1. How Things Work: Global Positioning System
Clay Graf
October 12, 2017

2. Contents Background of GPS What does it do? How does it do it?
GPS Structure and Operation
Space module
User module
Control Module
Setbacks – Ionosphere
Conclusion

3. Background GPS is an American owned Global Navigation Satellite System
Collection of over 30 satellites and 30 ground control stations
At least 4 satellites in direct line of sight at any time
Precise semi-synchronous orbit = reduced transmission power/cost
Conceptually established in the ‘60s
Military application, missile guidance
Overcoming timing issues – test of General Relativity
Handling ionospheric disturbances

4. What does it do?
GPS is used primarily for location, navigation, tracking, etc.
Atomic clocks onboard the satellites are used for precision timing and synchronization on the ground.
Applications range from military to civilian, private to corporate:
Air traffic control
Oil and gas mining
Agriculture
Missile tracking
Precision timing experiments

5. How does it do it?
Each GPS satellite continuously transmits a unique signal of where it is, when it is (this is frequently corrected)
A receiver obtains both position and time of transmission, along with a code from which time of arrival is deduced.
Distance between the satellite to receiver is calculated
“Trilateration” requires at least 3 satellites to obtain the height, latitude, and longitude of the receiver.
Another satellite necessary to account for clock delay on ground

6. Structure of GPS System
GPS is composed of 3 modules:
Satellite – transmits data
Receiver – processes data
Ground Control – corrects and monitors the data/satellite

7. Satellite - Transmitter
Continuously transmitting between 1.1 and 1.6 GHz (bands L1 – L5), each varying in application
L1 and L2 ( MHz and MHz) are modernized for civilian use.
Contain ranging code (C/A) and navigation message containing position and time of the satellites transmission

8. Transmitted signal
Below, the carrier wave is loaded with the C/A code and the navigation message before being transmitted.

9. Transmitted signal
The C/A code is used as PRN (pseudorandom noise) in order for receivers to distinguish between satellites (no correlation)
Also used to calculate the time of arrival from autocorrelation function

10. User - Receiver
The receiver contains the antenna (typically a tuned PIFA), processor(s), and a stable clock at the very least.
Necessary to have at least 4 (typically now) channels to deal with multiple satellite signals

11. Processing the signal
Upon receival, both the carrier wave and PRN code are demodulated, and the spatial and temporal satellite coordinates extracted.
Receiver location is ready to be established

12. Processing the signal
Using at least 3 satellites allows for trilateration of the receiver by computing the distance to each of the satellites at the time of transmission.
4th satellite provides time delay correction

13. Ground Control
Satellites need recalibration daily in order to stay precise
Ground stations talk to the satellites, updating the navigation message data, as well as resynchronizing the clocks if necessary
While the satellites are being corrected, a flag is set, notifying receivers not to use this satellite right now

14. Ionospheric Disturbances
This is the major source of error in GPS calculations
The signal undergoes dispersion (frequency dependent) as it passes through plasma layers
How is this overcome?

15. Ionospheric Correction
Modernized GPS is designed so the receiver listens to both L1C and L2C bands
The processor can then account for scintillations and refraction
Allows for correction and monitoring of the ionosphere in real time
Although effective, will not resolve the issue completely

16. Conclusion
GPS is composed of over 30 satellites, with at least 4 in direct LOS at any moment
Constant corrections are made on the ground to keep the satellites up to date
Unique signals are picked up by receivers which trilateration your position, and even derive your velocity
Assuming the signal is not disrupted by turbulent ionospheric plasma, we can all enjoy the feeling of never being lost again

17. Thanks
Garmin.com
Wikipedia
Google images

18. Questions?