CHAPTER 9
cruise missiles, to very large, extended targets, such as bombers. Target speeds vary from zero, for hovering heli- copters, to supersonic, for high-speed airplanes. Targets may travel straight, constant-speed flight paths, or they may perform evasive maneuvers expressly designed to cause the missile seeker to lose its track on the target or at least to cause a large miss distance. There may be one or more than one target within the seeker field of view at any given time, or if the seeker loses track, no targets are within the field of view.
When a target is viewed by a seeker from long range, it appears essentially as a point source. As the range horn the missile to the target decreases, the target appears larger and huger until it finally fills the field of view of the seeker.
Depending on the application, a target is represented in a scene simulation by a single point source, an area of radia- tion having a definable shape, or a number of discrete point sources.
Many seekers function most effectively when the image appears as a point source. The growth of the relative size of the target image as the missile approaches the target can affect the ability of a seeker to track accurately. For exam- ple, an EO seeker using a reticle (subpar. 2-2.1.1.1) modu- lates the target signal by alternately allowing the target signal to reach the detector and blocking it from the detec- tor. This passing and blocking of the signal is accomplished by the transparent and opaque areas of the reticle through which the signal must pass to reach the detector. When the target image is smaller than the individual transparent and opaque areas, the entire image is essentially contained within only one element of the reticle pattern at any given instant, as shown in Fig. 9-1(A). In this case the modulation of the target signal by the reticle is easily processed to fur- nish the relative position of the target within the field of view. However, when the target image becomes large enough to cause target radiation to pass through more than one transparent region at the same time, as shown in Fig. 9-
1(B), accurate signal processing is more difficult.
Although signal processing for RF seekers is entirely dif- ferent from that for EO seekers, RF seekers also experience increased tracking errors when the angular size of the target becomes significant relative to the size of the seeker field of view. These RF tracking errors are caused by the wandering of the apparent center of target radiation relative to the physical target center and this causes the seeker to adjust its pointing direction constantly.
The characteristics of the electromagnetic signals radi- ated from a target or reflected by it constitute the target sig- nature. The parameters of target signatures include the signal strength and spectral properties of the radiation and the effects of aspect angle and time on the target signal. For a given seeker only those target signature characteristics that are detectable by that particular seeker are of interest;
therefore, typical target signatures are applicable only within given frequency band limits, which must be specified for the signature to be meaningful.
9-2.1.1 Electro-Optical Signatures
As discussed in subpar. 2-2.1.1, the sources of EO radia- tion emitted from the target are the propulsion system, i.e., engine exhaust plume and hot tailpipe, the aircraft surface, i.e., heating by aerodynamics, solar energy, and thermal energy generated by internal components, and reflected energy, i.e., solar or laser illumination.
Since these various EO radiation sources have different temperatures, the predominant wavelengths of the respec- tive radiated power fall into different wavelength bands of the EO spectrum. Typical seeker detectors are-sensitive to only certain portions of the spectrum; therefore, not all of the EO power emanating from a target is detected by any given seeker. An example of the spatial distribution of an infrared (IR) signature in the azimuth plane of a typical tar- get and in the wavelength band of a typical EO seeker is given in Fig. 2-4.
Figure 9-1. Size of Target Relative to Reticle Pattern 9-2
Very high temperatures are required to radiate significant amounts of energy in the ultraviolet (UV) region of the spectrum; therefore, only small amounts of UV energy are generated by the target. The UV energy from sunlight is reflected from both the target and the sky background, and the W energy density from the sky may be greater or less than that reflected from the target. When the target W reflection is less than that of the background, the target appears to a UV detector as a hole in the relatively uniform radiation pattern of the sky background. In either case the W contrast of the target relative to the background can be employed by certain missile seekers that are designed to detect it.
EO power emitted from one target component can be masked by other target components. For example, much of the power of the exhaust plume is not visible to a seeker from the head-on direction because the power is masked by the airplane structure.
Variations in the IR signature of a target occur when the engine power setting of the target is changed because (1) changing the power setting changes the amount of power in the exhaust plume and (2) the resulting change in speed affects the aerodynamic heating of the surfaces of the air- craft. Sun glint from various surfaces of the target aircraft may affect the performance of an EO seeker.
9-2.1.2 Radio Frequency Signatures
Radio frequency radiation can be generated by electronic equipment onboard the target or it can be generated by an illuminator radar and reflected by the target.
When a complex target, such as an aircraft, is illuminated by an RF wave, power is dispersed in all directions from multiple points on the target. The apparent distribution of the radiating points of the target and the intensity of the tar- get signal vary nonlinearly with respect to range and aspect angle (Ref. 2). The spatial distribution of power reflected from the target depends on the size, shape, and composition of the target and on the frequency and nature of the incident radiation wave.
The intensities of the RF signatures of targets are charac- terized by a parameter called the scattering cross section.
The power reflected from a radar target in a particular direc- tion can be expressed as the product of an effective area and an incident radiation power density. In general, that effec- tive area is the scattering cross section of the target. Because scattered radiation fields depend on the attitude at which the target is presented to the incident wave, the scattering cross section fluctuates as the relative attitude changes. Thus the scattering cross section is not a constant but strongly depends on the aspect angles of the target relative to both the illuminator and the receiver radars. For directions other than back toward the illuminating radar, the scattering cross section is called the bistatic cross section, and when the direction is back toward the illuminating radar, it is called the backscattering cross section or simply the radar cross section (RCS) (Ref. 3). A typical RF signature in the azi- muth plane of an airplane is shown in Fig. 9-2.
The amplitude of the echo signal from a complex target may vary over wide limits as the aspect changes. If this vari- ation in signal amplitude occurs during the observation time
Figure 9-2. Typical Radar Cross Section in Azimuth Plane of Target
of a conical scan tracker, i.e., one revolution of the antenna beam, tracking errors are introduced that increase missile miss distance. The monopulse radar, on the other hand, determines the angular tracking error on the basis of a single pulse, and its tracking accuracy is not affected by changes in signal amplitude with time (Ref. 4).
At long range the receiver of the scattered RF energy views the target as essentially a single-point scatterer. At shorter ranges the apparent center of reflection tight not correspond to the target center. Changes in the target aspect with respect to the radar can cause the apparent center of radar reflections to wander from one point to another. In fact, it need not be confined to the physical extent of the tar- get and may be off the target a significant fiction of the time. This aimpoint wander, called glint, is perceived by the seeker as target motion and is a particularly important parameter in investigations of the miss distance of missiles with RF seekers. These angle fluctuations affect all tracking radars whether conical scan, sequential lobing, or monopulse (Ref. 4).