Le Bourget 2007 – Sixteen years after the launch of the first European scientific radar satellite ERS 1, this Earth observation technology will now be used for commercial purposes, too. In the coming five years, the German TerraSAR-X satellite is scheduled to supply data for a wide range of applications. TerraSAR-X is the culmination of a long and successful scientific and technological development in Germany which began at the end of the 1970s with the keen support of the German Aerospace Centre (DLR). It uses a radar system in so-called X-band a technology demanding high production standards. Based on many years of experience in this field, Europe’s leading space company Astrium (Friedrichshafen) has developed a technical masterpiece TerraSAR-X. Astrium could use proven technology for the satellite platform thus keeping down costs. The long-term objective is to regularly operate radar satellites in the Earth’s orbit, which has been practiced with the weather satellites for many years.
Radar beams scan the Earth
Passive Earth observation satellites have been well established for many years. They image the surface in the range of the visible light or infrared spectrum. Everybody knows their disadvantage from the meteorological images on TV: clouds often obstruct the view. This is the reason why researchers have developed the radar satellites. They emit radio waves which are reflected by the Earth’s surface and received again by the satellite.
This method has several advantages. Firstly, the waves penetrate the clouds and, secondly, they produce their own defined illumination. This is different from optical satellites where the appearance of a specific terrain depends on the current position of the Sun. Furthermore, the radar’s operation at night is just as good as at day.
The antenna of a satellite-based radar system emits a short high-frequency pulse and records the echo returned from the Earth after a specified period of time. As different materials have different reflectivity properties, the received signal contains a wealth of information about the type of the surface or its vegetation. To produce an image, a so-called Synthetic Aperture Radar (SAR) is needed. Its radar beam scans a wide surface strip during flight, and a computer reconstructs the surface from the reflected signal.
Already at an early stage, the German Aerospace Centre (DLR) has speeded up the development of next-generation radar technologies by substantial funds and studies. This included, in particular, the modern, state-of-the-art antenna demonstrator now enabling the implementation of the TerraSAR-X sensor. Already in 1978, the capability of SAR technology could be successfully demonstrated for the first time on a satellite; nine years later, the engineers of Astrium in Friedrichshafen started with the development of an X-band radar system on behalf of DLR. Up to that time, the radar satellites operated in C- or L-band. The radio signals emitted in this spectrum have wavelengths of 5.7 and 24 centimetres respectively. The X-band emits pulses with a wavelength of 3 centimetres. It is thus possible to obtain a higher image resolution (sharpness). However, the requirements of the X-band technology with regard to production are extremely demanding. In these fields, the Astrium specialists had to break new grounds in technology.
Thus, for example, the surface accuracy requirements of the antenna were extremely exacting because of the short wavelength in the X-band. Furthermore, despite the extreme temperature variations, the instrument must not change its shape and size in space. Finally, carbon fibre-reinforced plastics proved to be the ideal material. It is thermally stable, possesses a high mechanical stiffness and a low density. These properties save weight.
The complete satellite has a compact structure. The five-metre-long body has a hexagonal cross-section with the solar arrays on one side. The satellite’s orbit and orientation have been selected so that the cells are always on the sun-facing side. Transversally the flight direction, the radar antenna looks downwards in an inclined angle. The antenna with a length of 4.80 metres and a width of 80 centimetres consists of 384 individual sub-arrays which are technically called slotted waveguide radiators. The radio pulses are produced within the satellite, directed to the outside via waveguides and radiated towards the Earth through small slots.
As the slotted waveguide radiators consist of non-conductive carbon fibre material, they cannot transmit the radio waves themselves. This was only possible by applying an extra-thin silver coating to the surfaces. “It took as much as two years to develop an appropriate production method and to determine the proper layer thickness”, stated Wolfgang Pitz, TerraSAR project manager at Astrium. All in all, "X-SAR" technology involved about ten years of development work.
A “steerable” radar system without moving parts
The active radar is ground-breaking for all future radar satellites. It allows the electrical alignment of the beam perpendicular to the flight direction in a slewing range of 20 to 60 degrees without having to move the satellite. Pitz illustrates this method as follows: “This can be compared with a person walking while simultaneously raising his/her arm to different positions at the side of the body”. A minor displacement by 0.75 degrees is also possible in flight direction. The advantage is obvious: considerably more targets can be located from this orbit than with a fixed radar system.
Furthermore, the time of focusing the beam on an area during satellite fly-over can be increased with active radars. This enhances the intensity of the received signal which results in a higher resolution, i.e. detail accuracy. In the so-called Spotlight mode, an area of 5 by 10 square kilometres can be scanned with a resolution of 1 metre. This, for example, allows better identification of cars. In the medium Strip Map mode, a swath with a width of 30 kilometres and a length of up to 1,500 kilometres is scanned by the antenna with a resolution of three metres. The “wide-angle objective” (ScanSAR mode) provides a resolution of 16 metres in an area of 100 by 1,500 square kilometres. For comparison: Europe’s radar satellite ERS-1 provides images with a resolution of 30 metres.
