Addressing EMC in Harsh Environments

William D. Kimmel and Daryl D. Gerke

Equipment used in harsh environments, such as those found in military and avionics applications, needs special care when it comes to EMC.

There is a decided break between EMC in benign environments and harsh environments. This article defines benign environments as those including residential, commercial, industrial, telecom, and medical. These environments are in sharp contrast to harsh environments, which include military, avionics, aerospace, automotive, and marine.

Most of the harsh environments have factors in common. They all involve operation in a mobile or field environment. They also experience heightened risk of radio-frequency interference (RFI). Mobile vehicles rely on radio transmission and reception, often operating at extreme range.

There is also the double-edged problem of local electronics interfering with sensitive receiver reception, while nearby radio transmitters interfere with local electronics. Military platforms will often have multiple receivers and transmitters, so intermodulation is an important issue.

Mobile vehicles often have feeble power-generating capabilities. They also typically share the power bus with noisy sources in the vehicle. Power quality requirements for vehicles, therefore, are understandably severe. Mobile vehicles are more exposed to direct lightning and may be more exposed to electrostatic charge buildup.

This article examines key threats to EMC-sensitive equipment, how they fit into the various environments, and how they differ in the various disciplines.

In the Beginning

The foundation document for EMC requirements is MIL-STD-461. Actually, there were earlier standards, notably MIL-I-6181, but they are no longer available or relevant.

MIL-STD-461 comprises several standards under one cover. It accommodates environments ranging from commercial to aerospace. Several situations, including fixed installations, land vehicles, surface marine and submarine, airborne, and backpack requirements, are covered in MIL-STD-461.

Most commercial EMI standards are borrowed from MIL-STD-461, with requirements tailored to meet specific needs. Some standards are often much more detailed and can include issues not covered by MIL- STD-461. For example, RTCA DO-160D is used for avionics.

Military versus Commercial Electronics

In the early years of electronics, the military had the most advanced technology in the development of EMC protection. Integrated circuits were most valuable in mobile applications, where weight and volume were primary concerns. Missile development basically drove early electronics in the 1960s, and this quickly spilled over into avionics, land vehicles, and later into the commercial arena.

In the mid-1970s, the commercial world began driving the technology. One of the issues with military applications is that because piece parts are ordered at lower volume, they are more expensive than in the commercial sector. This put the military into a new position of adapting commercial technology to military applications. It then became important to assess
the suitability of commercial electronics to the military environment.

Military electronics can be compared, at first glance, with their commercial counterparts. Likewise, military avionics can be compared with commercial avionics, trucks with commercial vehicles, and so on. Immediately, however, some major differences emerge. Military electronics, for example, make heavy use of radio communications and radar, so there is a significantly higher RF threat, at a much wider frequency range, than in commercial radio. And as communication devices become more prolific, requirements for intermodulation testing also become necessary. Very low frequency is a key concern for submarines and antisubmarine warfare craft. Tempest standards may also be a requirement to reduce emissions.

These very specific concerns are just not applicable when working in the commercial sector. Still, the overlap between commercial and military applications is considerable.

Land-Based Equipment

For the purposes of this article, building installations are classified as a benign requirement. However, equipment installed on an Air Force base, for example, is close to powerful radio and radar sets and may need additional protection. If there is a need to protect equipment functions or to keep data secure, supplementary EMI requirements may be invoked.

Vehicular Equipment

Land vehicles include automobiles, trucks, buses, etc. These vehicles can be expected to operate in virtually all surface environments. The automotive industry has requirements well in hand, and the commercial equipment often performs well in a tactical environment.

Some vehicles are equipped with onboard transmitters. Transmitters that generate 100 W are not uncommon, and there are reports of transmitters in the kilowatt range. Thus, automobile companies expect to see RF levels in the cab of up to 200 V/m. These vehicles could be operating at extreme reception ranges, so emissions from onboard electronics may jam the receiver. Functions that are not necessary for safe operation, such as an AM/FM radio, do not have as stringent requirements as functions that are mandatory for safe operation, such as antilock brake systems (ABS).

Radio communications are on practically every military vehicle. Nonvehicular electronics are also more prevalent in military versus commercial vehicles. And in a tactical environment, there are known radar frequencies that operate well into the gigahertz range.

