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.
