1. Introduction to GNSS
ABE 459

2. What is GNSS? GNSS (Global Navigation Satellite System)
Satellite navigation System
Accurately and precisely locate a point on Earth
US – GPS (Functional)
Russian – GLONASS (Functional)
European – Galileo (Non Functional)
Chinese – COMPASS (BeiDou-2) (Non Functional)
Indian Regional Navigation Satellite System –IRNSS (Non Functional)

3. How does it Work?
Each GPS satellite transmits data that indicates its location and the current time
Satellites can be used if they are in the sight of a receiver as long as they are 25⁰ or greater above the horizon
GPS receivers require unobstructed view of the sky and they only receive data, don’t transmit it

4. How does it Find Location?
Trilateration: The process of determining absolute or relative locations of points by measurement of distances, using the geometry of circles or spheres
Can be accomplished using information from 3 separate satellites

5. How GNSS Works
Okay, you’re lost, you have amnesia, and you have absolutely no idea where you are. You go up to someone, and like a good amnesiac you ask, “Where Am I?” This person tells you that you are 180 miles from Chicago. You say, sarcastically, “Wow, that’s some great information. Thanks Buddy.”
Chicago

6. We’re Getting Closer
You still have absolutely no idea where you are, so once again you go up to someone and ask, “Where Am I?” This person tells you that you’re 170 miles from Indianapolis. You just scratch your head, but then you start to remember what you learned in GPS class, and, with the US map in your pocket to guide you, you proceed.
Chicago
Indianapolis

7. Now We Know!
You see a another smiling face and ask “where am I” one more time. This time you get the reply “you are 150 miles from Quincy. The light pops on in your brain. It’s all clear now. You draw three circles extending in the radii of 180 miles from Chicago, 170 from Indianapolis, and 150 miles from Quincy. The circles intersect in Decatur! Aren’t you glad that you took this class?
Chicago
Indianapolis
Decatur
Springfield

8. 2D Trilateration Procedure
You know the location of 3 satellites
Given the distance you are from those 3 satellites
Draw a circle, center is the location of the satellite, radius is distance from satellite
Repeat for all satellites
Your location is where the 3 circles intersect

9. Trilateration (continued)
In 3-D you can use 4 satellites (using spheres opposed to circles) to locate a precise location and include elevation
The more satellites that are in range the more accurate GNSS can be, mathematical models are used by GNSS units to determine best accuracy.

10. Differential Correction
Raw GNSS is accurate only ~ 5-10 Meters
Differential Corrections adjust for errors.
Satellite Based
WAAS/EGNOS
Purchased
Ground Based
Real Time Kinematics

11. Satellite Differential Correction
DGNSS Site
Receiver

12. RTK Differential Correction
DGNSS Site
Radio/Cell
Receiver

13. Signal Interference Earth’s Atmosphere Solid Structures
Metal
Electro-magnetic Fields

14. Common GNSS Uses Military (Weapons precision)
Navigation (Civilian or Commercial)
Surveying
Drainage Design
Topography Measurements
Precision Agriculture

15. Precision Agriculture – Brief History
Guidance adoption balloons
Finding the market - (20 years)
“Wandering in the Wilderness”
Reagan announces GPS will be free for civilian use
1983
Customers begin
Yield Mapping
Satellite based
guidance show payback
Guidance Prices Decline and Expand Platforms; increasing ROI
GPS guidance ties location and machine controls for integrated business applications
Disabling of Selective Availability
Integrated Applications

16. Other GNSS Facts
GNSS time is theoretically accurate to about 14 nanoseconds
GNSS systems have settled disputes between land owners
GNSS data has revealed that Mt. Everest is getting taller!