The Role of the GPS/GNSS in Geodesy and Geodynamics

1
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
• National Space-Based Positioning, Navigation, And
  Timing (PNT) Advisory Board
• Doubletree Hotel, 1515 Rhode Island Avenue,
  Washington, D.C. 20005
• October 4-5, 2007

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The Role of the GPS/GNSS in Geodesy and
Geodynamics

• 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

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Geodesy and the Space Age

• Geodesy is based on and provides information for
• geometry and kinematics of/on Earth and in its
  environment,
• Earth orientation and rotation, and
• The Earths gravity field including its
  variability.
• 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
  1989
• IGS (International GNSS Service) in 1991/1994
• ILRS (Intl. Laser Ranging Service) and IVS
  (Intl. VLBI Service) around the year 2000.

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Space Geodesy GNSS
Densification of ITRF, high resolution ERPs,
enabling atmosphere and gravity field
determination
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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.

6
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
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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
  satellites.
• In addition the IGS provides
• invaluable archive of GNSS observations (since
  1991)
• satellite and receiver clock corrections (sub-ns
  accuracy)
• polar motion (PM) and length of day (lod) (cm
  accuracy)
• 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.

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IGS enabling great science
GOCE
GRACE A and B
CHAMP
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.

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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
  occur.
• System-specific systematic errors may be detected
  more easily (and hopefully removed).
• Better coverage for atmosphere sounding
  applications

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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.

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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)

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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
  receivers.
• 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.

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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
  services,
• International Earth Rotation Service (IERS) in
  1989
• IGS (International GPS Service) in 1991/1994
• ILRS (Intl. Laser Ranging Service) and IVS
  (Intl. VLBI Service) around the end of the 20th
  century.

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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
  (ITRF)
• 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.

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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
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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
  network.

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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
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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).

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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
  accuracy)

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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).

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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.

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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
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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.

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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
  checks
• using other space geodetic techniques or
• using measurements from independent GNSS (such as
  GALILEO, GLONASS)
• 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).

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SLR residuals for orbits using ROCK
G05
BLOCK II
G06
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SLR residuals for orbits using ROCK
G05 G06
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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)

28
SLR residuals for orbits using ROCK
G05 G06
(cm)
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SLR residuals for orbits using CODE07
G05 G06
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Impact of SRP models on Geocenter
ROCK
NONE
CODE