interference Waypoint
Earth’s Atmosphere
A waypoint is based on coordinates entered into a
GPS receiver’s memory
It can be either a saved position fix, or user
Solid Structures
entered coordinates.
Metal
Electro-magnetic Fields
How A Receiver Sees Your
Planning a Navigation Route Route
= Waypoint
Start
GPS Satellite Geometry Ideal Satellite Geometry
N
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.
W E
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.
S
Good Satellite Geometry Good Satellite Geometry
Poor Satellite Geometry Poor Satellite Geometry
N
W E
Poor Satellite Geometry
Differential GPS
Differential GPS (DGPS) is a relatively DGPS is a method of improving the
simple technique to improve positional accuracy of your receiver by adding a
accuracy. This technique was developed local reference station to augment the
in the early 1980s, information available from the satellites.
Differential Global Positioning It also improves the integrity of the
System (DGPS) is an enhancement to whole GPS system by identifying certain
Global Positioning System that errors.
provides improved location accuracy.
Differential GPS
•Differential GPS uses one unit at a x+5, y-3
known location. 30, 60
The stationary unit compares its x-5, y+3
calculated GPS location with the actual
location and computes the error
Receiver DGPS Receiver
DGPS Site
DGPS correction = (30-5) and True coordinates =
(60+3) x+0, y+0
True coordinates = 25, 63 Correction = x-5, y+3
WAAS
Several ground stations receive the GPS information
from the standard GPS satellite system and measure
Wide Area Augmentation the error based on the known locations of these
ground stations.
System
This corrected signal is then sent back into space to
special geostationary GPS satellites (these are
satellites that stay in the same position above the
earth (not travelling around the earth at 14,000
km/hr as the normal GPS satellites do).
Wide Area Augmentation System
WAAS Geostationary GPS Constellation
WAAS satellites
These WAAS GPS satellites then stream this
corrected signal back to the earth where WAAS
enabled GPS receivers are able to capture this
corrected signal giving high levels of accuracy
unattainable with the normal GPS system.
The WAAS requires the geostationary satellites to be
present and these are currently only over the USA
mainland. Europe and Asia are both developing
similar systems for increased GPS accuracy. WAAS Control
Local Area Station (East
WAAS Control System (LAAS) Coast)
Station (West Coast)
How good is WAAS?
With Selective Availability set
to zero, and under ideal
DGPS Error
conditions, a GPS receiver
without WAAS can achieve
fifteen meter accuracy most
+-15 meters Standard Positioning Service (SPS ):
of the time.*
+-
Civilian Users
3 meters Source of Error Amount
Orbital errors <1m Not Removed
Under ideal conditions a
WAAS equipped GPS Ionosphere 5.0 to 7.0 m Mostly Removed
receiver can achieve three
meter accuracy 95% of the Troposphere 0.5 to 0.7 m All Removed
time.* Receiver noise 0.3 to 1.5 m All Removed
Multipath 0.6 to 1.2 m Not Removed
* Precision depends on good satellite geometry, open sky view, and no user Satellite clocks 1.5 to 3.6 m All Removed
induced errors.
Future GPS
(a) GPS in USA
(Global Position System)
(b) GLONASS Program for Russia
(Globalnaya Navigazionnaya Sputnikovaya Sistema)
(c) GALILEO for European countries 15 m: Typical GPS position accuracy
(Named after the Italian astronomer Galileo Galilei) without SA.
(d) COMPASS for China 3-5 m: Typical differential GPS (DGPS)
(e) IRNSS for India position accuracy.
(f) QZSS for Japan < 3 m: Typical WAAS position accuracy.
