Mars Descent Imager (MARDI)

PI: Michael C. Malin, Malin Space Science Systems

Introduction

The Mars Descent Imager (MARDI) is a fixed-focus color camera mounted on the forward port side of the MSL rover. The optical axis points in the +Z direction (toward the ground). The camera can obtain 1600 × 1200 pixel images at a rate of 4.5 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. 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.

Objectives

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.

History

MARDI was descoped from the MSL payload in the summer of 2007. Using its own resources, Malin Space Science Systems (MSSS) completed the instrument and NASA has agreed to reinstate the instrument for flight. The flight instrument was tested and calibrated in June 2008 and the camera head was delivered to JPL in July 2008. It was subsequently integrated with the rover--mechanically in August 2008 and electronically in October 2008. MARDI then participated in cruise and EDL testing of the fully stacked spacecraft in November-December 2008.

Instrument Details

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 the Mastcam and MAHLI. The camera also uses an identical detector, a Kodak KAI-2020CM interline transfer CCD with 1600 by 1200 active 7.4-micrometer square pixels. Red/green/blue (RGB) color imaging similar to the colors the human eye sees (twice as much green as red and blue) is achieved using filtered microlenses arranged in a Bayer pattern.

The rectangular field of view (FOV) of the detector is inscribed within a 90° diameter circular FOV, yielding a 70° × 55° frame with the long axis transverse to the direction of motion. The instantaneous FOV 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 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). Integrated over the detector is a RGB Bayer pattern filter (RG/GB unit cell). For a landing at 3 PM (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. Lossy JPEG or lossless predictive compression is applied, and 200 × 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 while the vehicle is descending on its landing engines, 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.