Scientific Objectives
The IMP camera was designed to study martian geological processes and
surface-atmosphere interactions similar to what was observed at the
Viking landing sites. Through the use of panoramic stereo images
taken at various times of the day, it was also used to observe the
general landscape, surface slopes, and the distribution of rocks.
Changes in the scene over the lifetime of the mission, possibly
attributable to the actions of frost, dust or sand deposition, erosion
or other surface-atmosphere interactions, were monitored. A greater
understanding of the surface and near-surface soil properties was
sought. This was to be obtained partly with data acquired by the
Rover and APXS and partly by IMP imaging of Rover wheel tracks, holes
dug by the Rover wheels, and any surface disruptions caused by airbag
bounces or retractions.
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 IMP consists of three physical subassemblies: (1) camera head
(with stereo optics, filter wheel, CCD and pre-amp, mechanisms and
stepper motors); (2) extensible mast with electronic cabling; and (3)
two plug-in electronics cards (CCD data card and power supply / motor
drive card) which plug into slots in the lander's Integrated
Electronics Module.
The major components of the camera head and electronics cards are described in greater detail below.
Detectors
Azimuth and elevation drives for the camera head are provided by
stepper motors with gear heads, providing a field of regard of
±178° in azimuth and +83° to -72° in elevation,
relative to lander coordinates.
Stereo separation | 15.0 cm |
---|---|
Toe-in | 12.5 mrad (left); -24.5 mrad (right) |
AZ/EL step size | 0.553°, 1° hysteresis (backlash) |
Repeatability | < 5 mrad, when approaching from the same direction |
Step speed | 10 steps per second |
Pointing range | 360° azimuth, +90° to -67° elevation |
Data compression | 1.3:1 lossless up to 24:1 lossy (JPEG) (higher compression ratios achieved using pixel blocking) |
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.
Readout noise | 15 electrons |
---|---|
Full well | 125,000 electrons |
Readout time | 2 sec. for full array; 1 sec. for left eye only |
Exposure time | 0 - 32.7675 seconds; step size is 0.5 milliseconds |
Spectral range | 440 - 1000 nm |
Gain | 30 electrons/pixel |
ADC | 12 bits/pixel |
Frame transfer | 0.5 milliseconds (no mechanical shutter) |
SNR | <= 350 |
Pixel size | 23 x 17 micrometers; 6 micrometers for an antiblooming channel |
Optics
The stereoscopic imager includes two imaging triplets, two fold
mirrors separated by 150 mm for stereo viewing, two 12-space filter
wheels (one in each path), and a fold prism to place the images
side-by-side on the CCD focal plane. Fused silica windows at each
path entrance prevent dust intrusion. The optical triplets are an
f/10 design, stopped down to f/18 with 23 mm effective focal lengths
and a 14.4° field of view. The pixel instantaneous field of
view is one milliradian. There are a total of 24 filters (twelve on
each filter wheel) divided into the following categories: four stereo
geology channels, eleven monoscopic geology channels, eight monoscopic
channels for solar and atmospheric studies, and one magnifying
filter.
Resolution | 0.981 mrad/pixel (left); 0.985 mrad/pixel (right) |
---|---|
Focal length | 23 mm |
f/number | f/18 |
FOV | 14.4° x 14.0° |
Depth of Field | best focus, 1.3 m; DOF, 0.5 m to infinity |
Electronics
The IMP has three electronics boards, all of which are housed in the
lander chassis, in order to keep them warm (above -50° Celcius).
They are connected to the onboard computer via the VME bus link. The
first board is a copy of the DISR power supply, which provides all the
necessary voltages for the CCD system from the lander 28 V power
bus.
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
Center wavelengths are shown for both the left and right eyes, and are
measured in nanometers.
