Zurück

Sprache

When the water rises …
or the Earth subsides

Help for helpers: radar images from TerraSAR-X

Flood waters can be easily identified from space – on radar images. Bodies of water and flooded areas simply appear as black surfaces. Hence, the high-precision German radar satellite TerraSAR-X can detect the extension of flood waters in the metre range. It is orbiting the Earth at an altitude of 515 kilometres. Clouds, darkness, or rain do not compromise its function (or that of its twin TanDEM-X flying at short distance in parallel): radar waves (in the X band with a wavelength of three centimetres) can penetrate clouds and do not need light.

Making the invisible visible

Radar images make more things visible than can be seen with the eye – since objects reflect radar waves in a different way than light does. Independent of clouds and shadows, not the optical appearance of the Earth is captured but its structure and the properties of its surface – sort of a “floor plan” of the Earth that is continuously updated.

Revolutionary: with the two radar satellites TerraSAR-X and TanDEM-X, vertical and horizontal movements of the surface are measurable from space in the millimetre range. This opens up new, innovative research opportunities and applications for scientists and authorities: in disaster management and environmental protection or to warn people of threats due to collapsing buildings.

The Balkans, May 2014: the river Soave situated in the border area between Croatia and Bosnia and Herzegovina bursts its banks. Several dams near the city of Orašje have burst. The consequence: the worst flooding in this region for more than 120 years. The image shows the results of the fully automated flood water detection via TerraSAR-X, established by the Center for Satellite-Based Crisis Information (ZKI) within the DLR.

From the satellite image to a flood map in less than one hour: previously, radar satellite data on flood water situations were more or less evaluated manually by an image interpreter. The DLR/ZKI has further developed this process in the last years: now, satellite data are evaluated fully automatic, combined with other information, and made available for retrieval online.

Heading for early detection of volcano eruptions from space: Indonesia, volcano island Krakatau. In December 2018, the Anak Krakatau volcano collapsed, lost 300 metres in height, and huge soil masses dropped into the sea. A devastating disaster: the tsunami caused thereby made havoc of the coastal regions of Java and Sumatra. Hundreds of people died. A tsunami caused by a volcano eruption with landslide – the early warning systems on site were not prepared for this since they primarily capture submarine earthquakes. However, TerraSAR-X radar images and other satellite data deliver important data to precisely analyse and better understand volcanic activity – and to contribute to a global early detection system of volcano eruptions.

Santorin, Greek archipelago: volcano protruding into the sea, and not exactly inactive. The circumference of the crater has increased by 14 centimetres from 2011 to 2015. The main island (on the right in blue colours) proved to be quite stable while the smaller islands moved considerably. Yet, nothing bad has happened there but it is advisable to continuously monitor such regions.

Safe ice roads: Canada. Northwest. Winter. The only roads to remote locations being icebound lakes and rivers. Ice road truckers set out for the dangerous trip. With thin ice, it becomes critical. What impacts has the load of the trucks? Now it becomes obvious: fast is harmful; large shock waves emerge – invisible for the human eye but not for radar eyes. Two TanDEM-X radar images, taken ten seconds apart, create new knowledge, with which ice roads can be made safer in the future.

Understanding glaciers: view of the flow velocity of a glacier system at the Antarctic Ross Shelf Ice from radar-interferometry data. The red areas show high flow velocities of the glacier. This data delivers important insights for polar and climate research.

Tracking icebergs: what if a large ice mass breaks off the shelf ice and becomes an iceberg? That is also visible from space by radar – like shown here for the A68 iceberg, one of the largest icebergs ever observed. Huge icebergs are floating in the ocean, partly for years. The position and movement data are used to warn maritime traffic of icebergs.

Verifying glacier retreat: the Kangerdlugssuaq glacier is the largest glacier at the southeastern shore of Greenland. It keeps increasingly retreating. By means of a time series of 150 TanDEM-X elevation models, scientists of Swansea University in Great Britain have measured the extent of height loss of the glacier. Between 2016 and 2018, it retreated more into the interior of the country than ever before in 80 years of observation.

More safety for places where people are living and working

Preventing damage to buildings: Berlin Main Station. Its steel construction expands in summer due to heat and shrinks in winter due to cold. Areas shown in red shrink by two centimetres in the course of the year, areas shown in blue expand by two centimetres. In this way, spots become visible that structural engineers should closely watch to prevent long-term damages.

Identifying doubtful building land: Mexico City. In four-month intervals, the soil subsided partly by ten centimetres, in this area-covering precision only detectable via radar from space. One reason: withdrawal of ground water. Insight: in the red areas, the risk of collapsing buildings has considerably increased.

