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The Role of the GPS

International Association of Geodesy. 14-Mar-12. 2. 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 DevelopmentScope and ProductsI GPS Service ? I GNSS ServiceIAG/IGS Expectations concerning different GNSS .

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The Role of the GPS

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    1. International Association of Geodesy 15-Mar-12 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

    2. International Association of Geodesy 15-Mar-12 2 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

    3. International Association of Geodesy 15-Mar-12 3 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 Earth‘s 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.

    4. International Association of Geodesy 15-Mar-12 4 Space Geodesy: GNSS

    5. International Association of Geodesy 15-Mar-12 5 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. International Association of Geodesy 15-Mar-12 6 IGS Development

    7. International Association of Geodesy 15-Mar-12 7 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.

    8. International Association of Geodesy 15-Mar-12 8 IGS enabling great science 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.

    9. International Association of Geodesy 15-Mar-12 9 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

    10. International Association of Geodesy 15-Mar-12 10 IAG/IGS Expectations concerning GNSS The GNSS constellations differ considerably Different systems improve the geometry, help to understand systematic errors.

    11. International Association of Geodesy 15-Mar-12 11 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. satellite‘s 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)

    12. International Association of Geodesy 15-Mar-12 12 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.

    13. International Association of Geodesy 15-Mar-12 13 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.

    14. International Association of Geodesy 15-Mar-12 14 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 Earth‘s ionosphere with two-hours time resolution. The GNSS/ IGS help enabling modern gravity field determination & monitoring.

    15. International Association of Geodesy 15-Mar-12 15 Space Geodesy: Laser Ranging

    16. International Association of Geodesy 15-Mar-12 16 Space Geodesy: VLBI VLBI provides in addition precession, nutation and UT1, and contributes to scale of the terrestrial network.

    17. International Association of Geodesy 15-Mar-12 17 The IGS as an Official Service

    18. International Association of Geodesy 15-Mar-12 18 Monitoring Polar Motion The Earth‘s pole moves „in bad circles of slowly varying radius“ around the Earth‘s figure axis (once in 430 days=Chandler period).

    19. International Association of Geodesy 15-Mar-12 19 Monitoring Length of Day Length of day variation between 1993 and 2007 (daily estimates by IGS with few microseconds accuracy)

    20. International Association of Geodesy 15-Mar-12 20 Monitoring the Earth‘s 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).

    21. International Association of Geodesy 15-Mar-12 21 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.

    22. International Association of Geodesy 15-Mar-12 22 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 Earth‘s gravity field.

    23. International Association of Geodesy 15-Mar-12 23 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.

    24. International Association of Geodesy 15-Mar-12 24 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).

    25. International Association of Geodesy 15-Mar-12 25 SLR residuals for orbits using: ROCK

    26. International Association of Geodesy 15-Mar-12 26 SLR residuals for orbits using: ROCK

    27. International Association of Geodesy 15-Mar-12 27 Coordinate system (b,u)

    28. International Association of Geodesy 15-Mar-12 28 SLR residuals for orbits using: ROCK

    29. International Association of Geodesy 15-Mar-12 29 SLR residuals for orbits using: CODE´07

    30. International Association of Geodesy 15-Mar-12 30 Impact of SRP models on Geocenter

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