Scientific Objectives

• Climate and the role of water throughout the Martian history
• Evolution of volcanism on Mars
• Shaping of the Martian surface throughout time and geologic processes involved
• Potential resources on Mars
• Characteristics of past, present and future landing sites
• Interactions between atmosphere and surface
• Mapping 100% of the surface with a resolution smaller than 30 meter / pixel
• Mapping 50% of the surface with a resolution smaller than 15 meter /pixel
• Observation of Phobos and Deimos

Picture of the HRSC- camera

Overview

Camera Head (1)

The HRSC's task is the mapping of most of the Martian surface. The resolution is 10 meters / pixel at an altitude of 250 km (point of closest approach to Mars).

SRC Super Resolution Channel (2)

This is the high resoluting channel with an resolution of down to 2.3 m per pixel. SRC images will provide detailed information about areas of special interest, e.g. for the examination of future landing sites.

Instrument Frame (3)

The Instrument Frame is the structure for the Camera Head (HRSC) and the SRC and provides thermal decoupling from the spacecraft and mechanical stability.

Digital Unit (4)

Camera Control Processor (CCP)

The CCP controls the entire camera system as its central processing unit. It interprets and executes the serial telecommands and control data of the spacecraft. The CCP is based on a microprocessor (80C86), a boot PROM, a RAM for data handling and program execution, and an EEPROM for application software. The EEPROM can be up and downloaded via normal telecommand / telemetry links for updating the software.

Data Compression Electronics (DCE)

The DCE consists of 4 Compression Units (CU) and the microcontroller (MC). The input buffers of the 4 CUs are directly linked to the four signal chains of the Camera Head. Each CU works individually and uses a parameter table controlled JPEG compression algorithm. After compression, an 80C31 microcontroller combines the data streams via multiplexing to a serial output.

Interface Electronics (IFE)

The IFE comprises two boards:

The IFE01 includes the low data rate interfaces between Camera Head, the Data Compression Electronics, spacecraft and the Camera Control Processor.

The second board IFE02 includes the fast interface electronics used for the transfer of the scientific image data to the spacecraft mass memory.

Power Supply Subsystem (PSS)

The PSS supplies the DU itself and the Camera Head. The power converter of the SRC is also located within the DU.

Heater Control Electronics (HCE)

The heater control system controls the thermal condition of the camera, which is thermally decoupled from the spacecraft. The dissipated energy from the Camera Head is removed via cooling straps and an external radiator. Simultaneously the camera is heated to a temperature of around 20°C via a system of 6 independent analogue heater control loops comprising thermistors and heaters on the optical bench and inside the Front End Electronics. The heater control logic can be switched on independently from the camera electronics.

Orbit

The Mars Express will have an elliptical orbit. On that account there are two special points, one in a minimum distance to the Mars (Pericenter) and one at a maximum range (Apocenter). The camera is operated mainly around the Pericenter, having about 40 minutes of good exposure conditions there. The other part of the orbit is used to send the compressed data home to earth. Besides that, there is the break mode for health checking and software maintenance and the standby mode for setting up the imaging sequence and it's following actions.

Imaging principle HRSC / SRC

The imaging electronics of the HRSC are based on the principle of a linescan camera. This means, only a line is exposed to the light, and not an area (e.g. like for ordinary 35-mm film). One CCD-line of the HRSC consists of 5184 light-sensitive cells (pixels). The HRSC has 9 of these lines. This means each read-out process creates 9 independent lines. The CCD's are situated perpendicular to the flight direction and are read-out in at variable frequency, which is adjusted to the ground velocity of the spacecraft. During imaging operations this creates 9 independent image strips (push broom principle), one from each channel. Three of the channels are sensitive in the spectral range of red, green and blue. Another one obtaines data in near-infrared. Additionally there are three stereo-channels (Nadir, Stereo 1 and Stereo 2), which are used for the extraction of 3D-data, which result finally in a digital terrain model. The principle is to get a downward, backward and a foreward-view of an object. From this different views one can derive 3D information via photogrammetry. Furthermore there are two photometric-channels, delivering data for the physical analysis of the Martian surface.

The SRC is the second part of the HRSC camera system and is working with an area-sensor. This means, the light intensity is measured by a matrix of 1024 X 1032 elements. The result of each read-out is a picture of 1024 X 1032 pixels, in an altitude of 250 km this corresponds to a square on the Martian surface with the edges of 2.35 km. The pixel-size is 2.3 meters.

Operating HRSC and SRC

In the regular case the HRSC and the SRC are working simultaneously. The data stream of the SRC is being integrated in the HRSC's one. Because of the difficulty to locate the SRC-images on the Martian surface, it's normally operated together with the HRSC. Thus the high resolution images of the SRC are nested-in within the HRSC-strip, yielding very detailed information about areas of special interest. There are three command modes to be distinguished: the spot mode, the raster mode and the contiguous mode.

Data reduction and output

Combining the nine channels and the SRC together in full resolution, would result in an enormous data rate. The main antenna has a limited data rate and has a maximum of 8 hours per Earth-day for contact with the ground station. Thus there is a need of data reduction. This is performed by two methods - pixel summation and compression. The pixel summation creates averages of adjacent pixels. The 1X1 summation keeps the full resolution. The 2X2 summation creates an average of a square with an edge-length of 2 pixels. This decreases the data volume by a factor of four. Further options are a 4X4 and an 8X8 summation. The second method is a JPEG based data compression. After extensive tests with sample data, the compression factor has been determined to a variable factor between 4 and 10. This enables a compression of the data at acceptable losses in quality.