MESSENGER MDIS Narrow-angle Derived Data Record (DDRNAC)

DDRNAC – Narrow-angle Derived Data Record

Instrument: MESSENGER Mercury Dual Imaging System

PDS Data Set ID: MESS-E/V/H-MDIS-6-DDR-GEOMDATA-V1.0DOI: 10.17189/1520321

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

The Mercury Dual Imaging System (MDIS) Narrow-angle Derived Data Record (DDRNAC) data products include geometrical information for each image pixel for all valid, calibratable images acquired by the Narrow-angle camera (NAC) during flybys and orbit around Mercury, for which the target is Mercury and at least part of the image falls on the surface of the planet. There are 5 image planes for information for each pixel: planetocentric longitude, positive east, planetocentric latitude, solar incidence angle, emission angle, and phase angle.

For each spatial pixel, the equipotential surface intercept is calculated from the instrument gimbal positions. 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.

MDIS observing variables pertaining to the DDRNACs 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. The dimensions of the DDRNAC are controlled by the types of pixel binning applied to the source image.

  • 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. The position of the pivot is a key input to calculation of values in the DDRNAC image planes.

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 DDRNAC products have the following file names:

DNrnnnnnnnnnM_DE_v.IMG

where:

D = product type = DDR

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, DDRNAC products have the following product IDs:

DNRNNNNNNNNNM_DE_V