lesson 1 - antiradar introduction ( stealth technology )
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 plus the unknown. 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 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.
Challenge
- Survive and prosper in the future environment of improved sensors, dense countermeasures, antiradiation weapons, and emitter locators.
- Become invulnerable or invincible.
Approach
- Force the threats to use active sensors sparingly by employing antiradiation missiles and electronic countermeasures.
- Decrease predictability and increase ‘randomness to force the threats to increase complexity and cost of intercept receivers, surveillance, fire control, and missiles.
- Reduce active and passive signatures and increase ‘hiding’ to make weapon systems less visible.
- Use tactics that combine with the order of battle as well as the natural and man- made environment to enhance the effect f the reduced observables.
- Use prior knowledge and off-board sensor cueing to minimize on-board active and passive exposure.
Stealth is not one item but an assemblage of techniques, which makes a system harder to find and attack. Stealth radar and data link design involves the reduction of active and passive signatures. Active signature is defined as all the observables emissions from a stealth platform: acoustic, chemical ( soot and contrails ) communications, radar, IFF, IR, laser and UV. Passive signature is defined as all the observables on a stealth platform that require external illumination; magnetic and gravitational anomalies; reflection of sunlight and cold outer space; reflection of acoustic, radar, and laser illumination; and refection of ambient RF ( sometimes called as splash track ). Active radar and data link signature reduction requires the use of techniques that minimize radiated power density at possible intercept receiver locations. Active signature reduction also depends on the implementation of tactics that reduce exposure time during emission.
Passive signature reduction techniques are often called low observables ( LO). They require the development of radome, antenna cavity, and antenna designs as interactive elements of a common subsystem that yield low in- band and out-of-band by employing special antenna design techniques that minimize retroreflective echoes. Low probability of intercept system ( LPIS ) design is an engineering problem with a larger set of optimization constraints and hence is no different from every modern design challenge. The stealth designer must always create designs in which complete knowledge of the design isn’t much help to the threat. Conventional antenna designs have sidelobe and mainlode patterns that do not differ fundamentally from below figure ( will add soon ). There are few close-in sidelobes. And of course, the mainlobe, which are above isotropic. The remainder of the sidelobes average 3 to 6 dB below isotropic for a conventional antenna. On the other hand, an LO/LPI antenna has sidelobes that are -10 to -3o dB below isotropic and may average more than 20 dB below isotropic. These low sidelobes are realized at the expense of mainlobe gain and full utilization of the total aperture area. Similarly, conventional aircraft RCS may be noise-like, but it is generally well above 5m² in most directions. in most cases, very little effort in conventional platform design was devoted to reduction of the platform RCS. some of this lack of effort due to the belief that low-RCS vehicles would have undesireable aerodynamic, hydrodynamic, functional shapes. It i now known that this is not the case; it is lack of platform allignment in the direction of the threat that results in many RCS spikes. A typical stealth aircraft signature strategy have five main spikes contain most of the RCS signature. The strategy for a stealth aircraft signature is not fundamentally different from that for an LO/LPI antenna. the idea is to design a platform is such a way that there are only a few RCS spikes in carefully controlled directions. The vast majority of angle space is occupied by RCS that is substantially below that of a conventional platform. This can be thought of as similar to the radar signal ambiguity function where the total volume under the curve must be preserved. Others have pointed out that the average RCS of a smooth body is on order of 1/4 the total surface area. As a result, concentrating all of the reflections in a few directions can reduce the RCS in all other directions. RCS enhancement can be designed so that those spikes are not tactically useful to threat sensors, because the geometry s poor relative to either the threat radar horizon or the threat platform velocity vector. The first ''great thought" of stealth is platform alignment or spike alignment of all the major scatterers on a platform. Most RCS reduction comes from shaping. Radar-absorbing materials ( RAMs ) are applied only in areas where there are special problems, and they have very little t do with the average RCS. Furthermore, mismatches between RAM and free space create a first scatter that must not be be reflected in a tactically useful direction.
