New approach to resolve water level changes of small rivers with CryoSat-2 SAR altimetry
CryoSat-2, launched in 2010, is the first altimetry satellite partly operating in synthetic-aperture radar (SAR) mode. Compared to conventional radar altimeters, the SAR mode enables altimetry measurements at increased along-track resolution and with smaller footprint. This opens new possibilities for the determination of water levels also of smaller inland water bodies that could not reliably be observed by the classical altimetry satellites. With its repeat time of 369 days CryoSat-2 does not provide a good temporal resolution, but on the other hand this long repeat orbit configuration results in a very dense spatial sampling of the water level, e.g. along a river.
The first step in the calculation of the water levels from CryoSat-2 data is the classification of the recorded radar returns into signals from water and land surfaces. Usually the classification is based on a predefined land-water mask. Such masks are often invariant with respect to time, i.e. they neither account for seasonal variations of the water extent nor for inter-annually shifting river banks. The determination of dynamic land-water (e.g. from remote sensing images) is difficult in particular in regions with frequent cloud coverage, and usually satellite images are not available simultaneously with the altimetry measurements.
By example of the Mekong river basin, scientists of DGFI-TUM and DTU Space (Denmark) developed and validated a new method for the identification of water returns in CryoSat-2 data that is independent from a land-water mask and relies solely on features derived from the SAR and range-integrated power (RIP) waveforms. The new approach has proven its effectiveness especially in the upstream region of the basin that is characterized by small- and medium-sized rivers. Compared to an approach based on a land-water mask, the new method significantly increased the number of valid measurements and their precision (1700 with 2% outliers vs. 1500 with 7% outliers). The approach and results are described in the open-access publication River Levels Derived with CryoSat-2 SAR Data Classification - A Case Study in the Mekong River Basin (Remote Sensing, 2017, DOI:10.3390/rs9121238, [PDF]).
Next-generation global geodetic reference frames: The potential of Satellite Laser Ranging
Consistent and accurate global terrestrial reference frames (TRFs, i.e., the realization of coordinate systems attached to the Earth) are a fundamental prerequisite for navigation, positioning, and for Earth system science by referencing smallest changes of our planet in space and time. As Earth system research is increasingly relying on satellite-based Earth observation data, also the precise orientation of the Earth with respect to inertial space is required to relate the satellite observations to coordinates in the Earth reference frame.
Satellite Laser Ranging (SLR) is among the most important space geodetic techniques contributing to the determination of TRFs. SLR relies on the two-way travel time of laser pulses from stations on the Earth’s surface to satellites equipped with retro-reflectors. The SLR measurement principle allows for the realization of origin (Earth’s center of mass) and scale of global networks with very high accuracy. One of the major factors limiting the present accuracy of SLR-derived TRFs and Earth orientation parameters (EOP), however, is the currently unbalanced global SLR station distribution with a lack of stations particularly in the southern hemisphere.
The benefit of the potential future development of the SLR network for the accuracy of TRF (including origin and scale) and EOP has been investigated in the study Future global SLR network evolution and its impact on the terrestrial reference frame (Journal of Geodesy, 2017, DOI: 10.1007/s00190-017-1083-1). It compares a simulated SLR network including potential future stations to improve the network geometry with the current network. The study highlights that an improved SLR network geometry means a cornerstone to meet the ambitious accuracy requirements for reference frames defined by IAG’s Global Geodetic Observing System (GGOS).
Tracking openings in sea ice to increase our knowledge of the Arctic and Antarctic Ocean
The Arctic and Antarctic oceans are located in areas that are experiencing new conditions due to climate change (higher atmospheric temperatures, melting of ice sheets). Nevertheless, it is difficult to understand the changes in sea level, due to the fact that large areas are seasonally or permanently covered by sea ice. Scientists at DGFI-TUM have developed a new technique to spot the leads, i.e. openings in sea ice that uncover the sea surface, by analysing the data of ESA's successful Cryosat-2 mission.
The radar altimeter on board sends electromagnetic waves and collects the reflections from the ocean surface at different incidence angles. But the retrieval of meaningful sea level estimates requires not only the recognition of the leads. It also needs to be ensured that the openings are perpendicular to the satellite position (nadir position). As the sea is calm and flat, the leads act like a mirror for the satellite: they could be easy to recognise, but their reflection is so strong that it has a signature on the data also at other incidence angles. An undetected lead is a missed opportunity to measure sea level, but a lead detected when not at nadir can cause a wrong estimation.
