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Rabbit® Keeps Marine Vessels in Position

Offshore survey ships and rigs require expensive equipment to get the most accurate heading and positioning on the open seas. Stable navigation is critical for mapping and survey missions, since the data collected is used to develop safe navigating avenues. Specialized devices such as a true north-seeking fiber-optic gyrocompass help survey ships maintain accurate position and heading during offshore survey missions. Mechanical gyros are used to keep compasses pointing north even though the ship's direction may change rapidly due to unstable open waters.

Geonav Marine Systems specializes in the development of marine electronics and offshore positioning systems. Using GPS, fiber-optic technology, and Rabbit's products, Genonav offers accurate and reliable positioning technology to customers at an affordable price.

"We aimed to build a system that combined a GPS heading sensor with a fiber-optic rate gyro," explains Russell Morton of Geonav. "The object was to create a device with a performance close to the very expensive north-seeking fiber-optic and ring laser devices, but at a price close to a good quality mechanical gyro."

Geonav's I-Compass system is comprised of different key components with RabbitCore modules as the main processing units. It uses four RabbitCore RCM3600s and one PowerCore 3800. Two external NEMA4 boxes house the critical components. The first box is the Remote GPS module containing the RCM3600 and a GPS heading sensor, which are attached by an RS-232 connection. An optional magnetic sensor can be added to help track the northern position during long-term GPS outages. The second box is the Remote Gyro module, which contains the RCM3600, a fiber-optic gyroscope (FOG), and a pitch and roll sensor. These devices are also attached via RS-232 connection. The RCM3600 in both modules connects back to the PowerCore 3800 housed in the I-Compass using an RS-422 connection. The connections can be up to 150 feet depending on the installation.

During a survey mission, the GPS heading sensor finds the true north position. The FOG provides system stability, accounting for sudden movements and directional changes. The information is gathered by the RCM3600 modules and is transmitted to the PowerCore3800, which processes the data and sends compass information to the display. The processed data is also sent to another set of RCM3600 modules, providing four RS-232 or RS-422 system ports for connection to other navigational equipment.

The I-Compass system takes advantage of absolute heading values to convert a rate compass from a relative heading device to a true north heading sensor. It offers a higher level of stability than an autonomous GPS heading sensor and is also capable of performing at rates of turn up to 100 degrees per second without data interruption. In the initial design, Geonav considered an embedded PC for the I-Compass application.

"We looked at an embedded PC approach with external support processors for I/O. It proved to be complex in terms of hardware, and could not give us the data rates that we needed. We also wanted to use a vacuum fluorescent display, since it is true sunlight readable but can also be dimmed at night. This required a unique interface that would be difficult to support on the PC," says Morton.

The project's complexity required serious processing power, but at a low cost. Geonav found the ideal solution with Rabbit's RCM3600 and PowerCore 3800.

"Because of the complexity of our system and the processing throughput required, it made sense to use multiple processors. We ended up using four Rabbit 3000 based core modules and a single more powerful central core module for the main display and calculation functions. The cost versus performance of the core modules made this architecture especially practical," explains Morton.

The I-Compass needed to provide a rate FOG that outputs the incremental change in heading 100 times per second. The output must be integrated exactly to get a total heading value, and a PC could not handle this operation. Rabbit's Dynamic C® integrated development environment gave Geonav the ability to design a Rabbit 3000 based system easily and quickly.

Geonav's decision to use Rabbit was solidified by the comprehensive technical support and consultants available to customers.

Morton says, "Our initial experience with the Rabbit 3000 in our PC version demonstrated good technical support from Rabbit when we had questions. This was reinforced after attending a training seminar. We talked to several Rabbit Consultants, and put out a general inquiry seeking help on the project. We were confident from the responses that we could get development help."

As an initial offering, the I-Compass is geared toward the offshore oil industry for positioning and construction work. Later versions of the product will be geared toward the dynamic marine positioning industry as an alternate heading sensor, and eventually to accommodate large marine vessels during low-speed maneuvers such as docking. As applications develop for the I-Compass, Rabbit will continue to provide reliable products and services to keep marine vessels in position.

"The cost versus performance of the core modules made this architecture especially practical"
—Russell Morton, Geonav Marine Systems


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