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,