By tracking the signature that the leads leave on the collected data, it is possible to improve the detection capabilities. This is shown in the publication Lead Detection using Cryosat-2 Delay-Doppler Processing and Sentinel-1 SAR images (Advances in Space Research, 2017, DOI: 10.1016/j.asr.2017.07.011, [PDF]). The technique is also validated using radar images from Sentinel-1 (see picture above). This study will increase the reliability of the sea level analysis at high latitudes and thus contributes to improve the knowledge of the sea level dynamics in the Arctic and Antarctic oceans.
Monitoring the Arctic Seas: Satellite Altimetry traces open water in sea-ice regions
Open water areas in sea ice regions significantly influence the ocean-ice-atmosphere interaction. For the debate about Arctic climate change, the monitoring and quantification of such openings, so-called leads and polynyas, is of high relevance.
In a recent study, scientists from DGFI-TUM demonstrated the potential of high-frequency satellite altimetry data from the missions SARAL and Envisat for the detection of open water areas in the ice-covered Greenland Sea. In comparison with Synthetic Aperture Radar (SAR) images, they obtained a consistency rate of 76.9% for SARAL and 70.7% for Envisat. Some samples even resulted in true water detection rates of up to 94%.
The study is based on an innovative, unsupervised classification approach that relates the radar altimetry echoes (so-called waveforms) with different surface conditions, among them open water and sea ice. The algorithm has successfully been used for the detection of water areas with different spatial extent, and it can be applied to all pulse-limited altimetry data sets. The procedure and results are described in the article Monitoring the Arctic Seas: How Satellite Altimetry can be used to detect open water in sea-ice regions (Remote Sensing, 2017, available via open access).
Near real-time modelling of the electron content in the ionosphere
The rapidly growing number of terrestrial GNSS (Global Navigation Satellite System) receivers providing double frequency measurements in real-time and near real-time enables the computation of ionosphere parameters such as the Vertical Total Electron Content (VTEC) with increasing accuracy and decreasing latency.
Scientists of DGFI-TUM have now developed a comprehensive processing framework to compute VTEC maps in near real-time from low latency GNSS measurements using compactly supported B-splines and recursive filtering methods. Details and results are described in the article Near real-time estimation of ionosphere vertical total electron content from GNSS satellites using B-splines in a Kalman filter (Annales Geophysicae, 2017, DOI: 10.5194/angeo-35-263-2017).
Series expansions in terms of B-spline functions allow for an appropriate handling of heterogeneously distributed input data. Kalman filtering enables the processing of the data immediately after acquisition and paves the way of sequential (near) real-time estimation of the unknown parameters, i.e. VTEC B-spline coefficients and differential code biases. Under the investigated conditions, the validation tests of our near real-time products show promising results in terms of accuracy and agreement with the post-processed final products of the International GNSS Service (IGS) and its analysis centers which are usually publicly available with several days of latency.
Time series of sea level in the Mediterranean and North Sea with improved coastal performances
Satellite altimetry has been monitoring the sea level since more than 25 years. Measurements in the coastal zone, however, were routinely discarded due to poor quality. Recently, several studies addressed various techniques to improve the precision and the accuracy of coastal sea level measurements.
Scientists at DGFI-TUM are reprocessing the satellite signals using the ALES algorithm and have set up an experimental platform to distribute corrected sea level anomalies with a documented procedure. The COastal Sea level Tailored ALES (COSTA) dataset is now available in the Mediterranean and in the North Sea and provides the user with time series at each point along the satellite tracks of two satellite missions (ERS-2 and Envisat) covering the years 1996-2010. The COSTA dataset improves the precision of the standard product in over 70% of the domain. Details are provided in the recent presentation ALES Coastal Processing Applied to ERS: Extending the Coastal Sea Level Time Series (10th Coastal Altimetry Workshop, 21-24 February 2017, Florence, Italy). COSTA guarantees therefore not only improvements in the coastal data, but it means also a step forward in the precision of satellite altimetry in the open sea.
COSTA data is available for download in our Science Data Products section.
High-resolution river water levels based on multi-mission altimetry data
Classical approaches of inland altimetry determine water level variations of rivers at so-called virtual stations, i.e. fixed locations given by the crossings of altimeter tracks and rivers. Depending on the repeat cycles of the individual altimeter missions, the temporal resolution of these time series is limited to 35 or 10 days.
Now, scientists of DGFI-TUM have developed a method to provide a complete spatio-temporal description of the Mekong River based on multi-mission altimetry data. Details and results are described in the article Combination of Multi-Mission Altimetry Data Along the Mekong River with Spatio-Temporal Kriging (Journal of Geodesy, 2016, DOI: 10.1007/s00190-016-0980-z). The approach uses statistical robust interpolation methods to combine measurements of different altimetry missions, considering diverse flow velocities of the river as well as remote relationships between different catchment areas. With this approach, water level time series can be determined for arbitrary locations along the river at a significantly increased temporal resolution. For the test case of the Mekong river, a resolution of 5 days could be achieved, and the accuracy was improved by 23 to 34% compared to standard methods.
