Technological Principles and Policy Challenges of the Global Positioning System Marlee Chong

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Technological Principles and Policy Challenges of the Global Positioning System

  • Marlee Chong

  • May 6, 2013


  • History

  • Technology

  • Applications

  • Policy


  • History

  • Technology

  • Applications

  • Policy

Where am I? Where am I going?

  • Landmarks

  • Dead reckoning

  • Coordinate system

    • Latitude
    • Longitude


  • Size of the Earth

  • Describing celestial and planetary motion

  • Timekeeping

  • Measurement in motion

  • Reducing error


  • Long Range Aid to Navigation (LORAN)


  • Limitations

    • Accurate radionavigation required remaining within the line of sight
    • Transit required a latitude to fix position and 10-15 minutes of processing time: too slow for aircraft
  • Transit (APL, 1960): satellite navigation using orbits

  • Timation (US Navy, 1964): stable timing of space-based satellite clocks

  • System 621B (US Air Force, 1963): digital signals and global coverage

Developing GPS

  • Defense Navigation System (NAVSTAR) Global Positioning System

    • Designed for civil and military use to “drop five bombs in the same hole” for less than $10,000
    • Department of Defense combined elements from Transit, Timation, and System 621B
    • Spearheaded by US Air Force

Draper Prize

  • Awarded in 2003 “for their technological achievements in the development of the Global Positioning System (GPS)”

  • Bradford Parkinson and Ivan Getting


  • History

  • Technology

  • Applications

  • Policy


  • History

  • Technology

  • Applications

  • Policy


  • System Elements

  • Signal

  • Ranging

  • Vulnerabilities

System Elements

Space Segment

  • 24-31 satellites identified by space vehicle number (SVN) and PRN code

  • 20,200 km altitude

  • 12 hour cycle

  • 6 planes of

  • 4 satellites

  • Solar powered with

  • backup batteries

  • Rocket fuel limits lifespan

Constellation considerations

  • Global coverage

  • 4 satellite minimum, 6 practical standard in case of anomalies

  • Good geometrically distribution

  • Robust if a satellite fails

  • Inexpensive repositioning

  • Minimal maneuvering to remain in orbit

  • Reduce tradeoffs of power requirements by distance

Control Segment

  • 2nd Space Operations Squadron of the United States Air Force tracks satellites, monitors transmissions and sends commands and information

  • Master Control Station

  • Monitor Stations

    • 6 USAF
    • 10 National Geospatial-Intellignece Agency sites added in 2008
  • Ground Antennas

    • 4 ground antennas located with monitor stations
    • 8 tracking stations in Air Force Satellite Control Network

Control Segment Map


GPS receivers

  • Single- or dual- frequency receivers access L1 or L1 and L2 carrier frequencies

  • Tracking channels (9-12) track specific satellites


  • Unique Psuedo-random noise (PRN) Codes

    • Coarse Acquisition
    • (C/A code) L1
    • Precision Code
    • (P code) L1 and L2
  • Microwave signals

    • L2 = 120 ƒ
    • L1 = 154 ƒo
  • Clock carrier frequency

    • ƒo=10.23 Hz

Security Designs

  • Anti-Spoofing (AS)

    • P-code encrypted as Y-code; L2 unavailable
  • Selective Availability (SA)

    • March 25, 1990-May 1, 2000
    • Provided civilians with Standard Positioning Service, compared to military standard Precise Positioning Service
    • Decreased performance by factor of 7
      • Delta error: dither (add random noise to) all satellite clocks
      • Epsilon error: slowly varying orbital errors almost identical for users with short separation distances


Psuedorange errors

  • p=c[(Tu’ + tu)-(Ts+∂t)=∆t+c(tu-∂t+∂tD)

  • tu = receiver clock error

  • ∂t= satellite clock error

    • ∂t= clock bias + clock drift * (t-tclock reference time) + frequency drift* (t-tclock reference time)^2+relativistic correction
    • Relativistic correction: ~ 4.5*10^-10
      • SR ~ -8*10^-11; GR ~ 5*10^-10
      • Adjust satellite clock frequency by correction
  • ∂tD=∂tatmosphere +∂tnoise+∂tmultipath+ ∂thardware

Atmospheric errors

  • Ionospheric divergence

  • Signal info delayed

    • np=1-c2/f2-c3/f3-…
    • vp=c/np
  • Carrier phase early

    • ng=1+c2/f2+c3/f3+…
    • vg=c/ng

Atmospheric errors

  • Troposphere

  • Nondispersive

  • Refractivity of hydrostatic and nonhydrostatic components

Geometric Dilution of Precision

  • Effect of satellite geometry in estimating range error propagation


  • Signal

  • Infrastructure

  • Incidental


  • Commercially available parts and publicly available directions


  • Damage to control center, satellite network, and cyber attacks

    • Alternate master control center
    • Extra satellites in orbit


  • Target: ubiquitous and well-known

  • Spectrum interference

  • Solar flares: thickens ionosphere

    • Magnetic storm:
    • solar wind interacts
    • with magnetic field
    • uV increases,
    • ionizing and heating
    • thermosphere


