Mars Descent Imager (MARDI)
PI: Michael C. Malin, Malin Space Science Systems
The Mars Descent Imager (MARDI) is a fixed-focus color camera fixed-body-mounted to the fore-port-side of the MSL rover, even with the bottom of the rover chassis. The optical axis points in the +Z direction (toward the ground in the rover coordinate system). The camera will take 1600 × 1200 pixel images at ~5 frames per second throughout the period of time between heatshield separation and touchdown plus a few seconds (a period of about two minutes). The rover software issues a “start imaging” command and the camera operates autonomously until the rover software determines that landing has succeeded and issues a “stop imaging” command. The data are written into permanent flash memory in real time during acquisition for later transmission.
MARDI as installed on the MSL flight rover (September 2008).
The rectangular field of view (FOV) of the detector is inscribed within the 90-deg diameter circular FOV of the lens, yielding a 70° × 55° frame with the long axis transverse to the direction of motion. The IFOV of the camera is ~0.76 milliradians, which provides in-focus pixel scales that range from 1.5 m at 2 km altitude to 1.5 mm at 2 m altitude, and cover between 2.4 × 1.8 km and 2.4 × 1.8 m at these respective altitudes. At distances less than 2 m, out-of-focus blurring increases at the same rate that spatial scale decreases, resulting in a constant spatial sampling of 1.5 mm. Many hundreds of images will be acquired at ground sampling distances many times greater than available from orbit.
An 8 gigabyte internal buffer permits the camera to acquire over 4,000 raw frames (equivalent to 800 seconds of descent, which is many times the actual descent duration). Integrated over the detector is a RGB Bayer pattern filter (GR/BG unit cell). For a landing at 3 PM LMST (solar incidence angle of 55°) and a surface albedo of 0.2, the nominal SNR will be ~80:1 in the green and red, and > 50:1 in the blue. The camera is capable of losslessly compressing the images, or applying lossy JPEG compression, in realtime during acquisition and storage, although the intent is to take the images in raw format and apply compression just prior to downlink to Earth. Thumbnail images of 200 × 150 pixels can be created simultaneously with the processing and storage of the realtime images, but will likely be generated again just prior to downlink. Large angular rate motion while the vehicle is descending on parachute, and rocket thruster induced vibratory motion while the vehicle is descending on its landing engines, are likely to blur many of the images despite a 1.3 millisecond exposure time.
MARDI’s primary objectives are to determine where exactly the vehicle has landed and to provide a geologic and engineering-geologic framework of the landing site for early operations. The rover is expected to leave the area imaged by MARDI after the first few weeks of the mission. It is likely that only a subset of the images will have been transmitted by this time (e.g., thumbnails plus a sampling of full-frame images). Vehicle horizontal offset between images within the descent sequence may permit digital elevation models (DEMs) to be created from the descent images. Additional objectives of the investigation are to examine vehicle ground-referenced motion deviations from inertial measurement unit (IMU) derived inertial position during descent to extract lower boundary layer wind velocity, and to help develop and test algorithms for future autonomous landing and hazard avoidance systems.
Although MARDI was descoped from the MSL payload in the summer of 2007, NASA permitted Malin Space Science Systems (MSSS) to use its own resources to complete the instrument. Subsequently NASA reinstated the instrument for flight. MSSS will process the data and archive them within the PDS.