Successfully dealing with EMC in space systems requires strong EMC engineering
Daryl Gerke, PE, and William Kimmel, PE
Many of us in the EMC business came of age during the “space race” of the 1950s, 60s, and 70s. We well remember names like Sputnik, Mercury, Gemini, and of course, Apollo. Some of us in the EMC business have even been fortunate enough to spend some (or all) or our careers dealing with space, that “final frontier.”Thanks to increased defense spending, there has been an increased interest in space programs. Last year’s IEEE EMC Symposium (Santa Clara, CA – 2004) even featured a special session on EMC in space, titled “Aerospace EMC at the Centennial of Flight.” Unlike the traditional theoretical sessions, this one focused on practical issues. It was great to hear key industry experts share their pragmatic insights from several different perspectives, and we hope the EMC Society will continue with these types of industry-oriented sessions. As EMC consulting engineers, we’ve worked on several space projects over the years. Some were defense-related, and others were related to ongoing space research. Like other industries, space programs have many of their own special EMC problems and concerns. In this article, we’ll share some of our experiences and perspectives on this exciting industry. Although tutorial by nature, we hope that even the “EMC space experts” will enjoy our efforts.Space EMC Vs Conventional EMC
After working on hundreds of EMC problems in a wide range of industries, we’ve come to a simple conclusion. Consistent with Paretto’s Law, about 80% of the EMC issues are common across the electronics industry, while probably 20% are unique to each individual industry. Much of the time, however, the unique problems are also the most interesting and challenging.This 80/20 split can be frustrating to those who move from one industry to another, even those with considerable EMC experience. We’ve seen this happen several times when commercial EMC “experts” moved into other areas like medical, automotive, military, avionics, and, of course, space systems. (That frustration was the catalyst for our 2003 article in Interference Technology, “Military EMC and the Revival of EMC Systems Engineering.”) The solution is to learn and to understand the EMC impact of the unique 20%, while building on the experience of the common 80%.So what makes EMC in space unique? First, there are severe constraints, and second, there are several special problems. In addition, many space programs involve numerous contractors so the management, integration, and partitioning of requirements becomes crucial to success.EMC Constraints in Space
These include weight, reliability, environment, and cost. Here are some comments on each of these constraints.Weight
It typically costs over $10,000 per pound to launch hardware into space. As such, standard earth based solutions like cable ferrites or heavy shielding are usually impractical.Reliability
After launch, on-site repairs are usually out of the question. (A notable exception was the space shuttle mission to repair the Hubble telescope.) Most space systems must work for several years without any human intervention, other than perhaps reloading software from the ground. Even a slight degradation in performance, such as a satellite losing a payload sensor or a communications channel, can be devastating to mission success. Harsh Environments
Space systems are typically exposed to three separate environments—pre-launch, launch, and operational. All three environments can be vexing. During pre-launch, systems are typically assembled in clean rooms and are subjected to a wide range of environmental tests. Following that, they must be transported to the launch site. During the actual launch, vibration and high-G forces will be encountered, as well as high RF levels from nearby radar and communications transmitters. When in space, temperature and vacuum can be formidable, along with special problems like plasma charging or ionizing radiation.Cost
In the “good old days,” cost was not a major factor, but NASA and the military now emphasize “better, faster, cheaper” over “reach for the stars.” There is a strong emphasis on using legacy designs for subsystems such as communications, navigation, and power. This approach not only saves money by avoiding costly redesigns, but also reduces risk and improves reliability by using equipment with proven track records.EMC Problems in Space
These include both unique problems and general problems. The unique EMC problems include plasma charging, magnetic cleanliness, passive intermodulation, nuclear effects, HIRF (high intensity RF), and TEMPEST (compromising emissions.) The general EMC problems include the old standard EMC issues: conducted emissions (CE), conducted susceptibility (CS), radiated emissions (RE), and radiated susceptibility (RS).Plasma Charging
As a spacecraft moves through charged particles in space (such as the ionosphere or solar wind), it will accumulate charge on exposed surfaces. Voltage potentials can form across surfaces with differing conductivities that will ultimately break down by arcing. Like personnel-induced ESD events, these discharges can cause equipment upset or damage.Design solutions include conductive bonding between different surfaces and applying conductive surface treatments to nonconductive surfaces. In the latter case, even modest amounts of conductivity (similar to conductive surfaces in factories for ESD prevention) may be adequate. Magnetic Cleanliness
