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The Role of the GPS/GNSS in Geodesy and Geodynamics


The Role of the GPS/GNSS in Geodesy and Geodynamics G. Beutler Astronomical Institute, University of Bern Member of IAG Executive Committee and of IGS Governing Board – PowerPoint PPT presentation

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Title: The Role of the GPS/GNSS in Geodesy and Geodynamics

The Role of the GPS/GNSS in Geodesy and
  • G. Beutler
  • Astronomical Institute, University of Bern
  • Member of IAG Executive Committee
  • and of IGS Governing Board
  • National Space-Based Positioning, Navigation, And
    Timing (PNT) Advisory Board
  • Doubletree Hotel, 1515 Rhode Island Avenue,
    Washington, D.C. 20005
  • October 4-5, 2007

The Role of the GPS/GNSS in Geodesy and
  • International Association of Geodesy (IAG) and
    Space Age
  • The International GNSS Service (IGS)
  • History and Development
  • Scope and Products
  • I GPS Service ? I GNSS Service
  • IAG/IGS Expectations concerning different GNSS

Geodesy and the Space Age
  • Geodesy is based on and provides information for
  • geometry and kinematics of/on Earth and in its
  • Earth orientation and rotation, and
  • The Earths gravity field including its
  • It is thus necessary to define, realize and
    maintain unique reference systems on Earth and in
    the sky, and to monitor the transformation
    between them.
  • The space age brought a revolution in geodesy and
    led to the creation of four important services,
  • International Earth Rotation Service (IERS) in
  • IGS (International GNSS Service) in 1991/1994
  • ILRS (Intl. Laser Ranging Service) and IVS
    (Intl. VLBI Service) around the year 2000.

Space Geodesy GNSS
Densification of ITRF, high resolution ERPs,
enabling atmosphere and gravity field
IGS International GNSS Service
  • The creation of the IGS was initiated in 1989
    with I.I. Mueller, G. Mader, B. Melbourne, B.
    Minster and Ruth Neilan as protagonists
  • The IGS became an official IAG service in 1994.
  • The IGS first was a pure GPS Service, it became
    the International GNSS Service in 2004.
  • Today the IGS truly is an interdisciplinary
    service in support of Earth Sciences and Society
    committed to use the data from all GNSS.
  • Since its creation the IGS Central Bureau is
    located in the USA with Ruth Neilan as director
    standing for continuity and leadership.

IGS Development
Monitor station motion in real time
IGS Network in 2007
In 1992 the IGS was based on about 20 geodetic
receivers, 400 receivers are active and their
data retrievable today
IGS Products
  • In 1992 the IGS started off as an orbit
    determination service (dm accuracy) for about 20
    GPS satellites.
  • Today, the IGS provides ephemerides (accuracy of
    2-4 cm) for about 30 GPS satellites and for 10-17
    GLONASS satellites, i.e., for all active GNSS
  • In addition the IGS provides
  • invaluable archive of GNSS observations (since
  • satellite and receiver clock corrections (sub-ns
  • polar motion (PM) and length of day (lod) (cm
  • coordinates and velocities for 200 sites (cm /
    mm/y accuracy)
  • atmosphere information
  • The IGS products are accurate, reliable and
    robust, available in a timely manner.

IGS enabling great science
GNSS/IGS-derived positions con-tribute to gravity
field estimation! (lower degree order harmonics)
  • The new age of gravity field determination was
    initiated with the launch of CHAMP in July 2000.
    GRACE, launched in 2002, explores the use of
    inter-satellite mea-surements (1-d-gradiometer)
    to study the time variability of the gravity
    field, GOCE will make use (starting 2007) of the
    3-d-gradiometer to derive the best possible
    stationary gravity field.

IAG/IGS Expectations concerning GNSS
  • The scientific community will not switch from one
    GNSS to another, but combine the measurements
    from all systems (the IGS is already doing that
    with GPS and GLONASS).
  • It is assumed that at least the same information
    as for GPS today will be openly (without fees)
    available for all GNSS and made available by
    the same receivers.
  • The obvious advantages of combining GNSS are
  • With n different GNSS the common parameters shoud
    at least improve by a factor of n1/2 ...
  • Inconsistencies in the reference frames cannot
  • System-specific systematic errors may be detected
    more easily (and hopefully removed).
  • Better coverage for atmosphere sounding

IAG/IGS Expectations concerning GNSS
July 7, 2006 sub-satellite tracks of GPS G06,
daily repeat orbit and GLONASS R06, repeating
after 8 days.
  • The GNSS constellations differ considerably
  • Different systems improve the geometry, help to
    understand systematic errors.

IAG/IGS Expectations concerning GNSS
  • In order to be really useful for science the GNSS
    system providers should make available the full
    technical information concerning the space
    segment and the signal structure, including
  • Offsets/patterns of antenna phase centers of
    satellites transmitting antennas w.r.t.
    satellites center of mass
  • Information related to the satellites attitude.
  • Information to generate an a priori radiation
    pressure model of good quality
  • SLR reflector arrays and corresponding
    information (including offset w.r.t. CoM)

IAG/IGS Expectations concerning GNSS
  • The scientific community, organized in IAG, will
    do its best to exploit the full potential of all
    Global Navigation Satellite Systems
  • by combining the measurements of all systems in
    the same analysis
  • stemming from combined GPS/GLONASS/GALILEO
  • This kind of analysis is already performed by the
    IGS for GPS and GLONASS, where the number of
    observations is not at all balanced
  • The IGS provides leadership in the scientific
    exploitation of the GPS and other GNSS since more
    than 15 years.
  • This IGS role should be acknowledged and the
    US/GPS contri-bution to the IGS strengthened
    through the PNT Advisory Board.

