Introduction to the Global Positioning System Introduction to the Global Positioning System

In the 1970s, the Department of Defense began using highly accurate atomic clocks placed in orbit in artificial satellites for navigation. The Omega Navigation System, used by the U.S. Navy, was the first worldwide radio navigation system. Limitations of the system drove the need for a navigation solution with greater accuracy. The History of GPS

 Also in the 1970’s, the Air Force began using a radio-navigation system called MOSAIC (Mobile System for Accurate ICBM Control)  Designed to increased accuracy for Air Force bombers as well as ICBMs.  Updates from the Navy system were too slow for the high speeds of Air Force operations. The History of GPS

 With these parallel developments, it was realized that a superior system could be developed by synthesizing the best technologies from both projects.  Over the Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon to discuss the creation of a Defense Navigation Satellite System (DNSS).  It was at this meeting that the real synthesis, that became GPS, was created. The History of GPS

 Later that year, the DNSS program was named Navstar.  The constellation of Navstar satellites was called Navstar-GPS, which was later shortened simply to GPS. The History of GPS

 The current GPS consists of three major segments.  Space segment (SS),  Control segment (CS),  User segment. How the GPS Works

Space Segment

 Space segment As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation 2 older satellites, retired from active service satellites kept in the constellation as spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. About 8 satellites are visible from any point on the ground at any one time. How the GPS Works

 Space segment Each satellite constantly broadcasts 3 very important pieces of data 1. Time – using highly accurate atomic clock 2. Satellite number 3. Location in space How the GPS Works

 Military.  Search and rescue.  Disaster relief.  Surveying.  Marine, aeronautical and terrestrial navigation.  Remote controlled vehicle and robot guidance.  Satellite positioning and tracking.  Shipping.  Geographic Information Systems (GIS).  Recreation. User Segment Who uses GPS Receivers?

 User segment GPS receivers are composed of 1. an antenna, tuned to the frequencies transmitted by the satellites, 2. processors 3. highly stable clock (often a crystal oscillator). How the GPS Works

 User segment GPS Receivers do split-second algorithms and trigonometry calculations using 1. the receiver location, and receiver time (on the Earth surface) 2. Satellite numbers, location and time signal sent. 3. Triangulation How the GPS Works

Signals From Two Satellites Latitude and longitude, but not specific to hemisphere (N, S, E, W)

Three Satellites (2D Positioning) Latitude and Longitude – Correct hemispheres But - No Altitude!

Three Dimensional (3D) Positioning Using 4 satellites, the receiver can determine Latitude, Longitude, and Altitude.

Kwajalein Atoll US Space Command Control Segment Hawaii Ascension Is. Diego Garcia Cape Canaveral Ground Antenna Master Control Station Monitor Station

 Control segment ( Ground-based) The control segment is composed of Master control station (MCS), Alternate master control station, 4 dedicated ground antennas and 6 dedicated monitor stations. Wide Area Augmentation System (WAAS) – Satellites in geosynchronous orbit over the surface of the Earth (To aid in accuracy) How the GPS Works

Wide Area Augmentation System Geostationary WAAS satellites GPS Constellation WAAS Control Station (West Coast) Local Area System (LAAS) WAAS Control Station (East Coast)

How good is WAAS? meters +-15 meters Under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.* Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.* * Precision depends on good satellite geometry, open sky view, and no user induced errors.

Control Segment Space Segment User Segment Three Segments of the GPS Monitor Stations Ground Antennas Master Station

Selective Availability (S/A)  Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded ("Selective Availability", SA).  The Defense Department intentionally distorted the satellite time message, reducing position accuracy to some GPS users.  S/A was designed to prevent America’s enemies from using GPS against us and our allies.

Selective Availability (S/A)  This changed with U.S. President Bill Clinton ordering Selective Availability turned off at midnight May 1, 2000, improving the precision of civilian GPS from 300 meters (about 1000 feet) to 20 meters (about 65 feet).  Later, in 2000, the Pentagon reduced S/A to zero meters error.  S/A could be reactivated at any time by the Pentagon.  The U.S. military by then had the ability to deny GPS service to potential adversaries on a regional basis.

Sources of Signal Interference Earth’s Atmosphere Solid Structures Metal Electro-magnetic Fields

Planning a Navigation Route Start = Waypoints