Global Positioning System - GPS

The global positioning system is a location determination network that uses satellites to act as reference points for the calculation of position information. These man-made reference points can be viewed as aerial lighthouses that are visible to user equipment and can also transmit additional information that can provide extremely accurate location information to the GPS function within location determination devices.

Several orbiting satellite systems are used for global positioning, which include:

• United States Global Positioning System (GPS)

• Russian Global Navigation Satellite System (GLONASS)

• European Galileo system

The USA GPS System

The USA Global Positioning System (GPS) is a location determination system that was developed by the Department of Defense's (DOD) Ivan Getting, and Massachusetts Institute of Technology (MIT).

This system, which consisted of eleven satellites, was called NAVSTAR (Navigation System with Timing and Ranging) and launched between 1978 and 1985. By 1980, 18 satellites were part of the NAVSTAR constellation and used by the military for GPS.

In 1983, President Ronald Regan declassified GPS technology, allowing for public use. This was prompted by the take down of Korean Airline 007 by Russian Military jets when the commercial airliner drifted into Russian airspace. Since then, public use of the satellites for commercial purposes was allowed and by July of 1995, 24 satellites were in place, completing the full system. It was off limits to the public between 1990 and 1993, during the first Gulf War, to allow for exclusive use by the military. Although the constellation currently has 29 satellites in orbit, only 24 are required for normal operation leaving five spares in case of lost operation.

GLONASS

The GLONASS system is a global positioning system that is operated by the Russian Republic, (State Unitary Enterprise of Applied Mechanics). The system has continued to evolve, providing more accuracy and precision, and much like the US system, it also has 24 satellites. Unlike the US satellites, which are launched individually, GLONASS satellites can be launched in groups of three. Additionally, the GLONASS system operates at a higher altitude than the US system. There are some technical trade offs to this approach such as better accuracy in the northern hemisphere at the cost of less accuracy in the southern hemisphere. Since GLONASS serves the Russian Republic, this is in their favor.

European Galileo System

The European Galileo System is a global navigation system similar to the US GPS system, which was started in May 2003 by the European Union and the European Space Agency The system was to be built between 2006 and 2008 at the cost of 3 billion Euros, and is expected to be complete by 2013 with a cost of $3.4 Billion Euro, as of this printing. This system will use 30 satellites for a complete constellation and will be for civilian use only. China, Korea, Israel and other non-EU countries are invested in the project and will use the system as well. The EU system aims to be the most accurate of the world’s systems.

Beidou Satellite Navigation System

The Beidou System is a Chinese regional satellite positioning system. It is a two-way satellite system that allows mobile devices to request position information from the satellite network.

Terrestrial and Other Positioning Systems

Terrestrial positioning systems are position location systems that use land-based transmitters or reference points to act as reference points for the calculation of position information. Some of the terrestrial based positioning systems include long range aid to navigation (LORAN), dead reckoning (DR), and inertial navigation systems (INS).

How GPS Works

Recall that there are 24 satellites being used in the US GPS system at all times. These satellites orbit the earth in such a way that at any given time and location, at least four satellites are visible to a GPS driven device. Each satellite is equipped with an extremely accurate atomic clock so the satellite is always aware of the current time on Earth at the Prime Meridian. The satellites are also aware of their own positions with the assistance of ground stations that give continuous updates. The 24 satellites orbit the earth transmitting their time and position.  These pieces of data are received by the antennas attached to radio receivers inside a GPS device. 

As a GPS device starts up, it must scan its radio tuner for very faint GPS satellite signals. Once it has collected data (the position of a satellite and the time the satellite sent the position) from at least three satellites, a location fix can be made.

Differential time of arrival and triangulation are the methods used to determine location in a GPS system. Dead reckoning may also be used when satellites are not visible. 

Differential Time of Arrival

Differential time of arrival is the method used to determine how far each satellite is from a GPS device. Although each satellite transmits its position and the time it was at that position, it takes time for that signal to reach the Earth. The receiver contains a very accurate clock, which can determine the difference in time between the current time and when the satellite sent the signal.  With this differential time and the speed of radio waves, the distance from each of the three satellites can be determined using the simple formula:

Rate x Time = Distance

Trilateration

Trilateration is a method that is used to determine position on Earth in three dimensions. GPS deals with three-dimensions rather than two. Since the distance from the Earth to a satellite results in a sphere rather than a flat circle, the calculation is a bit complex. 

Using trilateration, rather than draw circles to determine position we need to draw spheres. For example, if the first acquired satellite is 25,000 miles from position one cannot simply draw a circle around that satellite and determine a position 25,000 miles from it. A sphere must be plotted, extending toward Earth and away from Earth. A second satellite is calculated to be 25,001 miles from position, resulting in another sphere. The two spheres intersect, creating a perfect circle. A circular plane now exists, extending down through the earth and out into space.  A large number of potential positions have now been eliminated, but there is not yet an exact location. Many potential positions still exist and a third satellite is needed to define a sphere that intersects with the two current spheres resulting in two points that define possible position. One point is in space and one is on earth. Since the world is roughly a sphere, the point in space can be eliminated and the approximate position of the GPS receiver is located on Earth. A fourth satellite is necessary to account for altitude and provide an exact fix of the location. The plotting of a fourth sphere provides the exact location and altitude of the receiver at the time the four measurements were taken.

This figure shows a global positioning satellite (GPS) system. This diagram shows how a GPS system receives and compares the signals from orbiting GPS satellites to determine its geographic position. Using the precise timing signal based on a very accurate clock, the GPS receiver compares the signals from 3 or 4 satellites. Each satellite transmits its exact location along with a timed reference signal. The GPS receiver can use these signals to determine its distance from each of the satellites. Once the position and distance of each satellite is known, the GPS receiver can calculate the position where these distances cross. This is the location. This information can be displayed in latitude and longitude form or a computer device can use this information to display the position on a map on a computer display.

