This expanded, revised and updated new edition of Introduction to RF Stealth covers two major topics: Low Observables and Low Probability of Intercept (LO and LPI) of radars and data links, collectively sometimes called Stealth. Each chapter includes examples, student exercises and references. Worked simulations are available that illustrate the techniques described. Chapter 1 provides an introduction and history of RF/microwave LPI/LO techniques and some basic LPI/LO equations, expanded from the first edition with more information on new and current systems, including more on infrared and hypersonic missile signatures. Chapter 2 is a new chapter, covering radiation absorbing materials and shaping, focused on materials, meta-materials and detailed platform shaping and structures including ships. Chapter 3 covers interceptability parameters and analysis with corrections, updates and simulations. Chapter 4 covers current and future intercept receivers and some of their limitations with more information and tracking techniques. Chapter 5 surveys exploitation of both the natural and the threat environment with extensive threat table updates including Russian S300, S400, S500 and more information on cellular systems. Chapter 6 deals with LPIS waveforms and pulse compression with new material and simulations of new codes. Chapter 7 introduces some hardware techniques associated with LO/LPIS low sidelobe / cross section antenna and radome design with emphasis on active electronic scan arrays. Chapter 8 is a new chapter on RCS testing of subsystems and platforms.
Other keywords: emitter footprints; stealthy pulse compression design; IR signatures; emitter location accuracy; system engineering perspective; pulse compression sidelobes; RF stealth; ambient spectra; terrain obscuration; infrared signature; radiofrequency stealth; target visibility; ambient pulse density; active electronic scanned antennas; emitter interceptability; filtering; detection performance; stealthy antenna design; radar cross section
The objective of stealth is to keep the adversary guessing until it is too late. Over the past few decades, 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.
Before one can decide what materials and shapes a platform should use, one must identify which regions represent high and low threats for all parts of the electromagnetic spectrum. Then, a design should align specular and bright regions on the platform with the low threat directions. In general, shaping is much less expensive and more successful than adding materials. Special materials add weight, cost, and maintenance. The initial design should avoid dihedrals and inadvertent multiple bounces which may create dramatic increases in the signature. A simple rule is that a bad fit and finish will result in bad signatures. Normal platform construction materials can be applied in electromagnetically inventive ways. Once there are areas on a platform which cannot be improved by shaping then radiation absorbing materials (RAM) should be applied sparingly. One must start with RAM based on circuit analog sandwich materials, since they are usually lighter and more reproducible. Finally, apply traditional bulk RAM as a last resort.
Low probability of intercept (LPI) radar or data link performance is a complex function of many variables. There are three main elements of low probability of intercept system (LPIS) performance that affect a stealthy system's success: 1. The actual design features incorporated into the emitting system specifically for LPI. 2. The electronic support measure (ESM) strategy and corresponding implementation utilized by threat forces. Hostile forces' ESM strategy and implementation are very important factors affecting any LPI design. Design strategy begins with analysis of individual threat receiver characteristics against which the LPIS may be deployed. The electronic order of battle (EOB) deployment and location strategy for intercept receivers must also be considered in order to create the most successful design features. Additionally, the issue of whether or not individual interceptors are netted has a strong influence on the strategies an LPIS might use. 3. The geometry between the area of regard (AOR) and threat receivers.
A low probability of intercept system (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 which will continue well into the future. First, Chapter 4 will provide an overview of the general operating techniques for six separate types of intercept receiver implementations. Second, intercept receiver performance limitations will be discussed. Third, possible future threats will be analyzed. Finally, two LO/LPI countermeasures for intercept receivers will be suggested.
A significant part of stealth strategy is concerned with exploiting the environment in every way possible, consistent with the mission requirements. This consists of exploiting not only the natural environment (atmospheric attenuation, clutter, etc.) but also the man-made environment (such things as the electronic order of battle (EOB) and electronic countermeasures (ECM)). In the last part of this chapter, an example low probability of interceptor (LPI) scenario analysis will be performed. This analysis will show the essential elements of most LPI mission assessments. In the fourth chapter, there was a survey of the performance and parameters of current intercept receivers. This chapter will show among other things that the environment limits passive and active detection of stealth platforms, not raw sensitivity.
The primary unique criterion for stealth waveform design is reasonably flat total operating frequency band coverage. This objective is not 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. Low probability of intercept (LPI) requirements, in addition to signal-to-noise ratio (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. Another result based on stealth requirements is that the instantaneous (not just the average) bandwidth of the transmitted signal is as large and as uniform as possible. For each geometry, the power must be managed to the lowest level consistent with acceptable performance or bit error rate (BER). It is obviously desirable to keep the preprocessing and the bandwidth to a minimum; therefore, the waveforms chosen should result in the lowest possible data rate prior to compression or decompression. Lastly, LPI time and frequency constraints require noncontinuous or burst transmission for both data links and radars. Some systems naturally operate in a burst mode such as the JTIDS, which uses a TDMA format. The stealth platform whether aircraft, ship, or vehicle must move between transmissions to create an uncertainty volume.
The purpose of this chapter is to summarize the results of antenna and radome technology which are of particular interest for antenna stealth. Shape is everything when it comes to both the active and passive signatures of antennas and radomes. Once the shape is right then things such as element pattern, amplitude weighting, thickness, and edge treatment become important. First, the relationship between the radiation pattern (beamwidth, side-lobes, etc.) and the current distribution across the antenna aperture is discussed. This is followed by descriptions of the various types of antennas which have been applied to radar and datalinks, including reflectors, lenses, and arrays. Several methods of pattern synthesis are discussed. The effect of broadband signals and errors in the aperture distribution on the radiation and RCS patterns is also considered. This chapter covers both the active and passive signatures of antennas and radomes. The chapter concludes with brief discussions of radome and antenna near field interaction.
Clearly, a stealth platform must be tested across the chemical, acoustic, infrared (IR), visible (EO), laser and radar cross section (RCS), and ultraviolet (UV) spectrum. Many of these tests are done on static test ranges. Some tests can be done at the complex component and subsystem level. Acoustic and EO testing using ground sensors is usually adequate. RCS, IR, and UV testing with ground sensors is necessary but not usually adequate because of multipath, ground, and airborne/ spaceborne interferers. Ground interactions will skew many results. Final RCS, IR, and UV tests must be in the actual environment, for example, at-sea, in-flight, and in-space. Air to air RCS, IR, and UV instrumentation exists for such tests. All the RCS test examples in this chapter use large targets since most RCS signatures of real vehicles are highly classified. In the early days, outdoor ranges were used for RCS and EW tests. The USAF Ratscat, Navy, Grey Butte, and Tejon Ranch outdoor ranges were used in the 1960s and 1970s. Even with many upgrades, these ranges were inadequate for many reasons including incidental interference, weather, exposure to overhead surveillance, ground vibration, etc.
Appendices for 'An Introduction to RF Stealth, 2nd Edition'