Global Positioning System Alex Liang and Akhil Maddukuri

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

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

Space Segment (Continued) Each satellite broadcasts navigation message based upon data uploaded from Control Segment and adds the message to a 1.023 MHz Pseudo Random Noise (PRN) code sequence Satellite converts the code sequence onto a 1575.42 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

Visual Representation

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

Visual Representation

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

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

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

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

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

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

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?