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 International Association of Geodesy

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 International Association of Geodesy

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. International Association of Geodesy

4. GPS: USA , about 30 satellites in 6 planes GLONASS: 10-17 satellites in 3 planes GALILEO: one test satellite (GIOVE A) in orbit GPS GLONASS GALILEO Space Geodesy: GNSS Densification of ITRF, high resolution ERPs, enabling atmosphere and gravity field determination International Association of Geodesy

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. International Association of Geodesy

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 International Association of Geodesy

7. IGS Products In 1992 the IGS started off as an orbit determination service (dm accuracy) for about 20 GPS satellites. Today, the IGS providesephemerides (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. International Association of Geodesy

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. GOCE GRACE A and B CHAMP GNSS/IGS-derived positions con-tribute to gravity field estimation! (lower degree & order harmonics) International Association of Geodesy

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 GPSand 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: • Withn 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 bedetected more easily (and hopefully removed). • Better coverage for atmosphere sounding applications International Association of Geodesy

10. 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. International Association of Geodesy

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) International Association of Geodesy

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. International Association of Geodesy