Introduction to the NAVSTAR Global Positioning System GPS

Introduction to the NAVSTAR Global Positioning System (GPS)

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

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

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: l l l Space Segment Control Segment User Segment Slide 4

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

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

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

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

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

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

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 Slide 11

Triangulation Satellite 1 Satellite 3 Satellite 2 Satellite 4 Slide 12

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

Signal Structure L 1 Carrier Wave 1575. 42 MHz C/A Code 1. 023 MHz Navigation Message 50 Hz Precise Code 10. 23 MHz Slide 14

Signal Structure L 2 Carrier Wave 1227. 6 MHz Navigation Message 50 Hz Precise Code 10. 23 MHz Slide 15

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

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

3 vs 4 Satellites Slide 18

Satellite Locations Cartesian Coordinate System u 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 l l l X Axis Coordinate Distance in meters from the prime meridian at the origin; positive from 90º E Long to 90º W Long Y Axis Coordinate Distance in meters from 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 Hemisphere negative in the southern X 0º Long Z (X, Y, Z) Y 90°E Prime Meridian Slide 19

Common Problems - Errors r na Re fle c Di t. S c e Sig i R GPS Antenna al n g ted d cte e l f e al n g Si l Satellite Hard Surface Slide 20

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

Anti-Spoofing (A-S) • Protects military receivers from receiving a “fake” P-Code • P-Code modulation on both L 1 and L 2 P-Code +W-Key Y-Code • No plans to phase out • Continuously on since January 31, 1994

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 • Do. D working on electronic warfare enhancements to deny disruption and spoofing. – Direct Y-Code Receivers – Theater jamming capability

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

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

GPS Multipath Errors Effects of Multipath on the GPS Signal l gn a S Re GPS Antenna fle ct Di t c e r l a ign Si ed t c le Ref al n Sig ed • Avoid Reflective Surfaces • Use A Ground Plane Antenna • Use Multipath Rejection Receiver Satellite Hard Surface Slide 26

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

Dilution Of Precision (3) u PDOP = Position Dilution Of Precision (Most Commonly Used) u VDOP = Vertical Dilution Of Precision u GDOP = Geometric Dilution Of Precision u HDOP = Horizontal Dilution Of Precision u TDOP = Time Dilution Of Precision g n i in n QUALITY DOP n bta a l O P Very Good 1 -3 n to P o i al DO Good 4 -5 s s tic d i Fair 6 M ri oo C G Suspect >6 Is Slide 28

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

Scatter plot of horizontal accuracy 2 May 2000 Slide 30

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

GPS Accuracy - PPS Specifications and Derived Values PPS CEP/50 % DRMS 2 DRMS/95% Position Horizontal 8 m 10. 5 m 21 m Vertical 9 m 14 m 28 m Spherical 16 m 18 m 36 m 0. 07 m/sec 0. 1 m/sec 0. 2 m/sec GPS 17 nsec 26 nsec 52 nsec UTC 68 nsec 100 nsec 200 nsec Velocity Any Axis Time

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

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

DGPS Positioning

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

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

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 $30 M each • Launches began Jan 97

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 – Do. T option to add L? ? ? & L? ? ? Frequencies – Unique ground control (more autonomous)

GLONASS u 8 l l SVs in each of 3 inclined circular orbits 11. 25 hour period 19, 900 km altitude u Life time of SVs is 3 years u Uses SGS-85 Datum (within 20 m of WGS-84) u Five Satellites visible at all points on the globe u Satellites broadcast 2 signals l l Standard Precision Navigation Signal (Civil) High Precision Navigation Signal (military)

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