Introduction to GPS

Introduction to GPS : 

Introduction to GPS

What is the GPS? : 

What is the GPS? Orbiting navigational satellites Transmit position and time data Handheld receivers calculate latitude longitude altitude velocity Developed by Department of Defense

History of the GPS : 

History of the GPS 1969—Defense Navigation Satellite System (DNSS) formed 1973—NAVSTAR Global Positioning System developed 1978—first 4 satellites launched

History of the GPS : 

History of the GPS 1993—24th satellite launched; initial operational capability 1995—full operational capability May 2000—Military accuracy available to all users

Three Segments of the GPS : 

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

Components of the System : 

Components of the System Space segment 24 satellite vehicles Six orbital planes Inclined 55o with respect to equator Orbits separated by 60o 20,200 km elevation above Earth Orbital period of 11 hr 55 min Five to eight satellites visible from any point on Earth

The GPS Constellation : 

The GPS Constellation

Control Segment : 

Kwajalein Atoll US Space Command Control Segment Hawaii Ascension Is. Diego Garcia Cape Canaveral

User Segment : 

Military. 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

Four Basic Functions of GPS : 

Position and coordinates. The distance and direction between any two waypoints, or a position and a waypoint. Travel progress reports. Accurate time measurement. Four Basic Functions of GPS

Working of GPS : 

Working of GPS Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.

Determining Position : 

Determining Position A GPS receiver "knows" the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect.

Signal From One Satellite : 

Signal From One Satellite The receiver is somewhere on this sphere.

Signals From Two Satellites : 

Signals From Two Satellites

Three Satellites (2D Positioning) : 

Three Satellites (2D Positioning)

Three Dimensional (3D) Positioning : 

Three Dimensional (3D) Positioning

Components of the System : 

Components of the System User segment GPS antennas & receiver/processors Position Velocity Precise timing

GPS Communication and Control : 

GPS Communication and Control

How does GPS work? : 

How does GPS work? Satellite ranging Satellite locations Satellite to user distance Need four satellites to determine position Distance measurement Radio signal traveling at speed of light Measure time from satellite to user

Ideal Satellite Geometry : 

Ideal Satellite Geometry N S W E

Good Satellite Geometry : 

Good Satellite Geometry

Good Satellite Geometry : 

Good Satellite Geometry

Poor Satellite Geometry : 

Poor Satellite Geometry N S W E

Poor Satellite Geometry : 

Poor Satellite Geometry

Poor Satellite Geometry : 

Poor Satellite Geometry

Schematics of a GPS Receiver : 

Schematics of a GPS Receiver

GPS Receiver Selection : 

GPS Receiver Selection Application Operational Environment Accuracy Power Requirement Cost Data Exchange Format (RINEX – Receiver Independent Exchange Format)

Some Definitions : 

Some Definitions SPS - Standard Positioning System (uses C/A code) PPS - Precise Positioning System (uses P code) Almanac Data - Stored Ephemiris data in the receiver Carrier Phase Measurements - Difference in Phase between Satellite and Receiver and is not constant due to changing Sat – Receiver geometry. This is resolved in the receiver by post processing of data GPS Ephemiris + Post Processed Data = Precise Ephemiris

Advantages and Disadvantages : 

Advantages and Disadvantages Accuracy Positioning Any Time/Any Climate Less Man Work Less Time Inaccessibility Errors Initial Cost Skilled User Battery Life

Different Surveys and Accuracies : 

Different Surveys and Accuracies Survey A – Scientific Survey (< 1ppm) Survey B – Geodetic Survey (1-10ppm) Survey C – Commercial Surveys (>100ppm)

ERRORS : 

ERRORS Selective Availability Satellite Geometry Satellite Orbits Multipath Effect Atmospheric Effects Clock Inaccuracies and Rounding Errors

Selective Availability : 

Selective Availability Artificial falsification of the time in the signal transmitted by the satellite. The ephemeris data are transmitted with lower accuracy. In this way an inaccuracy of the position of 50 – 150 m can be achieved for several hours.

Satellite Geometry : 

Satellite Geometry If a receiver sees 4 satellites and all are arranged for example in the north-west, this leads to a “bad” geometry and causes an error up to 150 m. DoP – Dilution of Precision

Satellite Orbits : 

Satellite Orbits Although the satellites are positioned in very precise orbits, slight shifts of the orbits are possible due to gravitation forces of Sun, Moon and other celestial bodies.

Multipath Effect : 

Multipath Effect The multipath effect is caused by reflection of satellite signals (radio waves) on objects.  For GPS signals this effect mainly appears in the neighbourhood of large buildings or other elevations

Atmospheric Effects : 

Atmospheric Effects The ionosphere at a height of 80 – 400 km have large number of electrons and positive charged ions due to the ionizing force of the sun. This layer refract the electromagnetic waves from the satellites, resulting in an elongated runtime of the signals. These errors are mostly corrected by the receiver by calculations.

Clock Inaccuracies and Rounding Errors : 

Clock Inaccuracies and Rounding Errors Despite the synchronization of the receiver clock with the satellite time during the position determination, the remaining inaccuracy of the time still leads to an error of about 2 m. Rounding and calculation errors of the receiver sum up approximately to 1 m.

Sources of Signal Interference : 

Sources of Signal Interference

Sources of GPS Error : 

Sources of GPS Error Standard Positioning Service (SPS ): Civilian Users Source Amount of Error Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability - P & C/A code depends User error: Up to a kilometer or more Errors are cumulative and increased by PDOP.

Types of Positioning : 

Types of Positioning Static Positioning Kinematic Positioning

Static Positioning : 

Static Positioning Static Positioning

Slide 42: 

Rapid Static

Kinematic Positioning : 

Kinematic Positioning Stop and Go

Slide 44: 

True Kinematics