   Related Global Positioning System - GPS Definitions

3 Dimensional Mapping
Absolute Positioning
Acquisition
Acquisition Assistance
Aerial Guidance Systems
Aeronautical Radionavigation Satellite Service
Aeronautical Radionavigation Service
Air Navigation
Air Navigation
Airborne Mapping
Ambiguity Bias
Ambiguity Resolution
Antenna Offset
Antenna Swap Method
Apogee
Asset Tracking
Assisted Global Positioning System - A-GPS
Assisted GPS
Atmospheric Correction
Atmospheric Layers
Atomic Clock
Attitude Control System - ACS
Augmentation Systems
Automated Machine Guidance
Automated Machine Guidance
Automated Vehicle Location - AVL
Automatic Location Identification - ALI
Automatic Vehicle Location - AVL
Autonomous Navigation - Autonav
Base Repeaters
Bhangmeters
Block I Satellites
Block II and IIA Satellites
Block IIR Satellites
Block IIF Satellites
Broadcast Ephemeris
Cadastral Surveying
Carrier Phase Measurement
Clock Bias
Clock Drift
Clock Errors
Computational Assistance
Confidence Region
Coordinate Reference System
Coordinated Universal Time - UTC
Correlator
Cross Correlation
Cycle Slip
Dead Reckoning - DR
Differential Global Positioning Service - DGPS
Differential Long Range Navigation - Differential LORAN
Dilution of Precision - DOP
Displacement Effect
Double Difference Mode GPS
Dual Frequency Code receiver
Dual Frequency Receiver
Dynamic Positioning
Ellipticity
Emergency Location Services - ELS
Enhanced Location Method
Enhanced Long Range Navigation - eLORAN
Ephemeris
Ephemeris Error
Error Budget
Fleet Management
Geographic Fencing - GeoFencing
Geographical Information System - GIS
Geosynchronous Earth Orbit - GEO
Global Navigation Satellite System - Glonass
Global Navigation Satellite Systems - GNSS
Global Positioning System - GPS
GPS Base Repeater
GPS Base Station
GPS Errors
GPS Error Sources - GPS Errors
GPS Interfaces
GPS Operational Control Segment - OCS
GPS Readings
GPS Satellite Orbits
GPS Satellites
GPS Surveying
GPS System Time
GPS Time
Ground Segment
Group Repetition Interval - GRI
Guidance Systems
Gyroscope
Hardware Delay
I5-Code
Inertial Measurement Unit - IMU
Inertial Navigation System - INS
Initial Operational Capability - IOC
Integer Cycle Ambiguity
International Atomic Time - TAI
Ionosphere
Ionospheric Correction
Ionospheric Delay
Ionospheric Map Exchange Format - IONEX
Ionospheric Model
Jamming Resistance
Julian Day - JD
Kepler's First Law of Planetary Motion
Kepler's Second Law of Planetary Motion
Kepler's Third Law of Planetary Motion
Kinematic Positioning
Klobuchar Model
Known Baseline
L1 Carrier
L2 Carrier
L2 Civil Long - L2 CL
L2 Civil Moderate - L2 CM
L5 Carrier
Laser Range Finder - LRF
Last Known Location
Leap Second
Life Span
Local Position Determining Entity - LPDE
Location Based Services - LBS
Location Monitoring
Long Range Aid to Navigation - LORAN
Low Earth Orbit - LEO
Marine Navigation
Master Control Station - MCS
Medium Earth Orbit - MEO
Mesosphere
Mobile Position Center - MPC
Momentum Wheel
Monitor Station
Multichannel Receiver
Multi-Channel Receiver
Multipath Effect
Multipath Mitigation
Navigation Warfare - NAVWAR
Navigation Warfare - NAVWAR
Near Far Problem
Occupation Time
Off Network Connections
Offset Function
Orbital Period
Perigee
Personal Navigation
Phase Windup
Photogrammetry
Position Determining Entity - PDE
Position Tracking
Positioning Methods
Post Processed Kinematic - PPK
Post Processing - Postprocessing
Precise Code - P-Code
Primary Phase Factor - PF
Propagation Time
Precise Positioning Service - PPS
Precision Farming
Propagation Time Measurement
Pseudorange Measurements
Pseudo Satellites - Pseudolites
Q5-Code
Radio Detection And Ranging - Radar
Range Error
Ranging Code
Receiver Clock Offset
Receiver Velocity
Reference Station
Remote Receiver
Repeater Station
Residual Errors
Rover
Rover Receiver
Satellite Acquisition
Satellite Almanac
Real Time Kinematic GPS - RTK GPS
Receiver Initialization
Relative Positioning
Rover Station
Satellite Acquisition
Satellite Attitude Control System - ACS
Satellite Clocks
Satellite Constellation
Satellite Health
Satellite Perturbing Forces
Satellite Power System
Satellite Tracking
Satellite Visibility
Seafloor Mapping
Seismic Surveying
Single Frequency GPS Receiver
Standard Positioning Service - SPS
Structure Deformation Monitoring
Vehicle Navigation
Volume Surveying
Workforce Location Tracking
 

   Global Positioning System - GPS Books

more details

Global Positioning System - GPS Book This book covers satellite position location technology and the GPS system has evolved. You will learn the functional parts of GPS systems, how they work and work together to provide position measurements that are accurate to within centimeters. $19.99 Printed, $16.99 eBook

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