Section 1.1: Space Geodetic Techniques

The vision of our work in section 1.1 is to use the Global Navigation Satellite Systems (GNSS; GPS, GLONASS, BeiDou and Galileo) as powerful tools for geosciences and to optimally exploit their potential for the observation of the complex system Earth. The diversity of our scientific activities ranges from precise monitoring of continental plates movement with sub-mm/year accuracy, over regional and global atmospheric and ionospheric sounding, up to the monitoring of water and ice surfaces. In addition, we also adapt navigation satellite receivers for geoscientific applications.

The scientific work of GFZ section 1.1 is focused on the geophysical applications of GNSS. We use precise geodetic receivers to record the movement of lithospheric plates to an accuracy of sub-millimeters per year or to monitor in real-time geophysical processes during an earthquake. We also measure vertical land movements resulting from changes of the load of covering snow or water, or the land uplift as a relic of the last ice age.

Section 1.1 contributes to the GNSS tracking network of the International GNSS Service (IGS) by operating 26 multi-GNSS stations distributed globally. As an IGS analysis center of the IGS we provide high accurate satellite orbit and clock products for GPS&GLONASS and in the framework of the IGS Multi-GNSS Experiment and Pilot Project for all GNSS. Section 1.1 provides an advanced realtime analysis center as part of the IGS Real-time service. The registration and evaluation of GNSS data in real time play an important role here. This is complemented by the so-called multi-sensor fusion, i.e. the simultaneous use of GNSS and other sensors such as IMU (Inertial Measurement Units).

We also exploit the propagation properties of the GNSS signals. On their way from the GNSS satellites (at around 20000 km above Earth's surface) to the receivers on the ground, the signals are affected by the ionosphere and also the troposphere. This effect is an error source for precise navigation and positioning but also allows to obtain information about the troposphere/ionosphere. We apply this idea with radio occultation measurements from low Earth orbiting (LEO) satellites like CHAMP (2000-2010) and several international missions, as GRACE, GRACE-FO, TerraSAR-X, TanDEM-X, Metop, or COSMIC. Hereby we analyse the GPS signals that can be recorded on the LEO, when it "sees" a GPS satellites setting behind the horizon. The main result of such measurements are globally distributed and very precise vertical profiles of temperature and water vapor in the atmosphere, which are used to improve numerical weather forecasts or to identify climatological variations of the Earth's atmosphere. Information about the electron density in the ionosphere and space weather can also be obtained.

The GNSS radio signals in L-band (1.2 and 1.5 GHz) are reflected by water and ice surfaces and we investigate how these reflected or scattered radio waves can be used scientifically. One example is the use for precise height determination of water surfaces. One advanced application is the mapping of glacier surfaces or sea- and shelf-ice in Greenland and Antarctica. Another application is the derivation of wind speed and direction of the reflecting surface, e.g., the open ocean.

The very long baseline interferometry (VLBI) is a space geodetic technique that allows to determine key geodetic parameters. As an Analysis Center of the IVS (International VLBI Service for Geodesy and Astrometry) we contribute to the terrestrial and celestial reference frames, ITRF and ICRF, which are essential products of global geodesy. Our work on VLBI allows a lot of scientific investigations, such as determination of atmospheric parameters from VLBI and the study of radio source structure effects on VLBI observables.

A new focus of our work is the combination of various space geodetic techniques (GNSS, VLBI, SLR, DORIS) in the sense of GGOS, the Global Geodetic Observing System of the IAG (International Association of Geodesy). Several studies have been carried out about the additional benefit of co-location in space by satellite(s) that carry several of the techniques on board such as a VLBI transmitter.

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