Downwelling irradiance measurement

ERDC-TR-05-1
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Neither spectral channel bandwidth modification nor spatial binning is available,

but each pixel of the focal plane is digitized as single pixel. Therefore, mode A

requires long integration times. This mode is mainly for calibration, testing, and

demonstration. However, it can be used in low velocity or high-altitude aerial

surveys.

Mode B. Provides full spatial resolution (i.e., smallest pixels) with reduced

spectral resolution. This mode offers 384 spatial pixels with no binning, although

spatial binning (i.e., summing of 2, 4, or 8 pixels to output larger pixels) is

available for low signal level measurements.

Spectral sampling is programmable between 1.5- to 9.4-nm bandwidths

within a total wavelength range of 450 nm. Individual spectral bands are pro-

grammable. Therefore, the total number of channels is controlled by the selected

spectral bands and selected bandwidths.

Mode C. Provides full spectral resolution, with reduced spatial resolution.

This mode uses the full hyperspectral capabilities of the sensor (288 bands), but

with only 47 pixels in the cross-track direction (as compared to 384 with Mode

B). Further spatial binning (i.e., decreased spatial resolution) is still available.

Mode C is suitable for hyperspectral applications that require varied spatial

resolutions.

Mode D. Provides the greatest variation in both spectral resolution and spatial

resolution. While Mode D minimizes the amount of data to be collected and

allows high frame rates, it does not offer the highest spatial resolution.

2.3.2

Downwelling irradiance measurement

The quality of airborne hyperspectral remotely sensed data is enhanced by

using downwelling irradiance measurements together with the actual target

measurements. In AISA this is implemented using a hemisphere (or flat) reflector

attached to the aircraft roof receiving direct solar radiation. In the best case, there

is a clear, sunny sky. The correction is more difficult if atmospheric conditions

are cloudy or partly cloudy. The resulting signal is fiber-optically connected to

the spectrograph such that it covers a few pixels from the swath width (i.e., a few

columns on the detector). Thus, there is a simultaneous measurement of down-

welling irradiance with the same wavelength range as is captured by the ther-

moelectrically stabilized charge-coupled device (CCD). This is a great advantage

to systems requiring mechanical movement of the instrument or even a separate