Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Determination of spectral signatures of natural water
by optical airborne and shipbome instruments
D.Spitzer & M.R.Wemand
Netherlands Institute for Sea Research, Den Burg, Texel
ABSTRACT: Interpretation of remote sensing imagery of watermasses requires knowledge on the spectral
absorption and backscattering coefficients of the suspended and dissolved materials. Spectral (ir)radiance
measurements are to be performed in order to determine the temporally and locally dependent signatures
of diverse watertypes. Underwater, airborne and portable instruments ASIR (Advanced Spectral Irradiance-
meter), CORSAIR (Coastal Optical Remote Sensing Airborne Radiometer) and PFC (Portable Four Channel radio
meter) were developed, calibrated, tested and applied for numerous measurements in and above various
coastal and oceanic watertypes. Large sets of data, both optical and seatruth, were obtained particularly
during the IMERSE (Indonesian Marine Environment Remote Sensing Experiments) - Snellius II campaign.
RESUME: L'interpretation des images, obtenues par la télédétection exige une connaissance des spectres
d'absorption et de rétrodiffusion des matières suspendues et dissoutes. Des mesures de l'éclairement doivent
être réalisées afin de déterminer les signatures, variables en temps et localité, des masses d'eau diverses.
Des instruments aquatiques, aériens et portatifs ASIR (Advanced Spectral Irradiancemeter), CORSAIR (Coastal
Optical Remote Sensing Airborne Radiometer) et PFC (Portable Four Channel radiometer) ont été développés,
calibrés, testés et appliqués pour un grand nombre de mesures dans et au-dessus des diverses masses d'eau
côtières et pélagiques. De grands ensembles de données, optiques ainsi que les concentrations des matières
optico-actives, ont été obtenus, en particulier pendant l'IMERSE (Indonesian Marine Environment Remote
Sensing Experiments) - Snellius II campagne.
1 INTRODUCTION
Investigation and experiments in the past decade
have demonstrated applicability of the remote
optical measurements for the study and monitoring
of the distribution of the materials suspended and
dissolved in the seawater. Diverse theoretical
models, (semi)empirical approaches and statistical
methods provide different algorithms for the
retrieval of the concentrations of the phyto
plankton and other suspended materials and of the
dissolved organic matter. This is not surprising,
considering the diversity of the aquatic conditions,
algal populations and atmospheric and environmental
influences. Since no general spectral behaviour of
the substances suspended and dissolved in the sea
water can be expected and predicted, the problem
of the radiative transfer in and above water is
not generally analytically solvable and thus no
universal algorithms can exist.
Despite this, the optical remote sensing of the
oceanic and coastal processes can supply striking
results if supported by the bio-chemical and
optical in situ, measurements (sea truth) combined
with the knowledge of the oceanography of the
sensed region. Instruments must be employed
specifically designed for the optical measurements
in and above the waterbodies. Measuring conditions
and specific objectives of the research determine
spectral, spatial and time resolution of such
instruments and data acquisition systems.
2 INSTRUMENTS
For the interpretation and correction purposes of
satellite imagery, special requirements are put on
the in situ measuring procedures. Design and
application of the underwater and of the abovewater
(low altitude) instruments substantially differ.
2.1 Underwater measurements
The investigations on the spectral properties (i.e.
the absorption and scattering signatures) of natural
waters, resulting into establishment of the
relationships between the upwelling optical signals
and the composition of the watercolumn, so called
"colour algorithms", are preferably to be performed
near surface underwater. Doing this, no influence
of the surface reflection (glitter) and of the
atmosphere is accounted, though experimental con
strains are introduced by wave motion. Optimal
depth of the measurements must be chosen, depending
on the seastate, possible stratification of the
watercolumn and on the solar conditions. Both, up-
welling and downwelling spectral irradiance must be
measured in real time, giving then the quasi-
inherent reflectance. The spectral behaviour of the
reflectance depends on the composition of the sea
water (sea truth). Inversly, the absorption and
scattering signatures and hence the concentrations
of the suspended and dissolved materials can be
derived from the reflectance. Short duration of a
spectral scan is crucial, with respect to the
horizontal and vertical instability of the watermass
and to the variability of the incident solar
radiation.
The developed Advanced Spectral Irradiancemeter
(ASIR) can scan simultaneously 22 spectral channels
between 400 nm and 720 nm within several seconds.
Spectral bandwidth of each channel is within 10 nm.
Radiation is collected by two cosine diffusers at
each side (up and down) of the instrument mounted
in gimbals. The upwelling and downwelling irradiance
is simultaneously detected and recorded on board
by means of an HP data acquisition and storage
system controlled by microcomputer. Variations of
the incident solar radiation are recorded by a
separate instrument mounted at the top of the mea
suring platform (vessel). Irradiance depth profiles,