Many spacecraft use the earth’s DC magnetic field for navigation to determine altitude above the earth. In space, the magnetic fields can be quite low, often in the range of a few milliGauss. Thus, any DC magnetic field on the spacecraft (caused by either DC current flows or permanent magnetism) could corrupt those navigation measurements by masking the measured field.Design solutions include using nonmagnetic materials, as well as careful attention to power routing and grounding. In the latter case, single point grounds are usually mandatory for the primary DC power. Also, special attention may be needed during manufacturing. Passive Intermodulation
Almost all spacecraft have multiple onboard radio transmitters and receivers for command, control, navigation, and communications. The frequency mixing effects of these co-located transmitters can cause unwanted passive intermodulation products.Design solutions usually include frequency management to assure that no unwanted intermodulation products occur on or near any intended receiver frequency. Bonding may help mitigate this problem, but should not be regarded as a completely effective solution, as unwanted mixing can also occur within the communications equipment.HIRF (High Intensity Radio Frequency)
Spacecraft electronics can be upset or damaged by HIRF energy, either continuous or in a short burst. This could be accidental (being illuminated by a radar during launch or tracking) or intentional (weapons effects).Design solutions include shielding, as well as identification and protection of vulnerable circuits. Radio receiver inputs, low level analog sensors, and power electronics are particularly vulnerable to this threat.Nuclear Effects
Spacecraft electronics can be affected by both ionizing and non-ionizing radiation. The ionizing effects include gamma and neutron radiation, which can occur naturally or can be caused by the detonation of a nearby nuclear weapon. These effects can upset or damage solid-state components. The non-ionizing effects include EMP (electromagnetic pulse), an intense transient electromagnetic field caused by a high altitude nuclear weapon detonation, or interaction of charged particles with the spacecraft itself. Like HIRF or ESD, these effects can cause upset or damage.Design solutions include shielding, transient protection, and using radiation hardened components. Circumvention techniques may also be employed—e.g., temporarily shutting down critical systems until the threat is past. Since nuclear effects can be cumulative, they often limit the useful life of spacecraft systems.TEMPEST (Compromising Emissions)
Since command/control and communications functions may contain classified data, TEMPEST must be addressed in most military spacecraft systems.Design solutions typically include shielding and filtering of critical subsystems. The design solutions are often applied at the module level.Conducted Emissions/immunity
Transients and other perturbations can propagate through a spacecraft power distribution system. The goals are to maintain adequate margins between any EMC sources and “victims” and to assure proper operation within normal power parameters.Design solutions include filters, transient protection, adequate power regulation, and attention to the overall power bus design.Radiated Emissions/immunity
Low level unintended electromagnetic radiation can cause problems to sensitive communications receivers. Likewise, higher level intended electromagnetic radiation can cause problems to sensitive electronics, such as analog sensors. The goal is to maintain adequate margins between potential “victims” and typical EMC sources and external threats.Design solutions usually rely on a combination of shielding and filtering. The shielding can be applied to individual cables or modules, or it can be applied at higher levels of integration. To stay within weight limits, thin metallic foils are often used.EMC Requirements in Space
Most space EMC requirements are derived from MIL-STD-461 and are often highly tailored for the specific system. Unlike the commercial world, where EMC requirements are typically well defined, space EMC requirements are often flexible, negotiable, and subject to change.To validate the design approach, extensive EMC testing is usually employed. Given the extensive degree of tailoring, the test levels may be modified from standard levels, or tests may be added or deleted. The objective is to identify specific EMC threats and vulnerabilities without over-designing the system.As an alternate to testing, modules and subassemblies may be validated by analysis. For example, existing EMC test data are often used if a module has been tested and used on earlier spacecraft. Note that this approach is valid only if no updates or changes have been made, and only if the module is installed in exactly the same fashion; otherwise, the original test data may be suspect.A common technique is to partition, or “flow down,” the EMC requirements of different subsystems. For example, radiated emissions/susceptibility test requirements may be relaxed for modules installed in a separately shielded enclosure. From a high levels systems view, this approach works as long as the initial shielding assumptions are valid and if there are no changes to the shield over time.A word of caution on “flow down” requirements, particularly when used among different contractors. If the underlying assumptions are not valid, or if conditions have changed, the partitioning may no longer be valid. Consider the example in the previous paragraph. What if the shielded enclosure had openings that leaked at high frequencies? Or what if the shield is removed entirely to save weight?We’ve seen these “flow down” problems occur on several programs—that’s why we bring it up here. The problems can be particularly acute when different contractors are involved. Used carelessly, “flow downs” become akin to “passing the buck,” but ultimately the EMC “buck” must stop somewhere. While requirements are typically applied at the module level, the entire system operation must still be assessed for EMC.EMC Systems Engineering in Space
Because of the complexity of EMC space problems, a strong systems engineering approach is needed. This really is “rocket science,” not personal computer design. In this section, we’ll look at four key EMC systems interfaces from a space perspective—power, cables, grounding, and shielding.Power is an energy interface. This interface must provide adequate energy for equipment operation without degrading performance. Primary concerns are transients, sags and surges, and complete outages.Most space systems are closed systems that use solar arrays to charge batteries, which in turn provide DC power to electronics modules. A primary EMC concern is interaction among modules sharing the power distribution system. A secondary, but often very important, concern involves magnetic fields from the power system, which can adversely affect magnetic cleanliness.Systems analysis or testing may be needed to assure adequate margins between equipment emission levels and susceptibility thresholds at the power interface. Ideally, this should be done on every electronics module connected to the power distribution system. Don’t assume that just because a module has worked in the past, it will work in the future. Each power system should be treated as a new and unique entity.Design mitigation techniques include filters, transient protection, and adequate voltage regulation. In addition, special “low impedance” power bus designs are often employed with good results. In most environments (commercial, factory, vehicular, etc.), you have little control over the EMC characteristics of the power distribution system. In space systems, however, you often do have control and thus can optimize the power distribution system for EMC benefits.Solar arrays, and their associated electronics for energy management and battery charge control, usually deserve special EMC systems level attention. For example, are the solar arrays unwanted “antennas” for radiated emissions or susceptibility? Is the charge controller a “load” or a “source” to the power distribution system? How will solar arrays be simulated during EMC testing? These special issues must be addressed along with the more routine issues.Cables are a signal interface. This interface must provide clean signals with adequate bandwidth and noise margins. Primary concerns are cable and connector construction, cable shielding, cable filtering, crosstalk, and cable routing.Most space systems use a multiplicity of cables to connect various modules and subassemblies. These are usually defined in detail by the ICDs, or “interface control drawings.” We find the ICDs to be very useful for EMC systems analysis and design. A common EMC technique is to determine the “noise margin” on each interface and then establish a “noise budget” based on those margins. Those budgets can then be allocated to perturbations like crosstalk, reflections, ground shift, and externally induced noise. The typical goal is to demonstrate a 6-dB difference between the total noise and the overall noise margins.Design mitigation techniques include cable shielding, signal filtering, cable routing and segregation, and cable/connector construction. Since shielding adds weight, there is a strong emphasis on filtering versus shielding of signal interfaces. This choice is actually quite effective, as many modules are embedded controllers with relatively low bandwidth interfaces. In such cases, we usually prefer filtering to shielding anyway.Grounding has multiple interface duties. In addition to providing current paths and references for both the power and signal interfaces, grounding also is important for safety and ESD issues. Primary concerns are topology (single point versus multi-point) and bonding impedances.Most space systems are a classic mix of power, analog, and digital electronics. Frequencies can range from DC to UHF, and current amplitudes can range from nano-amps to hundreds of amps. As such, multiple techniques are usually needed, as one method does not solve all problems.For input power electronics, a “single-point ground” to the spacecraft frame is usually employed. This maintains a reference point to the frame, while minimizing power current “ground loops.” The former is needed for safety and fusing, and also prevents arcing between floating metal surfaces. The latter is needed to prevent jamming of analog sensors and to maintain magnetic cleanliness.For low frequency analog circuits, such as sensors, single point grounds are also employed on both circuits and cable shields to prevent ground loops. For high frequency digital circuits, multipoint grounding of circuits and shields is more common because of transmission line effects. In some cases, hybrid techniques (adding capacitors and/or chokes) must be applied if both low and high frequency threats are present.Bonding is also an important element of grounding. Low impedance connections between metal surfaces help mitigate problems with plasma charging, HIRF, EMP, and intermodulation. High quality bonds are mandatory for high frequency shielding of cables and enclosures.In all cases, a thorough assessment of grounding and bonding must be done at multiple levels—input power, secondary power, analog, digital, and radio frequency (transmitters and receivers). Failure to do so will almost invariably lead to EMC problems later in the program. For example, we had one case in which a ground loop in the secondary power had the potential to jam some very sensitive sensors. Without a detailed analysis, that problem might have gone undetected until too late—in space.Shielding is an electromagnetic field interface. This interface must contain internal electromagnetic fields while blocking external electromagnetic fields. In the former case, we are usually trying to protect systems communications receivers from low level emissions; while in the latter, we are usually trying to protect onboard electronics against high level emissions from radio transmitters (both systems and external).Most space systems use a mixture of individual electronics modules, connected though cables. Depending on the function, some are in well shielded boxes, and others are poorly shielded for EMC. Devices that use light (sensors, solar panels, lasers, etc.) or devices that must move (sensors on gimbals, for example) are usually among the most problematic for EMC shielding. Unfortunately, these devices are often among the most sensitive to EMC threats. As such, these types of subsystems deserve extraordinary EMC attention, right from the start of the program.Design solutions include individual shielding, shielded compartments, or overall spacecraft shielding. The latter two often lead to “flow down” partitioning, which may or may not be realistic. All shielding solutions must deal with the classic EMC shielding problems of seams and penetrations. For most space projects, if it isn’t water-tight, it isn’t “EMC-tight.”EMC Design Engineering in Space
Even though our emphasis has been at the systems level, good EMC design practices are still needed at the individual equipment level (box, board, and component) We regularly encourage EMC reviews at these levels, even when not formally required at the system level. It only take a few hours to check out the circuit boards or individual modules early in the program, while it make take weeks or months to chase problems at the end of a program.We like to pay particular attention to critical circuits on circuit boards. These include clocks, resets, control lines (memory read/write, interrupts), power regulators, low level analog circuits, and I/O circuits. The clocks and power regulators can contribute to radiated emissions. Resets, control lines, and interrupts are vulnerable to transients. Low level analog circuits and power regulators are vulnerable to radio frequency energy. I/O circuits are important because they provide the connection to the outside world.We also like to check out the power supplies for EMC issues. These include radiated and conducted emissions, as well as magnetic cleanliness problems. Power supplies can also be affected by ionizing radiation, which creates problems well beyond EMC. Space power supply design, like EMC design, occupies a special niche.Ultimately, all EMC problems begin and end at the circuit level. Consequently, it pays to check out these design issues early in the design phase, just as other, more mundane devices are checked in the earth-based world.Summary
Dealing with EMC in space systems can be extremely challenging, and the cost for failing to meet those challenges can be enormous. Success requires strong EMC engineering—with an emphasis on detailed understanding, ongoing assessments, extensive testing, and constant vigilance throughout the life of the project.References
Daryl Gerke and William Kimmel. “Military EMC and the Revival of EMC Systems Engineering.” Interference Technology, 2003 Annual EMC Guide. pp 99–105.
Daryl Gerke and William Kimmel. “Focus on EMC in Space.” Kimmel Gerke Bullets Fall 2002. pp 2–3.
Jim Lukash, et al. “Aerospace EMC at the Centennial of Flight.” IEEE EMC Symposium 2004 (Special Session MO-PM-WS-8). Workshop Notes. pp 258–301.
Daryl Gerke, PE, and William Kimmel, PE, are principals in Kimmel Gerke Associates, Ltd., an EMC consulting and training firm. Together, they share over 75 years of EMC experience and have served clients in a wide range of industries—military (including space), medical, commercial, industrial, automotive, avionics, telecommunications, and more. They both got started in military EMC over 35 years ago and have been full-time EMC consultants since 1987. Daryl lives in Mesa, AZ, and can be reached at [email protected]. Bill lives in West St. Paul, MN, and can be reached at [email protected]. Their website is www.emiguru.com.