This Part of the Standard provides guidance on the typical electromagnetic environment in which military equipment is deployed. 

It covers stationary and mobile military equipment including ordnance, deployed and operated by Land, Sea and Air Services on land, under and on the sea, and in the air. The environments considered include those with sources arising from natural phenomena as well as those generated by man-made activities, both civil and military. This Part of the Standard does not discuss the environment of materiel deployed outside the Earth's atmosphere. 

The electromagnetic environments due to Radio Frequency Weapons (RFW) such as High Power Microwave (HPM) and Ultra-Wideband (UWB) Weapons are not currently included in this Part of the Defence Standard. Where immunity to such threats is a requirement, specialist advice on such environments should be sought from the relevant authorities. 

This Part of the Standard only considers field magnitudes that are likely to cause malfunctions to materiel by unintentional coupling mechanisms (sometimes called "back-door" mechanisms). For this reason, many sources of EM radiation are ignored because they are evidently so low that it is inconceivable that they could cause an indirect threat to materiel. The environments described may also be used to assess the protection requirements for the "front-end" of a receiver to prevent damage. In addition, fields that may impinge on a weapon system as it approaches a hostile target and those caused by deliberate enemy jamming are not considered in this Part of the Standard. User and system requirement documents should specify if these threats need to be taken into account and the relevant scenarios should be defined by the Equipment Capability customer. 

This Part of the Standard contains descriptions of the EM environment in both the time domain and the frequency domain. When the description is in the frequency domain, the maximum frequency considered will be 40 GHz. (NOTE: above this frequency there are few deployed systems and currently no recorded problems. A system using a transmitter above 40 GHz should however include it in the environment definition for that system). Both the radiated and conducted environment (due to sources external to an equipment) are considered. For the conducted EM environment, levels will be platform specific and only the generic test limits as used in Part 3 of this Standard are discussed. Phenomena described in the frequency domain include static and quasi-static electric and magnetic fields, communications, radar and conducted emissions. Phenomena described in the time domain include Electro-Static Discharge (ESD), switching transients, Electro-Magnetic Pulse (EMP) and lightning. 

The depth of treatment for any given environment or related matter has been determined by the availability of information on the subject, in conjunction with its importance in the light of contemporary EMC problems. For this reason the following sources of EM radiation are not treated in this Part of this Standard:

Low power (<10 W) radio communications from a distant transmitter (>100m).

Cosmic / solar radiation.

Terrestrial magnetic field.

Environments are given in some detail, but the methods of minimizing degradation of performance and maximizing reliability are not covered here. Such descriptions can be found in Part 5 of this Standard and many textbooks.

This Part of the Standard recognizes that applying a worst case description of an environment to be used in all cases can lead to wasteful and unrealistic levels of hardening. There are two ways of overcoming this. The first is to give a statistical description of the field. This would be very difficult and time consuming to calculate and would be dependent on the operational scenario. The second is to give a maximum likely field for a variety of operational scenarios and where classification allows the rationale for each scenario. For systems with a different scenario it will then be necessary for revised figures to be produced. This will generally require the project to consult specialist authorities. This second approach has been adopted and is expected to offer a realistic solution to the problem of unnecessarily high field descriptions. However since the number of scenarios is limited it will be necessary for projects to determine whether the scenario is relevant to some or all of their system and define additional /alternative scenarios in terms of likely distances from various types of transmitter. These can then be transformed into EM environments using the information provided in this Part of the Standard and/or seeking guidance from relevant specialists.

Part 3 of this Standard includes a large number of EM susceptibility tests with a variety of limits which may be used for different classes of equipment. These limits have been established recognising the information contained in this part of the standard which may be relevant to common scenarios/conditions that exist in each service environment. Part 1 of this Standard calls for the limits to be tailored for each project to avoid under- or over-testing. The information provided in this part of the standard should be used to assist this tailoring process but for classification reasons it may be necessary to seek specialist assistance.