Guessing and knowing are two completely different things. The objective of stealth is to keep the adversary guessing until it is too late. Over the past few years, stealth platforms, especially aircraft, have come into public consciousness. However, stealth research work was conducted in earnest beginning in the mid-1970s and was spearheaded by the Defense Advanced Research Projects Agency (DARPA) in both U.S. Air Force and Navy programs. Most of those programs are still shrouded in secrecy, but a few, especially the earliest, are now declassified, and the basic notions of Stealth technology can be described. For reasons of classification, nothing can be said about deployed aircraft such as the F-117A and the B-2, pictured in Figs. 1.1 and 1.2, respectively, but they grew out of those early programs. Several generations of technology have now passed, as embodied by such aircraft, but the basic stealth concepts remain the same.

An LPIS must be designed to counter many different current intercept receivers. Any LPIS design also must anticipate future intercept threat trends. There are several trends in both airborne warning receivers and ground-based receivers that will continue well into the future. This chapter provides an overview of the general operating techniques for six separate types of intercept receiver implementations. Second, intercept receiver performance limitations are discussed. Third, possible future threats are analyzed. Finally, two LO/LPI countermeasures for intercept receivers are suggested.

The primary unique criterion for stealth waveform design is reasonably flat, total operating frequency band coverage. This objective isn't always compatible with best data link or radar mode performance. Some obvious criteria are stated in this section. These criteria are then applied to various spread spectrum strategies such as frequency diversity, discrete phase codes, linear FM, and hybrid waveforms. LPI requirements, in addition to SNR considerations, dictate the use of high duty cycle waveforms. This result has two implications: (1) That the transmitted pulse period must be incrementally variable and (2) that large expansion/compression ratios usually are involved.

Stealth signal processing consists of squeezing the maximum performance out of the signal processed, regardless of the amount of “number smashing” that may be required. This includes minimizing straddling losses, reducing RF interference, and limiting self-noise. It also means choosing a waveform that has lowest inherent losses. Furthermore, the output signal-to-noise ratio must be the minimum acceptable, not the best available. Even so, there are inevitably cost-performance trade-offs that must be made, which revolve around word length and total number of arithmetic operations.