1. Off-Road Equipment Management TSM 262: Spring 2016
LECTURE 10: Global Positioning Systems I
Off-Road Equipment Engineering
Dept of Agricultural and Biological Engineering

2. Homework, Lab and Technical Sessions

3. Learning Objectives: GPS
Students should be able to:
Explain how GPS works
Identify and explain the functions of the space, control and user segments of GPS
Identify and analyze GPS errors
Explain the purpose of differential GPS

4. What is GPS?
A system of satellites and receiving devices used to compute positions on the Earth
a navigational system involving satellites and computers that can determine the latitude, longitude and altitude of a receiver on Earth by computing the time difference for signals from different satellites to reach the receiver

5. Applications of GPS in Agriculture?

6. Positioning Systems

7. Satellite-Based navigation system
DoD created in early 80's, but fully operational Apr 27, 1995
Applications for military, civilian and commercial

8. GPS Components
Space Segment
Control Segment
User Segment

9. Space Segment 30 + NAVSTAR Satellites (some spares)
Orbit of 20,200 km above the earth’s surface
2 passes/day
6 orbital passes with 4-5 satellites in each orbit
At least 4 satellites will be “above the horizon” 24 hours a day
Radio transmitter based ( MHz)
Atomic clocks (4 on-board, 2 cesium and 2 rubidium)

10. Space Segment
30+
4-5

11. Space Segment
At least 4 satellites will be "above the horizon" 24 hours a day

12. Control Segment
Tracking, monitoring and management of the satellites by station network around the world
Falcon Air Force Base, Colorado
Springs, Master Control Monitor
Track exact orbits of each satellite

13. User Segment
GPS receivers that detect the transmitted signals.

14. User/Receiver Segment
Components
Antenna
Receiver
Display
Firmware
Input
Graphic from

15. User Segment
Components include antenna, receiver, input, datalogger, software & display
Antenna collects GPS radio signals
Receiver trilaterates a position (4 sats = 3D) typically in latitude/longitude/altitude
Display shows info - sat position, data and current position
Firmware - software that controls commands, setup, and menu structure
Input – buttons to enter commands
Memory stores position and possibly attribute data

16. How Does GPS Work?
Satellite positions in space at any instant are precisely documented in almanac downloaded to GPS from a satellite
As the GPS signal travels through the Ionosphere and the earth’s atmosphere, it gets delayed. This issue and other errors complicate the process
GPS measures distance to a satellite using the travel time of a radio message
To measure the travel time, GPS needs very accurate clocks
Triangulation using the distance from receiver to the GPS satellites is the basis of the system

17. Steps to locate a GPS Receiver
Distance to satellite determined by measuring how long it takes signal to reach receiver
Assume satellite and receiver are generating same pseudo random code at exactly the same time
Compute time for satellite signal to reach receiver by comparing how late the satellite’s pseudo-random code is compared to the receiver’s code
Assume speed of signals is 299,792,458 m/sec
Compute distance to each of (at least) 4 satellites
Locate the position of the satellites from an electronic almanac
Determine position by the intersection point for the 4 spheres around each satellite

18. Satellite Orbits
To calculate your position you not only need to know the distance to four or more satellites you also have to know where those satellites are in space
Gravitational pull and other factors cause the satellite to constantly vary from its standard orbit
GPS ground stations track the variations in the satellite’s orbit upload these variations or satellite ephemeris to the satellite for broadcast to the user

19. How Important is Time?
A timing error of just 1 nsec (10-9 sec) equals approximately 1 foot of measurement error
When a GPS satellite is directly overhead it takes about 6/100th of a second for the signal to reach us
GPS is all about time!

20. Pseudo-random Codes
GPS signal travel time

21. How to Measure Time Accurately
GPS satellites can keep very accurate time because they have atomic clocks onboard
Receiver clocks don’t have to be as accurate as the satellite clocks because an additional measurement can be used to solve for the receiver time error
A fourth satellite measurement is needed to solve for timing errors

22. Overview of Satellite Ranging
Pseudorange - An estimate of the position or distance to a satellite from a receiver.
True range = Pseudorange + signal transmission errors + errors in the receiver clock + errors in satellite clock and position
4 satellites provide estimates of position, vertical position and time (X, Y, Z, T).

23. Errors Inherent in GPS System1
Pseudorange
1 - Errors in satellite
clock or position
2 - Signal transmission
errors
3 - Errors in receiver
clock
True Range 2 3

24. Satellite Ranging
(Source: