Mars Orbiter Camera (MOC)

Description

The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) experiment is designed to obtain images of the surface and atmosphere of Mars for studying the meteorology/climatology and geology of Mars. Its primary objectives are to: (1) obtain global, synoptic views of the Martian surface and atmosphere on a daily basis in order to understand the meteorological and climatological changes during the mission; (2) monitor surface and atmospheric features for changes on temporal scales from hours to years and on a spatial scale necessary for resolving the details of their morphology; and, (3) systematically examine local areas at high spatial resolution so that surface/atmosphere interactions and geologic processes which operate on short time scales can be quantified.

The MOC is the flight spare for the Mars Observer Camera, and hence is essentially identical. In order to acquire images to satisfy these three objectives, the MOC consists of a narrow-angle assembly and a wide-angle assembly. These two assemblies are contained within a single cylindrical structure, approximately 80 cm in length and 40 cm in diameter. The narrow-angle assembly, which comprises the principal part of the instrument, is a 35 cm aperture, 3.5 m focal length (f/10) Ritchey-Chretien telescope with a 0.4 degree field of view, filtered to operate in a band pass from 500-900 nm. The focal plane contains two 2048 element, 13 micrometer pixel CCD line arrays. This system is capable of providing an image resolution of 1.41 m/pixel at a nominal altitude of 380 km with an expected resolution of better than 1.5 m/pixel during the course of the mission. The wide-angle assembly consists of two cameras mounted on the side of the narrow-angle assembly. One wide-angle camera, optimized to operate in a band pass from 400-450 nm (``blue'') has a focal length of 11.4 mm (f/6.3). The other, designed to operate in a band pass from 575-625 nm (``red''), has a focal length of 11.0 mm (f/6.4). The wide angle instruments have fields of view of 140 degrees, with resolution of 280 m/pixel at nadir and 2 km/pixel at the limb. The focal plane contains two 3456 element, 7 micrometer pixel CCD line arrays. All three cameras are mounted in a fixed position on the nadir panel so that, during the mapping portion of the mission, they are always pointed toward Mars.

The electronics for the MOC are completely redundant and the narrow-angle and wide-angle cameras can be operated by either set of electronics and all three cameras can be operated simultaneously. The camera is controlled by a 32 bit (10 MHz, 1 MIPS) SA3300 microprocessor with four ASICs, 128 kbytes EPROM, and 192 kbytes SRAM. Because of the high volume of data which imaging experiments can generate, the MOC electronics contain not only a large amount of memory (~12 MB DRAM buffer) for processing and storing the images, but also have the capability of utilizing a number of data compression techniques (both lossless and lossy). Further, the MOC can transfer these data (either to the on-board recording system or via real-time transmission) at any rate of which the spacecraft is capable. As a result, the equivalent of two, four, or eight (depending on mission phase) 2048 x 2048 pixel images can be processed on record-only days and, once every three days on average, fourteen such images can be processed and sent during an eight-hour real-time pass. In-flight calibration of the MOC is extremely limited due to the fixed pointing of the instrument. Some opportunities for in-flight calibration are possible during regional or global dust storms. Otherwise, pre-flight measurements made to characterize the instrument performance comprised the bulk of the instrument calibrations. Available data rates are 700, 2856, 9120, and 29260 real-time bits/sec.

Finally, due to the MOC's large capacity for the storage and processing of images, it is intended to be used as part of the Mars Relay, which will provide a communications link between other Mars missions and the Earth.