Overview

Knowing how deep the water is under one's vessel has been a must for safe navigation for thousands of years. With the rise of maritime trade and naval warfare, highly valuable and strategic charts derived from seafloor mapping became closely guarded secrets. Today, governments, militaries, telecommunications and petroleum companies, academic institutions and many more continue to chart the seafloor for a variety of reasons.

 

Figure 1: The USS Wingina, photo and scan.

 

The stage thus set, enter Norbit's new iWBMSc mulitbeam sonar, featuring NovAtel's Synchronized Position Attitude Navigation (SPAN) system.

“There are really three major components here,” explains NovAtel's Ryan Dixon. “The Inertial Measurement Unit (IMU) is manufactured by Sensonor. NovAtel manufactures the GNSS and interface cards, and the real technology element that NovAtel provides is the SPAN system, an Inertial Navigation Solution (INS) using both GNSS and the Sensonor IMU. Norbit then makes the multibeam sensors, which incorporate all of these components.”

The IMU Dixon is referring to is Sensonor's STIM300, a Micro Electromechanical Systems (MEMS) device for weight and size constrained environments, replacing previous bulky and cumbersome systems.

“The finished product is vastly simpler for an end customer to install,” says Dixon. “Instead of having a totally separate INS system to plug in alongside the existing items like the computers and multibeam, all of the navigation equipment is directly and invisibly embedded into the multibeam equipment. It's like going from an aftermarket GPS glued to the dashboard to a built-in unit.”

Bathymetry 101

Bathymetry is the marine equivalent of topography and it generally involves the use of a sonar transducer that transmits a sound pulse from the surface and records the returning signal as it bounces back from the bottom.

Bathymetric surveys need to be completed within a reasonably short period of time in order to be cost-effective. A dedicated survey vessel is expensive to operate, especially in rough and remote deep-sea environments where the ship must be large enough to ride out rough ocean weather and where a highly skilled crew and scientific staff may be lodged for days or weeks at a time.

Multibeam sonars can transmit and receive a large number of 'pings' at once, thus covering a wide swath of seafloor in a single sweep. While being the instrument of choice in today's mapping applications, to be most accurate, measurements must take into account the movement of the instruments on the surface of the water.

Horizontal and vertical positioning is generally measured using GNSS receivers and IMUs. The IMUs need to be mounted very close to the transducer to minimize any potential error due to the offset between them.

 

Figure 3: Underwater canyons revealed.

The new Norbit iWBMSc eliminates many of the problems associated with conventional multibeam sonar; because the SPAN system is completely integrated into the sonar unit, there is no additional error as when these units are mounted in different parts of the vessel. The system directly senses the motion of the sonar because it is actually a part of the sonar unit.

Dixon describes the advantages: “The IMU needs to be precisely mounted-already done for you; the distance between the IMU and antenna needs to be precisely measured-already done for you; and all sorts of cabling etc. needs to be run between all the different boxes-also taken care of. So from a simplicity point of view, there is much less that can go wrong with this system while still maintaining a very high degree of accuracy.”

Peter Koldgaard Eriksen is Business Development Director at Norbit Subsea US Ltd. His relationship with NovAtel goes back to his 'previous life' within the industry, he says, with another company within the industry. Since then he has kept NovAtel in his sights.

 

Figure 4: The iWBMSc in action.

“Norbit knew what it wanted for the new  multibeam sonar system,” he says. “We knew we needed GNSS in our system, and we wanted something very compact. But we also knew that conventional motion sensors would not fit inside our unit. And on top of that, we were very pricesensitive.”

At various meetings and every single trade show, he says, “we could see what NovAtel was doing and it was very clear that we would eventually work with them on our new system; NovAtel was definitely on our road map.”

Norbit understood the potential implications of putting a small IMU inside their multibeam sensor head, and like NovAtel they were really just waiting for the right IMU to come along. When Sensonor, based in Norway, created that IMU in the STIM300 and NovAtel demonstrated its potential in its SPAN system, Norbit could see the sonar unit they had always wanted to create was now possible.

“It was a matter of time,” Eriksen says. “Looking at the GNSS sector, those units just keep getting cheaper and cheaper, and smaller and smaller. With motion sensors as well, the gap was narrowing.”

 

Figure 5: Norbit Subsea Magnus Andersen fits a seagoing kayak for bathymetric surveying.

 

“We did look at different options, but for our mulitibeam sonar, the NovAtel solution came out on top. This is the first fully MEMS-based system of this type, and it came with a reasonable price tag,” says Eriksen.

“NovAtel's OEM technologies provide us with robust, compact solutions to further enhance our offerings to our customers. Through this collaboration, we are opening new possibilities and industry-changing innovation which will set us apart and benefit the people who use our equipment.”

 

Figure 6: 3D Hillshade image of the Columbia River upstream of The Dalles Dam, Oregon. Depths between 4m and 76m below water level.