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Global positioning system

The Global Positioning System (GPS) is a satellite-based navigation system that provides precise 3D location information around the world. It consists of 24 satellites orbiting 20,200 km above the Earth that transmit radio signals. GPS receivers on the ground use these signals to calculate the user's position by triangulating distances to four or more satellites. The system is operated and maintained by the U.S. Department of Defense.

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Global Positioning System GPS Dhaval A . Jalalpara
What is GPS ? A very precise positioning system • Developed and maintained by the US Department of Defense (DOD) • Satellite Based * 24 satellites * 20,200 km high orbit
 GPS stands for Global Positioning System which measures 3-D locations on Earth surface using satellites  GPS operates using radio signals sent from satellites orbiting the earth  Created and Maintained by the US Dept. of Defense  System as a whole consists of three segments ◦ Satellites (space segment) ◦ Receivers (user segment) ◦ Ground stations (control segment)
 At least 4 satellites are above the horizon anytime anywhere  GPS satellites are also known as “NAVSTAR satellites”  The satellites transmit time according to very accurate atomic clocks onboard each one  The precise positions of satellites are known to the GPS receivers from a GPS almanac Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
 Satellites have accurate atomic clocks onboard and all GPS satellites transmit the same time signal at the same time ◦ Think “synchronize your watches”  The satellite signals contain information that includes ◦ Satellite number ◦ Time of transmission
 Receivers use an almanac that includes ◦ The position of all satellites every second ◦ This is updated monthly from control stations  The satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satellite  Trilateration (similar to triangulation) is used to determine location from multiple satellite signals
 Satellite errors ◦ Satellite position error (i.e., satellite not exactly where it’s supposed to be) ◦ Atomic clocks, though very accurate, are not perfect  Atmospheric ◦ Electro-magnetic waves travel at light speed only in a vacuum ◦ Atmospheric molecules, particularly those in the ionosphere, change the signal speed  Multi-path distortion ◦ The signal may "bounce" off structures before reaching the GPS receiver – the reflected signal arrives a little later  Receiver error: ◦ Due to the receiver clock or internal noise  Selective Availability ◦ No longer an issue
Segments of GPS 1. Space Segment A constellation of 24 satellites 2. Monitor Station A network of earth-based facilities 3. Users & Equipment Source:Trimble
GPS Receivers
 Differential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver)  Any errors in a GPS signal are likely to be almost the same among all receivers within ~ 300 miles of each other ~ 300 miles (~ 480 km) or less Base station (known location) Rover receiver
Methods of data collection Three methods of positioning • Autonomous 10-20 meters • Differential 2-5 meters • Phase Differential centimeter 2-5m10-20 m cm
How accurate is GPS? Depends on some variables • Design of receiver • Relative positions of satellites, often known as DOP (Dilution of Precision) • Postprocessing • Time spent on measurement
What is a PDOP? • Position Dilution of Precision Good PDOP Poor PDOP
GPS for Navigation
• Generating mapped data for GIS databases • Collecting field data - travel to the field and capture location & attribute information • Other uses (many in real time): • 911/firefighter/police/ambulance dispatch • Car & boat navigation • Roadside assistance • Business vehicle/fleet management • Mineral/resource exploration • Wildlife tracking • Recreational (fishing, hunting, hiking, etc. • Ski patrol/medical staff location monitoring
 Easy To Incorporate into Project  Once trained, just about anyone can use it  Cheap  Widely Available
 Does require a training component  Accuracy Issues  Differential Correction may not be an option in many parts of the world
 Identify Your Accuracy Needs  Identify Error Correction Methodology ◦ Point Averaging  How long will points be collected? ◦ Differential Correction  Find a base station  Identify Point Collection Methodology ◦ Where will points be collected? ◦ Contingency plans ◦ Data backups
Global positioning system