  • Receivers and users detecting anomalies

  • Firewalls and cyber security

  • Alternative technologies and redundancy systems for must vulnerable and critical


  • History

  • Technology

  • Applications

  • Policy


  • History

  • Technology

  • Applications

  • Policy


  • Original mission: military navigation technology for positioning and navigation

  • Designed with dual use in mind

Assessment Scheme

  • Highly Critical: application able to perform some functions with alternative technologies and/or systems after severe disruption and consequences

  • Moderately Critical: application able to perform most functions with existing alternative technologies and/or systems with compromised accuracy and precision

  • Not Critical: application able to perform all functions and can utilize alternative technologies and/or systems with minimal disruption


  • Smart Bombs: ex. Joint Direct Attack Munition

    • JDAM receiver finds
    • position of bomb
    • Aircraft receiver finds
    • position of target
    • Guidance kit monitors
    • bomb position as
    • control computer
    • adjust tail fins
    • 40 feet accuracy

Highly Critical Positioning

  • Inertial guidance system

  • as an alternative

  • Loss of accuracy and

  • precision leads to

  • collateral damage

Nuclear Test Detection

  • US Atomic Energy Detection System

    • Global network of sensors to detect nuclear events operated by US Air Force
    • Sensors aboard the GPS
    • satellites monitor space
    • and the atmosphere

Moderately Nuclear Test Critical Detection

  • GPS system failure likely leaves satellites and sensors in orbit

  • Other satellites could

  • host future sensors


  • US Geological Survey measures the relative position of stations relative near active faults

    • Calculate strain, slip and ground deformation

Not Critical Positioning

  • USGS could easily substitute other methods to conduct surveys, such as LORAN-C, because the time resolution is large


  • Mobile phones with built-in receivers

    • Assisted GPS uses carrier network
  • Mobile apps

  • can access

  • GPS data

Moderately Navigation

  • Triangulation from cell towers

  • Vibrant community to code patches

  • or other apps

  • Spoofing

  • apps change

  • coordinates

  • read by other

  • apps


Highly Critical Timing

  • Without GPS

    • Trades not synchronized
    • Trading houses cannot obtain time from cellular networks from GPS
  • Without local receiver

    • Internet NTP has 2s offset
    • Resolution disrupts high frequency trades

Weather Forecasting

  • Radio Occultation

    • Measure atmospheric density from the refraction of signal passing through atmosphere to a low-Earth orbit satellite
  • Can compensate for lack of meteorological observation stations for oceans and the poles

Moderately Weather Critical Forecasting

  • Less complete models

    • Accuracy issues regardless
  • Less precise instrument calibration

  • Alternatives such as satellite imaging


  • History

  • Technology

  • Applications

  • Policy


  • History

  • Technology

  • Applications

  • Policy


  • Mandate

  • Governance

  • Funding

  • Interoperability

  • Privacy

  • Spectrum protection

  • Replacement?


  • Presidential policy (2010 National Space Policy)

    • “The United States must maintain its leadership in the service, provision, and use of global navigation satellite systems”
  • Law (Title 10 of the U.S. Code, Section 2281)

    • Created in National Defense Authorization Act for Fiscal Year 1998
    • Responsibility of Secretary of Defense


  • National Executive Committee for Space-Based Positioning, Navigation, and Timing

    • National Coordination Office runs policy with working groups
    • NASA advisory board
    • Defense (co-chair), Transportation (co-chair), State, Interior, Agriculture, Commerce, Homeland Security, Joint Chiefs of Staff, NASA

Funding: $1.5 billion

  • Dept of Defense

  • Develop

  • Acquire

  • Operate

  • Sustain


  • Replacements delayed

    • Relaxed, insufficient oversight
  • Options

    • Use retired satellites
    • Speed up production
    • Power management of secondary missions


  • UN’s International Committee on Global Navigation Satellite Systems (ICG)

    • Compatibility and interoperability
    • Cooperation: coordinate and share information and save costs
    • Position, navigation and timing information helps society’s security and environment
  • Working groups and individual agreements


  • United States vs Jones (Jan 2012)

    • Supreme Court: warrant needed to secretly install a tracker
    • United States vs Katzin
      • Challenging decision regarding tracking, which was not considered a search
  • Geolocation Privacy and Surveillance Act [Sen. Wyden D-Ore, Rep. Chaffetz (R-Utah), and Sen. Kirk (R-Ill)]

    • Introduced to Senate in 2011, House in May 2012
    • Proposes a standard for government access

Spectrum protection

  • Commercial products: FCC and NTIA

    • Lightsquared barred from using neighboring spectrum
    • Importing, marketing, selling, and operating jammers is illegal (up to $100,000 fine)


  • DARPA’s All Source Positioning and Navigation (ASPN)

    • Phase 2 June 2012 to design algorithms
    • “low cost, robust, and seamless navigation solutions for military users…with or without GPS”


  • Thank you all for coming

  • Special thanks to Prof. Venky Narayanamurti and Dr. Tolu Odumosu for their mentorship and assistance

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