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

Technological Principles and Policy Challenges of the Global Positioning System

• Marlee Chong

• May 6, 2013

Overview

• History

• Technology

• Applications

• Policy

Overview

• History

• Technology

• Applications

• Policy

Where am I? Where am I going?

• Landmarks

• Dead reckoning

• Coordinate system

• Latitude
• Longitude

Challenges

• Size of the Earth

• Describing celestial and planetary motion

• Timekeeping

• Measurement in motion

• Reducing error

Radionavigation

• Long Range Aid to Navigation (LORAN)

Opportunity

• 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

Overview

• History

• Technology

• Applications

• Policy

Overview

• History

• Technology

• Applications

• Policy

Technology

• 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

• http://www.gps.gov/systems/gps/control/

GPS receivers

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

• Tracking channels (9-12) track specific satellites

Signals

• 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

Pseudoranging

• Inherently indeterminable due to bias in system and user time measurements

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

Vulnerabilities

• Signal

• Infrastructure

• Incidental

Signal

• Commercially available parts and publicly available directions

Infrastructure

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

• Alternate master control center
• Extra satellites in orbit

Incidental

• 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

Mitigation

• Receivers and users detecting anomalies

• Firewalls and cyber security

• Alternative technologies and redundancy systems for must vulnerable and critical

Overview

• History

• Technology

• Applications

• Policy

Overview

• History

• Technology

• Applications

• Policy

Applications

• 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

Positioning

• 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

Positioning

• 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

Navigation

• 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

Timing

• NYSE and Nasdaq data centers execute and timestamp trades

• Network Time Protocol synchronizes servers every second with GPS

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

Overview

• History

• Technology

• Applications

• Policy

Overview

• History

• Technology

• Applications

• Policy

Policy

• Mandate

• Governance

• Funding

• Interoperability

• Privacy

• Spectrum protection

• Replacement?

Mandate

• 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

Governance

• 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

Brownouts

• Replacements delayed

• Relaxed, insufficient oversight

• Options

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

Interoperability

• 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

Privacy

• 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)

Replacement?

• 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”

Acknowledgements

• Thank you all for coming

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