In DLR the miniaturized Wide- Angle Optoelectronic Stereo Scanner (WAOSS) was developed for the Russian Mars96 mission. The wide angle WAOSS imaging system is a development to fulfill both scientific tasks and deep space technological requirements. The hardware was 1996 on the technological edge and can handle 6 MPixels/s with 11-bit radiometric dynamic and an SNR better than 8 bit.
Description of the cameras WAOSS and WAAC
The Wide- Angle Optoelectronic Stereo Scanner WAOSS is a three line stereo scanner working in the pushbroom mode . According to the camera design the stereo information will be generated within the image plane of one single objective by means of three CCD lines (see fig. 1). Due to the movement of the satellite with the camera, each of these lines senses a certain object or area with a given time shift and under a different viewing angle. However the time shift is so small that the illumination condition may be considered constant. After reducing the amount of data by means of special compression methods, the three image strips received will be transmitted to Earth. There the data will be reconstructed and combined to form stereo images.
Fig. 1: The three- line principle and mechanical design of the WAOSS
Scientific goals of the WAOSS camera
WAOSS was developed for the Russian Mars96 mission with DARA sponsoring taking into account its major development drivers:
· different scientific objectives like
- synoptic imaging of the weather phenomena with coarse ground resolution
- global topographical mapping with good ground resolution
· real time control of the camera parameters in the highly elliptical orbit around Mars
· severe restriction of data rates and data volumes to be transmitted to the Earth
· High reliability for at least 2 years
· low mass, volume, and power consumption budgets
Based on the different scientific tasks 6 special camera modes had to be implemented in the Camera.
Scientific Tasks of WAOSS
The primary task of the Wide-Angle Camera, WAOSS, is to globally image the planet Mars with a ground resolution of few hundred meters. The main emphasis of this camera is on broad surface coverage, rather than on high spatial resolution, which is the focus of the other german camera, HRSC (High Resolution Stereo Camera).
WAOSS will be capable of global topographic mapping with a scale factor of 1:500000, as well as imaging temporal changesin the atmosphere and on the surface. The planned mission duration of one Martian year (about 2 Earth years) will enable observation during all of the Martian seasons. Therefore, seasonal and weather-related changes at the Martian surface (e.g., ice coverage, albedo patterns) and the generation and propagation of clouds and dust storms can be investigated.
The data to be retrieved from WAOSS in Martian orbit shall serve a number of scientific disciplines, especially:
Design concept of the WAOSS cameras
The Fig. 2 shows the hardware design concept of WAOSS. The camera fit the scientific requirements very well with the flexibility and modularity of the DSP and data compression architecture. WAOSS has a complete cold redundancy structure. The Special processing boards inside the camera handles the real time correction of the pixel data [PRNU, DSNU] and the lossy JPEG compression. The compressed data will be bundled and sent to the spacecraft mass memory. The special feature of the real time data compression control board (DCE) was driven by the maximum output data rate of 100 up to 500 kBit/s. The master DSP controls the generation of the DCE data formats. The scientist can chose on ground the data format of 11 Bit raw data, 8 Bit normalized data and JPEG data. The low output data rate was the hardest technical requirement for the system.
Fig. 2: Structure of the WAOSS Camera
The main advantage of the deep space WAOSS camera are the wide field of view for atmospheric studies, the high geometric and radiometric accuracy, the stereo capability for studies of clouds and the onboard real time data processing. Technische Daten:
The 3 Lines-Stereo Principle
According to the camera design the stereo information will be generated within the image plane of one single objective by means of three CCD lines. Due to movement of the satellite with the camera, each of these lines senses a certain object or area with a given time shift and under a different viewing angle. However the time shift is so small that the illumination conitions may be considered constant, After reducing the amount of data by means of special compression methods, the 3 image strips received will be transmitted to Earth. There the data will be reconstructed and combined to from stereo images.
Flexible Sensor Control by Camera Intelligence
Due to the anticipated high-elliptic satellite orbit around Mars, the images must be taken from different heights above the planet and at different orbital velocities. This requires a position-dependent control of the camera parameters, e.g., the repetition rate for imaging. The necessary processing capability and "intelligence" is ensured by an efficient multiprocessor system. This intelligent control guarantees that, despite the use of an objective with fixed focal length, the ground resolution can be kept constant (e.g., 1 km x 1km in the "Weather Mode") by electronic zooming" over a large altitude range of the satellite. The highest resolution from pericenter (lowest altitude from the Martian surface) will be 65 m. This resolution can be used for the "Cartography Mode".