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THEMIS VIS Projected Albedo Product (VISALB)

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VISALB – Projected Visible Albedo Product

 

Instrument: Thermal Emission Imaging System

PDS Data Set ID: ODY-M-THM-5-VISALB-V2.0 (in data directories odtva2_****)

For more information about all THEMIS GEO products, see the Geometric Processing Guide.

 

The Thermal Emission Imaging System (THEMIS) Projected Visible Albedo Product (VISALB) data products are generated by calculating albedo from the calibrated visible radiance (VISRDR) (as is done with the Visible Apparent Brightness Record (VISABR) products), and then projecting the data as described for the Visible Geometric Projection V2 (VGEO2) products.  The results are then stored for a single band in a standard PDS IMAGE object.

 

Lambert albedo (L) is calculated from the THEMIS visible radiance using

 

 

where        R = calibrated spectral radiance data (VISRDR) in [W m-2 st-1 μm-1].

 J = solar irradiance in [W m-2 μm-1] at the Mars-Sun distance (d) in AU,

 and i = incidence angle of the Sun on the surface of Mars.

 

The solar distance (d) and incidence angle (i) are computed at the image center.  The solar irradiance values were convolved to the THEMIS visible wavelengths and are given in the table below.  This albedo equation does not account for topography or shadows, so the calculated values are most accurate for surfaces with low to no slopes.

 

VISALB images are generated for a single band of most available VISRDR images with an incidence angle less than 80°, and located between ± 75° latitude.  VISALB images are usually derived from band V-3 (below); if that is not available, band V-4 or the first available band may be used.

 

Solar irradiance values in THEMIS visible wavelengths.

THEMIS Band

J [W m-2 μm-1]

V-1

1714.882

V-2

1854.915

V-3

1567.595

V-4

1268.324

V-5

981.8372

 

THEMIS VIS images are acquired in framelets of size 1024 pixels crosstrack by 192 lines downtrack.  The number of framelets is determined by the number of bands selected (five available, enumerated in the table below) and the spatial resolution selected (three summing modes available).  If spatial summing is not applied, either a single-band, 19-framelet (65.6 km) image or a 5-band, 3-framelet (10.3 km) image can be collected.

 

THEMIS available visible bands.

Band Numbers

Center (micrometers)

FWHM* (micrometers)

V-1

0.425

0.049

V-2

0.540

0.051

V-3

0.654

0.053

V-4

0.749

0.053

V-5

0.860

0.045

*Full-width, Half-maximum

 

THEMIS VISALB products have the following file names:

VnnnnnsssALB.IMG

where:

nnnnn = 5-digit mission orbit when the image was collected

sss = 3-digit image sequence number indicating the order that images were collected each orbit (001 = first image collected in the nnnnn orbit)

 

In ODE, VISALB products have the following product IDs:

VNNNNNSSSALB