Surface Stereo Imager (SSI)

Instrument Overview

The Surface Stereo Imager (SSI) was designed for a resolution of <1 mrad/pixel, equivalent to the Viking cameras (color: 2.1 mrad/ pixel, monochrome high-resolution: 0.7 mrad/pixel). The focal plane array (FPA), supplied by the Max Planck Institute for Aeronomy (MPAe), has heritage from the Imager for Mars Pathfinder (IMP) and the descent imager (DISR) on the Huygens probe. There are two identical 256 x 256 pixel arrays, normally subframed to 248 x 256 for each eye; each field of view (FOV) is 13.88 x 14.38. Since the SSI is a copy of IMP, it maintains the same eye separation of 15.0 cm, and the light path is folded by two sets of mirrors to bring the light to the FPA. Prior to compression, a single-eye SSI frame contains 248 x 256 x 12 bits, or 762 kb. Downlinking the data 'raw' (uncompressed) requires putting the 12-bit data into 16-bit words by adding 4 flag-bits, resulting in an effective compression ratio of 0.75:1 and a 1.00 Mb image. Both the lossy (JPEG) and lossless (Rice) compressors efficiently discard these extra bits, and compression ratios refer to the original 12-bit/pixel image (e.g., a 762-kb full frame compressed at 6:1 requires 127 kb of downlink). About 100 frames are combined into a monochromic panorama, which brings the data volume to 76 Mb; instrument and packet headers add another 5% for a total of 80 Mb. A typical day on Mars allows the return of 30 Mb of SSI data. Therefore compression is an indispensable tool for the early mission. A 6:1 compressed panorama has few artifacts, compresses to 13 Mb, and is easily returned during a single sol.

Information in this instrument description is taken from The MVACS Surface Stereo Imager on Mars Polar Lander for the Journal of Geophysical Research [SMITHETAL2001]. See this paper for more details.

Scientific Objectives

The scientific goals for SSI fall into five general areas: atmospheric studies, topographic mapping using stereoscopic views, multispectral imaging normalized to a target of known reflectance, true color mosaics and studying the magnetic properties of airborne dust. The scientific objectives for SSI fall into several broad categories: geomorphology, mineralogy, and atmospheric studies. Geomorphology, as always, is governed by the choice of landing site. The polar layered terrain affords the rare opportunity to learn the stratigraphic history of this circumpolar geologic unit. Local landforms are likely to show fine-scale layering that is likely to have been depositional in origin, but later modified by strong winds. Unlike the Viking Landers 1 and 2 and Pathfinder sites, few, if any, rocks are expected. Defining the role of ices in the formation of the layers is a prime goal for SSI. To aid in the study of the layers, the camera's multispectral imaging mode will be used. Subtle color differences between layers may indicate a different ice fraction, changes in particle size, or composition changes. In addition to the stratigraphy we will look at local features to determine the topography and geology of the landing site, its photometric characteristics, and its homogeneity over a wide range of spatial scales. The SSI will also search for long timescale variations produced by atmospheric effects, for instance, aeolian modification of dune structures. In addition, it will observe effects resulting from the landing of the spacecraft and operations of the robotic arm.

Calibration

The improved Sweep magnet experiment (iSweep) an improved version of the Sweep magnet experiments flown onboard the two Mars Exploration Rovers (MERs) Spirit and Opportunity. The Sweep magnet is ring shaped and is designed to allow only non-magnetic particles to enter a small circular area at the center of the surface of this structure. The iSweeps will be used as radiometric calibration targets for the cameras on Phoenix, including SSI. The iSweeps will provide a presumably constant radiometric color reference for calibration of images from the landing site [LEERETAL2008].

The SSI has been characterized and calibrated with respect to absolute responsivity, spectral response, image quality, flat fielding, stray light, and pointing accuracy. Possible changes in the SSI spatial response pattern, particularly due to dust on the entrance windows, require subsequent calibration from images of the Martian sky. There are a number of error sources that combine to reduce the absolute calibration accuracy of the instrument. The major inaccuracies are lamp drift from its calibration radiance, distance errors from the lamp to the spectralon, improperly corrected stray light, and temperature mismeasurement. SSI images will be calibrated by a combination of flight software (FSW) aboard the spacecraft and ground software (GSW) after the data have been downlinked.

