International Association of Geodesy (IAG): GPS and GNSS for Science

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International Association of Geodesy (IAG) GPS
and GNSS for Science

• 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
• Hilton Arlington
• 901 North Stafford Street
• Virginia 22203
• October 16-17, 2008

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International Association of Geodesy (IAG) GPS
and GNSS for Science

• The International Association of Geodesy (IAG)
• Global Navigation Satellite Systems (GNSS)
• The International GNSS Service (IGS)
• Leadership
• Strategic engagement
• Future challenges
• General remarks concerning the activities of the
  2007-2008 National Space-Based PNT Advisory Board
  from the IAG perspective

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About Geodesy and IAG

• Geodesy is based on three pillars
• geometry and kinematics of/on Earth and in its
  environment,
• Earth orientation and rotation, and
• The Earths gravity field including its
  variability
• Geodesy provides the metrological basis for
  positioning, navigation, surveyingmapping,
  global change studies.
• IAG, the International Association of Geodesy,
  coordinates International activities related to
  the above pillars.
• The space age brought a revolution in geodesy and
  led to the creation of four services relevant for
  GNSS,
• 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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Active and Emerging GNSS
In addition China is developing a global/regional
system COMPASS with geostationary and MEO
satellites.
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About Geodesy and IAG

• Since 1994 Global Navigation Satellite Systems
  (GNSS) play an essential role in geodesy to
• maintain and densify the International
  Terrestrial Reference Frame (ITRF, issued by the
  IERS)
• monitor Earth rotation
• monitor the atmosphere
• determine precise LEO orbits (LEOLow Earth
  Orbiter)
• determine the Earths gravity field (!)
• In geodesy GNSS will continue to play a leading
  role for next 20-30 years.
• The IGS (International GNSS Service) is acting on
  behalf of IAG for the scientific exploitation of
  all GNSS.

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About the IGS

• The creation of the IGS was initiated in
  Edinburgh in 1989 with I.I. Mueller, G. Mader, B.
  Melbourne, and Ruth Neilan as protagonists
• The IGS became an official IAG service in 1994.
• The IGS first was a pure GPS Service, it was
  renamed as the International GNSS Service in
  2004.
• Today, the IGS is a truly 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
  who stands for continuity and leadership.

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About the IGS
Monitor station motion in real time
IGS Network in 2008
In 1992 the IGS GPS network was based on about 20
geodetic receivers, 400 receivers are active and
their data retrievable today
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About the IGS

• Development of the IGS combined GPS/GLONASS
  network (from Dach et al., 2008 IGS Workshop)

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About the IGS

• In 1992 the IGS started off as an orbit
  determination service (decimeter accuracy) for
  about 20 GPS satellites.
• Today, the IGS provides ephemerides (accuracy of
  2-4 cm) for about 30 GPS satellites and for all
  GLONASS satellites (lt10 cm), i.e., for all
  currently active GNSS satellites.
• In addition the IGS provides
• archive of all globally relevant 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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About the IGS

• The IGS
• provides leadership to the Civilian GPS/GNSS
  community striving for the highest achievable
  accuracy by studying
• systematic errors (case for SLR reflectors)
• system peculiarities (case of 30 constellation)
• different GNSS and their combination to the
  benefit of users
• reaches out to the entire GNSS community by
  making available precise
• GPS
• GLONASS
• in future hopefully also GALILEO
• orbits and clocks in one and the same
  reference frame.
• identifies future challenges by its strategic
  planning

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Case for SLR reflectors
Satellites position w.r.t. the Sun

• ... Elevation of the Sun
• above the orbital plane
• u ... Argument of latitude
• (satellite Sun)

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Case for SLR reflectors
G05 G06
(cm)
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Case for SLR reflectors

• From Yang Fumin(1), Chen Wanzhen(1), Zhang
  Zhongping(1), Wang Yuanming(1), Zhang Haifeng(1)
• Zhao You(2), Fan Cunbo(2) and Han Xingwei(2)
• (1) Shanghai Observatory, Chinese Academy of
  Sciences, Shanghai, China
• (2) Changchun Observatory, Chinese Academy of
  Sciences, Changchun, China

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SLR Reflectors on GNSS Satellites

• SLR provides the only independent check (in the
  radial direction) of GNSS orbits determined
  with the GNSS signals and carriers.
• The payload required to enable SLR tracking is
  minor (about 10 kg, the SLA on Compass has 2.5
  kg), the costs marginal.
• SLR tracking to GNSS, internationally coordinated
  by the ILRS, was successfully performed for
• GPS (PRN 05 06)
• All GLONASS satellites
• GIOVE-A -B (GALILEO)
• COMPASS-M1 (launched in spring 2007)

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GNSS Peculiarities
July 7, 2006 sub-satellite tracks of GPS PRN
06, with daily repeat orbit and GLONASS R06,
orbit repeating after 8 days.

• The GNSS constellations differ considerably
  (inclinations, daily vs. 8-day repeat orbits for
  GPS and GLONASS, respectively)
• Different constellations improve the geometry,
  help to understand systematic errors

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GNSS Peculiarities

• Observing GPS and GLONASS from one site Mean
  inclinations of GPS and GLONASS ore different
  (Dach et al., 2008)

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GNSS Peculiarities

• Observing one GPS and one GLONASS satellite over
  ten days from two different sites (roughly) at
  the same latitude (Dach et al., 2008)

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Case for 30 Satellites

• PDOP (Positional Dilution of Precision) lt1
  excellent, gt 4 bad.
• Motivation (a) 30 constellation, (b) combine
  systems

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International Committee on GNSS

• The ICG was set up by the United Nations Office
  for Outer Space Affairs (UNOOSA)
• ICG is A forum to discuss Global Navigation
  Satellite Systems (GNSS) to benefit people around
  the world
• As opposed to the IGS the emphasis is not on
  scientific, but on outreach issues where system
  providers and users are working together.
• The Third Meeting of the International Committee
  on Global Navigation Satellite Systems (ICG) is
  organized by the Jet Propulsion Laboratory (Ruth
  Neilan), Pasadena, USA, 8 - 12 December 2008.

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Report on PNT 2007-2008 Activities

• From the IAG perspective the draft report on the
  2007-08 activities is already in excellent shape
  (in particular the Executive Summary).
• It is a concise summary of the Boards 2007-08
  activities and provides in itself the best
  argument to continue the Boards activities.
• The IAG is very pleased that the case of
• Commitment to a 30 constellation
• SLR reflectors on GPS satellites
• get appropriate attention and that
• International collaboration is viewed as an
  important issue.