IAG and its Services
  • Monitoring global geodetic / geophysical
    phenomena is a difficult task. In IAG such tasks
    are handed over to so-called Services.
  • IAG is willing to establish a service, provided
  • there are clearly defined products and
  • an important scientific user community.
  • The space age generated a revolution in geodesy
    and led to the creation of four relevant
  • International Earth Rotation Service (IERS) in
  • IGS (International GPS Service) in 1991/1994
  • ILRS (Intl. Laser Ranging Service) and IVS
    (Intl. VLBI Service) around the end of the 20th

GNSS contributions to Science and Society
  • The GNSS/IGS support Earth science society by
    providing accurate
  • satellite orbits for all GNSS
  • satellite and (selected) receiver clock
    corrections for all GNSS
  • The GNSS/IGS contribute and give (will give) easy
    acces to
  • the International Terrestrial Reference Frame
  • The GNSS/IGS relate GNSS-specific systems like
    WGS-84, PZ-90 and will relate the corresponding
    GNSS system times on the sub-nanosecond level.
  • The GNSS/ IGS monitor Earth orientation and
    -rotation with daily resolution.
  • The GNSS/ IGS monitor the Earths ionosphere with
    two-hours time resolution.
  • The GNSS/ IGS help enabling modern gravity field
    determination monitoring.

Space Geodesy Laser Ranging
... SLR provides the origin of the terrestrial
system, it contributes to the scale, Earth
rotation, calibrates/validates GNSS orbits. The
ILRS (International Laser Ranging Service)
provides mea-surements and products
Space Geodesy VLBI
Celestial reference frame is established by VLBI.
  • VLBI provides in addition precession, nutation
    and UT1,
  • and contributes to scale of the terrestrial

The IGS as an Official Service
Orbit consistency between 1994 and 2007 (mm per
coordinate) of individual AC solutions w.r.t.
combined product reaches 1cm level, the satellite
clock consistency a level of 0.05 ns
Monitoring Polar Motion
Polar motion monitored by the IGS between 1993
and 2007. Diameter of figure about 7m, accuracy
of daily estimates ltlt 1cm! Changing diameter
of PM due to beat period (of 6 years) of Chandler
and annual period
  • The Earths pole moves in bad circles of slowly
    varying radius around the Earths figure axis
    (once in 430 daysChandler period).

Monitoring Length of Day
LoD decreased between 1993 and 2005, increases
since. LoD should increase on the average by 2 ms
per century. Large variations due to complex
(inner) structure of the Earth.
  • Length of day variation between 1993 and 2007
    (daily estimates by IGS with few microseconds

Monitoring the Earths Ionosphere
  • Mean total electron content (TEC) of the
    ionosphere may be ex-tracted using the two (or
    more) carriers of the GNSS signals (left). Global
    maps of the mean TEC available every two hours
    since 1995, daily mean TEC is extracted since
    1995 (right).

GNSS and time synchronization
  • Comparison between clock-differences using only
    the GPS carrier phases (small blue dots) and
    TWSTFT measurements (red dots) for the clocks in
    Torino, Italy and Teddington, U.K., during a
    four weeks comparison campaign in 2004.

IGS/GNSS enable great science
  • The IGS enables great science.
  • Example
  • Gravity field determination is per-formed with
    satellites and con-stellations of satellites
  • at low altitudes and high inclinations (LEOs)
  • equipped with accelero-meters (or sets of them)
  • The IGS products are used to establish the
    kinematic LEO orbits (orbit differences) with cm
    to mm precision, which in turn allow an easy
    estimation of the Earths gravity field.

CHAMP in Orbit
GNSS/IGS enabling great science
  • The error (log scale) of the gravity field as
    determined in the years 1960-1999 (by SLR,
    astrometry) and with one year of CHAMP data
    relative to (one of the best known) gravity
    fields known today.
  • The CHAMP-derived field was established using the
    IGS products.

IGS Expectations concerning GNSS
  • The following example underlines that
    system-specific systematic errors on the level of
    few cm
  • in fact do occur
  • can only be detected if there are independent
  • using other space geodetic techniques or
  • using measurements from independent GNSS (such as
  • The example uses SLR (Satellite Laser Ranging
    observations) to validate GPS orbits.
  • The illustrations are taken from a Ph.D. work in
    progress by Mrs. Claudia Flohrer (former Ms.
    Claudia Urschl).

SLR residuals for orbits using ROCK
SLR residuals for orbits using ROCK
G05 G06
Coordinate system (b,u)
Satellites position w.r.t. the Sun
  • ... Elevation of the Sun
  • above the orbital plane
  • u ... Argument of latitude
  • (satellite Sun)

SLR residuals for orbits using ROCK
G05 G06
SLR residuals for orbits using CODE07
G05 G06
Impact of SRP models on Geocenter
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