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
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
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
Notas del editor
At the cost of 15 billion dollar of your tax money