New flyers available: DTRF2014 and DAHITI
Two new flyers have recently been issued: The first one contains background information and directions for data access for DGFI-TUM's most recent realization of the International Terrestrial Reference System, the DTRF2014 (for more information about the DTRF2014 see message below).
The second flyer advertises DAHITI, DGFI-TUM's Database for Hydrological Time Series of Inland Waters that has been operated since 2013. DAHITI provides time series of water levels of lakes, reservoirs, rivers, and wetlands derived from multi-mission satellite altimetry for more than 400 globally distributed targets.
Both flyers can be accessed by clicking on the images on the right.
DGFI-TUM releases a new realization of the International Terrestrial Reference System: DTRF2014
The DTRF2014 is DGFI-TUM’s new realization of the International Terrestrial Reference System (ITRS). It comprises positions and velocities of 1712 globally distributed stations of the space geodetic observation techniques VLBI, SLR, GNSS and DORIS as well as consistently estimated Earth orientation parameters. The DTRF2014 includes six additional years of data compared to the previous realization, i.e., the DTRF2008 (Seitz et al., 2012). Additionally, for the first time, non-tidal atmospheric and hydrological loading is considered in the DTRF2014.
In its role as an "ITRS Combination Centre" within the International Earth Rotation and Reference Systems Service (IERS), DGFI-TUM took the responsibility for providing realizations of the ITRS in regular intervals. An up-to-date ITRS realization at highest accuracy and long-term stability is an indispensable requirement for various applications in daily life (e.g., for navigation and positioning, for the realization of height systems and precise time systems or for the computation of spacecraft and satellite orbits). Furthermore, it is the backbone for Earth system research by providing the metrological basis and uniform reference for monitoring processes in the context of global change (e.g., ice melting, sea level rise). Read more about the DTRF2014 here (also available in German here).
The DTRF2014 is available for download in our Science Data Products section.
New surface deformation model for Latin America after the 2010 earthquakes in Chile and Mexico (VEMOS2015)
Strong earthquakes cause changes in positions (up to several meters) and velocities of geodetic reference stations. Hence, existing global and regional reference frames become unusable in the affected regions. To ensure the long-term stability of the geodetic reference frames, the transformation of station positions between different epochs requires the availability of reliable continuous surface deformation models.
Scientists of DGFI-TUM now published the new continental continuous surface deformation model VEMOS2015 (Velocity Model for SIRGAS [Sistema de Referencia Geocéntrico para Las Américas]) for Latin America and the Caribbean inferred from GNSS (GPS+GLONASS) measurements gained after the strong earthquakes in 2010 in Chile and Mexico. VEMOS2015 is based on a multi-year velocity solution for a network of 456 continuously operating GNSS stations covering a five years period from March 14, 2010 to April 11, 2015. The approach and results are described in the article Crustal deformation and surface kinematics after the 2010 earthquakes in Latin America (Journal of Geodynamics, 2016, DOI: 10.1016/j.jog.2016.06.005).
VEMOS2015 as well as the SIRGAS reference frame realization SIR15P01 (multi-year solution for 456 GNSS stations, including weekly residual time series) are provided in our Science Data Products section. More information on DGFI-TUM's research activities related to SIRGAS can be found here.
Innovative processing method for altimetry data allows for monitoring water level variations in wetlands
Over the last years satellite altimetry has proven its potential to monitor water level variations not only over the oceans but also over inland water bodies. DGFI-TUM provides altimetry-derived time series of water stage variations of various globally distributed rivers and lakes via its web service "Database for Hydrological Time Series over Inland Waters" (DAHITI; see below).
Now, scientists of DGFI-TUM have developed an innovative processing method for monitoring and analyzing water level variations in wetlands and flooded areas. The approach is based on automated altimeter data selection by waveform classification and an optimized waveform retracking. It is described in the article Potential of ENVISAT Radar Altimetry for Water Level Monitoring in the Pantanal Wetland (Remote Sensing, 2016, available via open-access).
