846 
platform, waves on water surface, changes in optical water characteristics, 
etc.), the absolute demand for detecting system is the capability for 
concurrent multichannel spectrum analysis of return signal. Two alternative 
approaches are the use of commercially available optical multichannel analyzer 
(OMA) or designing the specific system based on a set of photomultiplier tubes 
(PMTs) responsible for detection at specific wavelengths. The common feature 
of both systems is gating the detector unit by a short pulse (usually - 20-50 
ns) synchronously with laser pulse to increase the ratio of useful pulsed 
laser-induced signal to the continuous background irradiance caused by solar 
illumination. This provides the opportunity for reliable signal detection even 
in conditions of intensive solar illumination of water surface. 
The major advantage of OMA-based systems is their capacity for accurate 
high-resolution (512-1024 points) measurement of optical response in required 
spectral range. This feature is particularly essential in research lidar 
systems (Cecchi et al., 1994; Chekalyuk et al., 1992a; see also Fig.l), 
allowing studies of various objects in a wide range of environments (including 
case I and case II waters). Although typical OMAs with gated intensified 
detector possess a sensitivity of about 10-20 photons per channel, recent 
progress in detectors development has resulted in increase in sensitivity up 
to detection of a few photons, comparable with PMTs. The disadvantages of OMAs 
are the lack of capability for time-resolved spectrum analysis with nanosecond 
resolution and limited dynamic range. 
In PMTs-based systems the wavelength selection is achieved by attaching 
the PMTs to the focal plane of spectrograph with light guides (Hoge and Swift, 
1981), or by utilizing the wavelength-selective mirrors and filters after 
telescope (Hengstermann, et al., 1992). The system of that kind with single 
PMT was tested as early as in 1973 (Mumola et al., 1973) for remote sensing of 
algal Chl-a fluorescence at 685 nm with the prototype of airborne lidar. The 
major advantages of PMTs-based systems are high sensitivity and relatively 
simple design. Thus they may be considered as cost-effective solution for 
operational lidars aimed for monitoring of particular objects (e.g. oil 
pollutions or phytoplankton). Nonetheless some caution must be exercised when 
selecting spectral points for detection and developing algorithms for data 
interpretation. This is of particular significance for the systems utilized 
for monitoring in coastal zones due to high variability in spectra of optical 
response in these areas. 
An important feature of PMTs-based detecting unit is also the potential 
possibility of time-resolved measurements for vertical profiling in near- 
surface water column. The first results provided by implementation of this 
approach with shipborne lidar look promising (Reuter et al, 1994). The proto 
type system may include up to 16 independent detecting channels, the logarith 
mic amplifiers are utilized for widening the dynamic range of detection to 
overcome fast attenuation of optical signal in water. The vertical resolution 
of 0.2 m and optical depths of 4 to 6 attenuation lengths have been recently 
achieved in the field with laser excitation at 532 nm (Nd:YAG-laser). 
3.3. Moving Platforms 
There are two types of lidar-fluorosensors utilized for monitoring in the sea: 
the aircraft-based (e.g. Hoge and Swift, 1981; Hengstermann et al., 1992) and 
shipborne (Reuter et al., 1994; Chekalyuk and Gorbunov, 1992b) lidar systems. 
The advantage of the first group is the capability for rapid measurements at 
synoptic (100-1000 km) scale. This peculiarity is of particular significance 
when studying fast-developing processes in the sea distributed over large 
areas, as well as for environmental survey in coastal zones. 
From this point the use of a ship as a platform for lidar system may lead 
to some limitations because of relatively low speed of a ship. On the other 
hand, the shipborne lidar systems provide the unique opportunities for 
development of lidar technique and methodology due to principle possibility to