Global Positioning System Pratyush Rajput Rajat Gupta Rishabh Shukla

Global Positioning System What is GPS? Working of GPS GPS Functionality GPS Signals & Frequencies Accuracy – Issues & Methods for Improvement Applications

Overview Official name of GPS is NAVigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS). First developed by the United States Department of Defense. Consists of 30+ GPS satellites in medium Earth orbit (2000km - 35,000 km). Made up of two dozen satellites working in unison are known as a satellite constellation. This constellation is currently controlled by the United States Air Force. It costs about $750 million to manage and maintain the system per year. Mainly used for navigation, map-making and surveying, etc.

Working Of GPS A GPS receiver can tell its own position by using the position data of itself, and compares that data with 3 or more GPS satellites. To get the distance to each satellite, the GPS transmits a signal to each satellite.  The signal travels at a known speed.  The system measures the time delay between the signal transmission and signal reception of the GPS signal.  The signals carry information about the satellite’s location.  Determines the position of, and distance to, at least three satellites.  The receiver computes position using TRILATERATION.

Trilateration

GPS Functionality GPS systems are made up of 3 segments – Space Segment (SS) – Control Segment (CS) – User Segment (US)

Space Segment GPS satellites fly in circular orbits at an altitude of 20,200 km and with a period of 12 hours. Orbital planes are centered on the Earth. Each satellite makes two complete orbits each sidereal day. It passes over the same location on Earth once each day. Orbits are designed so that at the very least, six satellites are always within line of sight from any location on the planet.

Control Segment The Control Segment consists of 3 entities: – Master Control Station – Monitor Stations – Ground Antennas – NGA Monitor Stations – Air Force Satellite Control Network (AFSCN) Remote Tracking Stations

Strategic Locations

Master Control Station The master control station, located at Falcon Air Force Base in Colorado Springs, Colorado, is responsible for overall management of the remote monitoring and transmission sites. Performs the primary control segment functions, providing command and control of the GPS constellation. Generates and uploads navigation messages and ensures the health and accuracy of the satellite constellation. Monitors navigation messages and system integrity, can reposition satellites to maintain an optimal GPS constellation.

Monitor Stations Six monitor stations are located at Falcon Air Force Base in Colorado, Cape Canaveral, Florida, Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia, and in the South Pacific Ocean. Checks the exact altitude, position, speed, and overall health of the orbiting satellites. The control segment uses measurements collected by the monitor stations to predict the behavior of each satellite's orbit and clock. The prediction data is up-linked, or transmitted, to the satellites for transmission back to the users. The control segment also ensures that the GPS satellite orbits and clocks remain within acceptable limits. A station can track up to 11 satellites at a time. This "check-up" is performed twice a day, by each station.

Ground Antennas Ground antennas monitor and track the satellites from horizon to horizon. They also transmit correction information to individual satellites. Communicate with the GPS satellites for command and control purposes. Four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape Canaveral.

AFSCN Remote Tracking Stations Air Force Satellite Control Network (AFSCN) provides support for the operation, control, and maintenance of a variety of United States Department of Defense satellites. This involves continual Tracking, Telemetry, and Command (TT&C). It also provides prelaunch simulation, launch support, and early orbit support while satellites are in initial or transfer orbits and require maneuvering to their final orbit.

NGA Monitor Stations The NGA Monitor collects, processes, and distributes GPS observations, environmental data, and station health information. It also provides 24/7 data integrity monitoring.

User Segment The user's GPS receiver is the User Segment of the GPS system. GPS receivers are generally composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (commonly a crystal oscillator). They include a display for showing location and speed information to the user. A receiver is often described by its number of channels this signifies how many satellites it can monitor simultaneously.

GPS Signals Coarse/Acquisition code Precision code Navigation message Almanac Data updates

GPS Frequencies L1 ( MHz) L2 ( MHz) L3 ( MHz) L4 ( MHz) L5 ( MHz)

Frequency Information The C/A code is transmitted on the L1 frequency as a MHz signal. The P(Y)-code is transmitted on both the L1 and L2 frequencies as a MHz signal. L3 is used by the Defense Support Program to signal detection of missile launches, nuclear detonations, and other applications. L4 is used for additional correction to the part of the atmosphere that is ionized by solar radiation. L5 is used as a civilian safety-of-life (SoL) signal.

