Introduction to the NAVSTAR Global Positioning System (GPS)

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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
• 
  

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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

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Space Segment Description

• 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

• Very high orbit
• 20,200 km
• 1 revolution in approximately 12 hrs
• For accuracy
• Survivability
• Coverage

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Control Segment
Monitor and Control
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Control Segment

• Correct Orbit
• and clock
• errors
• Create new
• navigation message

• Observe
• ephemeris
• and clock

Upload Station
(5) Monitor Stations
Falcon AFB
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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
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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

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How the system works
Space Segment 24 Satellites

• Monitor Stations
• Diego Garcia
• Ascension Island
• Kwajalein
• Hawaii
• Colorado Springs

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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

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Triangulation
Satellite 1
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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
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Signal Structure
L1 Carrier Wave 1575.42MHz
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Signal Structure
L2 Carrier Wave 1227.6MHz
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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

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Measuring Travel Time
Time Adjustment
1
2
X
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3 vs 4 Satellites
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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)
Y90E
X0º Long
Prime Meridian
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Common Problems - Errors
Reflected Signal
Direct Signal
Reflected Signal
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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.)

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Anti-Spoofing (A-S)
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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

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Common Problems - Errors
Pseudo-Ranging Errors

• Satellite clock (S/A)
• Ephemeris/orbit (S/A)
• Atmospheric delays Ionosphere Troposphere
• Receiver computation
• noise

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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

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GPS Multipath Errors
Effects of Multipath on the GPS Signal

• Avoid Reflective Surfaces
• Use A Ground Plane Antenna
• Use Multipath Rejection Receiver

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Dilution Of Precision (DOP)
A Measure of The Geometry Of The Visible GPS
Constellation
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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
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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

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Scatter plot of horizontal accuracy 2 May 2000
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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

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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
16 m
18 m
36 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
100 nsec
200 nsec
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Error and Map Problems
50 m Map Error
Map coordinate determined by terrain association
X
X
GPS coordinate plotted on map
21m GPS Error
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Differential GPS
Types of Differential Coverage

• Coverage
• Local Area (Coast Guard)
• Wide Area (INMARSAT)
• Methods
• Real-Time (navigation/mapping)
• Post Processing (survey)

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DGPS Positioning
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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)

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

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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)

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GLONASS

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

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Summary

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