Highly accurate 3D map of the Earth comes from Germany

In 2018, DLR published an elevation model of the entire Earth with 90-metre scanning – freely available for scientific use. In addition, there are, for example, more accurate models available with 30- or 12-metre scanning. In total, over 2,400 scientists from 70 different countries are working with the radar data of TanDEM-X und TerraSAR-X. The data is accessible on http://download.geoservice.dlr.de/TDM90/

This worldmap of forests also comes from Germany

What about forest areas all over the world? DLR makes them visible. With a consistent worldwide forest map that delivers a coherent overview for the first time. The global TanDEM-X forest map with a resolution of 50 metres is freely available for the scientific community. Interferometric data of the global TanDEM-X elevation model, evaluated with intelligent algorithms, show where forests are located. If you compare the dataset with older or recent data, forest damage, deforestation and loss caused by natural disasters can be made visible worldwide.

Radar: on airports, in space, and in cars

A radar device emits electromagnetic waves, they hit objects, are reflected more or less, and these echoes, in turn, are received by the radar device. In this way – independent of lighting or cloud coverage – among other things contours are made visible and distances can be calculated (from the time elapsed between transmitting and receiving). Radar has been initially used (and is still used today, of course), to locate aircraft and ships, and for weather forecasts (precipitation, wind, and clouds). Today, it can be found in an increasing number of cars (adaptive cruise control radar) – and on board the satellites.

The selection of the frequency range of the radar transmitter determines the field of application

X band: 9.6 gigahertz, wavelength 3.1 centimetres.
This is the range used by TerraSAR-X/TanDEM-X.

L band: is used, among other things, for airspace monitoring. In DLR’s Tandem-L mission concept, a wavelength of 23.6 centimetres is used. Two radar satellites in the L band are to create the prerequisites for tomographic capture of the three-dimensional structure of vegetation areas (e.g., forested hills) and ice areas as well as the large-scale measurement of deformations with millimetre precision, or the soil humidity – thus contributing to a better understanding of the Earth’s system and its dynamics. The aim of Tandem-L is to interferometrically depict the land mass of the Earth, forests, and biomass in weekly intervals. Tandem-L is to enable for the first time that seven essential climate variables can be measured simultaneously within a satellite mission. Tandem-L is intended to be the first mission worldwide for systematic and high-resolution monitoring of dynamic processes in the bio-, geo-, cryo- and hydrosphere. More than 80 research institutes are highly interested in using Tandem-L data and are members of the scientific team of Tandem-L.

Tandem-L should facilitate the systematic observation of a multitude of dynamic processes on the Earth’s surface. By the use of modern radar technologies, the stringent scientific requirements for observation interval, resolution, and data quality can be optimally fulfilled. © DLR

Comparison of X band and L band

While radar waves in the X band (like with TerraSAR-X/TanDEM-X) are mostly reflected at the surface of the vegetation cover, the L band penetrates down to the soil. Only radar systems featuring a long wavelength (L, or even better: P band) are capable of penetrating the entire vegetation cover and thus of receiving signals from all areas of vegetation, with the help of which the volume and the biomass can be determined.

F-SAR: making use of the advantages of several frequency ranges

Traunstein forest lands: F-SAR image © DLR

Each radar frequency band has its advantages, and they can be jointly used. The solution: sending and receiving as many frequency bands as desired by the same satellite (or research aircraft). This means several radar devices have to be on board – and this is exactly what is tested by DLR scientists, for example in a forest near Traunstein as shown on the image. The F-SAR system of DLR’s Microwaves and Radar Institute allows for simultaneous measurement in different wave lengths. To scan the upper area of the treetops in the forest, radar sensors in the C band and X band are used, whereas the L band penetrates the vegetation and reveals the forest soil, so to speak. With one flyover only, the F-SAR is capable of capturing different levels of an area – like near Traunstein as shown on the image.

W band: 75–100 GHz. Radar can increasingly be found on board of cars, for adaptive cruise control (ACC) and emergency brake assistants. They use the W band (to be precise: 77 GHz) to scan the environment. The wavelength is very short (about three millimetres), this being the reason that antennas are very small and fit into a car’s radiator cowling. The range in cars is about 150 metres. The unpleasant radar technology in road traffic – radar traps – mostly uses the Ka band (26.5–40 GHz) and the K band (18–26.5 GHz).

TerraSAR-X – the German radar eye in space

The radar images previously shown here are coming from TerraSAR-X, a German Earth observation satellite launched in 2007. With an X band radar sensor being operated in different modes, it takes images for research and development as well as scientific and commercial applications. The satellite is orbiting the Earth at an altitude of 514 kilometres on a polar orbit. Through its active antenna, it delivers radar data with a resolution of up to one metre, independent of weather conditions, cloud coverage, and daylight.

TanDEM-X – the Earth in three dimensions

Three years after, TerraSAR-X was followed by its “twin satellite” TanDEM-X. Both are flying in formation at a distance of a few hundred metres and thus allow for simultaneous images of the area from different angles of vision. From these images, precise elevation information is derived in a 12-metre grid and with a vertical accuracy of less than two metres.

