Global Positioning Systems

1
Global Positioning Systems and GIS An Introduction
Tom Vought and Katie Franke
2
GPS and GIS

• GPS Global Positioning System
• Geocaching an entertaining adventure game for
  GPS users.
• Individuals and organizations set up caches all
  over the world and share the locations of these
  caches on the internet. GPS users can then use
  the location coordinates to find the caches.
• GIS Geographic Information System
• Any system for capturing, storing, analyzing,
  managing and presenting data and associated
  attributes which are spatially referenced to
  Earth.
• Data capture entering information into a GIS
• Needs to be done well garbage in ? garbage out
• Tends to be very time consuming
• Positions from a GPS can also be directly entered
  into a GIS

3
GIS Defined

• A geographic information system is a framework
  for understanding our world and applying
  geographic knowledge to solve problems and guide
  human behavior.
• A GIS can produce information that answers
  specific questions and allows you to share that
  information with others. By visualizing
  relationships, connections, and patterns in data,
  you can make informed decisions and increase
  efficiency throughout your organization.

4
GIS Defined

• Tool for visualizing and analyzing the
  interaction of spatial phenomena by overalying
  and displaying geodata layers
• Interaction of
• Software (ArcGIS)
• Data (shapefiles, rasters)
• Users (you)

5
GIS John Snows map of cholera in London 1854
6
What is a GIS 3 views
7
The Earths GraticuleSystem for locating places
on Earth
8
Where is Manhattan?
39o 12 N96o 35 W
2,705 miles north of the Equator 5,126 miles west
of the Prime Meridian 4,480 miles east of the
International Date Line
9
What is GPS?

• GPS Global Positioning System
• Space-based (satellite) positioning system for
  navigation, tracking, and mapping
• Designed and maintained by the U.S. Department of
  Defense
• Global and available 24-7-365

Your location is 37o 23.323 N 122o 02.162 W
10
Evolution of GPS 1970s

• Grew out of two US military projects that were
  combined into the Defense Navigation Satellite
  System (DNSS)
• US Navy Timation Project
• US Air Force 621B Project
• DNSS led to Navstar (Navigation System with
  Timing and Ranging) later referred to as Global
  Positioning System or GPS.
• First generation of GPS satellites (Phases I and
  II) deployed.

11
Evolution of GPS 1980s

• 1984 No-cost civilian access to the GPS
  guaranteed by the US government.
• 1989 First Phase III (Block II) satellites
  launched.
• 1993 IOC (Initial Operation Capability) achieved
  with 24 fully functional satellites.
• July 1995 FOC (Full Operational Capability)
  achieved, 24 fully functional Block II satellites.

12
Block II NAVSTAR GPS Satellite
Name NAVSTAR Manufacturer Rockwell
International Altitude 12,544 Miles Weight 1900
lbs. Size 17 ft. (solar panels
extended). Orbital Period 12 hours Orbital
Plane 55 to equatorial plane Planned Lifespan
7.5 years Thrusters for orbit adjustments Current
Constellation 29 Block II satellites
13
GPS ConstellationNot all satellites shown
14
Other Systems

• Beidou Chinas regional system proposal to
  expand to global system called COMPASS
• Galileo global system proposed by EU for
  2011-12
• GLONASS Russia global system being restored
  with India
• IRNSS proposed regional system by India
• QZSS Japanese proposed regional system

15
How Does GPS Work?
Distance Velocity x Time

• Velocity of GPS radio signal is constant
  186,000 mi/sec
• Need to measure travel time of signal from
  satellite to ground receiver.
• GPS satellites have 4 onboard atomic clocks 2
  rubidium and 2 cesium.

16
Introducing TrilaterationWith one data source
Three people are driving to the same destination,
but we dont know what the destination is. The
first person starts at the red dot and drives in
a straight line at 60 mph for 10 hours (600
miles), so they would be somewhere on the edge of
this circle.
600 miles
17
Introducing TrilaterationAdd a second data
source
The second person starts at the green dot and
also drives in a straight line at 60 mph for 10
hours (600 miles), so they would be somewhere on
the edge of their circle. This means that the
destination must be on one of the two yellow
dots.
600 miles
600 miles
18
Introducing Trilaterationand a third data
source
Finally the third person starts at the purple dot
and also drives in a straight line at 60 mph for
10 hours (600 miles), so they would be somewhere
on the edge of their circle. This means that the
destination must be on the yellow dot.
600 miles
600 miles
600 miles
19
Making Trilateration Harder!
Even if the speed of and distance traveled by
each vehicle was different the process for
identifying the destination would be the same.
What is this distance?
20
GPS Trilateration Same as previous example, but
backwards. Destination is known, but where
were the three starting points? Velocity x Time
How do we measure the distance between person on
the ground and each of three satellites?
21
How Does GPS Work?
Distance Velocity x Time

• Velocity of GPS radio signal is constant
  186,000 mi/sec
• Need to measure travel time of radio signal from
  satellite to ground receiver.
• Distance from three satellites mandatory, four
  gives a more accurate position fix.

22
One-Way Satellite RangingDistance Velocity x
Time
90 milliseconds
40 milliseconds
50 milliseconds
0.050 x 186,000 9,300 miles 0.090 x 186,000
16,740 miles 0.040 x 186,000 7,440 miles
23
Sources of GPS Error

• Slight errors in time measurement introduce large
  errors in distance very precise clocks needed
  to measure time in billionths of seconds.
• 1 millisecond mistake 186 miles!
• Major Sources of Positional Error
• Satellite/Receiver Clock Error
• Receiver Noise
• Atmospheric Delay
• Multipath Error
• Selective Availability

24
Atmospheric Delay
GPS signals are delayed as they pass through
the atmosphere
Ionosphere
Troposphere
Less Error
More Error
25
Selective Availability (S/A)

• Government introduces artificial errors to reduce
  GPS position accuracy
• With S/A on no better than 100 meter accuracy
• Turned off in 2000, but could be turned back on
  at any time

100 meters
100 meters 40 meters 20 meters
26
Current GPS Status

• U.S. Coast Guard Navigation Center
• http//www.navcen.uscg.gov/gps/status_and_outage_i
  nfo.htm
• Latest GPS Status Message
• http//www.navcen.uscg.gov/ftp/GPS/status.txt

27
GPS and GIS

• GPS Global Positioning System
• Geocaching an entertaining adventure game for
  GPS users.
• Individuals and organizations set up caches all
  over the world and share the locations of these
  caches on the internet. GPS users can then use
  the location coordinates to find the caches.
• GIS Geographic Information System
• Any system for capturing, storing, analyzing,
  managing and presenting data and associated
  attributes which are spatially referenced to
  Earth.
• Data capture entering information into a GIS
• Needs to be done well garbage in ? garbage out
• Tends to be very time consuming
• Positions from a GPS can also be directly entered
  into a GIS