By using this
1. The surface
IS.
itinely used. In
green channel,
r detecting oil
nd 780 nm are
ut this task.)
red and NIR).
ments in these
or climatically
termination of
ese parameters
and hence the
e calculated by
ters mentioned
lentification of
early warnings
issified (Hahn,
istries dealing
im to 1000 nm
m mixed gases
jicted. In order
sitioned within
s occurs. In the
types of cloud
es (right)
ater areas or at
nd is important
im of the green
located in this
side of the red
nnel. Since the
n the software
and centred at
concentrated in
this channel is
Ralf Reulke
required for identifying vegetation. The channel is located in the strong rise (red edge) of the reflectance curve for
green vegetation
e channel 5 - NIR: 870 nm +/-35 nm. In the ADS40 this channel has not been implemented. The disadvantage of this
channel is the wide shape of the point spread function. Hence, the spatial resolution in this channel is no longer the
same as in the red one. But this channel is necessary for some applications in retrieving plant parameters. Hence it
follows that over heterogeneous areas channel 4 should be used instead of channel 5.
Table 2. summarises the values for the selected channels and their main characteristics for the user.
Channel Centre Comments Characteristics
Wavelength A, [nm]
Blue 460 Pure CCD water (maximum absorption)
sensitivity maximum light scattering
oil (with NIR)
RGB
Green 560 water, vegetation
(green reflectance peak)
RGB
Red 635 discrimination of vegetation (with
NIR)
RGB
NIR 1 730 Balance between | discrimination of vegetation
H,Op + CCD determination of biomass
vegetation - state of health
Broad Point mapping shorelines
Spread Function | discrimination of vegetation and soil
(PSF) pavement
NIR 2 860 Broad PSF
Table 2. Selected channels and their main characteristics
32 Signal-to-Noise Ratio (SNR) — Calculation for ADS40 Spectral Channels
This calculation is mainly focused on the signal in relation to the camera and Poisson noise. In this case the signal
estimation can be reduced on the calculation of the number of generated electrons:
A
n tig —-cos* 0 | dà R@)-TU)- L(A) (1)
4k ~
Ao
L is the radiance in front of the sensor, 7, the integration time (1 ms), k the f/4 of the optics, cos“ the shading of the
optical system, T the transmittance of the optical system and R the responsivity of the detector element. The calculation
was made with the following parameters. The detector size is A=6.5%6.5 um”. The spectral range is defined by table 2.
The ADS40 optics have an f/# = 4. The noise of the electrical channel in electrons is rms=150 €. The radiance in front
of the sensor was calculated with the 6S program (Vermote ef al., 1996). The results are obtained with a mid-latitude
summer model, an urban aerosol model, visibility 23 km, solar zenith angle 30°, and nadir view angle. Test targets are
vegetation, sand and lake water. Table 3 shows the results of the calculation. The calculation indicates a reasonable
signal-to-noise ratio for all spectral channels.
Centre |FWHM | Vegetation | SNR| Sand | SNR |Water| SNR
Wavelength
0.46 0.06 17504 43 8055 46 16526 [38
0.56 0.05 8547 48 0191 I 6307 [37
0.635 0.05 5770 34 11369 61 5576 133
0.73 0.05 27332 122 114122 !73 [3242 bo
0.86 0.05 31147 134 117068 [85 1585 10
PAN 94567 276 81490 252 441458 1163
Table 3. Generated electrons and SNR for different test targets
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 247
