The re-design of current military equipment (formerly known as "legacy" equipment) is most often required to be "form, fit, and function retrofittable". This terminology is used by the military customer to indicate that a replacement is needed that will be interchangeable with the current item. That is, it will take the same:

• shape, material, interfacing, and often appearance (form)
• size, including connectors and weight (fit)
• perform the exact same functions, that is, provide the exact same outputs, given appropriate inputs (function)

In this manner of replacement, it is unnecessary to make changes to interfacing equipment. No perceivable change to the next higher assembly occurs.

Military operations of recent years, however, have encountered environments not previously anticipated. In order to address the new environmental issues, additional requirements have been placed upon any new or re-designed equipment as a reliability and safety precaution. For example, the sand encountered during Desert Storm operations has led AED, the Army Engineering Directorate, to require revised sand/ dust related testing on selected equipment. In many cases, the item being replaced could not have passed these new tests. The replacement item, however, must.

This presents a time and cost increase for the performance of additional tests, but moreover, it presents a new strategy to the design. More stringent testing implies that a hardier design is necessary in order to meet the increased requirements. The replacement must exceed some of the performance levels of, rather than be as good as, the current item.

The changes to electromagnetic interference (EMI) requirements indicate another example of the new testing required. EMI testing allows for the characterization of an item in terms of how incident electromagnetic radiation and other unwanted interference signals will affect the item, and the effect that signals emitted from the item will have on nearby equipment. The signal strengths and bandwidths possible in the defense environment increases as years pass and new threats become un-covered, and as the critical frequencies in use by US forces change. Here again, as in the sand example, current equipment was designed to previous requirements that differed from today’s, and was typically less stringent. Depending upon the age of the equipment, cases may be encountered wherein no EMI testing had ever been required.

Case Study: C6533 Communications Unit
An example of a legacy item re-engineering effort is the C6533 redesign. This included the design and development of a replacement for the Communications System Control unit of the U.S. Blackhawk and Chinook helicopters (Figure 1-1).

The unit (Figure 1-2) serves as a link among ground crew, air crew, command radio stations, as well as other aircraft. The previous design used 1969 transistor technology, in an assembly of seven boards and plates that were hard-wired together. The control unit chassis had significant gaps in metalwork through which wavelengths may pass unobstructed. As a simple example, a 10GHz signal may pass through a gap of only 1.25 inches as the wavelength of the signal is 1.18 inches (full sinewave). Precautions for less than quarter-wavelengths should be taken, so, in this example, gaps smaller than .295 inch would be appropriate, in an environment with 10 GHz present.

Two characteristics of electromagnetic waves are connected with interference: the amplitude and the frequency of the waves. The control unit chassis had significant gaps in the metalwork into which energy from electromagnetic waves resonating at a quarter wavelength of the signal could have been radiated. Note also that what may be perceived as a shielding issue may actually be a cavity resonance issue. Here, the use of absorptive materials in the cavity has also proven to be effective in damping resonance.

In the older configuration, if an EMI problem were to arise, a correction would have probably included the addition of wire mesh and adhesive-backed copper foil to fill the gaps. These shielding methods, test-bench fixes, may prove to be effective, but interference should be anticipated and prevented in the early design stages of new equipment. The mechanical design of the new communications unit incorporates a more modular assembly and EMI gasketing.

Testing to Show Conformance
MIL-STD-461 incorporates conducted and radiated, emissions and susceptibility tests. Emissions tests are performed to demonstrate that the device under test (DUT) will not emit levels substantial enough to interfere with other equipment. Susceptibility tests are performed to show whether signals in the surrounding environment may adversely influence the DUT. This influence may mean catastrophic failure, distortion of the intended outputs, mis-tracking of data, or other interference with the throughput function of the DUT.

Typical EMI tests
The tests required of the Communications System Control unit included MIL-STD-461 revision E, applicable conducted emissions (CE), conducted susceptibility (CS), radiated emissions (RE), and radiated susceptibility (RS) tests. These emissions tests indicate allowable limits or threshold levels that the unit may not exceed. The susceptibility tests differ from the emissions tests, in that they define the levels to be subject upon the DUT, its interfacing cables, and the power, return, and ground leads. Lines are broken during the test in order to inject or measure conducted levels, whereas, during radiated tests, various antennae and monitors are used at predetermined distances from the DUT. The communications unit was subjected to the EMI tests listed in Table 1-1.

The severity of the 18GHz and 40GHz were not found among the previous requirements for the unit. Figure 1-3 is a photograph of one of the many the test setups.

Comparison With Legacy Requirements
The re-engineered communications unit is replacing technology that is circa 1969. In its last build in 1996, still adhering to the revised requirements of 1974, the unit specification document did contain limited EMI requirements. At that time, the applicable MIL-STD-461 tests included CE01, CE03, CS01, CS02, CS06, RE02, and RS03. Most of these have a similar counterpart from among the revision E tests (i.e. CE101, etc., as listed in Table 1-1).

However, several of the older tests are not directly comparable to the current set. CS02 (30% amplitude modulation (AM) at 1 KHz of all signals 150KHz to 400MHz) has no true counterpart among the revised group of tests, and is taken into account by other CS tests. A test similar to an even older 'RS02' was performed - a radiated susceptibility test called 'induction field' that imposed 400 Hz, 5 Amp rms upon the power line. RS03 (30% AM at 1KHz of all signals 2MHz to 30MHz, 10V/m) compares with today's more stringent RS103 (see Table 1, item 10: 2MHz to 40GHz, 200V/m field strength). The requirements of the older unit indicate that it could not be expected to meet today's EMI requirements.

Joint Forces Benefit
The requirements of any branch of the military are increasingly becoming of interest to the cooperative joint forces initiatives. Much may be saved in terms of effort, cost, and time, by investigating commonality and avoiding redundancy in design efforts. Sharing of lessons learned always provides a "heads up" to the next re-design.

The re-design and test of the Communications System Control unit have added to the overall effectiveness of military helicopter operations by improving safety and reliability.

Conclusion
While the form, fit, and function of legacy equipment must be maintained in re-designed versions, the re-designed items must also adhere to more stringent environmental and EMI requirements.
New requirements and increased testing, such as the changes made to EMI, will help to ensure the survivability of today's defense systems. Becoming familiar with new test requirements, before initiating the design phase of a project, will help to assure that a product will be reliable, and will have minimal issues at the time of qualification.