Introduction
My reviews have covered MIL-STD-461 over the past few months so it’s time to step up to the system requirements. We realize that the MIL-STD-461 requirements apply to equipment items and subsystems as stated in MIL-STD-464. Revision “C” is the current edition, but a review is a little overdue. In this review, the discussion will focus on the various requirements and how those requirements affect individual equipment and MIL-STD-461 evaluations.
The system requirements consider the integration aspects of equipment items and how the environment, the Electromagnetic Environment (EME), creates the need to address unique issues. Things like deployments to cold and dry locations present issues associated with Electrostatic Discharge (ESD) much more commonly than deployments to tropical regions.
As indicated throughout the MIL-STD-461 reviews the requirements were generic and that tailoring was supported. Tailoring is usually prompted by system analysis that identifies some risk that needs to be addressed. For example, a ground facility generically has a 20 V/m RS103 susceptibility requirement, but requirements at a specific facility elevates that to 60 V/m in the 2-3 GHz frequency range because of using several high-power S-Band transmitters in the area. So, qualification to MIL-STD-461 would not meet the needs for that environment.
In this review, MIL-STD-464C will be the basis for discussion but reference to prior revisions may be included. If a prior revision is used, it will be identified.
System Definition
What is a system? MIL-STD-464 defines a system as:
A composite of equipment, subsystems, skilled personnel, and techniques capable of performing or supporting a defined operational role. A complete system includes related facilities, equipment, subsystems, materials, services, and personnel required for its operation to the degree that it can be considered self-sufficient within its operational or support environment.
Most of the evaluations consider subsystems as a system for the test and evaluation process but consideration for all system elements should be included. If a shielded enclosure to contain high-level emissions was part of the subsystem design, then personnel within the enclosure would be exposed to the radiation. This would make the system integration unacceptable unless the equipment radiation inside the enclosure met the human exposure criteria.
We often consider the platform as the system, especially when the platform is autonomous. Can a building be considered as a platform? In many cases, the building is treated as a system although power, water, and communications links are needed to support the system.
For the purposes of design and evaluation, a subsystem that can be isolated and perform a function is evaluated independently with an integration analysis to support specific elements affecting the complete system.
As you can see, it is not always a clear separation between a system and subsystem. Once integrated, evaluations are accomplished to verify the system performance and risks for system self-compatibility.
Data Item Descriptions (DIDs)
MIL-STD-464C refers to four DIDs and the applicable DIDs are listed on the contract data requirements. Let’s quickly discuss the listed DIDs.
DI-EMCS-81540B – Electromagnetic Environmental Effects (E3) Integration and Analysis Report. The DID provides detail on the content with a requirement to address each of the MIL-STD-464C requirements. The report should examine the risks and details on how each requirement should be satisfied. This should include special considerations and a guide to tailoring as appropriate. For instance, a contract for a system at the facility mentioned earlier would include a need for higher level radiated susceptibility requirements and provide the technical rationale for the elevated requirement. This would prompt the contract to specify a tailored limit to address the potential issue. Note that each of the MIL-STD-464C requirements should be addressed; even not applicable items should be included and marked as not applicable. Not applicable is different than an omitted topic.
DI-EMCS-81541B – Electromagnetic Environmental Effects (E3) Verification Procedures. This DID procedure provides guidance on how E3 compliance is to be verified. Each of the requirements of MIL-STD-464C is to be addressed and includes detailed step-by-step procedures for the verification process. A test is not always required for verification, so the procedure identifies how verification is to be accomplished and how multiple subsystems are to be operated for the evaluation. Acceptance criteria are to be included in the procedures.
DI-EMCS-81542B – Electromagnetic Environmental Effects (E3) Verification Report. This DID describes the content for reporting the results for the execution of the verification procedures. As with the procedures, each of the MIL-STD-464C requirements are addressed providing the detailed results for each evaluation.
DI-EMCS-81827 – Spectrum Certification Spectral Characteristics Data. This DID supports the requirement to authorize the operation of wireless transmitters throughout the development and operational phases of systems that include radio frequency transmitters. DoDI 4650.01 requires risk assessments during the development for spectrum-dependent systems. DD Form 1494 is associated with this DID and that form is used to document the spectrum usage approvals for various stages of the development.
Requirements
5.1 Margins – Safety and mission-critical systems must have a 6 dB margin. Electrically Initiated Devices (EIDs) shall have a 16.5 dB margin of the Maximum No-fire Stimulus (MNFS) for safety and 6 dB for other applications. Testing susceptibility at a 6 dB higher level are often implied, but that is not the intent as discussed in the MIL-STD-464C appendix. The test level selection column of MIL-STD-461G for safety critical levels are used and the levels do not require an increase. The 16.5 dB (15%) MNFS is relatively easy to manage but requires the ability to monitor the firing level without the test equipment causing issues.
5.2 Intra-system electromagnetic compatibility (EMC) requires system self-compatibility. Remember the big picture where we often evaluate subsystems and merge with other subsystems to form the system. The complete system must be self-compatible.
Inter-modulation Interference (IMI) presents issues particularly aboard ships where conductors (normally unintentional conductors like bolts with corrosion) form a non-linear junction and when exposed to radiated signal may produce IMI and degrade receiver performance. Additionally, below deck environments form reverberation chambers, so limiting the radiation power to below deck transmitters is very important. Radiated power limits are provided in the standard. These issues may appear on other platforms such as aircraft and vehicles although not mentioned in MIL-STD-464C.
Multipaction is a consideration for space systems where an RF field may accelerate free electrons in a vacuum that may collide with surfaces releasing secondary electrons. This presents a risk of discharges and damage.
The induced level at antenna ports of antenna-connected receivers from unintentional RF emissions must be controlled to prevent degradation of receiver performance. Note that limits are not cited, so the Integration Analysis should identify risks and provide tailoring guidance for additional emission control if risks are present. I recall working an analysis where an equipment item was placed near the back lobe of a space vehicle antenna. Everything tended to be compatible until the detailed test report for the equipment showed emissions at the receiver operating frequency. The equipment test report stated that it met contract requirements, but the contract allowed a waiver for the over-limit emissions. The issue was discovered before launch and control measures were added.
5.3 External RF EME requires that the system operate in the environment. This requirement presents several tables where threat levels are presented for various environments. If multiple environments are applicable, then evaluate based on a composite table.
5.4 High-power microwave (HPM) sources requires that the operational performance be met if the requirement is specified in the contract. The contract must provide the threat test levels associated with the HPM requirement.
5.5 Lightning requires meeting the performance requirements for both direct and indirect lightning effects. Figures and tables are in the standard to identify the requirement characteristics. The performance requirements may allow operational interruptions during a strike for certain applications but normally survival is required. Examine the performance requirements to determine acceptance criteria.
5.6 Electromagnetic pulse (EMP) requires meeting the operational performance after being subjected to the EMP environment. The test limits are provided in MIL-STD-2169 and the requirement is applicable is specified in the contract.
5.7 Subsystems and equipment electromagnetic interference (EMI) requires that individual items meet MIL-STD-461 requirements. Non-developmental and commercial items may be acceptable based on other EMI/EMC compliance standards if the system performance is met. Shipboard DC magnetic field environment is also specifically identified to verify that ship degaussing does not impair system performance.
5.8 Electrostatic charge control requires that control and dissipation of electrostatic charges be managed to prevent performance degradation or damage. Several high-level threat environments and the associated requirement levels are provided. Included is personnel borne ESD invoking the same requirements as for subsystem evaluation per MIL-STD-461.
5.9 Electromagnetic radiation hazards (EMRADHAZ) requires that control be incorporated to protect personnel (HERP), fuels (HERF) and ordnance (HERO) from the effects of radiated energy. DoDI 6055.11 supports HERP requirements. HERF requires that ignition of the particular fuel vapor be used to evaluate the requirement. HERO levels are provided in MIL-STD-464C.
5.10 Life cycle, E3 hardness requires performance from the Electromagnetic Environmental Effects (E3) throughout the rated life cycle of the system. The ability for maintenance, repair, surveillance, and corrosion control be provided. The primary issue is providing surveillance because EMI control may not be necessary all the time but should work when needed. For example, how would we know if a transient suppressor failed until the lightning event damaged the equipment? However, the means must be provided to assess the functionality of the transient suppressor as part of a repair action. The dilemma is present, so the system manuals need to address EMIEMC and control measures.
5.11 Electrical bonding requires that bonding be adequate to meet the E3 requirements. General statements indicating that the power return meet the power quality standard, that the antenna installation meet the antenna performance, mechanical interfaces be connected to prevent voltage differences and conductivity of fault current to prevent shock or flammable vapor ignition.
Fuels are often considered when thinking about ignitable vapor but don’t omit cleaning solvents, lubricants, certain inks or other substances the contact current handling junctions where the resistance allows heating.
Mechanical junction bonding has a few specific values; 1) 10 milliohms from enclosure to system structure; 2) 15 milliohms from cable shields to equipment enclosure and 3) 2.5 milliohms across individual faying interfaces. Historically the bonding has been misinterpreted, requiring 2.5 milliohms or less from any equipment point to the structure – this is NOT the requirement.
For safety, MIL-HDBK-2036 requires 100 milliohms or less for equipment using hazardous voltages between the equipment chassis and the system ground reference.
Realistically the integration analysis needs to examine the bonding system to be used and the equipment/subsystem evaluation needs to use that during test and evaluation.
5.13 TEMPEST requires that national security information not be compromised by emanations from electrical/electronic equipment. TEMPEST specific requirements are provided in separate standards identified by contract.
5.14 System radiated emissions requires radiation control to operate with other co-located systems and limit capability to detect and track the system. Emission control (EMCON) specifies specific levels on system emissions. Note that it is feasible to meet MIL-STD-461 RE102 emission limits and not comply with the EMCON levels.
5.15 EM spectrum supportability requires compliance with DoD national and international spectrum regulations. See DoDI 4650.01 for details on spectrum-dependent system requirements.
Summary
Exact requirements are not always specified in the standard but the associated DIDs are critical to identifying what constitutes compliance. The system integration and analysis report develops the critical information that must flow into the contractual requirements and flow down to individual items.
Self-compatibility is a must, and all operational performance requirements need attention. The environment may be severe and can easily affect performance – but is limited performance allowed for certain environmental effects. Keeping the requirements to a level truly needed can prevent over-design and cost escalation.
Remember that MIL-STD-464C is a system document and often sees a call-out for equipment items. Equipment items should use the protection provided by system integration to avoid placing control measure on top of each other driving cost throughout the life cycle.
Questions – I always welcome questions – keeps me learning things every day!
