MESSENGER MASCS VIRS Derived Data Record (VIRDDR)

VIRDDR – Derived Data Record

Instrument: MESSENGER Mercury Atmospheric and Surface Composition Spectrometer

PDS Data Set ID: MESS-E/V/H-MASCS-4-VIRS-DDR-V1.0

For more information about MASCS VIRDDR products, see the VIRS CDR/DDR/DAP Product SIS.

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Visible and Infrared Spectrograph (VIRS) Derived Data Record (VIRDDR) data products consist of VIRS derived data. The VIRS experiment is a fixed concave grating spectrograph with a beam splitter that simultaneously disperses the spectrum onto two photodiode arrays. VIRDDRs are generated from on-planet (flyby and orbit) Calibrated Data Records (VIRCDRs).

One VIRDDR data product contains all the derived reflectance data from one observation. Not all VIRCDRs were converted to VIRDDRs; lamp calibrations, dark calibrations, data looking solely out into space, and star calibrations do not provide meaningful reflectance information. Thus there is not a one-to-one correspondence between VIRCDRs and VIRDDRs, but there should be a correspondence between VIRCDRs and VIRDDRs that target the surface of planets (Earth, Moon, Venus, Mercury). VIRCDRs are converted to VIRDDRs by first filtering out non-planet-targeted observations, then converting to reflectance by dividing the solar irradiance spectrum out of the MASCS radiance data. At high temperatures in the extended mission, the VIRS shutter sometimes failed to operate, resulting in bad interleaved dark corrections. Observations with frozen shutter anomalies have not been processed to VIRDDRs.

MASCS VIRS data are represented in the following coordinate systems:

• Planetocentric body fixed: The MBF coordinate system is defined by the planetocentric position; Cartesian X, Y, Z coordinates related to the planetocentric distance; latitude measured positive northward from the equator; and longitude measured positive eastward from the prime meridian.

• Cartographic: Surface observations use IAU planetocentric system with East longitudes being positive for planetary surfaces. The IAU2000 reference system for cartographic coordinates and rotational elements was used for computing latitude and longitude coordinates of planets.

The principal parameters when observing with the MASCS VIRS are shown below:

Parameter	Description
Integration Time	The amount of time that the array detectors will integrate photon counts.  Unit is 50 milliseconds.
Integration Count	The amount of integrations that will be taken.
Period	The interval time after which another integration will be started.  Integrations are started at the top of every period.  Time between integrations is the period time minus the integration time.  Unit is 0.05 seconds.
Dark Frequency	The frequency at which dark counts will be collected during a VIRS observation.  For an entered number n, dark counts will be collected every nth integration.
NIR Gain	Sets of the gain of the NIR array detector, to either low, high, or N/A when the VIS detector is enabled.
NIR Lamp On	The NIR flat field lamp is powered on or off.
VIS Lamp On	The VIS flat field lamp is powered on or off.
Binning	The number of pixels that were binned together in the data.
Start Pixel	Start pixel of data captured by the detector array.  For VIS, the value can be from 0-511.  For NIR, the value can be from 0-255.
End Pixel	End pixel captured by the detector array.  The end pixel value must be greater than the start pixel value.

MASCS VIRDDR products are named according to the following file naming convention:

VIRSdD_mmm_yyddd_hhmmss.DAT (with detached PDS label)

where:

d = detector

F = FUV

M = MUV

V = VIS

mmm = mission phase

MF1 = Mercury 1 flyby

MF2 = Mercury 2 flyby

ORB = orbit insertion until end of nominal orbit mission

OB2 = second year of orbital operations

OB3 = third year of orbital operations

OB4 = fourth year of orbital operations

OB5 = fifth year of orbital operations

yy = 2-digit year in UTC converted from the first Mission Elapsed Time (MET) in the EDR

ddd = 3-digit day of year in UTC converted from the first MET in the EDR

hhmmss = 6-digit hour, minute, second in UTC converted from the first MET in the EDR

In ODE, VIRDDR products appear under the following product IDs:

VIRSDD_MMM_YYDDD_HHMMSS_DAT