1. GPS Basics What is GPS? satellites (space segment)
GPS stands for Global Positioning System which measures 3-D locations on Earth surface with the aid of satellites
• Created and Maintained by the US Dept. of Defense and the US Air
Force
• System as a whole consists of three segments
satellites (space segment)
receivers (user segment)
ground stations (control segment)

2. Satellites

3. Satellites (space segment)
24 NAVSTAR satellites (21 operational and 3 spares)
orbit the Earth every 12 hours
~11,000 miles altitude
positioned in 6 orbital planes
orbital period/planes designed to keep 4-6 above the horizon at any time
controlled by five ground stations around the globe

4. GPS – User Segment (Receivers)
Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once
Determine their position using the time it takes signals from the satellites to reach the hand-held unit
Calculations result in varying degrees of accuracy that depend on:
quality of the receiver
user operation of the receiver
local & atmospheric conditions
current status of system

5. Ground Stations (control segment)
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
Five control stations
master station at Falcon (Schriever) AFB, Colorado
monitor satellite orbits & clocks
broadcast orbital data and clock corrections to satellites

6. GPS - Satellite Signals
How It Works (p. 1)
Satellites have accurate atomic clocks and all 24 satellites are transmitting the same time signal at the same time
The satellite signals contains information that includes
Satellite number
Time of transmission
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. How It Works (p. 2)
Start by determining distance between a GPS satellite and your position
Adding more distance measurements to satellites narrows down your possible positions

8. How It Works (p. 3) Three distances = two points
Intersection of Four spheres = one point
Note:
4th measurement not needed
Used for timing purposes instead (discussed later)

9. How It Works (p. 4) Distance between satellites and receivers
determined by timing how long it takes the signal to travel from satellite to receiver.
How?
Radio signals travel at speed of light: 186,000 miles/second
Satellites and receivers generate exactly the same signal at exactly the same time
Signal travel time = delay of satellite signal relative to the receiver signal
Distance from satellite to receiver =
signal travel time * 186,000 miles/second
1sec
Satellite signal
Receiver signal

10. How It Works (p. 5)
How do we know that satellites and receivers generate the same signal at the same time?
satellites have atomic clocks, so we know they are accurate
Receivers don't -- so can we ensure they are exactly accurate? No!
But if the receiver's timing is off, the location in 3-D space will be off slightly...
So: Use 4th satellite to resolve any signal timing error instead
determine a correction factor using 4th satellite
(like solving multiple equations...will only be one solution that satisfies all equations)

11. Error Sources Satellite errors satellite position error
atomic clock, though very accurate, not perfect.
Atmosphere
Electro-magnetic waves travels at light speed only in vacuum.
The ionosphere and atmospheric molecules change the signal speed.
Multi-path distortion
signal may "bounce" off structures nearby before reaching receiver – the reflected signal arrives a little later.
Receiver error: Due to receiver clock or internal noise.
Selective Availability

12. GPS - Sources of Error Satellite Coverage in Sky
Position Dilution of Precision (PDOP)
Poor
Ideal

13. 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
Based on Clinton’s order, Selective Availability turned off early May 2, 2000

14. GPS - Error Budget
Example of some typical observed using a consumer GPS receiver:
Typical observed errors
satellite clocks meters
orbit error meters
receiver errors meters
atmosphere meters
Total meters
Multiplied by PDOP (1 - 6)
meters
Meters
Atmosphere
Receivers
Orbit Error
Satellite Clocks 6 12 18 24 30

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

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

17. 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 the same among all receivers within 300 miles of each other
~ 300 miles (~ 480 km) or less
Base station (known location)
Rover receiver

18. 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

19. 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
Real-time differential correction
Done in real time by receiving a broadcasted correction signal (usually expensive), requiring other hardware (not just a consumer GPS receiver)

20. GPS Applications Generating mapped data for GIS databases
“traditional” GIS analysts & data developers
travel to field and capture location & attribute information cheaply (instead of surveying)
Other uses (many in real time):
911/firefighter/police/ambulance dispatch
car navigation
roadside assistance
business vehicle/fleet management
mineral/resource exploration
wildlife tracking
boat navigation
Recreational
Ski patrol/medical staff location monitoring

21. Garmin’s cheapest receivers
Garmin’s Forerunner 201: A watch that uses GPS to determine current speed, average speed, exact distance traveled, etc. ( ) Basic features also available in the Forerunner 101 ($115).
Garmin’s iQue 3600 PDA:

22. Garmin’s Outdoor GPS Receivers: etrex series
Basic GPS
eTrex®
eTrex Camo®
eTrex Summit®
eTrex Venture
     
GPS 76
GPS 72
GPS 12
GPS 12XL
   
Geko™ 101
Geko 201
Geko 301
    
Foretrex™ 101
Foretrex 201
                            
         
Garmin makes a host of GPS receivers for outdoor sports enthusiasts.

23. Garmin’s Outdoor GPS Receivers:
Etrex Legend C ($375)
“Along with the Etrex Vista C, is one of Garmin's smallest, least expensive products to combine a color TFT display and advanced GPS routing capabilities in a waterproof design.”
--is WAAS enabled
--has USB port for downloading maps from Garmin’s MapSource CD library
Etrex Vista C ($430)
--has a TFT (thin-film transistor, with 1-4 tranistors controlling each pixel; it is the highest-definition flat-panel technique) display
--WAAS enabled
--has USB port for downloading maps from Garmin’s MapSource library

24. Bluetooth GPS Receivers
Teletype’s Mini-bluetooth GPS receiver ($175)
Teletype’s USB GPS receiver for Laptops ($170)

25. HP’s Ipaqs and other PDAs with GPS software
Hewlett-Packard’s new iPAQ h1945 PDA
Now comes equipped with a hp GPS receiver and navigation system ($500)
Garmin’s iQue 3600 PDA: