High Resolution Stereo Camera (HRSC): The Red Planet in 3-D
The High Resolution Stereo Camera (HRSC) is Germany's most important contribution to the European Space Agency (ESA) Mars Express mission. The main objective of this mission, which reached Mars in late 2003) is the search for traces of water and signs of life. During the expected four-year mission, seven instruments on board the orbiter will gain new insights into the composition and geology of the surface of Mars and the atmospheric composition by means of various remote sensing experiments. The instruments on board the orbiter have been designed specifically to map the surface of Mars in high resolution for photo-geological and mineralogical investigations, and to study the Martian atmosphere and its interaction with the interplanetary medium.
The High Resolution Stereo Camera, developed at the DLR Institute of Planetary Research, is a unique experiment. For the first time, a special camera on board a spacecraft is continuously acquiring images of the planetary surface in three dimensions and in colour. The results will enable researchers to answer fundamental questions about the geological and climatic history of the Red Planet. The spatial resolution of the stereo images surpasses previous topographic data of the surface of Mars from a distance, and allows geoscientists to analyse details down to between 10 and 30 metres in three dimensions. As a special feature, the camera contains an additional, ultra-high-resolution telephoto lens. With this Super Resolution Channel (SRC), imaging objects two to three metres in size - embedded in the stereo image data - is possible. This allows, for example, identifying boulders the size of a garage or stratification in sedimentary rocks.
The HRSC camera system, which only weighs 20 kilograms, has two camera heads: the High Resolution Stereo head, which consists of nine CCD line sensors mounted in parallel behind a lens, and the SRC head, which is composed of a mirror telephoto lens and a CCD array sensor. The High Resolution Stereo head works on pushbroom mode: sensors image a line on the planet surface perpendicular to the ground track of the spacecraft and rely on the orbital motion of the spacecraft to reposition them as they record a sequence of images known as an image swath. In this case, each sensor records the same object on the surface at a different angle. Three-dimensional images are generated by five of the image strips. The remaining four of the nine line sensors are equipped with special colour filters for recording multi-spectral data.
The shortest distance from the spacecraft to Mars is 270 kilometres. At this altitude, the resolution of the 9 image strips is 12 metres for each of the 5184 seven-micron square pixels. The image swath is 52 kilometres and the minimum strip length 300 kilometres. The latter depends on the spacecraft's data storage and transmission capacity. The Super Resolution Channel (SRC) is used as a magnifying glass. At the pericentre, it provides pictures 2.3 kilometres by 2.3 kilometres wide in the centre of the image strips; the surface details are imaged with a resolution of 2.3 metres per pixel. The SRC recordings provide a geological context of the area, which is supplied by the high-resolution images acquired with the stereo head.
Before the data is transmitted, the images are compressed in the HRSC unit and stored in the spacecraft. On Earth, Mars recordings are first systematically processed at the DLR Institute of Planetary Research, and then distributed for further processing and analysis to the over 40 co-investigators from 33 institutions and 10 countries that make up the HRSC science team.
Since 1997, two versions of the HRSC been modified for aircraft use. These have already demonstrated the robustness of the HRSC design and the scientific value of HRSC technology in various flight campaigns.
The HRSC camera on Mars Express was developed by the DLR Institute of Planetary Research, which is responsible for its operation as well as the processing and distribution of the image data throughout the entire mission. The systematic processing of the HRSC image data is carried out at DLR and scene processing is carried out by the PI-group at the Institute for Geosciences of the Freie Universität Berlin in cooperation with DLR's Institute of Planetary Research, Berlin. The German HRSC-SRC experiment on board Mars Express is headed by Principal Investigator (PI) Prof. Ralf Jaumann, from the DLR Institute of Planetary Research.