The goal of the TanDEM-X mission (TerraSAR-X add-on for Digital Elevation Measurement) is a high-precision, three-dimensional image of our Earth in uniform quality and hitherto unprecedented precision. This goal has been achieved. For large parts of the Earth, only rough, patchy, or fragmentary elevation models were available from different data sources and collection methods. TanDEM-X closes these gaps and provides a homogeneous elevation model as an indispensable prerequisite for many commercial applications and scientific questions.

Soft coal opencast mining (near Juelich): Here for comparison, the elevation model of the Shuttle Radar Topography Mission (SRTM) of 2000 and that of TanDEM-X of 2010. The higher precision and the significant progress in opencast mining are obvious.

Monitoring the growth of rice: the video shows the growth of rice on fields near the border of Turkey and Greece in the period from July to September 2012 precisely to a few centimetres. With such evaluations, for example farmers will be able to evaluate the suitability of soils, fertilisation and watering.

TerraSAR-X & TanDEM-X Science Service:
data for science all over the world

Researching on climate change: permafrost observation of the Lena delta in Siberia.

TerraSAR-X and TanDEM-X are realised on behalf of DLR, funded with means provided by the Federal Ministry for Economic Affairs and Climate Action. They are the first German satellites that have been realised within the framework of a so-called Public Private Partnership (PPP) between the DLR and Airbus Defence and Space: the utilisation of the data for scientific purposes is the responsibility of the DLR. Scientists from all over the world file project suggestions at the DLR and will receive data at very low cost, for example for projects like the following:

Monitoring of permafrost regions
Monitoring of sea ice, glaciers, and icebergs
Biomass monitoring of rain forests
Monitoring of the surroundings of rubbish dumps
Observation of dangerous soil movements, like, for example, landslides and topplings
Monitoring of infrastructure
Monitoring of the implications of mining

WorldDEM™: the new standard for global elevation models

Airbus Defence and Space takes over the commercial merchandising of the data. One offer: WorldDEM™. Its precision exceeds that of any other global satellite-based elevation model presently available and sets a new industry standard – with a complete coverage from pole to pole.

In addition, the WorldDEM Digital Terrain Model (DTM) is available that builds on the WorldDEM™ Digital Surface Model (DSM). Vegetation and human-made objects are deleted to exclusively show the mere surface of the Earth. Such data are needed, for example, for road construction or the management of natural resources.

Radar echoes for more precision

To achieve such a high precision with TerraSAR-X and TanDEM-X, theoretically a radar antenna with a length of 15,000 metres would be required. This is why the scientists applied a technical trick to get along with an antenna with a length of five metres only. The satellite (and thus the antenna) passes objects on the ground along their trajectory. In doing so, impulses are transmitted, and their echoes received in regular intervals. Then the data are radioed to Earth, where the echoes of the radar impulses are put together in a data processing centre to form images. This calculation process is called “aperture synthesis”.

Technical data

Height: 5 metres, diameter: 2.4 metres
Weight: 1.3 tons
Solar cells: 5.25 square metres
Power consumption: 800 watts
Radar antenna: length 5 metres, width 80 centimetres
Radar frequency: 9.65 gigahertz (X band, wave length 3 centimetres)
Resolution: e.g. 10 x 10 kilometres with a resolution of 1 x 2 metres or a band with a length of 100 kilometres with a resolution of 16 metres
Precision of position data: 0.5 metres
Time between two observations of the same region: 2–11 days
Data rate: max. 300 Mbit/s
Launch: June 15, 2007 (TerraSAR-X) / June 21, 2010 (TanDEM-X)
Altitude of orbit: 514 kilometres
Data reception, mission operation: DLR (Neustrelitz, Oberpfaffenhofen, Weilheim), in addition: Inuvik (Canada), Kiruna (Sweden), Svalbard (Norway), O’Higgins (Antarctic)

Secondary payloads

Laser Communication Terminal (LCT): data transmission via laser, up to 5.6 Gbit/s over 5,000 kilometres. With this data rate, it would be possible to send about 200 HD TV channels simultaneously to Earth. For comparison: on Earth, such velocities are also possible in normal glass fibre networks, however, with a maximum reach of 80 kilometres.

Tracking, Occultation and Ranging Experiment (TOR): high-precision trajectory determination of the satellite with an accuracy up to ten centimetres based on the dual-frequency GPS receiver IGOR as well as a laser reflector unit.

For a quick overview: Sentinel-1A and -1B

Besides the TerraSAR satellites, new radar satellites complement the data offer: Sentinel-1A (in space since 2014) and Sentinel-1B (since 2016). They are part of the European Copernicus programme and provide large-scale, freely accessible data that enable monitoring of the development of glaciers or volcanic fields.

However, the Sentinel-1 satellites do not scan the Earth in the X band with a wavelength of three centimetres (as the TerraSAR satellites do) but in the C band with six centimetres. This is not such a high resolution, but the data volumes are not that high and can be faster processed on Earth, e.g., in case of flood waters. In some fields of research, the data from X and C band can be combined to gain new insights.

When the water rises … or the Earth subsides