Radiocommunications Agency  EMC Awareness

Marine

Description

The experiences of the crew of the research vessel (R/V) Deep Scan, a privately owned research and recovery ship, offer some insight into the complexities of integrating commercial off-the-shelf (COTS) computing equipment into a shipboard electromagnetic environment.

R/V Deep Scan is constructed as a commercial vessel with many of the electrical characteristics of military mine-clearing ships. Its hull and deck structures are constructed from wood, closed cell foam and fibreglass, and it shares EMI/EMC problems common to non-metallic ships.

Computing equipment on board is said to be compliant with FCC Part 15 for radiated emissions. A commercial workstation processes sonar and navigation track data from multiple transducers. A 386 PC processes both electromagnetic survey data from multiple detection transducers and data for navigation. Navigation data is provided by COTS GPS (Global Positioning System) and LORAN-C receiver systems. Depth information is provided by COTS depth sounding equipment. Heading data is provided by a COTS fluxgate compass.

Operating the marine VHF transmitter at more than 1W begins to corrupt collected data, and any use of HF SSB transmission causes the COTS computing equipment used for magnetic data collection and navigation to enter states that challenge rational explanation.

FCC rules limit the levels of unintentional electromagnetic radiation, but the close proximity of COTS computing equipment (the vessel is under 60 feet long) to the antennas used for data collection and communications is largely responsible for disruption of operations due to the EMI the COTS equipment generates.

EMI generated by the switching power supplies in the COTS equipment slightly degrades the LORAN-C signal-to-noise ratio through radiated coupling. COTS computing equipment generates sufficient radiated interference on the HF bands to render HF communications impractical. Broadband interference and harmonics from COTS computing equipment interfere with communications reception on selected VHF channels, in some cases enough to prevent useful communications.

Daily operations on board R/V Deep Scan are influenced by the EMI and susceptibility problems associated with the use of COTS computing equipment. Responding to a call on the VHF radio presently requires the crew to wait for a logical break in survey operations, or requires termination of survey operations. During survey operations, monitoring some VHF channels is not possible, HF transmission is impossible and HF reception is seriously degraded.

Commentary

Operations on board the ship were impaired by both the emissions and susceptibility of the COTS equipment. It is of course possible to modify the COTS equipment to overcome these problems, but this would negate the cost reasons for the choice of COTS instead of higher specification equipment.

The design of the vessel, specifically its non-metallic construction to permit the use of sensitive electromagnetic sensors, were a contributory factor, although non-metallic hulls are typical of smaller vessels.

The overall message from this article is that COTS (commercial/industrial) EMC standards and regulations are probably inadequate when used in very close proximity to radio receivers, radio transmitters, especially those operating in the HF (short-wave) bands. To be fair, the EMC standards listed under the EMC Directive generally point out these shortcomings. Had the computing equipment met the standard for marine navigation equipment (IEC 60945) then most or all of these issues would have been addressed.

Commercial/industrial EMC standards have to strike a balance between cost and risk, and it is seen as inappropriate to make the majority of products and systems comply with EMC specifications that only a few of them will ever experience. This means that whenever one is considering buying COTS equipment for a special project it is not sufficient to look for a general statement of compliance.

Instead, assess the electromagnetic environment that the equipment will be required to operate in, including the sensitive equipment that it must not interfere with, and compare it with the EMC tests the equipment is claimed to have passed.

References and links

“Electromagnetic susceptibility and shipboard computing” by Bruce D Salati, ITEM 1994, pages 67, 70 and 71, http://www.interferencetechnology.com.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
All of the mitigation techniques could be applicable, depending on the equipment or system concerned