Commercial and military vehicles share similar lightning specs and similar power quality specs, even though military vehicles have traditionally run on 24 V. But since 12 V is ubiquitous in the commercial sector, military vehicles may have dual-voltage systems, if only to make replacement parts easier to purchase.

Power disturbances for vehicles can be severe. Equipment needs to be protected from the numerous loads that are all drawing power from a fairly small alternator-and-battery combination. The worst case is the so-called load dump, which occurs when inadvertently lifting the battery terminal under heavy load and charging conditions.

Finally, there is electrostatic discharge (ESD). Electrostatic charge builds up when rubbing two materials together, as when sliding across the car seat. Test levels of up to 25 kV are performed. In the United States, SAE J1113 and J551 are used, but the auto industry has its own specifications as well. In Europe, CISPR 12 and 25 are relevant.

There is reasonable expectation that the commercial automotive equipment will work satisfactorily in a tactical environment—but it may need shoring up in some way.

Marine Equipment

Away from the pier, boats and ships are kept mostly well away from other floating vehicles, so most EMI originates within the platform. Primarily, the equipment that needs the most protection is for communication and navigation. Emissions from equipment near the receiving antennas should receive careful scrutiny. Large platforms, including ships and offshore drilling rigs, have powerful radio and radar transmitters.

Small craft experience power problems much like an automobile—not much reserve. Larger ships, however, have ac generation equipment rivaling that of a small city.

ESD tends to be a minor issue in marine applications. The humidity in maritime climates is generally high enough to minimize ESD. However, a notable exception is helicopter landing pads, where ESD is a major issue.

Lightning also becomes a significant factor once a craft is in open water.

Military ships are floating radio and radar stations, so any equipment installed and operating above deck can be expected to get a heavy dose of RFI. Similarly, sensitive receiving devices (or, rather, the antennas) will be operating at extreme range, and emissions should be kept to a minimum.

Below the deck of a metal hull, EMI requirements are significantly relaxed, as the hull provides significant shielding.

Avionics Equipment

Commercial avionics is driven by RTCA DO-160D. This standard draws heavily from the military environment, but concentrates specifically on avionics. The document is comprehensive, covering many situations likely to be encountered in commercial avionics. Requirements that are missing from MIL-STD-461E, regarding lightning, power quality, and ESD, have representation in RTCA DO-160D. Clearly, lightning can strike a military aircraft as easily as a commercial aircraft, so the requirements are quite similar.

As in other military applications, the radio frequencies have wider range than those in commercial applications. The high-intensity RF (HIRF) requirements will likely be higher. Navy aircraft often suffer from inadvertent exposure to airport
radar.

P-static is another issue that arises in using avionic equipment. P-static is a relatively continuous discharge of static charge buildup that can interfere with radio-receiving devices.

For military applications, ESD is also a big concern. All aircraft pick up large amounts of charge while traveling through the air at high speeds. For fixed-wing aircraft, the static buildup will dissipate by the time the aircraft comes to a stop. But helicopters do not have the benefit of a time lag, especially in Army applications. A helicopter often flies at high speeds then comes to a complete stop. The static charge it carries after such maneuvers could be lethal.

Aerospace Equipment

Aerospace applications are, by any measure, the most varied of all the environments. From start to finish, equipment may be subjected to transport environments, prelaunch, launch, and finally, outer space. Even in space, the environments vary widely, and EMI environments are unique. Aerospace environments can be quite severe from both a military and a commercial standpoint.

Satellites tend to be one-of-a-kind, so each new vehicle needs careful consideration. The possible events must be identified and managed before the launch. Obviously a rocket or spacecraft cannot be recalled once it has been launched.

Environments in space include three main categories: prelaunch, launch, and orbiting. Most possibilities can be described in terms of these three categories.

Prelaunch is defined as the period between final assembly and launch. At the beginning of the equipment’s life cycle, the factory where the final assembly occurs may not have particular EMI requirements in place, although it will have ESD requirements.

The first real standard is encountered during shipping. It may seem surprising to find EMI requirements at this stage. After all, it is unlikely that the equipment will be in operation during shipping. Nonetheless, it needs to be able to survive the trip, and EMI environments may be encountered along the way. Commercial craft do not generally need shipping requirements, but aerospace applications often include very sensitive instruments, such as an atomic clock or a magnetometer. It may be better to have an overly strict standard in place, rather than find out the hard way that the equipment was damaged during transport.

