1. The Global Positioning System
GPS
Part : 1 By
Mohammed Chessab Mahdi
Al-Furat Al-Awsat Technical University

2. The History of GPS Feasibility studies begun in 1960’s.
Pentagon appropriates funding in 1973.
First satellite launched in 1978.
System declared fully operational in April, 1995.
Mohammed Chessab Mahdi

3. Some Definitions
Latitude (Parallels) are formed by the intersection of the surface of the earth with a plane parallel to the equatorial plane
Longitude (Meridians) are formed by the intersection of the surface of the earth with a plane containing the earths axis.
Mohammed Chessab Mahdi

4. Latitude, Longitude and Altitude
One of many coordinate systems used to described a location on the surface of the earth
Latitude — parallels measured from the Equator.
North is “+”
Longitude — meridians measured from Greenwich Observatory.
East is “+”
Altitude — measured above reference datum: MSL
Normally Up is “+”
Mohammed Chessab Mahdi

5. GPS Coordinate Frames Inertial Frame of Reference –
Defined to be a non-accelerating or rotating coordinate frame of reference e.g., Earth Centered Inertial (ECI) . Required for analysis of satellite motion, inertial navigation, etc. Not convenient for terrestrial navigation
Coordinate systems you will mostly encounter in GPS are Earth Centered
Earth Fixed (ECEF) , East-North-Up (ENU) Coordinates
Mohammed Chessab Mahdi

6. Geometry of Earth Crude Approximation
A sphere
R0 = km
A spherical model is only good for “back of the envelope” type of calculations
Need a more precise model for navigation applications (especially inertial navigation)
A more accurate model is an ellipsoid
Parameters of the mathematical ellipsoid are :
Semi-major axis ( a ) = km
Semi-minor axis ( b ) = km
Mohammed Chessab Mahdi

7. Orbital Mechanics Kepler’s Law
Based on observations made by Tycho Brahe ( )
First Law: Each planet revolves around the Sun in an elliptical path, with the Sun occupying one of the foci of the ellipse.
Second Law: The straight line joining the Sun and a planet sweeps out equal areas in equal intervals of time.
Third Law: The squares of the planets' orbital periods are proportional to the cubes of the semi-major axes of their orbits.
Explanation came later – Isaac Newton ( )
Universal Law of Gravitation, where combined with his second law leads to
Mohammed Chessab Mahdi

8. Six Keplerian Elements
Recast the two-body equation of motion.
Characterize orbital ellipse
Semi-major Axis (A)
Eccentricity (e)
Characterize orbit’s orientation in space
Inclination (i)
Right Ascension of the Ascending Node (W)
Characterize ellipse’s orientation in orbital plane
Argument of Perigee (w)
Position of the satellite in the orbit
True anomaly (n)
Mohammed Chessab Mahdi

9. Kepler's second law
Illustration of Kepler's second law. The planet moves faster near the Sun so the same area is swept out in a given time as at larger distances, where the planet moves more slowly.
The green arrow represents the planet's velocity, and the purple arrows represents the force on the planet.
Mohammed Chessab Mahdi

10. GPS Orbital Parameters
Perturbed Orbits - quasi-Keplerian 15 element set
Non-central gravitational force
gravitational fields of the sun and moon
solar pressure
Additional 9 parameters
Three to account for the rate of changes:
Right Ascension of the Ascending Node (W-dot)
Inclination (i-dot)
Mean motion (n-dot)
Three pairs (6 parameter total) to correct
Argument of latitude
Orbit radius
Inclination angle
Pertubative Torque caused by Earth’s Equatorial Bulge
Figure from Bate, Mueller and White, Fundamentals of Astrodynamics (1971), pp. 156
Mohammed Chessab Mahdi

11. GPS Definition Global Positioning System
Network of 24 satellites (with spares)
Developed by Department of Defense
Operational 24 hours/day
Available worldwide
Land, sea and air
Works in all weather conditions
It is FREE!!
Mohammed Chessab Mahdi

12. Global Navigation Satellite Systems (GNSS)
NAVSTAR (Navigational Satellite Timing and Ranging)
USA
GLONASS
Russians
Galileo
Europeans
Mohammed Chessab Mahdi

13. Basic Functions of GPS Position and coordinates.
The distance and direction between any two waypoints, or a position and a waypoint.
Travel progress reports.
Accurate time measurement.
Mohammed Chessab Mahdi

14. GPS Segments / Components
The GPS consists of three major segments.
Space Segment (SS)
Control Segment (CS)
User Segment (US)
Mohammed Chessab Mahdi

15. GPS Segments / Components
Space Segment
User Segment
Control Segment
Ground Antennas
Master Station
Monitor Stations
Mohammed Chessab Mahdi

16. Space Segment (SS)
The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV).
The GPS design originally called for 24 SVs, eight each in three circular orbital planes.
This was modified to six planes with four satellites each.
Mohammed Chessab Mahdi

17. Space Segment (SS)
Mohammed Chessab Mahdi

18. Space Segment (SS) The orbital planes are centered on the Earth.
The six planes have approximately 55° inclination and are separated by 60° right ascension of the ascending node.
The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.
Satellites are orbiting at an altitude of approximately 20,200 kilometers.
Mohammed Chessab Mahdi

19. Space Segment (SS)
Each SV makes two complete orbits each sidereal day, repeating the same ground track.
As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation.
The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements.
Mohammed Chessab Mahdi

20. Control Segment (Monitor Stations)
The GPS-System is controlled by the US Army. The “master control station” and four additional monitoring stations were set up for monitoring the satellites.
The Control Segment is composed of
A Master Control Station (MCS)
An Alternate Master Control Station,
Four dedicated Ground Antennas
Six dedicated Monitor Stations
Mohammed Chessab Mahdi

21. Control Segment (Monitor Stations)
Mohammed Chessab Mahdi

22. Control Segment (Monitor Stations)
The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in
Hawaii
Kwajalein
Ascension Island
Diego Garcia
Colorado Springs
Monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.
Mohammed Chessab Mahdi

23. Control Segment (Monitor Stations)
During August and September 2005, six more monitor stations of the National Geospatial-Intelligence Agency were added to the grid. Now, every satellite can be seen from at least two monitor stations.
This allows calculating more precise orbits and ephemeris data. For the end user, a better position precision can be expected from this.
Mohammed Chessab Mahdi

24. Control Segment (Monitor Stations)
In the near future, five more stations will be added so that every satellite can be seen by at least three monitor stations. This improves integrity monitoring of the satellites and thus the whole system.
Mohammed Chessab Mahdi

25. User Segment
The User Segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service.
In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock.
Mohammed Chessab Mahdi

26. User Segment Military. Search and rescue. Disaster relief. Surveying.
Marine, aeronautical and terrestrial navigation.
Remote controlled vehicle and robot guidance.
Satellite positioning and tracking.
Shipping.
Geographic Information Systems (GIS).
Recreation.