2:00pm - 2:20pm
Keynote: Geodetic Space Sensors for Height System Unification and Absolute Sea Level Determination
Technical University of Munich, Germany
Sea level research nowadays is based on a number of geodetic space sensors, which all have their own characteristics and deliver different type of observations. Over the oceans, satellite altimetry is the main information source providing the absolute sea level at a specific location and at a specific time with respect to a geometric reference frame. By analysis of these measurements in the space and time domain one can derive mean sea surfaces over specific areas and periods and/or its change in time. At the coasts, sea level is usually observed with tide gauges delivering instantaneous sea surface heights relative to a zero marker of the tide gauge station. In order to get absolute sea level heights at tide gauges two additional quantities are required. These are observation of the relative motion of the zero marker with respect to a global geometric reference frame delivering instantaneous geometric heights and the gravimetric equipotential surface going through the tide gauge zero marker and its difference to an equipotential surface defining a global height reference system. By scaling the potential difference with gravity accelerations one can obtain physical heights of the tide gauge zero marker. Subtracting these physical heights from the geometric sea surface height one can finally get absolute sea level heights at tide gauge stations in case consistent geometrical and physical reference frames are applied.
Height system unification and absolute sea level determination therefore requires observations from a number of different geodetic space sensors delivering geometric and physical quantities in consistent reference frames. Geometric heights are delivered from satellite altimeters, GNSS stations and synthetic aperture radars (SAR), while physical heights need knowledge about the Earth gravity field as it is delivered from GOCE, GRACE and GRACE-FO, complemented by surface and airborne observations. As of today the observing systems exhibit several weaknesses, which are crucial to height system unification and absolute sea level determination. For example, only a few tide gauge stations are equipped with permanent GNSS receivers. Therefore, new approaches for the geometric station monitoring need to be implemented, which for example can be based on SAR absolute and relative positioning techniques. Further-on, a high resolution geoid at the tide gauge station is needed in order determine physical heights with the required accuracy. Finally, in all processing steps involved, it has to be ensured that the geometric and physical reference systems are compatible and that all sensors deliver consistent information.
The paper introduces the problem of height system unification and absolute sea level determination, investigates potential problems in the joint use of geodetic sensor data for this purpose and identifies possible research topics to be addressed by future studies. Special emphasis will be given to absolute point positioning with SAR techniques and its possible benefit for this application. It concludes with some ideas on how to set up a global height system/sea level observing system.
2:20pm - 2:40pm
Keynote: GNSS-Augmented Radar Transponders for InSAR Datum Connection
Delft University of Technology, Netherlands, The
Ramon Hanssen, Hans van der Marel, Freek van Leijen and Karsh Patel.
Geodetic Missions Workshop
20-24 March 2017
Banff, Alberta, Canada
InSAR deformation estimates form a ‘free network’ referred to an arbitrary datum, e.g. by assuming a reference point in the image to be stable. Consequently, the estimates of any measurement point in the image are dependent of these postulations on reference point stability, and the estimates cannot be compared with datasets of other types of measurement (e.g. historical levelling data or sea-level changes).Yet, some applications require ‘absolute’ InSAR estimates, i.e. expressed in a well-defined terrestrial reference frame (TRF). We achieve this using collocated InSAR and GNSS measurements, achieved by rigidly attaching phase-stable millimetre-precision compact active transponders to permanent GNSS antennas. The InSAR deformation estimates at these transponders are then estimated in a TRF using the GNSS measurements. Consequently, deformation estimates at all other scatterers are now also defined in the same TRF.
Here we report on latest experiments with a new type of low-cost radar transponders.
2:40pm - 2:55pm
Potential of Global SAR Positioning for Geodetic Applications – Lessons Learned from TerraSAR-X and Sentinel-1
1Chair of Astronomical and Physical Geodesy, TUM, Germany; 2Remote Sensing Technology Institute, DLR, Germany; 3German Space Operations Center, DLR, Germany
With our implementation of geodetic techniques for data processing and data correction, spaceborne Synthetic Aperture Radar (SAR) has attained the possibility of fixing global positions of dedicated radar points at the low centimeter accuracy level. Such points can be created by passive radar corner reflectors, and the positioning method relies on the inherent ranging capabilities of SAR sensors. Thus, we may refer to the method as SAR imaging geodesy or geodetic SAR.
