Mars Descent Imager Camera (MARDI)
The Mars Descent Imager (MARDI) is a fixed-focus color camera mounted on the forward port side of the Mars Science Laboratory (MSL) rover. The optic axis points in the +Z direction (towards the ground). The camera can obtain 1600 x 1200 pixel images at a rate of 4.5 frames per second throughout the period between heatshield separation and touchdown plus a few seconds (a period of about two minutes). The rover software issues a start imaging command (that includes the frame rate and the number of frames; substantial margin is commanded to take into account unusually long descent durations) and the camera operates thereafter autonomously. The data are written into permanent flash memory in realtime during acquisition for later transmission. Hundreds of images will be acquired at scales many times greater than available from orbit.
The MARDI science investigation primary objectives are to determine where exactly the MSL 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. 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 wind velocity from the lower boundary layer, and to help develop and test algorithms for future autonomous landing and hazard avoidance systems. Although not an original requirement or objective of the investigation, additional images may be taken during rover traverses for visual odometry and geologic mapping.
The MARDI, like the MSL Mastcams and MAHLI, consists of 2 parts: a camera head, mounted on the rover body, and a Digital Electronics Assembly (DEA), housed inside the warm electronics box of the rover's chassis. The DEA and camera head electronics are the same design as those of MAHLI and Mastcam. The camera, like the Mastcams and MAHLI, uses a Kodak KAI-2020CM interline transfer CCD with 1600 by 1200 active 7.4 micron square pixels. Red/green/blue (RGB) color imaging similar to the colors the human eye sees is achieved using filtered microlenses arranged in a Bayer pattern.
The rectangular field of view (FOV) of the detector is inscribed within a 90 degree diameter circular FOV, yielding a 70 degree by 55 degree frame with the long axis transverse to the direction of motion. The instantaneous field of view (IFOV) of the camera is ~0.76 milliradians per pixel, which provides in-focus pixel scales that range from about 1.5 m at 2 km altitude to 1.5 mm at 2 m altitude, and covers between 2.4 x 1.8 km and 2.4 x 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 scale of 0.75 mm/pixel (calibration demonstrates the potential for acquiring 1.5 mm resolution images of the surface after landing).
An 8 Gbyte 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). For a landing at 3 p.m. local time (incidence = 55 degrees) and an albedo of 0.2, the nominal SNR will be ~80:1 in the green and red, and >50:1 in the blue. Lossy JPEG or lossless predictive compression is applied, and 200 x 150 pixel thumbnails are created, during read out from the buffer.
Large angular rate motion while the vehicle is descending on parachute, and rocket thruster induced vibratory motion, are likely to blur some of the images despite a 1.3 millisecond exposure time.
A white swatch of beta cloth is affixed to the interior surface of the MSL heat shield to serve as a MARDI white balance target as the heat shield falls away during descent to the martian surface.
The rover entry, descent, and landing (EDL) sequence, operating within the rover computer, issued a start imaging command to the MARDI, triggered on actual event times during EDL. That sequence told the MARDI to take 1504 images as fast as it could (frame rate is a complex function of image size, image sub-framing, camera clock speeds, and charge flushing; the final frame rate was 3.88 frames per second), and started that sequence ~13 seconds after parachute deployment, while the heat shield was still attached. These images were un-illuminated. Heat shield jettison occurred ~6 seconds later (between the 26th and 27th MARDI image). A total of 626 images were acquired while the vehicle was in flight, with an additional 878 acquired on the surface after landing. Of the 626 acquired in-flight, ~140 were at pixel scales better than 35 cm.