Table of Contents

• Instrument Overview
• Scientific Objectives
• Subsystems
• Detectors
• Optics
• Electronics
• Filters
• Operational Modes
• Calibration
• Operational Considerations
• Related Information

The Imager for Mars Pathfinder or IMP is a stereo imaging system with color capability provided by twenty-four selectable filters for the two camera channels. It is derived from the descent imager/spectral radiometer (DISR) instrument aboard the Cassini Huygens probe.

Scientific Objectives

An investigation was made of atmospheric aerosols and water vapor. This included the determination of the aerosol optical depth as a function of wavelength above the landing site, the size and density distribution of the aerosols, a characterization of the shape of the aerosol particles, the vertical distribution of the aerosols, and the imaginary refractive index of the particles.

The IMP experiment also included a magnetic properties investigation. A set of magnets of differing field strengths were mounted to a plate and attached to the lander. Images taken over the duration of the landed mission were used to determine the accumulation of magnetic species in the wind-blown dust. Multispectral images of these accumulations were used to differentiate among the several proposed mineral compositions.

The IMP investigation included the observation of wind direction and velocity using wind socks mounted on the ASI/MET mast.

Subsystems

The major components of the camera head and electronics cards are described in greater detail below.

The extensible mast was built by AEC Able Corp, and is a continuous longeron, open-lattice type used for magnetometers. It was held in its stowed position during cruise by a pyro-activated pin puller. Upon deployment, the camera system, mounted at the top of the mast, sprung up to a height of 62 cm above its stowed position, or roughly .9 meters above the surface of Mars.

Detectors

The focal plane of the IMP consists of a CCD mounted at the foci of two optical paths where it is bonded to a small printed wiring board, which in turn is attached by a short flex cable to the preamplifier board. The CCD is a front-illuminated frame transfer array with 23 micrometer square pixels. Its image section is divided into two square frames, one for each half of the stereo FOV's. Each has 256 x 256 active elements. A 256 x 512 storage section (identical to the imaging section) is located under a metal mask. The IMP focal plane and electronics are nearly identical copies of the comparable subsystem employed in the Huygens Probe Descent Imaging Spectroradiometer (DISR), using the Loral 512 x 512 CCD. The entire CCD subsystem is provided by the Max Planck Institute for Aeronomy.

Optics

Electronics

The second board sends clock pulses to the CCD. The twelve bit ADC also receives the analog signals back from the pre-amp board and converts them to digital signals over a period of two seconds, to be stored in the frame buffer chip on the third board.

The frame buffer board is connected to the VME backplane and is controlled by a field programmable gate array functioning as a state machine for command decoding. This third board also phases the steps and drives the three motors.

Filters

Bandwidths shown are likewise for both left and right eyes, and are also measured in nanometers.

Responsivity (R) as a function of temperature (T) is shown for each filter and each eye as the parameters of a quadratic, where:

R(T) = a1 + (a2 * T) + (a3 * T2)

Both lossless and lossy data compression are available. The lossless compression (with a compression rate between 1.3:1 and 2:1, depending on the busyness of the scene) employs a Rice algorithm developed at JPL. For cases where lossy compression is acceptable, compression rates between 6:1 and 24:1 can be obtained using a modified JPEG compressor, which uses arithmetic coding developed at the Technical University of Braunschweig. This compression is enhanced by local cosine transform prior to the JPEG-specific discrete cosine transform and made robust against data dropouts. Higher compression ratios are achieved using pixel blocking.

Additional alternatives for reducing the amount of data include image subframing and row and column averaging. Subframing is primarily useful when imaging targets like the Sun; most Sun images are returned as 31 x 31 pixel blocks. Row and column averaging can be used for sky images, providing a gradient and the edges of cloud features but not the high-resolution of a normal image.

For more details on IMP data compression, please see the IMP EDR archive dataset object or [RUEFFERETAL1995].

Calibration

Operational Considerations

h = MAST + BASE + ISA + AIRBAG LAYER

where

	MAST =	the distance between the stowed IMP elevation axis and the deployed IMP elevation axis (0.61825 m)
	BASE =	height of the stowed IMP elevation axis above the Integrated Support Assembly (ISA) (0.17550 m)
	ISA =	height of the ISA box (0.436576 m)
	LAYER =	the vertical distance between the local Martian surface and the bottom of the ISA box: includes the petal, the airbag gas generator, the airbags, rocks, and anything else between the ISA and the flat Martian surface. Current best estimate is 0.30 m +/- 50% uncertainty.

(Minor note: The optical axis is displaced ~0.012 m above the elevation axis.)

Therefore, the height of the camera in the stowed and deployed positions is:

STOWED_HEIGHT_ABOVE_SURFACE = 0.62 + ~ 0.30 = ~ 0.92 m DEPLOYED_HEIGHT_ABOVE_SURFACE = 1.24 + ~ 0.30 = ~ 1.54 m

Given a roughly 50% uncertainty in the thickness of the AIRBAG LAYER, the uncertainty in the stowed and deployed heights of the camera are around 20% and 10%, respectively.

The flight model IMP has some measured characteristics which differ from the ideal, but which do not prevent success of the goals for mission science. These characteristics are:

1. Unequal toe-in between the two eyes.

2. Some vignetting is present, primarily about three image lines in the left eye due to the left mirror mount.

3. The left eye points downward about 0.8 milliradians compared to the right eye.

4. The eyes have image scales differing by about 0.4%.

5. Aliasing is significant at all wavelengths.

6. Alignment changes by about 0.5 milliradian as a function of the filter selected.

7. Camera pointing can be inexact due to a backlash effect.

Mission

Instrument Host

Targets

References