Determining accurate long-term global positions of objects on the Earth’s surface is typically associated with Global Navigation Satellite Systems (GNSS) and one of the core elements of modern space geodesy. In order to do so, high-grade geodetic equipment with constant power supply, as well as the possibility for data transfer are required, limiting dense application on a large scale and poses difficulties for very remote areas with little or no infrastructure. Whereas certain regions like Japan or the San Andreas Fault are densely covered by GNSS such coverage may not be achievable everywhere on the globe.
To improve the situation, we present a concept of jointly using SAR and GNSS for expanding geodetic positioning to applications requiring long-term coordinate monitoring. In future, the use of cost-effective passive reflectors in X-band SAR or low-cost battery-powered active transponders, which are currently in development for C-band SAR, could provide global coordinates anywhere where SAR imagery is acquired under multiple incidence angles. The main requirements are precise orbit determination, processing of the SAR imagery omitting geometric approximations, as well as the rigorous correction of perturbations caused by atmospheric path delay and signals of the dynamic Earth. If a reflector or transponder already has known reference coordinates, e.g. from co-location with GNSS, the perturbing signals can be mitigated for surrounding radar points by applying differential SAR positioning techniques similar to differential GNSS, provided that all the points are included in the same radar image. In this contribution we discuss these geodetic SAR methods with respect to our experiences gained from the TerraSAR-X mission, and present first results of experiments carried out with Sentinel-1 data.
2:55pm - 3:10pm
European Gravity Service For Improved Emergency Management
1University of Bern, Switzerland; 2University of Luxembourg, Luxembourg, now University of Hannover, Germany; 3German Research Centre for Geosciences, Germany; 4Technical University of Graz, Austria; 5Deutsches Zentrum für Luft - und Raumfahrt, Germany; 6Centre National D'Etudes Spatiales, France; 7University of Hannover, Germany; 8Géode & Cie, Toulouse, France
The European Gravity Service for Improved Emergency Management (EGSIEM) is a project of the Horizon 2020 Framework Programme for Research and Innovation of the European Commission. It shall demonstrate that observations of the redistribution of water and ice mass derived from the current GRACE mission, the future GRACE-FO mission, and additional data provide critical and complementary information to more traditional Earth Observation products and shall open the door for innovative approaches to flood and drought monitoring and forecasting. We give an overview of the current status of the project and present the latest results from the three key objectives that EGSIEM shall address: 1) to establish a scientific combination service to deliver the best gravity products for applications in Earth and environmental science research based on the unified knowledge of the European GRACE community, 2) to establish a near real-time and regional service to reduce the latency and increase the temporal resolution of the mass redistribution products, and 3) to establish a hydrological and early warning service to develop gravity-based indicators for extreme hydrological events and to demonstrate their value for flood and drought forecasting and monitoring services.
3:10pm - 3:25pm
Near Realtime Mass Transport Products for Monitoring of Hydrological Extreme Events
1GFZ Potsdam, Germany; 2TU Graz, Austria; 3University Luxembourg, Luxembourg
The nominal time delay of the GRACE Level-1 instrument data (11 days) and of the derived monthly global Level-2 gravity field products (60 days) makes the application of GRACE for monitoring of e.g. hydrological extremes difficult. Flood forecast models need, e.g. near-real time (NRT) information to estimate the probable development of the event in terms of flood stage or river discharge with typical lead times of a few days for larger river basins.
To enable the application of GRACE (and later GRACE-FO) mass redistribution data for rapid monitoring of hydrological extreme events, the EU funded project EGSIEM (European Gravity Service for Improved Emergency Management) has established a NRT and Regional Service, that aims to reduce the time delay of mass transport products to less than 5 days, to increase the time resolution from one month to one day, and to improve the quality by providing regional solutions based on alternative representations of the gravity field, e.g. space-localizing radial base functions. The quality of the NRT mass transport products will be tested using GNSS loading and ocean bottom pressure data as well as hydrological flood events. Assuming that GRACE Quick-Look data (provided by JPL) will still be available in April 2017 an operational test run of the NRT Service is planned for about 6 months. Here the NRT products will be provided on a daily basis to the EGSIEM Hydrological Service which derives NRT flood indicators to be used within DLR´s Center for Satellite-based Crisis Information.
The presentation will focus on the current status of the project in view of the upcoming milestone “Operational Service Readiness”.