GPS technology and how it works!

1. Introduction to the Global
Positioning
System
by
Dr. Stephen C. Brown
K-State Research and Extension
Brown County
And
Mandy Kern
Hiawatha High Agriculture Educator

2. WHAT IS GPS?
• Satellite based radio positioning system
• Developed and maintained by the
U.S. Department of Defense
• Made up of three segments:
Space
Control
User or Ground receivers
• Used for obtaining accurate locations
anywhere on earth

3. Space Segment

4. GPS Is An Excellent
Mapping Tool
Instantly gives:
Altitude
Speed
Longitude and Latitude

5. How It Works….
Trilateration

6. Math Connection
Step 1
•Suppose we measure our
distance from a satellite and
find it to be 11,000 miles.
•Knowing that we're 11,000
miles from a particular
satellite narrows down all the
possible locations we could
be in the whole universe to
the surface of a sphere that is
centered on this satellite and
has a radius of 11,000 miles.

7. Math Connection
Step 2
•Next, say we measure
our distance to a second
satellite and find out that
it's 12,000 miles away.
• That tells us that we're
not only on the first sphere
but we're also on a sphere
that's 12,000 miles from
the second satellite. Or in
other words, we're
somewhere on the circle
where these two spheres
intersect.

8. Math Connection
Step 3
•If we then make a
measurement from a third
satellite and find that we're
13,000 miles from that one,
that narrows our position down
even further to the two points
where the 13,000 mile sphere
cuts through the intersection of
the first two spheres. •To decide which one is our true
location we could make a fourth
•So by ranging from three measurement. But usually one of
satellites we can narrow our the two points is a ridiculous
position to just two points in answer (either too far from Earth or
space. moving at an impossible velocity)
and can be rejected without a
measurement.

9. LAND •USER SEGMENT
SEA
AIR
NEED
THREE SV’s.

10. Control Segment
• Maintained by the Department of Defense

11. Sources of Error
 Selective Availability (SA Error)
 Atmospheric
 SV Ephemeris
 Multipathing
 Poor SV Geometry

12. EPE
(Estimated Position Error)
The higher the EPE,
the greater the
Positioning error!
Good EPE’s range from 10-20 feet.
Proper pre-mission planning prevents poor positioning!

13. Shape of the Earth
(datum)

14. PRACTICAL APPLICATIONS
•Navigation
•Sailing, hiking and aviation
•Collecting and locating way-points
•Locating Features
• Stocking locations or stream surveys
• Endangered or sensitive species locations
• Locating trails or physical structures
•GIS
•Collecting GPS data along with attribute data
• Mapping and database update
•Surveying
•Locating Control Points quickly

19. LOW END UNITS
•Capabilities
•20 ft accuracy
•1-12 channel receiver
•Collect only way point data
•Cost
•$80 - $600
•Limitations
•Poor reception under tree canopy
•No file collection
•Applications
•Navigation for hiking or sailing

20. MID-RANGE UNITS
•Capabilities
• Sub-meter accuracy with DGPS
• 6-12 channel receiver
• Collects static, mobile and attribute data
•Cost
• $2,500 - $15,000
•Limitations
•Not accurate for surveying
•Applications
• Navigation
• Mapping
•GIS Update

21. HIGH END UNITS
•Capabilities
• cm accuracy with DGPS
• 12 channel receiver
• Collects static points
•Cost
• $15,000 - $100,000
•Applications
• Surveying

22. Garmin GPS Receivers
 Inexpensive and downloadable
 Available locally for less
than $150
 Most GPS receivers similar
in operation

23. Okay, lets see how it
works!
Zoom out/in Page Button
Button
Power
Button
Find
Button
Thumbstick

24. Pages In the GPS Unit

25. SV Visibility
clear view obstructed view
clear view obstructed view

26. Satellite Status
Page
 Satellite status
appears while the
unit “finds” the
satellites
 Acquisition can
take several
minutes

27. Satellite Status
Page
Satellite elevation/position
Signal strength
Receiver position

28. Main Page
 The main page
allows user to edit
waypoints, tracks
and routes or
configure system to
personal preference

29. Map Page
 Allows you to view
map of current
location,
waypoints and
tracks

30. Waypoints
4
track
2
3
waypoint 1

31. Creating Waypoints
 Marking waypoints
allows you to
remember and return
to a specific location
 Waypoints are
created by pressing
and holding
thumbstick

32. Finding Waypoints
 Pressing “Find”
button will bring up
a list of waypoints
 Entering a waypoint
will result in “as the
crow flies”
directions to that

33. Finding Waypoints
Name of Waypoint
Distance to Waypoint
Bearing to Waypoint
Heading
Speed

34. What is
Geocaching?
Geocaching is a worldwide “scavenger hunt”
using GPS to find “caches” of treasure. The
only rules are to leave something if you take
something and do not disturb the cache

38. Links to using GPS into the
Classroom
 http://worldofagriculture.org/WOA-precag.htm

39. Let’s Practice!!
 Waypoint #1 – N 39°51.102
W 095°31.536
 Waypoint #2 – N 39°51.081
W 095°31.590
 Waypoint #3 – N 39°51.113
W 095°31.417
 Waypoint #4 – N 39°51.090
W 095°31.400