Pinpointing images precisely

The HRSC will take more accurate and detailed images of the surface and atmosphere of Mars than any other camera before. It will image the entire planet in full colour, in three dimensions and at high resolution. Its greatest strength, however, is its unprecedented pointing accuracy, which it achieves by combining images at two different resolutions. This makes it particularly useful for, among other things, assessing potential landing sites for future missions to Mars.

The High/Super Resolution Stereo Colour Camera.

The camera, which weighs 21.2 kg will take 3D colour pictures of the Martian surface and of atmospheric phenomena, such as cloud cover and dust storms, at a resolution of 10-30 m. At the same time, a super resolution channel will allow it to home in on selected sites of particular interest for imaging at 2 m per pixel resolution. "As the 2 m resolution image is nested in a 10 m resolution swath, we will know precisely where we are looking," says Gerhard Neukum, Principal Investigator for the HRSC in Berlin. Although cameras on other spacecraft have imaged small areas at high resolution, or large areas at low resolution, they have never before combined the two and hence have been unable to determine the location of high resolution images to better than a few kilometres.

"The 2 m resolution channel will just allow us to pick out Beagle 2 (the Mars Express lander) on the surface," says Neukum. That means the resolution will be sharp enough to see whether a potential landing site that looks smooth and flat at the lower resolution is, in fact, strewn with small boulders or crevices that could upturn or damage a small spacecraft landing on top of them.

Full colour and 3D

The HRSC's images will be in 3D and full-colour. The 3D imaging will provide a measurement of the height of features within an image relative to each other. The vertical resolution will be similar to the horizontal resolution. The camera's imaging capabilities will be achieved by ten channels, each consisting of a CCD (charge coupled device). One will be the super resolution channel and the other nine will record the same image simultaneously at the lower resolution. Four of these channels will record the image at different wavelengths (colours), and five will view it from different angles to provide the 3D effect.

When the camera is 300 km above the surface, its footprint will be 62 km by 206 km. The diagram shows the CCD lines except for the super resolution channel. The five white lines are used to provide the stereo effect.

A super resolution pixel inside a high resolution pixel. At 300 km above the surface, the high resolution pixel is 12 m x 12 m and the super resolution pixel is 2.75 m x 2.75 m. One super resolution image covers 4% of one high resolution image.

Stereo-imaging is achieved by viewing the same target from different positions.

During the four Earth-year lifetime of the nominal and extended Mars Express missions, the camera will gradually build up a full-colour, 3D image of the entire surface of Mars at the lower resolution. About 1% of the surface will be imaged at the higher resolution. The complete image will provide an unprecedented map of the topography of the Martian surface, which will be an excellent tool to complement many types of study including understanding the interior of Mars through gravity measurements and understanding the climatic and geological evolution of the planet. "We will be able to see the cloud, the fog, the dust devils (swirls of dust sometimes reaching kilometres in height) and the ice cap coverage," says Neukum, adding that "the morphology of the northern water ice cap and its surrounding layered terrain and dune fields, are particularly important for understanding climate history."

Although the HRSC will provide 3D images allowing estimates of the relative heights of features on the surface, it will not measure surface heights relative to the centre of Mars as accurately as the Mars Orbiter Laser Altimeter (MOLA) on board NASA's Mars Global Surveyor spacecraft, which is now in orbit around the red planet. The combination of MOLA data with HRSC images will be "absolutely fantastic", says Neukum and should settle, among other issues, the question of whether an ancient ocean once filled most of the northern hemisphere.

Viking image of Kasei Vallis (left), the largest outflow channel on Mars, and a digital terrain model (right) derived from Viking stereo images.

Coherent, contiguous coverage

Two different interpretations of MGS images in 1999 claimed to have found evidence and no evidence for an ancient coastline. The existence of a coastline, however, is difficult to see using MGS alone because the images, although high resolution, are discontinuous. According to Neukum, the "coherent, contiguous coverage" provided by the HRSC should make a coastline clearly visible, if it exists. "You need to see it with your own eyes, if you are to believe it," he says.

Each member of the HRSC's 35-strong team of co-investigators from nine countries will be putting in requests for sites to home in on with the 2 m channel. Many targets will be chosen for their potential to solve the mystery of what happened to the water on Mars. "One major target will be the Kasei Vallis which ends in a river delta at the western end of the Chryse Basin. It's the biggest outflow channel on Mars," says Neukum. "The canyon system is also very important because it tells us about the tectonic and volcanic activity on Mars," he adds.

Remote sensing closer to home

Since the failure of Mars 96, for which the HRSC was originally developed, the HRSC team have kept busy refining the qualification model for remote sensing on Earth from an aircraft. As an aircraft flies at a lower altitude than a satellite, resolutions of 10-20 cm are possible. Such resolution, combined with contiguous coverage, could not be achieved on Mars, at least in the foreseeable future, as the optical systems would be too large to fly on a spacecraft. Also the global positioning system of satellites (GPS) in orbit around the Earth enable very precise determination of the position and orientation of the camera with respect to a terrestrial reference system. Mobile phone companies, in particular, are finding the highly accurate terrain models generated by HRSC flights useful for planning where to put transmitters.

Stereo image of the Reichstag in Berlin taken with the HRSC on an aircraft flying at a height of 3 km.

Principal Investigator: Professor Gerhard Neukum, Freie Universität, Berlin, Germany

For further information, see related links.

	ASPERA: Energetic Neutral Atoms Analyser
	MaRS: Mars Radio Science Experiment