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 Earths 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

• 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
  (?), 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-collecte
  d data from Intl 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
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 (GNSSs)
  (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 GNSSs should be encouraged to do the same.