1. Global Positioning System (GPS)

2. GPS Basics
GPS stands for Global Positioning System which measures 3D 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)

3. GPS History Development began in 1973
First satellite became operational in 1978
Declared completely functional in 1995
A total of 52 satellites have been launched in 4 phases
30 satellites are currently functional
Managed by the U.S. DOD
Originally developed for submarines
Now part of many modern applications
GPS History

4. Satellites
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 data
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. 4

5. Satellites cont. The satellites are in motion around the earth
Like the sun and moon satellites rise and set as they cross the sky
Locations on earth are determined from available satellites (i.e., those above the horizon) at the time the GPS data are collected
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. 5

6. Receivers
Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once
Geographic position is determined using the time it takes signals from the satellites to reach the GPS receiver
Calculations result in varying degrees of accuracy that depend on:
Quality of the receiver
User operation of the receiver (e.g., skill of user and receiver settings)
Atmospheric conditions
Local conditions (i.e., objects that block or reflect the signals)
Current status of system

7. Ground Stations Control stations Responsibilities
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
Ground Stations
Control stations
Master station at Falcon (Schriever) AFB, Colorado
4 additional monitoring stations distributed around the world
Responsibilities
Monitor satellite orbits & clocks
Broadcast orbital data and clock corrections to satellites

8. How GPS Works: Overview
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
Explain Triangulation 8

9. How GPS Works: Overview
Receivers use an almanac that includes
The position of all satellites every second
This is updated timely 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
Explain Triangulation 9

10. How GPS Works: Signal Processing
Distances between satellites and receivers is determined by the time is takes the signal to travel from satellite to receiver
Radio signals travel at speed of light (186,000 miles/second)
All satellites send the identical time, which is also generated by the receivers
Signal travel time = offset between the satellite signal and the receiver signal
Distance from each satellite to receiver = signal travel time * 186,000 miles/second
1sec
Satellite signal
Receiver signal

11. How GPS Works: Trilateration
Start by determining distance between a GPS satellite and your position

12. How GPS Works: Trilateration
Adding more distance measurements to satellites narrows down your possible positions

13. How GPS Works: Trilateration

14. How GPS Works: Trilateration
The 4th satellite in trilateration is to resolve any signal timing error
Unlike GPS satellites, GPS receivers do not contain an atomic clock
To make sure the internal clock in the receiver is set correctly we use the signal from the 4th satellite

15. GPS Error Sources 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

16. 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

17. Satellite Clock & Satellite Position
Sources of Error
Satellite Clock & Satellite Position
Atomic clock errors
+/- 2 meters of error
Satellite is not in precise orbit
+/- 2.5 meters of error 17

18. Atmospheric Delays/Bending
Sources of Error
Atmospheric Delays/Bending
+/- 5 meters or error 18

19. Sources of Error
Multi Path Interference (signal bouncing off of buildings, trees, etc.)
+/- 1 meter of error 19

20. Receiver Timing/Rounding Errors
Sources of Error
Receiver Timing/Rounding Errors
+/- 1 meter of error (depends on the quality of the GPS receiver)
Quadruple Redundant Atomic Clocks
Accurate to Nanoseconds
2:02:
Powered by 4 AA Batteries
2:02: 20

21. GPS - Selective Availability
A former significant source of error
Error intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasons
Selective Availability turned off early May 2, 2000 21

22. Satellite Coverage: Position Dilution of Precision (PDOP)
GPS Error: Position Dilution of Percision
Satellite Coverage: Position Dilution of Precision (PDOP)
Remember that satellites are moving, causing the satellite constellation to change
Some configurations of satellites are better than others
Poor PDOP Good PDOP

23. GPS - Error Budget
Example of typically observed error from a consumer GPS receiver:
Typical Observed errors (meters)
satellite clocks 0.6
orbit (position error) 0.6
receiver errors
atmosphere
Total
Multiplied by PDOP (1-6)
Total error ~ meters
Meters
Atmosphere
Receivers
Orbit Error
Satellite Clocks 6 12 18 24 30

24. Differential Correction
GPS - Error Correction
2 Methods:
Point Averaging
Differential Correction 24

25. Point Averaging
Point Averaging is one of the simplest ways to correct GPS point locations
Collect many GPS measurements at the same location and then average them to get one point
The averaged point should have greater accuracy than a single point measurement
Accuracy varies with this method but you should have a position that is within 5 meters of its true location 95% of the time 25

26. GPS - Point Averaging Averaged Location
This figure shows a successive series of 3-D positions taken using a receiver kept at the same location, and then averaged 26

27. GPS - Differential Correction
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 27

28. GPS - Differential Correction
The base station knows its own location
It compares this location with its location at that moment obtained using GPS satellites, and computes error
This known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment
Example: Base Station File
Time
GPS Lat
GPS Long
Lat. error
Long. error
3:12.5
3:13.0
3:13.5
3:14.0
3:14.5
3:15.0
35.50
35.05
34.95
36.00
35.35
35.20
79.05
78.65
79.55
80.45
79.30
79.35 .5
.05
-.05
1.0
.35
.20 .5
-.35
.55
1.45
.30
.35 28

29. GPS - Differential Correction
GPS error when using differential correction: 1 – 3 meters
There are two ways that differential correction can be applied:
Post-processing differential correction
Does the error calculations after the rover has collected the points
Requires downloading a base-station file
Real-time differential correction
Done in real time by receiving a broadcasted correction signal
May require additional hardware

30. Strengths of GPS Easy To Incorporate into Project
Once trained, just about anyone can use it
Cheap
Widely Available 30

31. Weaknesses Does require a training component Accuracy Issues
Differential Correction may not be an option in many parts of the world 31

32. Other satellite navigation systems in use or various states of development include:
GLONASS – russia's global navigation system. Fully operational worldwide.
Galileo – a global system being developed by the european union and other partner countries, planned to be operational by 2016 (and fully deployed by 2020)
Beidou – people's republic of china's regional system, currently limited to asia and the west pacific

33. COMPASS – people's republic of china's global system, planned to be operational by 2020
IRNSS – india's regional navigation system, covering india and northern indian ocean
QZSS – japanese regional system covering asia and oceania

34. Thanks…. 34

35. Thanks 35