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Introduction to the NAVSTAR Global Positioning System (GPS)

Introduction to the NAVSTAR Global Positioning System (GPS). Agenda. GPS Lineage What is GPS How Does It Work Errors and Accuracy's in the GPS system Future Initiatives . GPS Lineage. Phase 1 : 1973-1979 CONCEPT VALIDATION 1978- First Launch of Block 1 SV Phase 2 : 1979-1985

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Introduction to the NAVSTAR Global Positioning System (GPS)

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  1. Introduction to the NAVSTAR Global Positioning System (GPS)

  2. Agenda • GPS Lineage • What is GPS • How Does It Work • Errors and Accuracy's in the GPS system • Future Initiatives

  3. GPS Lineage • Phase 1: 1973-1979 CONCEPT VALIDATION 1978- First Launch of Block 1 SV • Phase 2: 1979-1985 FULL DEVELOPMENT AND TESTS • Phase 3: 1985-Present PRODUCTION AND DEPLOYMENT 1993-IOC 1995-FOC

  4. What is GPS The Global Positioning System (GPS) is a Constellation of Earth-Orbiting Satellites Maintained by the United States Government for the Purpose of Defining Geographic Positions On and Above the Surface of the Earth. It consists of Three Segments: • Space Segment • Control Segment • User Segment

  5. Very high orbit 20,200 km 1 revolution in approximately 12 hrs For accuracy Survivability Coverage 24+ satellites 6 planes with 55° Inclination Each plane has 4 or 5 satellites Broadcasting position and time information on 2 frequencies Constellation has Spares Space Segment Description

  6. Colorado Springs Kwajalein Ascension Islands Hawaii Diego Garcia Master Control Station Monitor Station Ground Antenna Control Segment Monitor and Control

  7. Control Segment • Correct Orbit • and clock • errors • Create new • navigation message • Observe • ephemeris • and clock Upload Station (5) Monitor Stations Falcon AFB

  8. User Segment • Over $19 Billion invested by DoD • Dual Use System Since 1985 (civil & military) • Civilian community was quick to take advantage of the system • Hundreds of receivers on the market • 3 billion in sales, double in 2 years • 95% of current users • DoD/DoT Executive Board sets GPS policy PLGR

  9. Common Uses for GPS • Military Specific: • Navigation • Surveying • Target acquisition and destruction • Missile Guidance Systems • Joint Direct Attack Munition (JDAM) • Tomahawk III • Joint Stand Off Weapon (JSOW) • Data Collection • Integration with INS for High dynamic environment • Search & Rescue Ops

  10. The Current Ephemeris is Transmitted to Users End User GPS Control Colorado Springs How the system works Space Segment 24+ Satellites • Monitor Stations • Diego Garcia • Ascension Island • Kwajalein • Hawaii • Colorado Springs

  11. Distance Measuring • The whole system revolves around time!!! • Distance = Rate x Time • Rate = 186,000 miles per second (Speed of Light) • Time = time it takes signal to travel from the SV to GPS receiver

  12. Satellite 2 Satellite 3 Satellite 4 Triangulation Satellite 1

  13. Distance Measuring Transmission Time The Carrier... combined with Satellite The PRN code... produces the Modulated carrier signal which is transmitted... demodulated... Receiver Producing the same code at the user, but delayed... Time delay

  14. C/A Code 1.023 MHz Navigation Message 50Hz Precise Code 10.23 MHz Signal Structure L1 Carrier Wave 1575.42MHz

  15. Navigation Message 50 Hz Precise Code 10.23 MHz Signal Structure L2 Carrier Wave 1227.6MHz

  16. Measuring Travel Time • SV Clocks • 2 Cesium & 2 Rubidium in each SV • $100,000-$500,000 each • Receiver Clocks • Clocks similar to quartz watch • Always an error between satellite and receiver clocks (  t) • Require 4 satellites to solve for x, y, z, and  t

  17. 4 YY ZZ 8 nano seconds (wrong time) XX 3 9 nano seconds 7 nano seconds (wrong time) (wrong time) Measuring Travel Time Time Adjustment 1 2 X

  18. 3 vs 4 Satellites

  19. Satellite Locations Cartesian Coordinate System • Three dimensional right coordinate system with an origin at the center of the earth and the X axis oriented at at the Prime Meridian and the Z at the North Pole • X Axis Coordinate Distance in meters from the the prime meridian at the origin; positive from 90º E Long to 90º W Long • Y Axis Coordinate Distance in metersfrom 90º E longitude at the origin; positive in the eastern hemisphere and negative in the western • Z Axis Coordinate Distance in meters from the plane of the equator; positive in the northern Hemispherenegative in the southern Z (X,Y,Z) Y90°E X0º Long Prime Meridian

  20. Satellite GPS Antenna Hard Surface Common Problems - Errors Reflected Signal Direct Signal Reflected Signal

  21. Selective Availability (S/A) • Implemented on Block II satellites, but turned off 2 May 2000 for the foreseeable future: • Requires military to develop Direct Y Code receivers and local jamming capability • Introduces deliberate errors into satellite ephemeris (SV location) and clock parameters on the C\A code • Degrades horizontal positional accuracy to 100m 2DRMS (95% of the time.)

