The German RADAR Eye in Space

Figure 1. One of the most dangerous volcanoes of the world, the Merapi on Java in Indonesia: TerraSAR-X data are important bases for rapid mapping activities in the case of natural disasters.

Figure 2. Viedma Glacier in Patagonia: Remote sensing data provided by TerraSAR-X obtained from space complete Earth-based measurements, and are essential for monitoring the polar ice caps to analyze the effects of climate change.

Thomas Feske, Agnès Mellot,
Dr. Jürgen Drescher
German Aerospace Center (DLR)
Washington, D.C.

TerraSAR-X is Germany's first national remote sensing satellite implemented in a public/private partnership between the German Aerospace Center (DLR) and Infoterra GmbH/EADS Astrium, with a significant financial participation from the industrial partner. This commercial radar satellite will acquire high-quality radar data for scientific observation of Earth, for disaster monitoring, and for geoinformation purposes for at least five years. At the same time, it is designed to satisfy the steadily growing demand of the private sector for earth remote sensing data in the commercial market.

TerraSAR-X (TSX) was launched in June 2007 on a Russian/Ukrainian DNEPR-1 launch vehicle from the Baikonur Cosmodrome in Kazakhstan. It delivered TerraSAR-X into a 514-km high, near-polar orbit. The orbit was selected so that the satellite flies in a sun-synchronous, dusk-dawn orbit. This means that the satellite moves along the day-night boundary of the earth and always presents the same face to the sun. While TerraSAR-X circles the earth, the satellite can take images of all regions of the earth with a resolution up to 1 meter in a spotlight mode, scanning Synthetic Aperture Radar (SAR) and strip-map manner, until after each 11 days it returns back to its original position and begins a new cycle. With different angles of view, each point of the earth can be targeted within two to four days.

The TerraSAR-X satellite is operated from the German Satellite Operation Center (GSOC) in Oberpfaffenhofen near Munich. The remote sensing data downlink is provided to the ground station at DLR Neustrelitz. A special law covering aspects of data security and the commercial distrib-ution of high-resolution remote sensing data was put in place after negotiations on an intergovernmental level.

Scientific Utilization

In the 21st century, it is difficult to imagine our modern life without satellite services, whether the service is the daily weather report using meteorological satellites, the live transmission of an ongoing event from remote areas using modern communication satellites, or the navigation of airplanes, cars and ships on the world's oceans by means of global posi-tioning system satellites.

These services that in recent times have become more and more part of our daily lives are possible thanks only to technologically advanced satellite missions operating in space combining remote sensing, navigation and communication services.

Figure 3. Tourist and agricultural land use on Mount Egmont in New Zealand: TerraSAR-X supports agricultural mapping services through multitemporal and multipolarization observations and facilitates large area assessments.

Sensor systems based in space are used for applications on science and earth observation, be they for research focusing on alterations of land surfaces, oceans, and the earth's atmosphere; for climate research; for the monitoring of disasters for geological investigations; or for many more uses. Figure 1 reflects volcano activity in Indonesia, where an immediate observation can be vitally important.

Remote sensing data obtained from space thus complement and complete ground-based measurements. They are always essential if a global view of the earth is required, as is the case, for example, with weather monitoring or with investigation of the polar ice caps. Figure 2 shows the Viedma Glacier in Patagonia.

With the new TerraSAR-X radar satellite, land masses of the earth, as well as ocean surfaces, are monitored and closely inspected. With increasing technical cap-abilities of these sensors, geoinformation data can be extracted from satellite images with ever more precision and with broad applications.

Because of one of the outstanding features of TerraSAR-X, the high spatial resolution that has been achieved with this civilian radar system will offer completely new perspectives for the monitoring of detailed ground infrastructures and resources. Figure 3 is an example for land use classification on Mount Egmont in New Zealand. Examples are monitoring of vegetation and the separation of different plant species, as well as the precise analysis of urban environments in towns and villages. Figure 4 shows copper surface mining in South America.

Radar technology provides velocity information of moving objects, and uses the Doppler shift effects in the signal analysis. So traffic monitoring on the ground is an additional feature and a useful tool, broadening the application spectrum of TSX. Figure 5 demonstrates TerraSAR-X capabilities for detection and tracking of moving objects.

Figure 4. The biggest man-made hole, a copper surface mining operation in the center of the Atacama Desert on the west coast of South America: TerraSAR-X offers completely new perspectives for the monitoring of urban environments, as this example shows, observong long-term surface displacement.

Figure 5. The highly frequented Strait of Gibraltar in Spain: Along Track Interferometry allows, among other things, the detection of moving objects such as cars or ships.

