MESSENGER MDIS Wide-angle Map-Projected Regional Targeted Mosaic (RTMWAC)

RTMWAC – Wide-angle Map-projected Regional Targeted Mosaic

Instrument: MESSENGER Mercury Dual Imaging System

PDS Data Set ID: MESS-H-MDIS-5-RDR-RTM-V1.0DOI: 10.17189/1520399

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

The Mercury Dual Imaging System (MDIS) Wide-angle Map-projected Regional Targeted Mosaic (RTMWAC) data products include regional mosaics of target observations directed at regions of interest. RTMWAC observations are one of three types: WAC 3-color targeted observations acquired mainly during the primary mission, to complement 1 km/pixel 8-color mapping with a higher spatial resolution 3-color product; WAC observations of targets that were observed repeatedly at different photometric geometries in order to improve photometric models of Mercury (these were observed initially in 8-color and since early 2013 in 11-color); or WAC 8- or 11-color science targeted observations. Each mosaic is map=projected in an orthographic projection. WAC color products contain 3 backplanes: solar incidence angle, emission angle, and phase angle.

The observations that make up the RTMWAC data set are scattered throughout the Calibrated Data Record (CDRWAC) data set linked by a Site ID in their PDS label. In this data set, the images for each Site ID are collected, coregistered if they are multispectral, map-projected, and photometrically corrected to a standard geometry of incidence angle i=30°, emission angle e=0°, and phase angle g=30°. All map-projections are orthographic and near the native resolution of the images to preserve spatial resolution.

The sequence of processing for creation of an RTMWAC from CDRWACs and Derived Data Records (DDRWACs) is the following:

  1. Experiment Data Records (EDRWACs) are assembled from raw data.

  2. Radiance images are created from the EDRWACs and calibration files.

  3. Radiance is converted to I/F CDRWACs by dividing by [pi*(solar flux at 1 AU)/(heliocentric distance in AU)2].

  4. I/F is photometrically corrected to i=30°, e=0°, and g=30°.

  5. Gimbal positions are extracted from the spacecraft housekeeping.

  6. DDRWACs are created. The surface intercept on a model of Mercury’s surface is calculated for each spatial pixel. The angles of this pixel relative to the equatorial plane and reference longitude constitute the latitude and longitude of the pixel. For that latitude and longitude, solar incidence, emission, and phase angles are determined.

  7. I/F corrected to i=30° and e=0° is map-projected into RTMs using the latitude and longitude information in the DDRWACs. The same procedure is used on DDRWACs to assemble the backplanes with derived information. They are appended to the image band in the following order:

    1. Solar Incidence Angle

    2. Emission Angle

    3. Phase Angle

MDIS observing scenarios pertaining to the RTMs are the following:

  • Pixel Binning: Some RTM 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, only where needed to increase image cadence and along-track overlap. Main Processor (MP) binning was used only for WAC photometric targets to control data volume.

  • 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). Typically, WAC multispectral images are 12 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, but any value from 1 to 32 can be used. Lossless compression was used when downlink allowed, especially for WAC multispectral observations.

  • 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 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. Most images in RTMs were acquired using automatic exposure, with an upper limit on exposure time to limit image smear.

  • 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 targeted observations, the pointing of the pivot varied depending on the type of target and whether the observation was coordinated with MASCS or MLA. WAC targeted 3- or 11-color image sequences are typically targeted at low solar incidence angles with constrained emission angles. WAC 8- or 11-color image sequences targeted for photometry have whatever pointing is required in order to meet specified bounds on incidence, emission, or phase angle for the particular region of interest.

  • Filter selection: The WAC imager contains a 12-position filter wheel to provide spectral imaging over the spectral range of the CCD detector. WAC filter 7 (750 BP 5) was chosen to complement the Narrow-angle camera (NAC) because its bandpass within that of the NAC lessens any discontinuities that might result from regional variations in spectral slope. RTMWACs may consist of WAC images in 3, 8, or 11 filters. The usage of filters in types of WAC observations is given in the table below.

Usage of filters in types of WAC observations:

Filter Number

Filter Filename Letter

Center wavelength (nm)

Bandpass width

(nm)

3-color

8-color

11-color

1

A

698.8

5.3

 

 

X

2

B

700.0

600.0

 

 

 

3

C

479.9

10.1

 

X

X

4

D

558.9

5.8

 

X

X

5

E

628.8

5.5

 

X

X

6

F

433.2

18.1

X

X

X

7

G

748.7

5.1

X

X

X

8

H

947.0

6.2

 

 

X

9

I

996.2

14.3

X

X

X

10

J

898.8

5.1

 

X

X

11

K

1012.6

33.3

 

 

X

12

L

828.4

5.2

 

X

X

MDIS RTMWAC products have the following file names:

MDIS_RTM_Wbb_ssssss_nnnnnnn_v.IMG

where:

MDIS = instrument = Mercury Dual Imaging System

RTM = product type = Regional Targeted Mosaic

W = camera = WAC

bb = bands = 03, 08, or 11 depending on type of observation (as shown in the table above)

ssssss = 6-digit integer giving the unique Side ID of the region covered by the product

nnnnnnn = image observation ID of the first image (lowest ID)

v = version number

In ODE, RTMWAC products have the following product IDs:

MDIS_RTM_WBB_SSSSSS_NNNNNNN_V