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Multibeam Airborne Lidar Coastal Mapping Advances

The Rapid Airborne Multibeam Mapping System, RAMMS, was developed over a period of three years before it was officially launched in the summer of 2018. Senior hydrographer Richard Goosen describes how Fugro used the system for coastal and nearshore mapping at the paradise Turks and Caicos Islands.

From an article in Hydro International by Richard Goosen.

In July 2018, Fugro embarked on a large-scale project to map the Turks and Caicos Islands for the United Kingdom Hydrographic Office (UKHO). This landmark effort called for integrated, high-resolution bathymetric, topographic and orthoimage datasets to support nautical chart updates and coastal zone management activities. To achieve the sizeable bathymetric portion of the survey, Fugro debuted a new airborne lidar system known as RAMMS: Rapid Airborne Multibeam Mapping System.

A New Answer to an Old Problem
When it comes to shallow water, airborne lidar bathymetry (ALB) has long been proven to be a fast, safe and cost-effective method for accurately defining nearshore water depths. Traditionally, ALB systems have been specialised to deliver either high point densities or good depth penetration, but not usually both, due to sensor design limitations. As such, ‘high-resolution’ systems are typically used in areas shallower than 15m, and ‘deepwater’ systems are used in areas down to 50m (water clarity permitting). While it is possible to merge datasets acquired by high-resolution and deepwater systems, doing so is time-, cost- and resource-intensive.

In recent years, international hydrographic agencies have challenged contractors to improve the efficiency and quality of ALB data acquisition and deliverables. Specifically, they have pressed for a solution that would support inshore nautical charting standards.

Fugro accepted this challenge, identifying and leveraging decades of shallow-water mine detection technology developed by Areté Associates, then refining it to meet hydrographic surveying needs. The system, known as RAMMS (Rapid Airborne Multibeam Mapping System), was developed over a period of three years before it was officially launched in the summer of 2018.

System Development
RAMMS is based on third-generation lidar technology and differs from conventional ALB sensors in the way energy is transmitted and received from the sensor. Traditional scanning systems use a pulsing laser that transmits discrete beams of laser light, which are then directed to the seafloor in a ‘pseudo swath’ using scanning or rotating mirrors. In contrast, RAMMS is a solid-state pushbroom system with no moving parts. Instead, it transmits a single diffuse pulse of laser light that generates a ‘true swath’ of energy approximately equal in width to the flying height of the aircraft. The returning signal is then focused into a streak tube receiver where it is beam-formed into a maximum of 900 individual slices (beams) comprising the full waveform.

This swath-coverage methodology closely parallels the workings of vessel-based multibeam echosounder systems and produces data of a similar quality. RAMMS delivers 24,000 range observations per second while achieving 3-Secchi disk depth penetration, making it possible to meet International Hydrographic Organization (IHO) Order 1 survey standards. This lightweight system can also be combined with other remote sensing technologies to address a wide range of bathymetric, topographic and imagery needs from a single airborne mission.

After testing RAMMS on land, off the California coast and at an established lidar-testing facility in Fort Lauderdale, Florida, Fugro performed a proof-of-concept project in Belize. The RAMMS data acquisition tied in with a multibeam echosounder survey that was already underway for the UKHO and provided favourable results. With that, RAMMS was ready for commercialisation.

Mapping in Paradise
In July 2018, the UKHO awarded Fugro a contract to survey 7400km2 of the Turks and Caicos Islands in support of the Foreign and Commonwealth Office’s Overseas Territory Programme. The project called for full bathymetric, topographic and orthoimagery data, collected to IHO Order 1b specification. With sparse existing survey coverage and shallow water depths (0m at shoreline to 40m at the reef edge), ALB was deemed the most cost-effective option for acquiring the project’s bathymetry data. Fugro proposed using RAMMS and was subsequently awarded the contract by the UKHO. To capitalise on the system’s availability in the region, the UKHO awarded Fugro a second shallow water survey comprising 2500km2 off the northern coast of Belize. This add-on work was performed immediately after completion of the primary Turks and Caicos mission in support of the UK’s Commonwealth Marine Economies Programme.

As is expected for any new technology, challenges arose during this first large-scale deployment of RAMMS, with data management chief among them. Since RAMMS records the full waveform for each pulse, raw data capture is very large. Additionally, the high-resolution orthoimagery coverage meant that each 4- to 5-hour collection flight resulted in approximately 1TB of preprocessed data. Raw data deliverables for Turks and Caicos alone totalled more than 82TB! Due to limited local infrastructure, the data could not be uploaded to Fugro’s Houston data centre via a data link but had to be shipped by hard drive instead.

The project’s large size and diverse survey area also tested the system. The Turks and Caicos project required long flight lines, introducing several processing challenges which impacted the ability to create LAS files in the field. Staff also needed to update the field-processing module several times during the project to account for the dynamic environment that spanned the shallow, extremely reflective bank, its topography, and a very steep drop-off along the edge of the barrier reef that surrounds almost the entire island chain. The nature of the software meant that each time a significant update was applied, all the data had to be reprocessed to ensure data cohesion. Similarly, new data artefacts were discovered that had not been encountered during development, requiring some lines to be reflown to fill unexpected data gaps.

These issues aside, by the time the Turks and Caicos part of the deployment had been completed, the system workflow, software, and processing routines had been refined back to the original design intention, which was to achieve initial LAS upload to the data centre in a 1:1 (flight time to processing) timeframe.

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