The Satellite

With TerraSAR-X, a modern radar system is used for Earth remote sensing purposes. The technology of SAR is able to produce high-resolution images of the earth's surface, similar to photographic images. A SAR has a number of advantages compared with optical systems; for instance, radar is independent of any illumination by the sun. The measurements can be taken around the clock at any time of day or night under any weather conditions. This radar sensor technology is to a large extent independent of weather conditions such as cloud coverage. This attribute contributes significantly to the operation, application and reliability of the system, qualities that are increasingly requested by many users, since data are often required at a certain point in time regardless of weather or time of day.

With SAR, the earth's surface is "illuminated" with short pulses radiated by a radar antenna. The radar pulse is reflected from the earth's surface, and the so-called radar echo is received by the antenna and recorded. In order to achieve a high spatial resolution, a technical trick is applied: The satellite with the SAR instrument moves at high velocity over the earth's surface. During the over flight, the echoes of many radiated radar pulses are summarized. The result is equivalent to a very large radar antenna (synthetic aperture), proportional to the distance the satellite traveled in this period of time. With this technique the spatial resolution is increased in the flight direction, since it depends upon the size of the antenna.

A SAR system like TerraSAR-X can be operated in different imaging modes, in order to achieve various results. The active antenna enables users to switch rapidly among different imaging methods:

  1. In the spotlight mode, the radar image records an area of 5 to 10 by 10 km size. In this manner a maximum resolution of up to one meter is achieved.
  2. In the strip map mode, the satellite images a strip of 30 km width and a maximum length of 1,500 km. The resolution is three meters.
  3. In the ScanSAR mode, a strip of 100 km width and 1,500 km maximum length is scanned with a resolution of 16 meters.

The approximately 1.3-ton Terra-SAR-X spacecraft is based on the EADS Astrium Flexbus concept and has an extensive heritage from the successful Challenging Mini-satellite Payload "CHAMP" and from Gravity Recovery and Climate Experiment "GRACE" missions. The 5-meter-long and 2.4-meter-wide satellite bus features a structure with a hexagonal cross-section. One of the six sides carries the 5-meter-long and 80-cm-wide radar antenna. The electronic boxes of the SAR instrument and the satellite bus are also fitted on the side faces of the structure, in just the same way as the satellite's solar generator, 5.25 square meters in size, which ensures the supply of energy by means of gallium arsenide solar cells.

The data recorded by the SAR instrument are transferred via a downlink antenna to the ground receiving station. The antenna is secured to a 3.3-meter-long mast in order to avoid interferences caused by the radar antenna. The mast is folded up during the launch and is extended only after positioning of the satellite into its orbit. It allows simultaneous acquisition of new data by the radar and transmission of previously stored data to the ground. The technology of the active, phase-controlled antenna enables a high flexibility and mission efficiency.

While in the case of a passive system, the whole radar antenna or even the satellite must be rotated in order to align the antenna onto the target area, the active antenna of TerraSAR-X can steer its radar pulses in a specific direction. The antenna is 4.8 meters long and 80 centimeters wide. The satellite is designed in a way that it can be installed, together with its antenna at its full size, on the launch vehicle. In this way, a complex unfolding mechanism can be avoided.

With radar instruments, various frequency ranges of the radar spectrum can be observed, the so-called bands. Terra-SAR-X is operated in the X-band, which is lying at a frequency around 9.65 GHz, corresponding to a wavelength of about 3 cm.

Thanks to the exact information concerning the contours of the earth—gained with the TerraSAR-X—scientists can, for example, predict along which routes the water will flow on the surface of our planet. By means of "virtual flooding" of the digital landscape on the computer, it is possible to simulate the effects of long duration rainfalls and to predict flooded areas and their effects on the environment.

When exact height data is combined with information concerning the natural cover and the surface structures created by people, such as streets and buildings, construction companies are better able to plan power lines, oil pipelines, and also railway lines and bridges on the computer. Here the digital map replaces or supports the land surveyor on site.

With the aid of a landscape model, mobile telephone companies can simulate the propagation of the radio waves and determine the optimum positions for their antenna masts. While still in the planning phase, it is possible to expose "radio holes" and remove them by careful positioning of the masts. By now, one year after the launch of TerraSAR-X, because of these technical possibilities there are many requests from private companies for the digital data.

Public/Private Partnership

In the past, space projects have been almost exclusively financed by the state, due to their high costs and global mission objectives. With further technological development, particular areas of space activities have also begun to interest the private market. Today remotely sensed Earth data and the information extracted from this data are also increasingly required for private-sector applications alongside scientific utilization. It is therefore the objective of the national Earth remote sensing program to transfer the extraction of this type of data in the long term to the private sector, and to open a self-supporting, sustainable area of business.

