MESSENGER MDIS Wide-angle Map-projected Multispectral Data Record (MDRWAC)

MDRWAC – Wide-angle Map-projected Multispectral Data Record

Instrument:MESSENGER Mercury Dual Imaging System

PDS Data Set ID:MESS-H-MDIS-5-RDR-MD3-V1.0DOI: 10.17189/1520201

For more information about MDIS DDR products, see the CDR/RDR Product SIS.

The Mercury Dual Imaging System (MDIS) Wide-angle Map-projected Multispectral Data Record (MDRWAC) data products include a 3-color map of northern and equatorial latitudes acquired during MESSENGER’s Extended Mission, and includes I/F in the 3 filters used for this map. This product has higher spatial resolution than, and complements, an 8-color multispectral map acquired during the primary mission. This product is photometrically normalized to a solar incidence angle, i=30°, emission angle, e=30°, and phase angle, g=30°, at a spatial sampling of 128 pixels per degree. The map is divided into “tiles” and each tile is composed of 3 bands corresponding to 3 of the 11 WAC filters. Each tile also contains backplanes describing ancillary information. The subset of 3 of 11 available multispectral filters samples both albedo and spectral slope over MDIS’s wavelength range. The fewer colors than in the Primary Mission 8-color product is to balance the higher spatial resolution to manage data volume during acquisition.

MDIS 3-color imaging of Mercury is mosaicked into non-overlapping, 128 pixel/degree tiles. Each tile corresponds to the NW, NE, SW, or SE quadrant of one of the pre-existing Mercury non-polar charts or one of the two polar charts, defined by the U.S. Geological Survey (USGS).

Mercury Charts, as named at the end of mission delivery:

Quadrangle

Filename Identifier

Latitude (degrees)

Longitude (degrees east)

H-1 Borealis

H01

65 to 90

0 to 360

H-2 Victoria

H02

22.5 to 65

270 to 360

H-3 Shakespeare

H03

22.5 to 65

180 to 270

H-4 Raditladi

H04

22.5 to 65

90 to 180

H-5 Hokusai

H05

22.5 to 65

0 to 90

H-6 Kuiper

H06

-22.5 to 22.5

288 to 360

H-7 Beethoven

H07

-22.5 to 22.5

216 to 288

H-8 Tolstoj

H08

-22.5 to 22.5

144 to 216

H-9 Eminsecu

H09

-22.5 to 22.5

72 to 144

H-10 Derain

H10

-22.5 to 22.5

0 to 72

H-11 Discovery

H11

-65 to -22.5

270 to 360

H-12 Michelangelo

H12

-65 to -22.5

180 to 270

H-13 Neruda

H13

-65 to -22.5

90 to 180

H-14 Debussey

H14

-65 to -22.5

0 to 90

H-15 Bach*

H15

-90 to -65

0 to 360

*For MDRWACs there are no H15 mosaics because the 3-color mapping campaign did not include that portion of Mercury.

MDIS observing variables pertaining to the MDRWACs are the following:

  • Pixel Binning: Some images are unbinned and 1024 x 1024 pixels. Some images are 2x2 pixel binned in the focal plane hardware (also known as “on-chip” binning), resulting in 512 x 512 images. Images can be further compressed 2x2 using the Main Processor (MP), in addition to Data Processing Unit (DPU) binning, yielding 256 x 256 images. Images used in 3-color MDRWAC products are mostly not binned although some taken at the lowest altitudes are binned 2x2 “on-chip”. This less aggressive binning strategy than in the 8-color map results in nearly a 2.5x reduction in the average native pixel scale of the data.

  • 12-8 bit compression: Images are read off the detector in 12-bit format. 12 bit images may be converted to 8-bit images using one of eight lookup tables (LUTs). All WAC images collected as part of the 3-color multispectral map have been converted to 8 bits.

  • FAST/DPCM compression: All images are compressed losslessly using FAST/DPCM compression as they are read out of the DPU, to conserve recorder space. Once the data are written to the recorder, they can be uncompressed and recompressed more aggressively in the spacecraft MP.

  • Wavelet compression: Images may be integer wavelet transform-compressed in the MP, typically at 3:1 for color data. Color imaging was taken losslessly compressed when downlink volume permitted it.

  • Exposure Control: The exposure time of MDIS images can be set manually by command, or automatically by the software. In manual mode, exposure times from 1-989, 1001-1989, …, to 9001-9989 ms are available. In autoexposure mode the exposure time of the next image is computed by the DPU software, and cannot exceed 989 ms in duration. If the time of the next image occurs before the calculation can be completed, and pixel binning or filter position change, then the algorithm compensates for predicted changes in scene brightness and filter transmission using an onboard data structure. All images in MDRWACs were acquired using automatic exposure.

  • Pointing: The MDIS imagers are mounted on a pivot platform, which is itself mounted to the MESSENGER spacecraft deck. The pivot platform is controlled by a stepper motor, which is controlled by the DPU. The pivot platform can move in either direction. The total range of motion is 240 degrees, limited by mechanical “hard” stops, and is further constrained by “soft” stops applied by the software. The nominal pointing position for MDIS is defined as 0 degrees, aligned with the spacecraft +Z axis and the boresight for several other instruments. The range of the soft stops is set to 40 degrees in the spacecraft –Y direction (toward the MESSENGER sunshade) and +50 degrees in the +Y direction (away from the sunshade). The pivot position can be commanded in intervals of 0.01 degrees within this range. During acquisition of the global multispectral map, the pivot was used to point the WAC to low emission angles on the surface, at times when the solar incidence angle was close to the lowest values possible at that latitude.

  • Filter selection: The WAC imager contains a 12-position filter wheel to provide spectral imaging over the spectral range of the CCD detector. Three of the filters were chosen for the map and appear in the MDRWAC in order of increasing wavelength:

    • WAC filter 6 (filter F), 430 nm, BP 40

    • WAC filter 7 (filter G), 750 nm, BP 5

    • WAC filter 9 (filter I), 1000 nm, BP 15

MDIS MDRWAC products have the following file names:

MDIS_MD3_128PPD_Hxxddv.IMG

where:

MDIS = instrument = Mercury Dual Imaging System

MD3 = product type = MDRWAC

128PPD = resolution in pixels/degree

Hxx = Mercury chart designation

dd = quadrant within Mercury chart (NW, NE, SW, or SE), or a polar chart (NP, SP)

v = version number

In ODE, MDRWAC products have the following product IDs:

MDIS_MD3_128PPD_HXXDDV