In years past, E3 (Electromagnetic Environment Effects) control and Spectrum Supportability were considered separate entities and design and development efforts were often segregated. Over the last several years, the trend has shifted to a more consolidated approach and today the two areas go hand in hand. Spectrum Supportability starts with equipment spectrum certification, essentially a license to operate a system in a particular spectrum band. Spectrum certification is the authorization to develop or procure a spectrum-dependent system for operation in specific frequency bands. It is accomplished using the DD Form 1494, Application for Equipment Frequency Allocation, via the J-12 Process and is based on regulatory requirements. The DD Form 1494 provides the information required to determine whether the subject system meets the criteria established by the allocation tables. Potential impacts to current band users are also considered by approval authorities. The bottom diagram in Figure 1 maps the stages of the DD Form 1494 to the procurement cycle.
Defining the EME
From an EMC requirements perspective, one of the very first tasks is to define the operational EME for the platform or system of interest. It is extremely important to properly define the various EMEs in which the item is most likely to operate as this forms the basis of accurate E3 design and test requirements. The EME is the composite of electromagnetic energy, including man-made and natural sources, to which a system or subsystem/equipment will be exposed during its operational life cycle. It takes into account the expected areas of operation, other S-D (Spectrum Dependent) systems, including emitters that may be in that area. It also includes natural sources such as lightning and ESD. The EME is an ever changing evolution created by the demand for more radiated power and different or more common platforms in a given area.
Steps in this process include: y
- Identifying the major geographic regions in which the system will operate, that is, the U.S., Atlantic, Pacific, Europe, Middle East, or possibly, worldwide.
- Identifying the specific countries in each major region in which the item is likely to be deployed, since obtaining host nation approval to operate may be more challenging in some countries.
- Defining the theater and missions.
- Identifying individual host platforms and systems on or near the item to be deployed.
- Identifying types and characteristics of any S-D item present or planned that could possibly interact with the proposed item. This identification addresses both items affected by and those that affect the item. The identification must address both the military and commercial EME alike.
Specifying an EME that is too stringent may result in additional costs in design and test phases that are unnecessary by qualifying a system to an overly harsh EME. Each distinctive EME that an item will be exposed to during its life cycle should be defined before specifying its performance requirements. For example, a missile will be exposed to different EME levels during shipment, storage, checkout, launch, and the approach to a target. The specified E3 performance requirements should ensure the item’s performance is not adversely affected by any of the EME levels that will be encountered. The majority of military systems begin establishing E3 related requirements based on MIL-STD-464 (currently at Revision C).
MIL-STD-464C identifies five key activities that comprise an E3 integration approach.
a. Establish the external threat environment against which the system is required to demonstrate compliance of immunity. The external environments (EME, lightning and EMP) to which the system should be designed and verified are addressed in other sections of this appendix.
b. Identify the system electrical and electronic equipment performing functions required for operation during application of the external threat. Normally all functions essential for completing the missions are protected against the external threats.
c. Establish the internal environment caused by external electromagnetic effects for each installed equipment. All of the environments external to the system specified in this standard cause related environments internal to the system. The level of this internal environment will be the result of many factors such as structural details, penetration of apertures and seams, and system and cable resonances. The internal environment for each threat should be established by analysis, similarity to previously tested systems, or testing. The internal environment is usually expressed as the level of electrical current stresses appearing at the interface to the equipment or electromagnetic field quantities. These internal stresses are typically associated with standardized requirements for equipment (for example, MIL-STD-461). Trade-offs need to be made of the degree of hardening to be implemented at the system-level (such as shielded volumes or overbraiding on interconnecting wiring) versus equipment-level (more stringent electromagnetic interference requirements) to establish the most effective approach from performance and cost standpoints.
d. Design the system and equipment protection. System features are then designed as necessary to control the internal environment (including margin considerations) to levels determined from the trade-off studies and appropriate requirements are imposed on the electrical and electronic equipment. The equipment immunity levels must be above the internal environments by necessary margins to account for criticality of the equipment, manufacturing tolerances, and uncertainties in verification. Normally there are design and test requirements in MILSTD-461 applicable for each of the external environments, but they may need modification for the particular system application. For example, the external environment may result in internal environments above the susceptibility level specified in MIL-STD-461. If so, the limit must be tailored for the particular system, alternative requirements must be imposed or the internal environment must be reduced to an acceptable level. The system E3 design must be viable throughout the system life cycle. This aspect requires an awareness of proper application of corrosion control provisions and issues related to maintenance actions that may affect EMC. Examples are ensuring that electrical bonding provisions are not degraded, maintaining surface treatments in place for E3 control, and considering exposure of electronics to EMEs when access panels are open. Maintaining a viable system E3 design also requires an effective configuration management program for tracking and evaluating engineering changes to the system to ensure that the E3 design is not compromised.
e. Verify the protection adequacy. The system and equipment E3 protection design must be verified as meeting contractual requirements. Verification of the adequacy of the protection design includes demonstrating that the actual levels of the internal environments appearing at the equipment interfaces and enclosures do not exceed the qualification test levels of the equipment for each environment by required margins. All electronic and electrical equipment must have been qualified to their appropriate specification level. Systems-level testing is normally required to minimize the required margin demonstration. Analysis may be acceptable under some conditions; however,the required margins will typically be larger.