the basic information about the global positioning system.

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