Operational Considerations

The dust signature must be taken into account when interpreting calibration relative to the targets, especially later in the mission as dust accumulation becomes significant. For water vapor measurements, special care has been taken to control the temperature of the detector (heated to T = -20 degC) during these observations to reduce variations due to changes in the relative response near the long wavelength end of the silicon CCD response.

Detectors

The decision to include stereo plus the requirement for low mass led to a split of a single detector into left and right image areas and the avoidance of the extra complexity of two separate focal planes. Therefore there are two identical 256 x 256 pixel arrays, normally subframed to 248 x 256 for each eye.

Electronics

There are two electronic boards housed in the Payload Electronics Box (PEB), where their environment is temperature controlled. The two boards mount to opposite sides of a thermal plate of aluminum that acts as a heat sink and support bracket. The detector cables from both the SSI and the RAC cameras connect to the CCD Readout Board (CRB) and the motor and auxiliary readouts connect to the Frame Buffer Board (FBB). The FBB interfaces to the spacecraft through its Payload and Attitude Control Interface (PACI) board with a 1-MHz serial link controlled by a Field Programmable Gate Array functioning as a state machine for command decoding.

The camera system is controlled through a sequence of uplinked commands that are time tagged and stored in spacecraft RAM. The image command includes many optional parameters that control the exposure and processing. Everything from the exposure time to the amount and type of data compression is specified here and attached to the data set to be placed in the header. Subframing boundaries and pixel-averaging parameters can also be specified. After processing, the packetized images are stored in the telemetry buffer.

Filters

Each eye has 12 selectable filters between 440 and 1000 nm. Eight low-transmission filters are included for imaging the Sun directly at multiple wavelengths to give SSI the ability to measure dust opacity and potentially the water vapor content. Images of the Sun through low-transmission, narrowband filters will be converted to optical depth measurements. Filters near the 935-nm water band allow us to constrain water vapor concentration. Sky brightness scans reveal the scattering phase function of the aerosols, which can in turn be analyzed to provide the size distribution, index of refraction, and shapes of the aerosols.

Optics

The f/18 optics have a large depth of field, and no focusing mechanism is required; a mechanical shutter is avoided by using the frame transfer capability of the 528 x 512 CCD.

Location

For ground-based data, coordinates of the observatory

Operational Modes

Defining the role of ices in the formation of the layers is a prime goal for SSI. To aid in the study of the layers, the camera's multispectral imaging mode will be used. Subtle color differences between layers may indicate a different ice fraction, changes in particle size, or composition changes.

Subsystems

Images taken of the lander deck and solar panels will be valuable as troubleshooting guides when performance of various subsystems degrades.

Measured Parameters

The camera system is controlled through a sequence of uplinked commands that are time tagged and stored in spacecraft RAM. The image command includes many optional parameters that control the exposure and processing. Everything from the exposure time to the amount and type of data compression is specified here and attached to the data set to be placed in the header. Subframing boundaries and pixel-averaging parameters can also be specified.

The color capability of the SSI will be used to take panoramic images in the search for evidence of aeolian sorting on the surface. The fundamental SSI camera product is a stereo image pair (or an 'image cube' of pairs obtained with the same geometry but with different filters). By design, such a pair can be viewed directly to give a three-dimensional impression of a portion of the SSI landing site. In addition, quantitative photogrammetric processing is planned that will lead to several useful derived data products.

SSI can acquire multispectral image cubes consisting of up to 12 wavelengths, using the geology filters. The band centers of the filters are clustered to optimize discrimination of the two most important mineral groups detectable by SSI. The first objective is to identify the crystalline ferric oxides, oxyhydroxides, and the poorly crystalline or nanophase ferric oxides.

Views of the solar disk through narrowband, low transmission filters and of the sky through the regular filters give us a wealth of information concerning haze and cloud properties.