The Global Positioning System (GPS) is a satellite-based navigation system that provides precise 3D location information around the world. It consists of 24 satellites orbiting 20,200 km above the Earth that transmit radio signals. GPS receivers on the ground use these signals to calculate the user's position by triangulating distances to four or more satellites. The system is operated and maintained by the U.S. Department of Defense.

1. Global Positioning System
GPS
Dhaval A . Jalalpara

2. What is GPS ?
A very precise positioning system
• Developed and maintained by the
US Department of Defense (DOD)
• Satellite Based
* 24 satellites
* 20,200 km high orbit

3.  GPS stands for Global Positioning System which
measures 3-D locations on Earth surface using
satellites
 GPS operates using radio signals sent from satellites
orbiting the earth
 Created and Maintained by the US Dept. of Defense
 System as a whole consists of three segments
◦ Satellites (space segment)
◦ Receivers (user segment)
◦ Ground stations (control segment)

4.  At least 4 satellites are
above the horizon
anytime anywhere
 GPS satellites are also
known as “NAVSTAR
satellites”
 The satellites transmit
time according to very
accurate atomic clocks
onboard each one
 The precise positions of
satellites are known to
the GPS receivers from a
GPS almanac
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.

5.  Satellites have accurate atomic clocks
onboard and all GPS satellites transmit the
same time signal at the same time
◦ Think “synchronize your watches”
 The satellite signals contain information that
includes
◦ Satellite number
◦ Time of transmission

6.  Receivers use an almanac that includes
◦ The position of all satellites every second
◦ This is updated monthly from control stations
 The satellite signal is received, compared with the
receiver’s internal clock, and used to calculate the
distance from that satellite
 Trilateration (similar to triangulation) is used to
determine location from multiple satellite signals

7.  Satellite errors
◦ Satellite position error (i.e., satellite not exactly where it’s supposed to be)
◦ Atomic clocks, though very accurate, are not perfect
 Atmospheric
◦ Electro-magnetic waves travel at light speed only in a vacuum
◦ Atmospheric molecules, particularly those in the ionosphere, change the signal
speed
 Multi-path distortion
◦ The signal may "bounce" off structures before reaching the GPS receiver – the
reflected signal arrives a little later
 Receiver error:
◦ Due to the receiver clock or internal noise
 Selective Availability
◦ No longer an issue

8. Segments of GPS
1. Space Segment
A constellation of 24 satellites
2. Monitor Station
A network of earth-based facilities
3. Users & Equipment
Source:Trimble

9. GPS Receivers

10.  Differential correction collects points using a
receiver at a known location (known as a base
station) while you collect points in the field at the
same time (known as a rover receiver)
 Any errors in a GPS signal are likely to be almost
the same among all receivers within ~ 300 miles of
each other
~ 300 miles (~ 480 km) or less
Base station (known location) Rover receiver

11. Methods of data collection
Three methods of positioning
• Autonomous
10-20 meters
• Differential
2-5 meters
• Phase Differential
centimeter
2-5m10-20 m
cm

12. How accurate is GPS?
Depends on some variables
• Design of receiver
• Relative positions of satellites,
often known as DOP (Dilution of
Precision)
• Postprocessing
• Time spent on measurement

13. What is a PDOP?
• Position Dilution of Precision
Good PDOP Poor PDOP

14. GPS for Navigation

15. • Generating mapped data for GIS databases
• Collecting field data - travel to the field and capture
location & attribute information
• Other uses (many in real time):
• 911/firefighter/police/ambulance dispatch
• Car & boat navigation
• Roadside assistance
• Business vehicle/fleet management
• Mineral/resource exploration
• Wildlife tracking
• Recreational (fishing, hunting, hiking, etc.
• Ski patrol/medical staff location monitoring

16.  Easy To Incorporate into Project
 Once trained, just about anyone can use it
 Cheap
 Widely Available

17.  Does require a training component
 Accuracy Issues
 Differential Correction may not be an option in
many parts of the world

18.  Identify Your Accuracy Needs
 Identify Error Correction Methodology
◦ Point Averaging
 How long will points be collected?
◦ Differential Correction
 Find a base station
 Identify Point Collection Methodology
◦ Where will points be collected?
◦ Contingency plans
◦ Data backups