TERRESTRIAL REFERENCE SYSTEMS FOR GLOBAL NAVIGATION SATELLITE SYSTEMS

1. TERRESTRIAL REFERENCE SYSTEMSFORGLOBAL NAVIGATION SATELLITE SYSTEMS James A. Slater Basic and Applied Research Office National Geospatial-Intelligence Agency National Space-Based PNT Advisory Board Meeting October 4, 2007

2. Objectives of a “Reference System” • Satisfy the need to answer the questions: • Where am I (at some instant in time)? • What is the location of some object or someone else? • In absolute terms or in relative terms and at varying accuracies • For the military: • Missile launch sites, precision weapons and targets • Landmines • Battlespace coordination • For the general civilian population: • International borders • Car, ship or plane navigation • Mineral resources • For the scientific community: • Crustal motion • Sea level change • Satellite orbits

3. Need for a Terrestrial Reference System • Create a foundational structure that we call a Terrestrial Reference System to quantitatively and consistently specify position locations • Define a set of conventions, constants, models and parameters which form the mathematical basis for representing locations on, above or below the Earth. • Example: Construct 3-dimensional coordinate system, fixed to the Earth, with its origin at the Earth’s center of mass, oriented with the equator and the prime meridian. Model figure of the Earth as an ellipsoid that rotates with the Earth, whose center coincides with coordinate system origin, and whose axes are aligned with coordinate system axes.

4. Need for a Standard Global Terrestrial Reference SystemWhat happens if every country implements a different version of a geodetic reference system?

5. International Terrestrial Reference System • Scientific community rigorously establishes international standard for terrestrial reference system • International Earth Rotation and Reference Systems Service (IERS) maintains the standard • International Terrestrial Reference Frame (ITRF) defined (realized) to be • Geocentric coordinate system • Aligned close to mean equator of 1900 and Greenwich meridian (coordinate axes oriented to the BIH Terrestrial System at 1984.0 for historical consistency) • Set of reference points on topographic surface of the Earth • Based on multiple data sources: • Very Long Baseline Interferometry (VLBI) • Satellite Laser Ranging (SLR) • GPS • DORIS • Reference station coordinate solutions and velocities define frame at specific time • Solutions based on consistent set of conventions, constants and models

6. U.S. Department of Defense World Geodetic System (WGS) • Global Geocentric Terrestrial Reference System • 1950s • early space exploration offered first global view • satellite tracking and ICBMs required global coordinate systems • WGS 1960 provided first standard global coordinate system for Dept. of Defense (DoD) • WGS 1966 and 1972 answered the need for greater accuracy and broader application to DoD requirements • WGS 1984 represented significant improvement • DoD World Geodetic Systems have always conformed to and adopted international standards • Applied to all DoD products and services – maps, charts, airfields, features data, topography, satellite orbits, real-time positioning,…

7. Department of Defense World Geodetic System ω • Earth-Centered Earth-Fixed Coordinate System • Adopted ITRF definition • Standard Earth Model • Ellipsoid with mass and rotation rate of Earth • Center coincides with coord. system origin and axes coincide with those of coord. system • Earth Gravitational Model (EGM) • Mathematical representation of the gravitational field (current version EGM96, next version EGM07) • Global “mean sea level” surface (i.e. elevation = 0) for referencing topographic elevations (“geoid” surface) • International Standard Physical Constants and Models Adopted • Examples: Flattening (f) and semi-major axis of ellipsoid (a), speed of light (c), gravitational constant (GM), Earth rotation rate (ω), precession, nutation, … b a GM

8. “Realization” of WGS 84 Reference Frame • Defined (realized) by the coordinates of a globally-distributed set of reference points on the topographic surface of the Earth – constituted solely by a network of “permanent” GPS stations • WGS 84 reference frame periodically adjusted to maintain close alignment to ITRF: Positions of the reference points (DoD monitor stations) are estimated using GPS observations at these points combined with simultaneously-collected data from Int’l GNSS Service (IGS) stations roughly as follows: • Given: • High level of consistency between the WGS and ITRS conventions, constants and models • Known ITRF coordinates of IGS stations • Hold IGS station coordinates fixed* in the computations, solve forDoD station positions* and GPS satellite orbit parameters • Result: DoD station coordinates and by definition WGS 84 reference frame is coincident with the ITRF within some level of uncertainty *Note: Plate tectonic motion is accounted for.

9. DoD WGS 84 (G1150) Reference Stations

10. IGS Reference Stations for WGS 84 (G1150)

11. Operational GPS Orbits from DoD Refinements of WGS 84 Reference Frame (reference positions): • WGS 84 (G730) – June 1994  ~ 10 cm accuracy • WGS 84 (G873) – January 1997  ~ 5 cm accuracy • WGS 84 (G1150) – January 2002  ~ 1-2 cm accuracy Operational Implementation: • GPS observations from only DoD station network (NGA + AF) • DoD station coordinates ‘fixed’ to (ITRF-aligned) WGS 84 coordinates in the orbit computation • Result – Precise orbits and broadcast orbits in WGS 84 reference frame • Positioning and navigation based on these orbits  WGS 84 position coordinates (alternate realization of reference frame) WGS 84 Adopted by NATO, ICAO, and IHO

12. Exploiting GNSS in the Future – StandardizationMultiple Constellations • Want to optimize positioning and navigation performance from multiple constellations and signals • So many choices… • GPS III – new satellites with more signals • GLONASS – new satellites with more satellites • Galileo – new global constellation • Compass – new global and regional satellites • Space-based augmentations from India, Japan and U.S. WAAS • Users and Manufacturers want: • Interoperability, Compatibility and Standardization • Improved availability, Improved integrity and Higher accuracy (we hope)  Real-time, seamless operation • Standardization should start with: • Common geodetic reference frame • Common time reference

13. Effect on GLONASS Broadcast Orbits from Standardization of GLONASS Terrestrial Reference Frame PZ90.02 with ITRF2000 (Sept. 20, 2007)

14. Exploiting GNSS in the Future – Quality Assurance and Enhanced Performance for GPS III Long-term geodetic objectives: 1. Achieve a stable geodetic reference frame with an accuracy at least 10 times better than the anticipated user requirements for positioning, navigation, and timing. 2. Maintain a close alignment of the WGS 84 reference frame with the International Terrestrial Reference Frame (ITRF). 3. Provide a quality assessment capability independent of current radiometric measurements used to determine GPS orbit and clock performance. 4. Ensure interoperability of GPS with other Global Navigation Satellite Systems (GNSS’s) (e.g., GLONASS, Galileo) through a common, independent measurement technique. A Case for Laser Retro-reflectors on GPS III: • Help achieve long-term geodetic objectives • Achieve compatibility with GLONASS and Galileo • Allow direct ties between GPS and SLR reference frames • Contribute to identification of anomalous satellite behavior and improved modeling of long-term and long wavelength effects on satellite orbits • Potentially improve in combination of GPS, GLONASS and Galileo data resulting in improved positioning and navigation

15. Summary • A GlobalStandardTerrestrial Reference System is critical to future positioning and navigation with Global Navigation Satellites. • Exploitation of multiple systems to support increased demands of a wide range of users (millimeters to 10s of meters) and long-term stability would be facilitated by, if not require, use of interoperable reference systems consistent with conventions, constants and models of the International Terrestrial Reference System. • Accordingly, • The WGS 84 reference frame has been and will continue to be periodically re-aligned to the ITRF. • Galileo plans to define its operational reference frame based on the ITRF. • GLONASS has recently redefined its realization of the PZ90.02 reference frame based on the ITRF. • Other GNSS’s should be encouraged to do the same.