MESSENGER MDIS Narrow-angle Experiment Data Record (EDRNAC)

EDRNAC – Narrow-angle Experiment Data Record

Instrument: MESSENGER Mercury Dual Imaging System

PDS Data Set ID: MESS-E/V/H-MDIS-2-EDR-RAWDATA-V1.0DOI: 10.17189/1520380

For more information about MDIS EDR products, see the EDR Product SIS.

The Mercury Dual Imaging System (MDIS) Narrow-angle Experiment Data Record (EDRNAC) data products include the uncalibrated versions of all available images acquired by the MDIS Narrow-angle camera (NAC) during the cruise phase to Mercury and includes post-launch checkout images, flyby images of Earth, Venus, Mercury, images acquired from Mercury orbit and in-flight calibration images.

There is one EDR product per MDIS image. The EDR product consists of a binary image file with an attached PDS label. The PDS label describes the format of the file, and has all of the parameters describing the state of the instrument when the image was exposed. The label also describes the post-processing options selected to be performed by the MESSENGER Main Processor (MP). In addition, the label has parameters derived from the instrument state such as viewing geometry, lighting, filter selections, etc. An MDIS EDR can consist of up to five sub-frames, reassembled into a full frame image. An EDR can also be a regularly spaced set of individual columns reassembled into a full frame image (jailbar mode, described below). The image data have been uncompressed and reformatted with standard PDS labels, but are otherwise “raw”.

MDIS observing scenarios are constructed using a set of key variables (“configurations”) which include the following:

  • Compression: MDIS images may optionally be compressed in a number of ways, including pixel binning on-chip, 12-8 bit compression, and FAST/Differential pulse code modulation (DPCM) lossless compression, all carried out in the instrument hardware; further pixel binning, subframing, wavelet compression, and jailbars are all carried out in the spacecraft MP, either individually or in combinations.

  • Pixel Binning: MDIS images can undergo 2x2 pixel binning in the focal plane hardware (also known as “on-chip” binning), resulting in a 512 x 512 image. Images can also be compressed using the MP, either in addition to Data Processing Unit (DPU) binning, or instead of DPU binning. MP pixel binning options of 2x2, 4x4, or 8x8 are available.

  • 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 lookup tables (LUTs) designed to preferentially retain information at low, medium, or high 12-bit Digital Number (DN) values. Eight LUTs are available.

  • 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 MP.

  • Wavelet compression: Images may be integer wavelet transform-compressed in the MP, typically at 3:1 for color data and 4:1 for monochrome data, but any value from 1 to 32 can be used.

  • Subframing: In order to manage downlink resources, up to 5 portions of the image can be selected in the MP and downlinked as subframes. The subframes are allowed to overlap. During EDR construction the subframes are all mosaicked into the original image. During orbital operations this option is not regularly used.

  • Jailbars: Intended for data management during optical navigation, jailbars are selected columns of an image retained by the MP. Commanded column spacing values are not restricted and can be set to any integer value between 1 and 1024, but the spacing is fixed throughout the image. During EDR construction the jailbars are all mosaicked into the original image. During flight this option is not regularly used.

  • 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.

  • 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.

MDIS Narrow-angle Camera Specifications:

Parameter

Narrow Angle

Field of view

1.5° x 1.5°

Pivot range (observational)

-40° (Sunward) to +50° (Planetward)

Exposure time

1 ms to ~10 ms

Frame transfer time

< 4 ms

Image readout time*

1 s

Spectral filters

1

Spectral range

725-775 nm

Focal length

550 mm

Collecting area

462 mm2

NAC-WAC coalignment knowledge

0.01 deg (179 μrad)

Spacecraft pointing knowledge

0.1 deg (1.75 mrad)

Spacecraft knowledge

0.02 deg (350 μrad)

Detector-TH7888A

CCD 1024 x 1024, 14-μm pixels

IFOV

26 μrad

Pixel FOV

5.1 m at 200-km altitude

Quantization

12-bits per pixel

Compression

Lossless, multi-resolution lossy, 12-to-n bits

*Transfer to DPU; transfer from DPU to Solid State Recorder (SSR) limited to 3 Mbps (4s to transfer 1024 x 1024 image)

MDIS EDRNAC products have the following file names:

ENrnnnnnnnnnM.IMG

where:

E = product type = EDR

N = camera = NAC

r = spacecraft-clock-partition-number minus 1 [0, 1], associates with pre- or post-spacecraft-clock-reset

nnnnnnnnn = Mission Elapsed Time (MET) counter taken from the image header; NOTE: this is a spacecraft clock seconds counter, and the value in the filename corresponds to the LAST second of the exposure

M = filter wheel position = M for the NAC which has no filter wheel

In ODE, EDRNAC products have the following product IDs:

ENRNNNNNNNNNM