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How big is an OMI pixel?

M de Graaf, H Sihler, LG Tilstra, P Stammes

The Ozone Monitoring Instrument (OMI) is a
push-broom imaging spectrometer, observing solar radiation
backscattered by the Earth’s atmosphere and surface. The
incoming radiation is detected using a static imaging CCD
(charge-coupled device) detector array with no moving parts,
as opposed to most of the previous satellite spectrometers,
which used a moving mirror to scan the Earth in the across-
track direction. The field of view (FoV) of detector pixels
is the solid angle from which radiation is observed, aver-
aged over the integration time of a measurement. The OMI
FoV is not quadrangular, which is common for scanning
instruments, but rather super-Gaussian shaped and overlap-
ping with the FoV of neighbouring pixels. This has con-
sequences for pixel-area-dependent applications, like cloud
fraction products, and visualisation.
The shapes and sizes of OMI FoVs were determined pre-
flight by theoretical and experimental tests but never veri-
fied after launch. In this paper the OMI FoV is characterised
using collocated MODerate resolution Imaging Spectrora-
diometer (MODIS) reflectance measurements. MODIS mea-
surements have a much higher spatial resolution than OMI
measurements and spectrally overlap at 469 nm. The OMI
FoV was verified by finding the highest correlation betweenThe optimal overlap function between OMI and MODIS
reflectances is scene dependent and highly dependent on time
differences between overpasses, especially with clouds in the
scene. For partially clouded scenes, the optimal overlap func-
tion was represented by super-Gaussian exponents around 1
or smaller, which indicates that this function is unsuitable to
represent the overlap sensitivity function in these cases. This
was especially true for scenes before 2008, when the time
differences between Aqua and Aura overpasses was about
15 min, instead of 8 min after 2008. During the time between
overpasses, clouds change the scene reflectance, reducing the
correlation and influencing the shape of the optimal overlap
function.
MODIS and OMI reflectances in cloud-free scenes, assuming
a 2-D super-Gaussian function with varying size and shape to
represent the OMI FoV. Our results show that the OMPIX-
COR product 75FoV corner coordinates are accurate as the
full width at half maximum (FWHM) of a super-Gaussian
FoV model when this function is assumed. The softness of
the function edges, modelled by the super-Gaussian expo-
nents, is different in both directions and is view angle dependent.

Bibliografische gegevens

M de Graaf, H Sihler, LG Tilstra, P Stammes. How big is an OMI pixel?
published, Atmospheric Measurement Techniques, 2016, 9

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