For TerraSAR-X, a cooperation agreement forms the basis of the collaboration between the state and the private sector.This agreement was signed on March 25, 2002, by the DLR space agency and EADS Astrium GmbH. DLR placed a contract for the development, assembly and launch of the satellite to EADS Astrium. The DLR research institutes are undertaking the development of the satellite operating system, as well as the ground segment for reception of the radar data and its processing, archiving, calibration and distribution. Moreover, DLR is responsible for the operation of the satellite over a period of five years.

Apart from the direct financial contribution to the project, EADS Astrium GmbH is obliged to develop a portfolio of innovative TerraSAR-X-based products and services and to establish a global distribution network. For this purpose, EADS Astrium founded its 100% subsidiary Infoterra GmbH in 2001. Infoterra's portfolio comprises data products, applications, and distribution partnerships.

Commercial Utilization

These data products are of high interest on the free market. Infoterra offers Terra-SAR-X data in different levels of refinement. They range from Basic Image Products (raw image data that can be acquired in different imaging modes, polarizations, and geometric projections according to clients' specifications) to Enhanced Image Products (orthorectified images or mosaics from several scenes) all the way to Geoinformation Products that contain significant information such as change detections.

The concept for the commercial exploitation of TerraSAR-X includes the marketing of so-called Direct Access Services. These provide TerraSAR-X imagery either for immediate use by an end user, the Direct Access Customer, or for further distribution in a specific region of the world by a Direct Access Partner.

Direct Access Customers and Direct Access Partners around the globe will operate their own ground stations and be able to receive the data directly from the spacecraft. Infoterra GmbH establishes its distribution headquarters in Germany and provides data, specific geo-information, and TerraSAR-X applications to clients worldwide.

Legal Situation: Data Security

In order to carry out the mission successfully, various facilities are required on Earth–the so-called ground segment. The satellite is controlled from the DLR ground station in Weilheim, near Oberpfaffenhofen. Both the command channel and the data transfer will be encrypted to prevent unauthorized access. DLR Neustrelitz provides the TSX-data downlink capabilities.

Data acquired by TerraSAR-X have a quality that, until recently, could be produced only by classified military satellites. These data are security-relevant. Effects of weapons or political threats can be substantially strengthened by these earth remote sensing data.

In German law there had been no regulations concerning distribution of data or pictures of such quality. TerraSAR-X made a new law necessary. In addition, almost all efficient earth observing systems are dependent on U.S. construction units. The U.S. makes an export license for these construction units dependent on the fact that national regulations exist. Security interests must consider this when producing or distributing the earth remote sensing data.

Regulations of the new data security law in Germany are focused on examination of the distribution or accessibility of products of earth remote sensing data. This examination is necessary only if the data obtained from an earth remote sensing system were technically able to endanger the security interests of the Federal Republic of Germany. Great demands are made on operating with such high-quality earth remote sensing systems, demands that prevent satellites like the TerraSAR-X from being commanded by unauthorized entities and that protect important data from being inspected by unauthorized ones. For this reason, the administration of the earth remote sensing system must be approved and supervised.

The examination of the distribution or accessibility has to be arranged only in part by the provider of the data. By arranging this examination with sole responsibility, he could spread the data almost completely without official participation. Therefore, the provider of the data has to examine the data in connection with the concrete customer request, based on the possible endangerment of security interests. The examination takes place on the basis of specific criteria, which are given by the authority and which guarantee a fast, automated examination.

Relevant criteria are fourfold: the information content of the data; the person who inquires, as well as his customers; the requested target area; and the desired time. If the result of this examination is that no possibility of a security risk exists, then the data provider can distribute the data. Administrative proceedings are not necessary.

If the examination of the inquiry comes to the conclusion that security interests are possibly concerned, then the spread of data by the provider is subject to an official permission and certain limitations. The authority examines the inquiry and can permit it without change or can, in certain cases, limit the use and circulation of data of certain target areas, limit sending data to certain ground segments, limit permissible sensor operating mode or limit the quality of manufacturing of the data.

The Future

Designers are already planning a follow-up system that will replace the satellite at the end of its operational life, anticipating that this follow-up system will be financed solely by the industry from the profits achieved with TerraSAR-X. The project, TerraSAR-X add-on for Digital Elevation Measurement, (TanDEM-X), is intended to generate a global digital elevation model of all terrestrial landmasses with an accuracy that has not yet been reached. This precision can be achieved by complementing TerraSAR-X with the additional TanDEM-X satellite of nearly the same design in a tandem orbit configuration. The two satellites will fly in a tight formation with only a few hundred meters of separation in approximately the same orbit. TanDEM-X is designed for a mission duration period of five years.

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