In Spotlight mode, the image can be taken from many different orbit positions. The maximum access time for any place on Earth is four days. In ScanSAR mode, the area can be imaged in the desired format from a few orbit positions only. Therefore, the maximum access time is eleven days.
In addition to the variable mode of operation, the technical design is another fascinating feature of the active radar. To change the viewing direction, it is neither necessary to swivel the antenna nor to turn the whole satellite; this would need too much time. “Turning” is performed by simply superimposing the signals of the individual transmit antennas. In this case, the waves in one direction area destroy each other (destructive interference) whereas they add up in another direction (constructive interference). In this way, depending on the desired direction, some parts are decreased and other ones increased.
A specialty of TerraSAR-X is the “Dual Receive Antenna Mode”. This is an experimental mode allowing the detection of motions on the ground. Technically, this mode is obtained by the following trick. Almost all on-board elements are available in duplicate so that the task can be assumed by a spare part in the event of a component failure. With the exception of the antenna, this applies to the complete radar electronics. In dual-receive mode, the antenna is split into two parts, and each part is operated by one of the radar electronics systems. Thus, two antennas are operating independently of each other and their data are recorded separately. The satellite operates as if it had two eyes for viewing the movements occurring between two pulses on the ground.
It is intended to use this so-called along-track interferometry for traffic flow measurement on motorways, for example. Another possible application is the tracking of movements of ships and other vehicles. Already in 2000, the functional capability of this technology was demonstrated in the scope of the Shuttle Radar Topography Mission (SRTM), a joint mission between DLR and NASA with the German astronaut Gerhard Thiele on board. Astrium was prime contractor for the X-SAR system.
Another technical detail is the possibility of operating in two polarisation directions. This means that all emitted waves oscillate either horizontally or vertically in the same level. This oscillation direction changes upon reflection from the ground; this provides additional information about the object features or the ground composition.
The measured data are stored in a fixed 256 Gbit memory on board, and each time there is a contact with the ground, they are sent to the DLR ground station in Neustrelitz with 300 Mbit per second. The experimental data transmission unit “Laser Communication Terminal“ is also on board. This instrument financed by DLR and developed by TESAT Spacecom (Backnang) shall be used for testing rapid data transfer with several Gbit per second via laser beam.
Cost-saving design
Costs had to be saved to a large extent in order to ensure the commercial success of TerraSAR-X – but, of course, without a loss in quality. This objective could only be reached by long-term and substantial promotion of radar technology by DLR. Astrium, on the one hand, could benefit from its many years of experience in radar construction. On the other hand, the company could make use of proven satellite technology. Thus, the complete satellite is based on the AstroBus concept. AstroBus is a sophisticated basic architecture for the satellite bus combining components which have already been proven in space missions and individual mission-specific elements. To a large extent, TerraSAR-X makes use of the AstroBus of the geo-satellites CHAMP and GRACE. This was the only way to build such a complex satellite with a finance volume of approx. 130 million euros.
About Astrium:
Astrium, a wholly owned subsidiary of EADS, is dedicated to providing civil and defence space systems and services. In 2006, Astrium had a turnover of €3.2 billion and 11,000 employees in France, Germany, the United Kingdom, Spain and the Netherlands. Its three main areas of activity are: the business units Astrium Space Transportation for launchers and orbital infrastructure, and Astrium Satellites for spacecraft and ground segment, and its wholly owned subsidiary Astrium Services for the development and delivery of satellite services.
EADS is a global leader in aerospace, defence and related services. In 2006, EADS generated revenues of €39.4 billion and employed a workforce of more than 116, 000.
Press contacts:
Mathias Pikelj +49 (0) 7545 8 91 23
Hendrik Thielemann +49 (0) 89 607 27244
TerraSAR-X at a glance
Height: 4.88m
Diameter: 2.4m
Launch mass: 1,230kg
of which payload: approx. 400kg
Radar frequency: 9.65 GHz
Power consumption: 800 Watt (average)
Resolution: 1m, 3m, 16m (depending on image size)
Launch vehicle: Dnepr 1 (former SS-18)
Launch site: Baikonur, Kazakhstan
Orbit altitude: 514km
Tilt angle towards equator: 97.4° (Sun-synchronous)
Life time: at least 5 years
TerraSAR-X is the first German satellite implemented in a so-called Public-Private Partnership (PPP) between DLR and Astrium: Europe’s leading satellite specialist. Astrium contributes to the costs of development, construction and deployment of the spacecraft. The scientific exploitation of TerraSAR-X data is the responsibility of the German Aerospace Centre (DLR), while Infoterra GmbH, a subsidiary of Astrium, is responsible for commercial marketing. Surrounding the Earth on a polar orbit at an altitude of 514 kilometres, TerraSAR-X with its active antenna will collect new-quality X-band radar data of the entire planet. TerraSAR-X operates independent of weather conditions, cloud cover and illumination and will be capable of delivering radar data with a resolution of up to one metre.