The next phase is the prelaunch checkout. At this stage, the equipment must operate in a normal environment, without functional anomalies. The equipment should function, at minimum, with radios and cell phones operating nearby. If the equipment is exceedingly sensitive, it will need to be protected during this phase.

Now that the equipment is functional in a normal environment, it can go to launch. Different systems—launch vehicles, spacecraft, or missiles, for example—have vastly different requirements. On the military side, launches are possible from tactical environments that, with submarine and mobile technologies, could be anywhere. Further, hostile environments may be encountered anywhere along the way.

For nonmilitary launches, a hostile environment is unlikely. The launch vehicle must withstand and operate through the launch environment, including the local radar and anything downstream. The space vehicle will not usually have to be operational during this time—it can wake up at the appropriate time, well away from earthborne interference sources.

Few launch sites are available for a nonmilitary endeavor, since there are not yet any commercial applications. But there may be in the near future. Vandenberg Air Force Base in California and the Kennedy Space Center in Florida, and a few other sites around the world, make the list for nonmilitary launch sites. Each of these will have its own substantial RF environment.

Once the vehicle is off the ground, there is a new set of environments that are, forgive the expression, alien to most EMC concerns. In addition to the standard EMI concerns (e.g., susceptibility to radiated and conducted emissions), space-related systems have several special EMI issues that must also be addressed. These include plasma charging, magnetic cleanliness, intermodulation, and lightning.

Plasma charging occurs when spacecraft move through charged particles in space (such as the ionosphere or solar wind) and accumulate charge on exposed surfaces. If these surfaces have different conductivities and are not properly bonded together, voltage potentials can occur that ultimately break down by arcing. Similar to earthbound ESD events, these discharges can upset equipment or cause damage to spacecraft electronics.

Spacecraft should also require magnetic cleanliness. Many vehicles measure the earth’s magnetic field to determine altitude above the earth. These weak magnetic fields may be in the range of only a few milliGauss. Therefore, any dc magnetic fields on the spacecraft, such as those from permanent magnetism or dc current flows, could mask the earth’s field and corrupt navigation measurements.

Almost all spacecraft have multiple onboard radio transmitters and receivers for command, control, navigation, and communications. The collocated transmitters can cause unwanted intermodulation products from frequency-mixing effects. Care must be taken to ensure that a strong intermodulation product does not land on or near any intended receive frequency.

Military space systems may also need to address HIRF, nuclear effects, and Tempest. HIRF spacecraft can be damaged by HIRF energy, either continuous or in a short burst. This could be intentional, such as weapons effects, or accidental, such as being illuminated by strong radar during launch or tracking.

Spacecraft can be affected by both ionizing and nonionizing effects. Ionizing effects include gamma and neutron radiation from a nearby nuclear detonation, which can upset or damage solid-state devices at the component level. Nonionizing effects include electromagnetic pulse, an intense transient field that results from a high-altitude detonation, which can cause upset or damage similar to HIRF effects. Both types of effects are a major concern for military systems.

Since both the control and communications functions may contain classified data that must be encrypted, tempest concerns must be addressed in most military spacecraft systems.

Conclusion

Modern commercial electronics are increasingly being pressed into service in military applications. In many cases, the environments are similar enough that the equipment can work with little or no modification. The biggest difference in military applications is that users may need to operate in higher RF environments and over a wider frequency range.

All equipment needs testing, and in all probability, some shoring up of the design. Make sure testing is done early in the game so that any necessary corrective action is accomplished in a timely manner.

Bibliography

Engineering Practice Study, March 2, 2001: “Results of Detailed Comparisons of Individual EMC Requirements and Test Procedures Delineated in Major National and International Commercial Standards with MIL-STD-461E.” Conducted by the DoD/Industry Electromagnetic Environmental Effects Standards Committee. Available on the Internet: www.jsc.mil/jsce3/emcslsa/stdlib/lib.asp (downloadable as an engineering guide).

For information on military EMC- related documents, visit www.jsc.mil and www.dsp.dla.mil.

William D. Kimmel, PE, and Daryl D. Gerke, PE, are partners in Kimmel Gerke Associates Ltd., an EMC consulting firm with offices in St. Paul, MN, and Mesa, AZ, that specializes in EMC design, troubleshooting, and seminars. Together, they have more than 70 years of EMC experience. The NARTE-certified EMC and ESD engineers may be reached by phone at 888-EMI-GURU or via their Web site at www.emiguru.com.