1. TERRESTRIAL REFERENCE SYSTEMS FOR GLOBAL 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 System What 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 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
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, … ω 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 for DoD 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
This is a more detailed view of the WGS 84 (G1150) TRF stations.
NIMA DGRS refers to the Differential GPS Reference Stations used by NGA field survey groups in very precise positioning in support of DoD test range activities.
NIMA MSN Test stations include one in St. Louis and one In Austin, TX The St. Louis station is used primarily for training NGA personnel and testing new monitor station software deployments. The Austin station is used by the Applied Research Laboratories of the University of Texas at Austin (ARL:UT) in the development and initial testing of new monitor station hardware and software. Data from these stations are not generally used in the computing NGA precise ephemerides for the GPS satellites.
The Naval Surface Warfare Center Dahlgren Division (NSWCDD) stations are used for developmental work by that organization.
All of the NIMA MSN stations will eventually feed data to the GPS Master Control Stations for use in integrity monitoring and filter computations.

10. IGS Reference Stations for WGS 84 (G1150)
These are the IGS stations with ITRF2000 positions that were used to “float” WGS 84 into better alignment with the ITRF and “tighten-up” the WGS 84 TRF accuracy. Most of these stations were held fixed at the ITRF2000 positions during the G1150 solution. Note that station velocities were used to “move” the stations to the same time as the GPS observations used to compute the G11500 solution.

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 – Standardization Multiple 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 Global Standard Terrestrial 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.