2. What is GPS??? GPS is short for Global Positioning System
Most simply, you use a hand held device that received signals from satellites in orbit around the earth to determine your position on earth’s surface

3. What is GPS??? + = GPS units Receive signals from satellites in space
Determine where an object is on earth’s surface

4. Very Accurate

6. How do GPS units work??
The basis of GPS is "triangulation" from satellites.
A GPS receiver measures distance using the travel time of radio signals.
To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
Along with distance, you need to know exactly where the satellites are in space.
Air Force maintains ground control
Finally you must correct for any delays the signal experiences as it travels through the atmosphere.

10. Only have 2 points in space where 3 spheres intersect
One will be on earth’s surface, other out in space

11. GPS Errors Signals travel at speed of light but not entirely true
Signals travel at speed of light but not entirely true
Slowed down in Ionosphere: charged particles
Slower down in Troposphere: water vapor
Solution: Modeling atmospheric interference, not easy, changing constantly

12. GPS Errors Multipath errors
Result when the direct path to your receiver is blocked
House,  roof,  trees, mountains, 
buildings, etc
The signal from the satellite is REFLECTED by some object. 

13. Differential Correction : Eliminate Errors
Base station, known position.
Used to calculate the error in the satellite signals.
Accomplished by finding the difference between the positions calculated from the satellite signals and the known reference position (base station)
Resulting differential corrections can be used to remove much of the error from the rover file data.

16. Who maintains the satellites??
U.S. Air Force's 2nd Satellite Operations squadron at Falcon AFB in Colorado maintains the GPS system
GPS is a high maintenance system
Satellites require regular tweaking
data uploads,
orbital positioning adjustments
clock maintenance
If the ground segment stopped, the system would "gracefully degrade" to complete uselessness in about two weeks time.
So, as each satellite whizzes along and completes one earth orbit every 12 hours, Air Force personnel at Falcon AFB talk to it every few hours.

17. Converting Units Degrees, Minutes: -96, 47.46732 or 44, 27.66789
-96, = -(96 + ( /60)) =
Decimal degrees:
44, = 44 + ( /60) =
Decimal degrees:
Degrees, Minutes, Seconds:
-96, 47, =
-(96 + (47 /60) + ( /3600)) =
Decimal degrees:

18. Converting Units 1. Decimal Degrees: -96.791122 -(96 + (0 .791122*60))
Degrees, Minutes:
-96,
2. Decimal Degrees:
(-96 + ( * 60)) = -96,
-96, 47, ( * 60)
Degrees, Minutes, Seconds
-96, 47,

19. Accuracy Different equipment have different levels of accuracy
Magellan units, less expensive, less accurate, Cabela’s
Trimble units, Longitude, Latitude, not so accurate in elevation, Ag132
Trimble RTK units, Longitude, Latitude, very accurate in elevation, more expensive

20. Different Use, Different Accuracy, Different Cost!!
Magellan units like today
Cost $200 – 250
Accuracy
Bring you back to within about 90 feet of same spot
Buy many places
In SD, Cabela’s, Scheels
Uses: Fishing, Hunting, Navigation

21. Different Use, Different Accuracy, Different Cost!!
IPAQs and GPS receivers
Agriculture:
Scouting, Collecting GPS for data points
Record Keeping:
Pocket Excel, Work, Internet
Accuracy:
About 2 to 5 meters
Cost: $350 for IPAQ, $180 for GPS receiver, $200 - $400 for GPS software to log points
GPS

22. Different Use, Different Accuracy, Different Cost!!
Trimble AG 132 GPS units
Accuracy: 6 to 10 in.
Cost: $3,000
Trimble Units
Mapping data

23. Different Use, Different Accuracy, Different Cost!!
Real Time Kinematic GPS or RTK GPS
Survey grade
Engineering roads
Navigation
Fishing
Airlines
Agriculture:
Fertilizer or seed applications
Accuracy:
< 1 inch
Cost:
$25,000 to $50,000

26. GPS in Agriculture

27. Yield Monitor

28. Mapping Weeds

36. Questions