ce Wr NW
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-
objects. The maximum level will most probably saturate
over clouds, hot spot situations, and glitter resulting from
reflections on the water surface. Applications related to
these reflectance levels (e.g. top-of-the-cloud BRDF
measurements) are explicitly excluded from the range of
APEX applications. An update to include these levels
would require a remodelling process including an albedo of
1.1 over clouds at altitudes up to 6.5 km.
The validation level is used to verify the SNR
specifications. The requirements for SNR are defined as the
average SNR over all channels covered by one detector
(e.g. VIS or SWIR) including the band positions in the
water vapor absorption bands. Atmospheric applications are
an important goal of APEX and therefore the specifications
include all band positions inside the absorption bands.
The minimum level is defined as 0.01 albedo. This is at the
same time the expected uncertainty of the RTC (Radiative
Transfer Code). Inland water monitoring is an important
application for the APEX instrument. Therefore the lower
end of the dynamic range is defined by this minimum level.
wm mmn ih :
: i y vos AA
T 100-7 nf
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5 Pe
"E 10 : ys...
=
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9 E i
z - --- Maximum Wu
Q 991 À | Validation SS LA
Zt... lur Minimum TU
0.001 TUUTUCTCUEBUTUUITIEUIETTTT rrr TTT TET
04 05 06 07 08 09 10 11 12 13
14 3.50145:04.75, 15.8:/(£:9:2:0:2.31522. 2:33:24. 12.5
Wavelength [nm]
Figure 3. APEX radiometric specifications for minimum, validation and maximum at-sensor-radiances
A major figure of interest in the expected performance on
an airborne imaging spectrometer is the signal-to-noise
ratio (SNR). The signal is calculated using an end-to-
end sensor model (DSS, 1997; Schaepman, 1998). This
allows to determine the total number of electrons arriving
per detector pixel based on the modelled at-sensor-
radiance using the RTC. The final estimation of the noise
is calculated using the manufacturer information of the
detector and the electronic noise parameters. The SNR is
then derived using the total number of electrons divided
by the noise-contributiong electrons.
WEST es
1000 4 ej %
100 HF |
= a
= jur ^ eet
$ P
10 7
olde Maximum
— Validation
ee Minimum
fr
| | | | I | I | |
04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Wavelength [um]
Figure 4. SNR for all APEX channels (maximum, validation and minimum level)
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
35
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