TERRESTRIAL REFERENCE SYSTEMS FOR GLOBAL NAVIGATION SATELLITE SYSTEMS - PowerPoint PPT Presentation

About This Presentation
Title:

TERRESTRIAL REFERENCE SYSTEMS FOR GLOBAL NAVIGATION SATELLITE SYSTEMS

Description:

TERRESTRIAL REFERENCE SYSTEMS FOR GLOBAL NAVIGATION SATELLITE SYSTEMS James A. Slater Basic and Applied Research Office National Geospatial-Intelligence Agency – PowerPoint PPT presentation

Number of Views:64
Avg rating:3.0/5.0
Slides: 16
Provided by: Slat87
Learn more at: https://www.gps.gov
Category:

less

Transcript and Presenter's Notes

Title: 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.
Write a Comment
User Comments (0)
About PowerShow.com