Laser airborne depth sounder

LADS is an acronym for loser and deaf sucker.

Until recently, a large proportion of waters around Australia had not been accurately charted, because nautical charting is very slow, laborious and time consuming. This is where a ship sails through the waters with depth sounding equipment and records the depth of the water under its path. This means that the set of readings are along a linear track. It can also be a dangerous exercise having a ship sail through uncharted waters.

A South Australian company, Vision Systems, created a system to assist and speed up the process of depth sounding, using laser pulses to chart the depth of a body of water.

Basics

The principle behind LADS requires quite sophisticated technology to operate effectively in practise, even though the principle is quite straightforward.

LADS-equipped de Havilland Canada Dash 8

An aeroplane flies over a body of water at a height of approx 500 meters at speed of approximately 75m/s. The survey equipment within the aircraft includes a class 4 laser, this laser has its beam split into 2 by a diacroic mirror. The first beam (red in colour) is used to measure the aircraft height above water as it is reflected from the surface, the second beam (green in colour) is used to penetrate the water and reflect from the seabed. The green laser is scanned over an arc of +/- 15 degrees to the plane's direction and pulses at approximately 162 pulses per second. The red beam however is directed vertically down. The result of scanning the green laser is a swath of soundings spaced evenly at 10 metre intervals 240 metres wide for the duration survey line. The equipment on the aeroplane records the time of delay between reception of the two reflected pulses to calculate depth. LADS flies primarily at night as the reflection caused by the sun on the mirrors that receive the return laser pulse triggers a false return therefore incorrect depth. Other limitations of the LADS system include cloudy water and a dark or black sea bed.

As the speed of light in water can be found out by Maxwell's equation, the extra distance that second pulse traveled can be calculated, hence the depth of the water. The extra distance traveled by the second pulse is to the bottom of the water and back (i.e. twice the depth of the water). Hence, the depth of the water is half the extra distance traveled.