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The Laser Airborne Depth Sounder
is a self-contained hydrographic surveying system that includes
comprehensive data acquisition, data analysis, and system support
facilities.
The system consists of the airborne
laser, navigation and data acquisition equipment mounted in
a Fokker F27-500 aircraft and a ground-based data analysis
and support system.
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A purpose-designed Nd:YAG infra-red
laser, operating at 990 pulses per second (990 Hz), is mounted
in an aircraft. Its output is frequency doubled to produce
visible green light with a wavelength of 532 nanometres
in one megawatt, five nanosecond duration pulses. These
pulses propagate well in clear ocean or coastal waters.
An optical coupler splits the output into infra-red and
green components. The green beam scans to either side
of the aircraft track using a scanning mirror.
The infra-red component reflects from
the surface. The green component penetrates the water
and reflects from the sea floor. The difference between
the lengths of the two beams can be used to determine
the depth of the water.
The infra-red pulses are emitted vertically
from the aircraft and reflect from the sea surface to
provide an initial reference. A scanning mirror directs
the green pulses to form a rectilinear pattern across
the survey track. These green pulses reflect from both
the sea surface and the bottom.
Returning pulses are collected by the scanning
mirror and directed to the green receiving
telescope which contain spectral, spatial and
polarising filters. The pulses are detected
by a sensitive photomultiplier with controlled
gain and propagation characteristics.
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LADS' unmatched combination of
speed and data density, leads the world in coastal water hydrographic
surveying and is a valuable addition to conventional echo-sounder
technology.
The picture to the above-right
shows a comparison between the LADS and the Ship Survey over
a 4 second interval.
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The Waveform
The image to the left is an example of a waveform
that the Ground-Based Unit receives after the LADS
aircraft surveys an area of the ocean.
100%
Survey Coverage
Typically, aircraft speed is 90m/s
(175 knots for a 5m pattern) allowing total area coverage to be rapidly
built up by overlapping survey runs, similar to the
use of swathe echo sounding in ships. However,
LADS' rate of progress is much faster.
Survey Mission
Planning
Survey sorties are
created in the system unit and are then used
in the airborne computers. The navigation computers
use the sortie data to provide tracking information
to the pilot for precise survey track and height
keeping under auto-pilot or manual flying. |
Navigation fixes for both the
sounding data and the aircraft survey track keeping are obtained
from GPS.
Ground-Based Equipment
The ground-based LADS equipment
provides facilities for mission and sortie planning, the processing
and validation of the raw survey data as well as system support
and maintenance requirements.
Mobile
Support
Ground power and
cooling for the Fokker aircraft are provided as truck mounted
units. Transportable support facilities ensure that the LADS
system has complete deployment flexibility.
Data
Processing
In the Ground System
(GS) raw digital data, transferred from the airborne acquisition
equipment on magnetic tape, is automatically converted to discrete
depth soundings for each sounding pulse of the laser, using
unique analysis software. Processing corrects each sounding
for system geometry, surface datum, refraction, depth bias,
tides and position. Each depth sounding is characterised by
a confidence vector which describes the sounding in terms of
the quality of the bottom return signal, the agreement between
adjacent soundings, the surface reference quality and the positional
accuracy.
The primary data, at the density
of the original soundings, is subsequently processed, at a
nominated scale of survey, to produce secondary data. This
process automatically selects and records hydrographically
significant shoal depths, together with the selection factors.
The GS computers ensure that data from each sortie is promptly
processed and available for on-site interpretation and validation. Primary,
secondary and raw data are recorded in a comprehensive
database. Sounding data can be viewed in a variety of formats.
Graphic, tabular and statistical formats are available on interactive
screens or as hardcopy plots. Software processing tools provide
ready comparison of survey data with defined benchmark areas.
On-Site Validation
Software tools enable the hydrographic
surveyor to work interactively with the presented depth data,
assess its integrity, its consistency with benchmark values
and other observations, and ensure that it meets the required
standards.
Validated data is then
available for inclusion in a hydrographic database for chart
production.
Lads | Introduction | How
it Works | Technical
Information
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