1. Global Positioning System
Alex Liang and Akhil Maddukuri

2. 3 Components The space segment - Actual GPS satellites
Control system - Operated by US DoD
User segment - Includes both military and civilian users and their GPS equipment

3. Space Segment
The US commits to maintain at least 24 operational GPS satellites, 95% of the time
Satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth
GPS satellites fly in medium Earth orbit at approximately 20, 200 km in altitude, with each satellite circling the Earth twice a day

4. Space Segment (Continued)
Each satellite broadcasts navigation message based upon data uploaded from Control Segment and adds the message to a MHz Pseudo Random Noise (PRN) code sequence
Satellite converts the code sequence onto a MHz L-band carrier to create a spread spectrum ranging signal
L-band is range of radio signals from 1 to 2 GHz
L-band waves penetrate clouds, fog, rain, storms, and vegetation - accurate signal transmission
Spread Spectrum modulation is a method in which a signal generated with one bandwidth is spread in the frequency domain to create a signal with wider bandwidth

5. Visual Representation

6. Control Segment
GPS Control Segment (CS) is comprised of four major components:
Master Control Station (MCS)
Backup Master Control Station (BMCS)
Four ground antennas
Six monitor stations
MCS is based in Schriever Air Force Base, Colorado - operated 24/7
Roles: Navigation data upload operations, Detection and response for service failures, satellite maintenance
CS's four ground antennas provide real-time Telemetry, Tracking, and Commanding (TT&C) interface between the GPS satellites and the MCS
Six monitor stations provide real-time satellite ranging measurement data to MCS

7. Visual Representation

8. User Segment
Used in everything from cell phones and wristwatches to bulldozers, shipping containers, and ATM's
Critical to U.S. national security - nearly all new military assets from vehicles to munitions are equipped with GPS

9. How Does GPS Work? Radio waves Travel at the speed of light
Calculates receiver position by how long signals take to reach
Velocity x time = distance
Satellites are constantly monitored and predictable
Speed of light 186,000

10. How Does GPS Work? “Pseudo-random” code as receivers
Digital Code - measures the timing delay from GPS to receiver
Football field reference
Special atomic clocks
Oscillations of an atom as 24 “metronome” provide the most accurate time
Receiver clocks aren’t as accurate
Football field 1 person on each side, measure delay of voice
Receiver clocks don’t need to be that accurate because of trignometric trick

11. How Does GPS Work? Around 21 satellites in orbit
Only need 3 to pinpoint location
Having more is more accurate but receivers must be developed to have the channels
Two channel compromise
1 performs time measurement calculations
Other moves on to the next one right away
Possibly more than 21 satellites
New Iphone so sophisticated it as 12 channels

12. Challenges Ionosphere delays Region of charged particles above earth
Predict an everyday average or looking at relative speeds of two different signals
Atmospheric delays
Clouds, water vapor, etc can also affect speed of light

13. Challenges Satellite errors Atomic clock errors
Constant monitoring by the DoD
Receiver errors
Electrical interference
Rounding off mathematical calculations
Iphone can be off by up to 100 yards or more
Depending on where you are, etc

14. Governmental Control of GPS
GPS is a global navigation satellite system, or GNSS
Few exist due to high cost and complex infrastructure of developing and maintaining a GNSS
All are controlled by nation-states
Only Russia’s GLONASS offers the equivalent full global coverage and a complete satellite constellation
EU’s Galileo system began in 2011 and will require until 2025 for full operability
China’s Beidou system currently offers limited service and will mature around same time
Other states want to establish political independence - problems of disunity?