Using the example of the Pantanal wetland in South America, this study demonstrates the capability and limitations of the ENVISAT radar altimeter for monitoring water levels in inundation areas. The accuracy of water stages varies between 30 and 50 cm (RMSE) and is in the same order of magnitude as reported for smaller rivers. Most areas of the Pantanal show clear annual water level variations with maximum water stages between January and June. The amplitudes can reach up to about 1.5 m for larger rivers and their floodplains. However, some areas of the wetland show water level variations of less than a few decimeters, which is below the accuracy of the method. These areas cannot reliably be monitored by ENVISAT. Further investigations will show if the usage of Delay-Doppler altimeter data (such as measured by the recently launched Sentinel-3 mission) might improve the results there.
IAG adopts a new conventional value for the reference gravity potential W0 of the geoid
The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) promotes through its Focus Area 1 Unified Height System the definition and realization of a global vertical reference system with homogeneous consistency and long-term stability. For the term 2011-2015 DGFI-TUM coordinated the Working Group Vertical Datum Standardization, which main purpose was to determine an updated value for the gravity potential W0 of the geoid to be introduced as the conventional reference level for the realization of a global height system.
The derived value was officially adopted by the IAG in its Resolution No. 1, July 2015, as the conventional W0 value for the definition and realization of the International Height Reference System. A detailed description about the DGFI-TUM computation strategy of W0, applied models, conventions and standards, as well as results is presented in the recent publication A conventional value for the geoid reference potential W0 (Journal of Geodesy, 2016, DOI: 10.1007/s00190-016-0913-x).
Read more about the background and DGFI-TUM's activities related to the determination of the new W0 value here.
New flexible combination approach for regional gravity field modelling applied to Northern Germany
Different measurement techniques of the Earth's gravity field are characterized by different spectral sensitivities, i.e they allow for detecting structures of the gravity field at different spatial scales. By combining the observations from various measurement techniques a data set of a broad spectral range can be obtained. Typically, high-resolution gravity data from regional measurements are combined with global satellite information of lower spatial resolution.
To exploit the gravitational information as optimally as possible, scientists of DGFI-TUM set up a regional modeling approach. It uses radial spherical basis functions and emphasizes the strengths of various data sets by a flexible combination of high- and middle-resolution terrestrial, airborne, shipborne, and altimetry measurements. The resulting regional models can serve as a basis for various applications, such as the refinement of global gravity field models, national geoid determination, and the detection of mass anomalies in the Earth’s interior. Details can be found in the recently published article Combination of various observation techniques for regional modeling of the gravity field (Journal of Geophysical Research, 2016, DOI:10.1002/2015JB012586).
Data portal DAHITI: Water level time series of rivers and lakes from multi-mission satellite altimetry
Scientists of DGFI-TUM have developed a new approach for the automated estimation of water levels of inland water bodies based on satellite observations from multi-mission altimetry. Time series of water stage variations of various globally distributed rivers and lakes are made available through DGFI-TUM's web service “Database for Hydrological Time Series over Inland Waters” (DAHITI).
The approach is described in the publication DAHITI – an innovative approach for estimating water level time series over inland waters using multi-mission satellite altimetry (Hydrology and Earth System Sciences, 2015, available via open-access).
The new method is based on an extended outlier rejection and a Kalman filter incorporating cross-calibrated multi-mission altimeter data from Envisat, ERS-2, Jason-1,Jason-2, TOPEX/Poseidon, and SARAL/AltiKa, including their uncertainties. The paper presents water level time series for a variety of lakes and rivers in North and South America. A comprehensive validation is performed by comparisons with in situ gauge data and results from external inland altimeter databases. The new approach yields RMS differences with respect to in situ data between 4 and 36 cm for lakes and 8 and 114 cm for rivers. For most study cases, more accurate height information than from other available altimeter databases can be achieved.
DGFI-TUM contributes to the implementation of an UN Resolution for a Global Geodetic Reference Frame
In February 2015, the UN General Assembly adopted its first geospatial resolution „A Global Geodetic Reference Frame for Sustainable Development“. This resolution recognizes the importance of geodesy for many societal and economic benefit areas, including navigation and transport, construction and monitoring of infrastructure, process control, surveying and mapping, and the growing demand for precisely observing our planet's changes in space and time. The resolution stresses the significance of the global reference frame for accomplishing these tasks, for natural disaster management, and to provide reliable information for decision-makers.
The United Nations Global Geospatial Information Management (UN-GGIM) Working Group on the Global Geodetic Reference Frame (GGRF) has the task for drafting a roadmap for the enhancement of the GGRF under UN mandate.
Based on its competence in the realization of reference frames DGFI-TUM is involved in this activity by contributing to the compilation of a concept paper in the frame of the International Association of Geodesy (IAG). The main purpose of this paper is to provide a common understanding for the definition of the GGRF and the scientific basis for the preparation of the roadmap to be accomplished by the UN-GGIM Working Group on the GGRF. [more]