Frequency L2C Launched in 2005, L2C is civilian GPS signal, designed specifically to meet commercial needs. L2C enables ionospheric correction, a technique that boosts accuracy. Delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power making it easier to receive under trees and even indoors. It is estimated L2C could generate $5.8 billion in economic productivity benefits through the year 2030.

Accuracy The position calculated by a GPS receiver relies on three accurate measurements: – Current time – Position of the satellite – Time delay for the signal The GPS signal in space will provide a "worst case" accuracy of 7.8 meters at a 95% confidence level. GPS time is accurate to about 14 nanoseconds. Higher accuracy is available today by using GPS in combination with augmentation systems. These enable real-time positioning to within a few centimeters.

Issues That Affect Accuracy Changing Atmospheric Issues: – Radio signals travel at different velocities through the atmosphere. – It changes the speed of the GPS signals unpredictably as they pass through the ionosphere. – The amount of humidity in the air also has a delaying effect on the signal.

Issues That Affect Accuracy (cont’d) Clock Errors : – Can occur when a GPS satellite is boosted back into a proper orbit. – The satellite's atomic clocks experience noise and clock drift errors. GPS Jamming : – It limits the effectiveness of the GPS signal. – GPS jammer is a low cost device to temporarily disable the reception of the civilian coarse acquisition (C/A) code.

Issues That Affect Accuracy (cont’d) Multi-path Issues : – The multipath effect is caused by reflection of satellite signals (radio waves) on objects. – The reflected signal takes more time to reach the receiver than the direct signal.

Methods of Improving Accuracy Precision monitoring – Dual Frequency Monitoring – Carrier-Phase Enhancement (CPGPS) – Relative Kinematic Positioning (RKP) Augmentation

A. Dual Frequency Monitoring Refers to systems that can compare two or more signals. These two frequencies are affected in two different ways. After monitoring these signals, it’s possible to calculate what the error is and eliminate it. Receivers that have the correct decryption key can decode the P(Y)-code transmitted on signals to measure the error.

B. Carrier-Phase Enhancement (CPGPS) CPGPS uses the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS works to within 1% of perfect transition to reduce the error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.

C. Relative Kinematic Positioning (RKP) Determination of range signal can be resolved to an accuracy of less than 10 centimeters (4 in). Resolves the number of cycles in which the signal is transmitted and received by the receiver. Accomplished by using a combination of DGPS correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests — possibly with processing in real-time.

Augmentation – Relies on external information being integrated into the calculation process. – Some augmentation systems transmit additional information about sources of error. – Some provide direct measurements of how much the signal was off in the past – Another group could provide additional navigational or vehicle information to be integrated in the calculation process.

Augmentation Systems Nationwide Differential GPS System (NDGPS) – Ground-based augmentation system that provides increased accuracy and integrity of GPS information to users on U.S. land and waterways. – The system consists of the Maritime Differential GPS System operated by the U.S. Coast Guard and an inland component funded by the Department of Transportation.

Augmentation Systems (cont’d) Wide Area Augmentation System (WAAS) – Satellite-based augmentation system operated by the Federal Aviation Administration (FAA), supports aircraft navigation across North America. Global Differential GPS (GDGPS) – High accuracy GPS augmentation system, developed by the NASA Jet Propulsion Laboratory (JPL) to support the real-time positioning, timing, and determination requirements of NASA science missions. – Future NASA plans include using the Tracking and Data Relay Satellite System (TDRSS) to transmit via satellite a real-time differential correction message.

Applications Civilian – Geotagging : Applying location coordinates to digital objects such as photographs and other documents. – Disaster Relief/Emergency Services – Vehicle Tracking Systems – Person Tracking Systems – GPS Aircraft Tracking – Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems.

Applications (cont’d) Military – Target Tracking: Tracking potential ground and air targets before flagging them as hostile. – Navigation – Missile and Projectile Guidance: Allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions – Reconnaissance – Search and Rescue: Downed pilots can be located faster if their position is known.

Applications (cont’d) Other Applications – Railroad Systems – Recreational Activities – Weather Prediction – Skydiving – And many more!