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)
Filter 0 | ||||||
---|---|---|---|---|---|---|
Filter Name | L440_R440 | |||||
Filter Application | Stereo, Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 443.3 | Right: | 443.0 | ||
Filter Bandwidth (nm) | Left: | 26.2 | Right: | 26.2 | ||
Responsivity, left eye | a1: | 128.8 | a2: | -0.387 | a3: | -0.0007 |
Responsivity, right eye | a1: | 117.9 | a2: | -0.392 | a3: | -0.0006 |
Filter 1 | ||||||
Filter Name | L450_R670 | |||||
Filter Application | Solar | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 450.3 | Right: | 669.8 | ||
Filter Bandwidth (nm) | Left: | 4.91 | Right: | 5.30 | ||
Responsivity, left eye | a1: | 0.246 | a2: | -0.0025 | a3: | -0.00001 |
Responsivity, right eye | a1: | 2.238 | a2: | -0.0058 | a3: | -0.00002 |
Filter 2 | ||||||
Filter Name | L885_R947 | |||||
Filter Application | Solar | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 883.4 | Right: | 945.5 | ||
Filter Bandwidth (nm) | Left: | 5.60 | Right: | 43.7 | ||
Responsivity, left eye | a1: | 14.5 | a2: | 0.0233 | a3: | 0.00002 |
Responsivity, right eye | a1: | 25.94 | a2: | 0.078 | a3: | 0.00005 |
Filter 3 | ||||||
Filter Name | L925_R935 | |||||
Filter Application | Solar | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 924.9 | Right: | 935.6 | ||
Filter Bandwidth (nm) | Left: | 5.03 | Right: | 4.91 | ||
Responsivity, left eye | a1: | 5.389 | a2: | 0.0193 | a3: | 0.00004 |
Responsivity, right eye | a1: | 9.738 | a2: | 0.028 | a3: | -0.000003 |
Filter 4 | ||||||
Filter Name | L935_R990 | |||||
Filter Application | Solar | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 935.4 | Right: | 988.9 | ||
Filter Bandwidth (nm) | Left: | 4.84 | Right: | 5.39 | ||
Responsivity, left eye | a1: | 10.42 | a2: | 0.0378 | a3: | 0.00006 |
Responsivity, right eye | a1: | 1.857 | a2: | 0.0064 | a3: | -0.000006 |
Filter 5 | ||||||
Filter Name | L670_R670 | |||||
Filter Application | Stereo, Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 671.4 | Right: | 671.2 | ||
Filter Bandwidth (nm) | Left: | 19.7 | Right: | 19.5 | ||
Responsivity, left eye | a1: | 575.3 | a2: | -0.570 | a3: | -0.0013 |
Responsivity, right eye | a1: | 557.3 | a2: | -0.575 | a3: | -0.0014 |
Filter 6 | ||||||
Filter Name | L800_R750 | |||||
Filter Application | Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 801.6 | Right: | 752.0 | ||
Filter Bandwidth (nm) | Left: | 21.0 | Right: | 18.9 | ||
Responsivity, left eye | a1: | 872.2 | a2: | 0.237 | a3: | -0.0029 |
Responsivity, right eye | a1: | 787.1 | a2: | -0.247 | a3: | -0.0019 |
Filter 7 | ||||||
Filter Name | L860_R-DIOPTER | |||||
Filter Application | Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 858.4 | Right: | 900.0 | ||
Filter Bandwidth (nm) | Left: | 34.4 | ||||
Responsivity, left eye | a1: | 1435 | a2: | 2.491 | a3: | 0.0035 |
Responsivity, right eye | a1: | 7596.9 | a2: | 9.057 | a3: | -0.0235 |
Filter 8 | ||||||
Filter Name | L900_R600 | |||||
Filter Application | Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 897.9 | Right: | 599.5 | ||
Filter Bandwidth (nm) | Left: | 40.8 | Right: | 21.0 | ||
Responsivity, left eye | a1: | 1120 | a2: | 3.006 | a3: | 0.0059 |
Responsivity, right eye | a1: | 592.7 | a2: | -0.598 | a3: | -0.0013 |
Filter 9 | ||||||
Filter Name | L930_R530 | |||||
Filter Application | Stereo, Ranging, Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 931.1 | Right: | 530.8 | ||
Filter Bandwidth (nm) | Left: | 27.0 | Right: | 29.6 | ||
Responsivity, left eye | a1: | 478.7 | a2: | 1.928 | a3: | 0.005 |
Responsivity, right eye | a1: | 578.6 | a2: | -0.893 | a3: | -0.002 |
Filter 10 | ||||||
Filter Name | L1000_R480 | |||||
Filter Application | Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 1002.9 | Right: | 479.9 | ||
Filter Bandwidth (nm) | Left: | 29.1 | Right: | 27.0 | ||
Responsivity, left eye | a1: | 213.4 | a2: | 1.606 | a3: | 0.0052 |
Responsivity, right eye | a1: | 368.1 | a2: | -0.668 | a3: | -0.002 |
Filter 11 | ||||||
Filter Name | L965_R965 | |||||
Filter Application | Stereo, Ranging, Geology | |||||
Filter Type | Interference | |||||
Center Wavelength (nm) | Left: | 968.0 | Right: | 966.8 | ||
Filter Bandwidth (nm) | Left: | 31.4 | Right: | 29.6 | ||
Responsivity, left eye | a1: | 395.8 | a2: | 2.027 | a3: | 0.0051 |
Responsivity, right eye | a1: | 393.5 | a2: | 2.185 | a3: | 0.0065 |
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
The preparation of the IMP calibration files and algorithms has not
yet been completed. Until it is, please see [CROWEETAL1997] and [WELLMAN1996A].
Operational
Considerations
There is some uncertainty in the height of the IMP camera above the
surface of Mars. If we assume a relatively flat surface under the
lander, then the height (h) of the elevation axis of the camera above
the Martian surface is given by:
h = MAST + BASE + ISA + AIRBAG LAYER
where
(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: