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SMC-T-008: “Tailoring for AIAA S-121A-2017, Electromagnetic Compatibility Requirements for Space Equipment and Systems”
SMC-T-008 is a tailoring of AIAA-S-121 and cannot be used without also having a copy of the AIAA document. Consider looking at its predecessor, SMC-S-008, as a useful supplement.
The Air Force Space Command has created its own set of EMC/EME requirements through the Space and Missile Systems Center. These documents are meant to apply largely to satellites and launch vehicles. In 2008 SMC-S-008 was developed as a way to provide verification methods for most of the requirements of both MIL-STD-461 AND MIL-STD-464. In 2019, SMC-T-008 was released, superseding the 2008 version. According to the front matter, one of the goals of the new edition was to reduce the number of “shall” statements that needed to be tracked. See figure below, where “Tailored AIAA-S-121” means SMC-T-008.
So SMC-T-008 eliminated as many “low risk” requirements as it could. It also reduced as much text as possible, and simply refers to each section of AIAA-S-121, which also encompasses both MIL-STD-461 and -464, with either a statement of “There are no changes to this section” (no shall statement!) or a statement on how to re-word AIAA-S-121 for their purposes, as in: “Insert “, subsystem, and unit” between “system” and “level” in the first sentence of AIAA S-121A-2017, Section 6.1.2.” (still no shall statement!). This certainly makes it a shorter document, but it cannot be used without having a copy of AIAA-S-121 to read in parallel, and the AIAA standard must be purchased. You can freely download SMC-T-008 here, or SMC-S-008 here, or purchase AIAA-S-121 here. [To be fair, I believe the “shall” count of Figure 1-1 includes applicable “shalls” found in AIAA-S-121, even if they’re not directly quoted in SMC-T-008 but are still applicable. But still!]
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SMC-S-008 is an excellent, publicly available document that shows methods of compliance for all of MIL-STD-464–not just the EMC-specific portions that the better known MIL-STD-461 test methods address. For instance, it’s one of the few places you’ll find a referenced test method to verify the multiplication requirement. (Multipaction is a destructive phenomenon that can affect high power RF systems. The verification method you’ll find in SMC-S-008, or AIAA-S-121 behind a paywall, but not in MIL-STD-464 is “ECSS-E-20-01A, paragraphs 4.4, 4.5, 5.1-5.6, 6.2-6.6.4, 7.1-7.3.2, 8.1-8.4.2, Annex B, Annex D and Annex E”.) Consider looking it up when trying to parse MIL-STD-464 sections and verification requirements.
BCI Testing: MIL-STD-461 CS114, RTCA DO160 Sec 20, and ISO 11452-4
Bulk Current Injection (BCI) testing is one of the best ways to ensure that operations will not be interrupted by low frequency noise via either radiated or crosstalk mechanisms of interference.
All Bulk Current Injection (BCI) tests are addressing the same risk: given a relatively long cable run mounted relatively close to conductive structure (vehicle chassis), the cabling can pick up electromagnetic noise from the surrounding environment. Cabling is especially good at doing this at lower frequencies, below 200-ish MHz. Various sources can be: crosstalk from co-routed cabling, noise from on-board RF transmitters, or noise from the external environment (radars, comms, HIRF, etc.). At higher frequencies, the risk is addressed by testing radiated susceptibility/immunity (e.g. RS103) in a semi-anechoic or reverb chamber. However there are drawbacks to that approach at lower frequencies: the longer distances of radiated testing doesn’t represent the crosstalk risk; reverb chambers are harder to spec at lower frequencies; radiated testing at 1 m is in the near field at lower frequencies, which can lead to a lack of replicability. Additionally, BCI testing can be done in a shield room instead of a more specialized (and often harder to schedule) ALSE or reverb chamber.
Thus BCI testing, where current is directly induced in the cable under test (CUT), is generally preferable below 200 MHz. There are three comparable standards that do this for defense, aerospace, and automotive industries that are worth examining:
MIL-STD-461G, CS114 (Defense and aerospace)
RTCA DO160, Section 20 (Civilian aircraft)
ISO 11452-4 (Automotive)
Differences between the three can be seen immediately from their frequency ranges and maximum induced current levels:
CS114: 10 kHz - 200 MHz, up to 109 dBuA
DO160: 10 kHz - 400 MHz, up to 109 dBuA
ISO 11452-4: 1 MHz - 400 MHz, up to 100 dBuA
DO160 mentions specifically that their frequency range is meant to overlap with their radiated susceptibility testing range, which starts at 100 MHz. ISO, focused on the automotive industry, doesn’t have as many strong threats in the kHz range (although given the increasing prevalence of HV systems in EVs switching in the kHz range, I wonder if this will change in the future).
Another thing to compare is the maximum amplitudes. MIL-STD-461G has a good rule of thumb that for wiring suspended 5 cm above a ground plane (as is specified in all the test testups), an incident E-field of 1 V/m will result in 1.5 mA of induced current. CS114 and DO160 have maximum levels of 109 dBuA, equivalent to 300 mA and assuming a maximum threat of 200 V/m. In both cases, HIRF (high intensity radiated field) is one of the driving concerns. If we apply the same formula to ISO 11452-4, we get an incident field of 67 V/m. This is a bit odd because the maximum vehicle level radiated immunity test in this frequency range goes up to 100 V/m, but there may be a presumption that the vehicle chassis provides some level of shielding to the unit.
The other thing to consider is the modulation of the injected current and the dwell times. All methods recommend dwelling for the response time of the unit under test (UUT) but have different minimums:
CS114: Amplitude modulated (AM) only, dwell time 3 sec
DO160: Both AM and continuous wave (CW), dwell time 1 sec
ISO 11452-4: Both AM and CW, dwell time 1 sec
Generally speaking a unit tested to DO160 BCI would be considered equivalent to one tested to CS114, but the different dwell times means there’s a small chance the DO160 test could miss something, while the fact that CS114 doesn’t cover 200 - 400 MHz means there’s a small chance something could be missed there.
In other respects, the test setups between the three are generally identical. They all have cables suspended 5 cm above a ground plane, with a monitor probe in addition to the injection probe during testing. They test cable bundles per connector, and if it is known that chassis will be used for current return the ground lead is excluded from the cable bundle. For CS114 and DO160, if there are cables with redundant purposes they should all be subjected to the BCI stimulus simultaneously; this situation generally does not arise on automotive modules.
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To save time and hassle later, test all cables unshielded but have a shielded version ready to swap in if there’s a serious failure. Especially on aerospace projects it is not uncommon to spec that all cables will be shielded and to do EMC testing that way–then later in the project to ditch the shielding for cost/weight purposes, rendering the earlier EMC testing meaningless. Testing without shielding gives you good confidence that the unit will be OK if shields are removed, or it will give you hard data to show that shields are necessary.
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GSFC-STD-7000b (as usual) has some excellent additional information on this test method in Section 2.5.3.3.6, which expands on some fundamental aspects beyond what you’ll find in the MIL-STD-461 appendices.
RTCA DO-160: “Environmental Conditions and Test Procedures for Airborne Equipment”
RTCA DO-160 is critical for qualifying electronics modules for use on aircraft.
RTCA DO-160 is key for getting electronics modules tested in order to ensure proper operation and support certification of aircraft. The current Rev is G and can be purchased here. Its history traces back to 1958, about the same time the precursors to MIL-STD-461 were coming into existence. It follows the same general principles as many MIL-STD-461-derived standards: test individual units very thoroughly, and when you integrate them into a larger system you have a good chance that will operate together successfully. In DO-160, EMC-related topics are a subset of a larger realm of environmental testing, including thermal, humidity, shock & vibe, sand & dust, salt spray, etc. Sections 16 - 23, plus 25, are the ones with the most EMC/E3 impact.
Section 16 specifies “Power Input” and can be thought of as a power quality spec. It controls things like normal operating voltages, both AC and DC, normal transients, ripple voltages, phase imbalance for AC power, and abnormal conditions as well. It has more in common with MIL-STD-704 than 461 or 464. This section assumes the equipment under test (EUT) is getting power in one of the following forms: 14, 28, or 270 Vdc, or 115 or 230 Vrms AC at 400 Hz.
Section 17 is for “Voltage Spike” testing. This is most similar to MIL-STD-461 CS06/CS106 testing, which was removed from the latest 461 Rev G. It can be found in the older versions of 461, as well as in GSFC-STD-7000.
Section 18, “Audio Frequency Conducted Susceptibility”. This is closely related to MIL-STD-461 CS101, which can be a tricky test to execute.
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Figure 18-1 of DO-160G shows an optional current monitor as part of the test setup. Always use this if you have the equipment–it gives you a lot of additional information for not a lot of extra effort. In particular it gives you the ability to calculate how the input impedance of the EUT is changing over frequency.
Section 19 covers “Induced Signal Susceptibility”. This doesn’t have a direct MIL-STD-461 analogue, but in general it is looking for susceptibility to low frequency stimulus induced in signal lines, as opposed to Section 18 focusing on power lines.
Section 20, “RF Susceptibility (Radiated and Conducted)” covers similar test methods to MIL-STD-461 CS114 and RS103. The radiated test includes both semi-anechoic and reverb methods.
Section 21, “Emission of RF Energy”, covers similar test methods to MIL-STD-461 CE102 and RE102. As in Section 20, the radiated test includes both ALSE and reverb methods.
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Use reverb testing whenever possible to save time. DO-160G Section 20 has excellent information about the mathematical and statistical techniques needed to ensure a chamber is set up correctly. The math is scarier, but there’s a significant savings in terms of testing time-in-chamber.
Sections 22 and 23 cover lightning testing (“Lightning Induced Transient Susceptibility” and “Lightning Direct Effects”). These share a lot of heritage with MIL-STD-464 as electromagnetic environmental effects tests. Section 25 covers ESD, and like most ESD specs basically follows IEC 61000-4-2.
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One of the things that separates DO-160 from other standards is the use of alphanumeric codes to describe the EUT and the applicable tests. Whenever you pin down a letter/number designation that applies to your EUT, write it down somewhere. Otherwise you’ll spend a lot of time flipping back and forth across many pages trying to find the right code again.
SpaceX Payload User’s Guides
SpaceX User’s Guides are particularly useful in summarizing EM environments for launch and ground handling.
SpaceX provides “User’s Guides” (Payload User’s Guides, or PUGs) for payloads going on Falcon 9, Falcon Heavy, or taking advantage of its Rideshare program. They’re all free to download from SpaceX. These are fairly comprehensive documents, including mission planning timelines and different environments: thermal, shock/vibe, pressure, etc., along with necessary mechanical and electrical interface specs. For our purposes, the section within “Environments” on “Electromagnetic” can be useful beyond only specific payload missions.
The purpose of this section is to let the user know what electromagnetic environments it might be exposed to, and also how it must limit its emissions in order to protect the RF and avionics systems of the launch vehicle. The limits are, on the whole, more lenient than the default limits found in MIL-STD-461, both for emissions and susceptibility, and can be very useful for tailoring EMC requirements and testing.
What I find particularly helpful is the set of limits in the image below. This is an envelope of the worst-case EM radiation environment between both the Eastern and Western launch ranges where SpaceX operates. (Presumably when Starship gets its own User’s Guide, the environment for the Texas base will be included as well.) This is incredibly helpful for planning what levels a piece of equipment should be robust enough to handle during all phases of operations leading up to launch: shipping, ground handling and checkouts, stacked and awaiting launch, as well as launch itself. This helps inform EMC radiated susceptibility test levels with a lot more granularity than the 20 V/m level set by MIL-STD-461, and is information that used to be hard to find.
ICNIRP: “ICNIRP GUIDELINES FOR LIMITING EXPOSURE TO ELECTROMAGNETIC FIELDS (100 KHZ TO 300 GHZ)”
ICNIRP 2020 Guidelines help determine limits on electromagnetic fields for human protection.
ICNIRP (often pronounced “ick-nerp”) is the International Commission on Non-Ionizing Radiation Protection. They are a global research group that has been studying the safety implications of electromagnetic radiation (as opposed to nuclear ionizing radiation, e.g. Chernobyl), from DC to 300 GHz, since 1992. (The commission was chartered in 1992, but includes research and researchers going back to 1973.) They publish a wide variety of freely available guidelines, statements, and papers on their website. The most recent major guideline document for RF energy (100 kHz - 300 GHz) was published in 2020 and can be downloaded here.
ICNIRP has published major guidelines on safe levels of electromagnetic energy for both the general public and specialist workers in 1998 and 2020. It has had a significant influence on related standards such as ANSI C95.1. The two main categories of potential EM field/human body interactions with RF fields are nerve stimulation (more prevalent at lower frequencies) and tissue heating. Probably the table that product designers will reference the most is Table 2 on basic restrictions (2020), as seen below.
SAR = specific absorption rate, which is the key metric when looking at exposure from devices such as cell phones.
After this table there is a lot more information on refining these levels for different scenarios. And as always, there is extremely useful information contained in the appendices.
There is a 2010 document that covers 1 Hz - 100 kHz for low-frequency effects and exposure to magnetic fields. There’s also a 2014 document with guidance on fields that vary at less than 1 Hz.
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In general, the exposure guidelines have relaxed over time, as additional research allows the commission to refine its knowledge of human effects and move away from worst-case assumptions. However, since many medical devices are still in circulation that were built assuming the stricter guidelines from 1998, if you need to set default limits, you should use the 1998 document as your guidance.
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ICNIRP has gotten drawn into numerous controversies and public panic about high voltage power lines, cell phones, 5G specifically, and other topics. If you have concerns about any of these topics, I highly recommend you go through their meticulously researched and documented (and freely available) publications on your specific concerns. I have learned a lot about EM interactions with human bodies through reading their research.
AIAA-S-121: “Electromagnetic Compatibility Requirements for Space Equipment and Systems”
AIAA-S-121 adapts MIL-STDs 464 and 461 specifically for space missions, including ground handling and launch vehicles.
AIAA-S-121 is an interesting beast. It is a tailoring of the combination of both MIL-STD-464 (system level) and -461 (equipment/module level) with an eye to making them specifically applicable to space systems. It can be purchased from the AIAA. It was reaffirmed in 2023, and there’s an effort underway to make it a joint standard with the IEEE. It’s something that the EMC Society standards committee and also Technical Committee 8 (Aerospace EMC) have been involved in, so if that’s of interest to you, please get in touch (standards@emcunited.com).
AIAA-S-121 draws from MIL-STD-1541 and SMC-S-008 (both freely available), and has similarities with GSFC-STD-7000. However it is very much its own document and should be read independently. Generally speaking, Section 6 follows MIL-STD-464, Section 7 follows MIL-STD-461 Section 4, and Section 8 follows MIL-STD-461 Section 5. Section 7 starts with a helpful table that explains many of its deviations from MIL-STD-461. Like MIL-STDs 464 and 461, it has appendices with excellent additional information that reward the thorough reader.
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Just because a unit is compliant to MIL-STD-464 and -461 does not guarantee that it will be successful in meeting AIAA-S-121. At a minimum, the radiated susceptibility levels are different. MIL-STD-461 RS103 for space systems specifies a threat level of 20 V/m, where AIAA-S-121 requires levels up to 50 or 100 V/m depending on frequency range and mission phase.
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For those concerned about Multipaction in space RF systems, MIL-STD-464 requires verification by test but does not specify or even recommend or reference a specific test method. AIAA-S-121 helps us out by pointing to “ECSS-E-20-01A, Space Engineering – Multipaction and Test, European Space Agency (ESA) for the European Cooperation for Space Standardization (ECSS), 5 May 2003.”
TIP: EverySpec.com
EverySpec.com is a site that just about every EMC engineer should have bookmarked.
Everyone knows about EverySpec.com, right? While it doesn’t have every spec, it sure has a lot of them. You won’t find illegal copies of the must-purchase ISO, CISPR, ANSI, etc. standards here. But you will find many of the publicly available ones. For me, being able to quickly and easily find MIL-STD and NASA standards has made this a go-to resource. Not only that, but the page for each standard has a nicely organized table where you can see not only the current revision of a document, but links to download each previous revision. That can be a real life-saver when you’re dealing with legacy systems and heritage hardware.
ECE Reg 10: “Uniform provisions concerning the approval of vehicles with regard to electromagnetic compatibility”
ECE Reg 10.06 is a key automotive standard for manufacturers planning to sell vehicles in Europe.
UN/ECE Regulation No. 10 is a key automotive standard for manufacturers planning to sell vehicles in Europe. It is officially on its 6th revision (which can be freely downloaded here) and has been amended twice, most recently in 2022. Revision 7 is currently in work. It is an official publication of the United Nations Economic Commission for Europe. Ultimately a vehicle manufacturer will apply for “type certification” for a given vehicle line to be sold in Europe. In addition to the paperwork, a representative vehicle will need to be tested to ECE Reg 10 in the presence of an official EU witness.
It covers immunity, emissions, and the specific hazards concerning plugin electric vehicles with concern to the power grid. The limits for different tests are found in Appendices 2 - 7. The test method details are found in Annexes 4 - 22. In general, ECE Reg 10 adopts the following international standards:
CISPR 12 for off-board radiated emissions
ISO 11451-2 for off-board radiated immunity
CISPR 25 for radiated emissions from modules
ISO 11452-2, 3, 4 or 5 for radiation immunity of modules
ISO 7637-2 for module-level immunity to transient disturbances
The following apply to plug in electric vehicles in their charging mode only, either at the component or vehicle level:
IEC 61000-3-2 and -12 for measuring the harmonic conducted emissions on the AC power lines
IEC 61000-3-3 and -11 for measuring voltage flicker on AC power lines
CISPR 16-2-1 to measure RF conducted emissions
CISPR 22 to measure RF conducted emissions on wire network port
IEC 61000-4-4 for conducted immunity to fast transient/burst disturbances
IEC 61000-4-5 for conducted immunity to power surges
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Because ECE Reg 10 has detailed test setups for adapting various international EMC standards to its exact automotive concerns, you can learn a lot about EMC standards and testing by reading through this freely available document.
Quasi-Peak Explainer
Peak and Average measurements are pretty self-explanatory, but what on Earth is Quasi Peak (QP)?
If you think about old-school over-the-air radio (AM/FM), you’ve almost certainly heard static on a weak channel. If you think about the noise that could be coming through your car stereo speakers, which is more annoying, background/hissing static, or a continuous shrill tone?
The idea that a continuous screech is more annoying than background pops and clicks is the rationale behind QP measurements. In this method, you dwell at each frequency step long enough to apply weighting to the signals you find there. If you have a continuous wave (CW) signal, that will come through as a single tone. However if you have transient noise that is bouncing around and only present during part of the dwell time, that would be the popping and hissing type of noise. QP measurements weight CW signals more heavily. Thus for FCC or CISPR 25 testing, you have to show measurements compared to the QP limit line, in order to be less annoying to the public.
In a world where all our noise sources are stable, which is what we’re ideally trying to achieve in testing, then there can’t be any signal “worse” than a CW signal when measuring for QP. However, a peak detector will easily capture the peak of a CW signal, no problem. And a peak sweep is much faster than QP, since you don’t have to dwell at each frequency so long. So what is typically recommended is to do a peak sweep that covers the full frequency range first. If you pass that, it’s accepted that you will obviously pass the QP measurements as well. If there is a peak value over the limit, then you can return to that specific frequency range and do a longer-dwell QP sweep only in that area, saving test time.
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I’d like to note that the rule that QP is always less-than-or-equal-to Peak only holds when testing conditions are static. If you are testing a more complex system that may have functions turning on and off during testing, it is possible to see QP values higher than Peak at the same frequency. That’s because the measurements are taken at different times, and the vehicle or other DUT may have turned on something in a later test that wasn’t there originally (e.g. a continuously running electric vehicle (EV) turns on its cooling system for thermal management partway through a test). That’s not to say that you should always run a full scan on QP detection; that would take forever. But if you see QP greater-than Peak, that can be an explanation.
LISN Explainer
LISNs are used in multiple different test methods, with different names and characteristics. What the heck are they, anyway?
I’m indebted to Ken Javor’s 2023 article “Line Impedance Stabilization is in its Seventieth Year and Still Going Strong”.
When we test equipment/boxes/modules for EMC, we are testing them in very different conditions than their installations. For instance, usually there’s only one module being powered by only one power supply. In a real installation, there would be power distribution points that feed many different modules (e.g., in a car, your average 12V module gets power from the body control module (BCM) instead of directly from the alternator or 12V battery, and the BCM may be sending power to dozens of modules). There’s a lot that will vary from installation to installation, depending on the platform, end use, construction, etc.
Enter the Line Impedance Stabilization Network (LISN), also sometimes known as an Artificial Network (AN). The clearest picture I’ve yet found to represent its purpose is the one below from GSFC-STD-7000B. The LISN is meant to represent Zs from the picture below.
If you assume that power is distributed via a single wire running 5 cm above structure, and structure is used for current return, then you can reasonably estimate 1 uH/m inductance from all that wiring. On a very large platform like a naval vessel, 50 m of wiring isn’t unheard of--and now you know the origin of the 50 uH LISN. The very first LISN design, from 1953, is the 5 uH LISN (aircraft in particular were smaller back then), and the 5 uH LISN is still used when appropriate today.
There are plenty of variations. For instance, the typical Goddard Space Flight Center project (JWST notwithstanding) is a small science satellite, a cube not much more than 2 m on a side, with a 28 Vdc battery recharged by solar arrays and never using structure for current return. That’s a very low inductance arrangement, and thus they use a stabilization network with a pair of 10 uF feedthrough caps and a 10,000 uF line-to-line cap.
LISNs also help in providing repeatability of measurements across tests and across different labs. Thus if you’re doing FCC testing, you’ll be using the LISN specified in ANSI C63.4 no matter what your final installation is.
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Take a moment to consider what flavor of LISN is most appropriate for the end installation your product will be used in. MIL-STD-461 Rev G Section A4.3.6 has tailoring guidance on this topic.