Inside the AHS
LADS II (Laser Airborne Depth Sounder)
For further information on LADS, please visit the FUGRO LADS website.
Originally developed by Australia's Defence, Science and Technology Organisation (DSTO) for the Royal Australian Navy, the Laser Airborne Depth Sounder (LADS), fulfils a crucial need. Two hundred years after Cook, Flinders and other explorers of the Southern Hemisphere started?? charting? Australia's vast coastline, nearly half of the continental shelf, or around one million square kilometres, remains unsurveyed or? incompletely surveyed. LADS was developed to help speed up surveying and charting progress.
The LADS system provides valuable data for The Australian Hydrographic Service. 990 soundings per second , spaced from 2 to 6 metres apart in a swath between 47 to 288 metres wide; LADS provides accurate, high density digital depth and positional data of coastal waters up to 70 metres in depth. Flying at speeds between 140 and 210 knots depending on the sounding pattern.
Operational sounding height is between 1200 and 2200 feet (~400 to 730m) above the sea, unhindered by reefs or shallows. ? LADS surveys the sea floor at a rate in excess of 50 square kilometres an hour.
LADS can be deployed in a variety of hydrographic tasks for depths down to 70 metres and topographic heights up to 50m when conditions are suitable for laser survey.
The LADS aircraft is deployed from landing strips within operating range of the survey site. With accurate ground position determined using a Wide Area Global Positioning System (WaGPS) and post processed with information from a known base station.? This is known as Post Processed Kinematic (PPK) and provides an accuracy better than the WaGPS alone.
LADS has been designed for isolated, self-contained operation, maintenance and support. .
7 plus hours of survey runs can be flown close to a deployment base. At a distance of 300 nautical miles, four hours of survey are available. LADS is used for both day and night operation.
Airborne data collection eliminates the difficulties associated with shipborne survey of shallow, dangerous or complex waters such as reef areas.? It also permits the fast identification of key areas of navigational significance, such as undiscovered channels and passages.
Powerful analysis and conversion capabilities provide timely turn-around of data. This enables efficient, flexible survey sortie operations.
The digital data recorded by the airborne system is processed in ground-based analysis equipment. "Ground truthing" reference data and tidal information are incorporated in the data analysis and validation process.? LADS data accuracy meets the International Hydrographic Organisation accuracy standards for hydrographic surveys.
LADS is operated with only a small number of personnel. A typical crew? consists of the flight crew, two LADS airborne operators/data analysis staff and two ground support staff.
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 DASH 8 aircraft and a ground-based data analysis and support system.
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.
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.
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.
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.
Ground power and cooling for the aircraft are provided as truck mounted units. Transportable support facilities ensure that the LADS system has complete deployment flexibility.
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.
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 airborne equipment is modularised and mountable via seat rails in most fixed wing aircraft. LADS airborne equipment is mounted in a DASH 8 for the Royal Australian Navy.
LADS Airborne Equipment
LADS airborne equipment is modularised and mountable via seat rails in most fixed wing aircraft. LADS airborne equipment is mounted in a DASH 8 aircraft.
|Sounding Platform:||2 axis stabilised;
?5? roll; -1.5? to ?6.0? pitch; ?20? yaw
|Console:||single station fully integrated graphical user interface (GUI); digital imagery acquisition, donward looking video recording, plasma data displays; calibrated video track display; depth sounding signal display|
|Scanning Mirror:||Major (transverse) ?20? at 9Hz; Minor (longitudinal) +0.5? at 18Hz||Digital Image Acquisition (DIA) Hard Drive||Removable 80GB HDD saves time synchronised digital images.|
|Laser:||Nd:YAG||Communications:||intercom (4 nets); HF; VHF, Satellite phone|
|Wavelength (IR):||1,064nm||Position Fixing:||Post Processed Kinematic Wide Area GPS|
|Wavelength (Green):||532nm||Sortie Plan:||ground planned survey and calibration objectives and system parameters|
|Laser Tuning:||automatic||Sortie Plan Media:||CD|
|Laser Safety:||eye safe to AS2211-1991 at survey height||Recorded Data:||raw depth soundings; position; system data and equipment status|
|Beam Geometry:||vertical 1,064nm and scanned 532nm beams||Recording Media:||400/800 GB LTO, 4 GB PCMCIA data card (GPS data only)|
|Sounding Rate:||900 per second||Video Data:||video of ground track with superimposed data. Includes recordings from audio intercom, HF, VHF and satellite phone communications|
|Swath Width:||47 - 288 metres||Video Media:||VHS PAL|
|Sounding Spacing:||2, 3, 4, 5 or 6m
Nominal is 5m
|Power Requirements:||+28V DC 7kW and 208V AC|
|Scan Pattern:||orthogonal to planned track||LADS Survey Crew:||1 or 2|
Airborne Equipment Layout in the Dash 8
A. Operator Position, B. LASER Cabinet and Laser Platform, C. System Control Cabinet
The laser beam geometry has been designed to cover the widest swath while maintaining performance and accuracy.
A 240m wide matrix of depth soundings at nominal 10m intervals is recorded on each survey run allowing total area coverage to be built up by overlapping survey runs. This provides significant benefits over conventional shipborne sonar.
Depth Sounding Rate:
|168 soundings per second||
|240m at 500m height||
Transit Cruise Speed:
|orthogonal to planned track||
|24 soundings/transverse scan; nominal 10m x 10m grid||
Soundings per sq km:
|LADS airborne equipment plus 18 passengers|
Soundings per hour:
Data Processing Time:
|approx 1:1 with survey time|
|on-line storage for secondary sounding data for 120 sorties|
Depth Sounding Accuracy:
|better than 0.3m (one standard deviation) over depth range 2-30m|
|2 to 50m||
Positional Accuracy of Soundings:
|54 sq km/hour||
|eye-safe to AS2211-1991 at survey height|
|7 + hours||
|self-contained with maintenance facilities for remote site operation|