Technological Principles and Policy Challenges of the Global Positioning System

1. Technological Principles and Policy Challenges of the Global Positioning System Marlee Chong May 6, 2013

2. Overview • History • Technology • Applications • Policy

3. Overview • History • Technology • Applications • Policy

4. Where am I? Where am I going? • Landmarks • Dead reckoning • Coordinate system • Latitude • Longitude

5. Challenges • Size of the Earth • Describing celestial and planetary motion • Timekeeping • Measurement in motion • Reducing error

6. Radionavigation Long Range Aid to Navigation (LORAN) Transit Timation System 621B

7. 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

8. 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

9. Draper Prize • Awarded in 2003 “for their technological achievements in the development of the Global Positioning System (GPS)” Bradford Parkinson and Ivan Getting

10. Overview • History • Technology • Applications • Policy

11. Overview • History • Technology • Applications • Policy

12. Technology • System Elements • Signal • Ranging • Vulnerabilities

13. System Elements

14. 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

15. 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

16. 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

17. Control Segment Map http://www.gps.gov/systems/gps/control/

18. GPS receivers • Single- or dual- frequency receivers access L1 or L1 and L2 carrier frequencies • Tracking channels (9-12) track specific satellites

19. 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

20. 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

21. Pseudoranging • Inherently indeterminable due to bias in system and user time measurements Geometric range time ∆t Satellite clock reads time Signal reaches receiver w/o error ∂tD tu ∂t time Ts Signal leaves satellite Ts +∂t T’u Signal reaches receiver T’u +∂t Receiver clock reads time Pseudorange time

22. 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 and interference

23. 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

24. Troposphere Nondispersive Refractivity of hydrostatic and nonhydrostatic components Atmospheric errors

25. Geometric Dilution of Precision • Effect of satellite geometry in estimating range error propagation

26. Vulnerabilities • Signal • Infrastructure • Incidental

27. Signal • Commercially available parts and publicly available directions Jamming • Intentional interference • Detectable and illegal Spoofing • Valid signal with time delay • No easy detection or solutions yet

28. Infrastructure • Damage to control center, satellite network, and cyber attacks • Alternate master control center • Extra satellites in orbit

29. 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

30. Mitigation • Receivers and users detecting anomalies • Firewalls and cyber security • Alternative technologies and redundancy systems for must vulnerable and critical

31. Overview • History • Technology • Applications • Policy

32. Overview • History • Technology • Applications • Policy

33. Applications • Original mission: military navigation technology for positioning and navigation • Designed with dual use in mind

34. 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

35. 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

36. Highly Critical Positioning • Inertial guidance system as an alternative • Loss of accuracy and precision leads to collateral damage

37. 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

38. Moderately Nuclear Test Critical Detection • GPS system failure likely leaves satellites and sensors in orbit • Other satellites could host future sensors

39. Positioning • US Geological Survey measures the relative position of stations relative near active faults • Calculate strain, slip and ground deformation

40. Not Critical Positioning • USGS could easily substitute other methods to conduct surveys, such as LORAN-C, because the time resolution is large

41. Navigation • Mobile phones with built-in receivers • Assisted GPS uses carrier network • Mobile apps can access GPS data

42. Moderately Navigation Critical • Triangulation from cell towers • Vibrant community to code patches or other apps • Spoofing apps change coordinates read by other apps

43. Timing • NYSE and Nasdaq data centers execute and timestamp trades • Network Time Protocol synchronizes servers every second with GPS

44. 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

45. 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

46. ModeratelyWeather Critical Forecasting • Less complete models • Accuracy issues regardless • Less precise instrument calibration • Alternatives such as satellite imaging

47. Overview • History • Technology • Applications • Policy

48. Overview • History • Technology • Applications • Policy

49. Policy • Mandate • Governance • Funding • Interoperability • Privacy • Spectrum protection • Replacement?

50. 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