Intro to GIS
Spring 2012
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GPS = Global Positioning System
GNSS = Global Navigation Satellite Systems
Satellite based technologies that give
location on Earth’s surface
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Navigation: knowing where you are and
where to go
Defense: precise locations for targets
Now: many uses – commercial, field mapping
and surveying, automobile travel, recreation
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Navigating the oceans
 Compass: points to magnetic north so know
direction traveling
 Sextant: instrument that can determine angles of
stars, moon and sun over horizon. Indicates your
latitude
 Chronometer: shipboard timepiece that indicates
your longitude
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Early 20th century: radio-based navigation
systems used during WWII
Limitations of ground radio systems
 Very accurate but doesn’t cover wide area
 Or, one that covers wide area but is not very
accurate
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NAVSTAR: operated by the US Department
of Defense, 1st satellite launched in 1978, last
satellite launched in 1994
GLONASS: Russian, little used internationally
Galileo: being developed by a consortium of
European governments and industries
Chinese Compass Satellite Navigation
System: in development
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http://www.aero.org/
Satellites
Receivers(users)
Control stations
24 satellites orbiting
the Earth
 About 20,000 km
above surface
 Complete orbit in 12
hours
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http://www.aero.org/
Each satellite is
carefully placed and
monitored in orbit
 Each contains a very
accurate clock – to 3
billionths of a second,
or 0.000000003
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http://www.aero.org/
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Each broadcasts a signal that includes:
 Pseudorandom code: unique to identify which
satellite
 Ephemeris data: identifies satellite position in
space at any given moment
 Almanac data: exact time signal was sent
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Electromagnetic radiation
 Low power radio waves that pass through clouds,
glass and plastic, but not Earth or buildings
 Speed of light
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Distance = velocity x time
Know velocity: signals traveling at speed of
light 3 x 108 m/s
Time: determine time between when signal
was sent by satellite and when received by
the receiver (GPS unit)
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Need precise clocks
Each satellite emits a
pseudorandom code
 Signal so complicated
that it looks random
 Receiver compares the
signal it receives with
the signal should be
exactly when it is
received
Source Bolstad, 2008. p. 180
•Satellite directly overhead: takes about
0.06 second to reach receiver
Detect, decode and process signals from
satellites within range
 Contain accurate clock, although not as accurate
as the ones on satellites
 Measure the distance between the time the
signal was sent and the time it was picked up by
the receiver.
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 Used to determine the distance to satellite
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Need signals from at least 4 satellites to
determine location and elevation
 More is better
Know location of
satellites (at least 4)
 Know distance of each
satellite from receiver
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Source Bolstad, 2008. p. 181
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5 ground stations around the world
 Hawaii, Ascension Island (South Atlantic Ocean),
Diego Garcia (Indian Ocean), Kwajalein (Marshall
Islands), and Colorado Springs
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Master station in Colorado, USA
Responsible for:
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Tracking
Communications
Data gathering
Integration
Analysis
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Cannot pass through buildings, underground, sides of
mountains, and dense foliage
Buildings and terrain can reduce visible sky and block
signal reception
Signals can reflect off buildings and outcrops, thus
increasing the length of time the unit receives the signal
Signal slows through the atmosphere. GPS uses a
correctional factor
Locations of satellites should be at wide angles to each
other
Use 2 receivers: 1 stationary
and the other roving (e.g.,
hand-held unit)
 Location of stationary
known. Can be used to apply
correction to signals due to
atmospheric interference
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 Works in reverse: known
location so will know the
length of time a satellite signal
should take to reach it
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If the roving unit is within a
few hundred kilometers, can
use the error correction
Source Bolstad, 2008. p. 186
Improves accuracy
Some GPS receivers
can receive correction
while collecting data
 Others require post
processing: corrections
are applied later in a
lab
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Source Bolstad, 2008. p. 188