  22. Protects military receivers from receiving a “fake” P-Code • P-Code modulation on both L1 and L2 P-Code +W-Key Y-Code • No plans to phase out • Continuously on since January 31, 1994 Anti-Spoofing (A-S)

  23. Resistance to Jamming • Low power signal is vulnerable to jamming • Intentional or unintentional jamming • Theater wide jamming • Local area jamming • The P-Code is phase modulated to provide better resistance to jamming • DoD working on electronic warfare enhancements to deny disruption and spoofing. • Direct Y-Code Receivers • Theater jamming capability

  24. Common Problems - Errors Pseudo-Ranging Errors • Satellite clock (S/A) • Ephemeris/orbit (S/A) • Atmospheric delays Ionosphere Troposphere • Receiver computation & noise

  25. Common Problems - Errors • Errors Caused By GPS Multipath Reflections • Use Ground Plane On Antenna • Move Away From Reflective Surfaces • Influences on the GPS Signal • Radar • Microwave • ILS or Radio NDB Equipment • ATC Radio Traffic • Misidentification of Thresholds and Other Features

  26. Satellite Reflected Signal Direct Signal Reflected Signal GPS Antenna Hard Surface GPS Multipath Errors Effects of Multipath on the GPS Signal • Avoid Reflective Surfaces • Use A Ground Plane Antenna • Use Multipath Rejection Receiver

  27. Good DOP Poor DOP Dilution Of Precision (DOP) A Measure of The Geometry Of The Visible GPS Constellation

  28. QUALITY DOP Very Good 1-3 Good 4-5 Fair 6 Suspect >6 Dilution Of Precision (3) • PDOP = Position Dilution Of Precision (Most Commonly Used) • VDOP = Vertical Dilution Of Precision • GDOP = Geometric Dilution Of Precision • HDOP = Horizontal Dilution Of Precision • TDOP = Time Dilution Of Precision Mission Planning Is Critical to Obtain Good DOP

  29. System Accuracy Standard Positioning Service (SPS) • Available to all users • Accuracy was degraded by Selective Availability until 2 May 2000 • Horizontal Accuracy: 100 meters 2 DRMS (40 meters CEP) • Now has roughly the same accuracy as PPS • Used by military receivers before Y-code lock is established

  30. Scatter plot of horizontal accuracy 2 May 2000

  31. System Accuracy Precise Positioning Service (PPS) • Only available to authorized DoD users • Decryption device and crypto key are required to decode A-S and remove SA • GUV Key (1 year) • CVW Key (1 week) • Accurate to 21m 2DRMS (8 m CEP) • 95% of the time, a receiver's computed horizontal position will be within 21 meters of its actual location

  32. GPS Accuracy - PPS Specifications and Derived Values CEP/50 % DRMS 2DRMS/95% PPS Position Horizontal 8 m 10.5 m 21 m Vertical 9 m 14 m 28 m Spherical 18 m 36 m 16 m Velocity Any Axis 0.07 m/sec 0.1 m/sec 0.2 m/sec Time GPS 17 nsec 26 nsec 52 nsec UTC 68 nsec 200 nsec 100 nsec

  33. Error and Map Problems 50 m Map Error Map coordinate determined by terrain association X X GPS coordinate plotted on map 21m GPS Error

  34. Differential GPS Types of Differential Coverage • Coverage: • Local Area (Coast Guard) • Wide Area (INMARSAT) • Methods: • Real-Time (navigation/mapping) • Post Processing (survey)

  35. DGPS Positioning

  36. DGPS Navigation Coast Guard Differential GPS System • Initial Operational Capability on 30 Jan 96 • Provides pseudo-range corrections over existing radio beacons • Corrections to NAD-83 (WGS-84) • Observed accuracy 1 to 3 meters out to 150 nautical miles from base station • Station sites available on the internet (WWW.NAVCEN.USCG.MIL)

  37. DGPS Positioning Wide Area Differential GPS Error correction message GPS signals Reference receivers Field receiver Real-time Corrections to Remove S/A etc.

  38. Future Developments Planned Replenishments - Block IIR • Some IIR improvements over Block II/IIA SVs: • More power/better batteries (Life EST 7.8 years) • More fuel • Two Atomic clocks on at all times • Re-programmable CPU, more autonomous • Cross Link Ranging - 180 day autonomy with no degradation • 21 SVs purchased from Lockheed Martin at $30M each • Launches began Jan 97

  39. Future Developments Planned Sustainment - Block IIF • Boeing awarded contract for production of 33 Block IIF SVs • Improvements over IIR • Larger Payload (more fuel, power, etc) • 10 year life span • DoT option to add L??? & L??? Frequencies • Unique ground control (more autonomous)

  40. Summary • History • GPS Applications • Three Segments of GPS • 5 Principles of GPS Operations • System Accuracy • Other Satellite Navigation Systems • Future Developments

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