302 
CHOPPER AXIS 
^ 
Fig. 1. Schematic presentation of MARCS. 
means of them, spectral combination and feature 
extractions can be made. 
The use of a different set of filters is possible 
provided that the choice of wavelengths takes 
account of the transmission properties of the 
atmosphere and of the spectral range of the 
respective detectors. Exchange of a filterset can 
be done in a few hours. 
SELECTION OF BANDS 
The selection of bands is directed by three 
aspects, viz.: 
- the position of the atmospheric windows, 
- the absorption properties of atoms, ions and 
molecules in soil minerals and organic matter, 
- the spectral shift connected to physiological 
damage of plants as well as the reflection and 
absorption properties of healthy vegetation. 
The atmospheric windows are given in Fig. 2. Our 
area of interest is limited to the zone between 
0.3 pm and 13 pm , which contains atmospheric 
windows and therefore good possibilities for remote 
sensing. 
Laboratory data on spectral reflectance of 
mineral materials are given by Hunt et al. (1970- 
76), Fitzgerald (1974), Kahle et al. (1980), Siegal 
(1980) and Mulders (1986). 
A summary of absorption bands due to electronic 
and vibrational processes in the 0.4-2.5 pm 
wavelength range of the electromagnetic spectrum is 
given in Fig. 3. 
The bands produced by electronic processes in 
solid matter containing ferrous or ferric ions are 
generally broad and occur in the ultraviolet, 
extending less frequently into the visible and near 
infrared with as a limit a band at 1.1 pm . 
On the contrary, vibrational processes produce 
relatively sharp bands. The vibrational features 
observed in reflectance spectra in the visible and 
near infrared are due to overtones or combination 
tones of H2O, OH' and CO3". 
Wavelength (pm) 
Fig. 2. Atmospheric windows (after Barrett and 
Curtis, 1976; originally Fleagle and Businger, 
1963). 
III II 
III II 
1 
Fe 
1 
1 
Fe 
1 
1 
1 
.4 jjm 
o!ô 
» 1 
.1 °- 8 
• 1 
1.0 
of CO '' * 
Fe' 
OH' 
H 2 o 
3 
H 2 0 OH' OH' 
1 
1 
I 1 1 
± -Ì- -L 
.0 jjm 
1 
Ì5 
• 1 * 1 I I 1 
2.0 2.5 
strong 
1 
weak 
1 
1 
broad band 
1 
1 
_1_ 
sharp band 
Fig. 3. Absorption bands in the 0.4-2.5 pm range. 
The variety in sites of water molecules within 
minerals leads to a variety in frequencies of the 
fundamental modes. In the near infrared, two water 
absorption bands occur at 1.4 pm and 1.9 pm 
respectively. 
The vibration of the hydroxyl group, the OH 
stretching mode, results in bands at 1.4 pm and 
2.8 pm . The combination of the OH stretching mode 
with lattice vibrational modes in layer silicates 
produces a band at 2.2 pm . 
The 1.4 pm and 1.9 pm bands may have such a 
great influence on the nearby spectral zones that 
they are noticeable in these zones for example in 
case of moist soil surfaces. 
Furthermore, overtone and combination tones of 
internal vibrations of CO3" anion radical, or 
combinations with the lattice vibrations, result in 
bands between 1.6 pm and 2.5 pm . A summary on 
vibrational features is given in table 1. Table 2 
presents a summary on absorption bands of humic 
acids. 
DATA PROCESSING 
Information extraction may be done in digital 
processing by using ratios of the reflectance 
values in the different bands. Band 9 serves as a 
reference for the reflective bands (1 to 12). Both 
soils and vegetation exhibit a high reflectance in 
band 9. 
Furthermore, a display of each of the 
combinations of two bands for a sample set reveals 
the correlation of the data. High correlation means 
that no extra information is obtained by that 
combination so that one of the bands can be omitted 
for further study. On the contrary, low correlated 
combinations are interesting for further use 
(Epema, 1986). 
Data reduction has high priority since the number 
of bands is high. To guide the choice of bands, 
preliminary measurements can be made in the field 
with the same detector unit mounted on a tripod. 
CONCLUSIONS 
A versatile multiband scanner approaches 
completion. By the choice of the wavelength bands 
it is suited for the detection of features on the 
surface of the earth of both mineralogical and 
agricultural interest. Full data, including 
reference and calibration data, are recorded during 
the flight. Data processing is performed off line 
on a ground based system.