De Re Aedificatoria (en latín) de Leon Battista Alberti

A smart weapon that contains MMW radar includes a number of key component items.

The seeker is generally the MMW radar (including the antenna gimbal system) and frequency down-conversion systems of the analog data to be digitized for the signal

and data processors. An illustration of a kinetic energy smart weapon, sometimes called a rod or penetrator munition is shown in Figure 3-10.

The weapon or munition depicted in the figure is powered; that is, it contains a propellant to advance its progress in the flight timeline after launch. For a glide muni-tion, there is no propellant. This allows more space for the warhead but reduces the available kinetic and potential energy of the platform. Depending on the design, this type of munition often leaves a ‘‘hole’’ in the center for electronics in front of a shaped charge warhead to allow minimal disturbance to the blast as it propagates toward the point of collision. Glide munitions contain ‘‘wings’’ much like airplanes to allow for stabilization and guidance throughout the flight profile.

A good illustration of a glide munition is the Scorpion system developed by Lockheed Martin Systems [10], which includes an option for an MMW seeker head as illustrated in Figure 3-11.

The seeker compartment contains the major MMW radar components, including:

● the antenna and RF transmit-and-receive section,

● the IF section,

● a digitizer, and

● the signal data processor.

Figure 3-12 shows a generic block diagram of the MMW seeker component.

In the seeker itself, many discrete components are required to perform open- and closed-loop tracking functions. In modern systems, many of the discrete functions are combined into solid-state transceivers. Since the wavelength is small, microstrip com-ponents have matured that embed ‘‘waveguide’’ materials directly onto substrates, thus allowing higher levels of component integration into single devices.

Wing Kit

MMW SEEKER PROCESSOR FALSE TARGETS FALSE ALARMS DATA PROCESSINGGUIDANCE

VALID TARGETS

DISCRIMINATION LOGIC

PARALLEL PROCESSINGSQUARE LAW DETECTOR • LENGTH • TOT • • THRESHOLD DETECTOR NOISE, CLUTTER

TARGETS PRF

AGC TRANSMITTER MODULATORTIMER SEEKER PLATFORM, SENSOR, AND TRACKING ELECTRONICS

PRF

MIXER IFRF

CIRCULATORANTENNAATMOSPHERE TARGETS BACKGROUND

SAMPLE AND HOLDIF AMPLIFIER AND FILTERS RANGE GATES LOCAL OSCILLATOR

RF AMPLIFIER TRANSMITTER/RECEIVER FIGURE3-12¢BlockDiagramofGenericMMWSeeker.

3.6.1.1 Components

Each key component in the MMW seeker is discussed in the following sections.

Radomes While generally not part of the MMW radar compartment, one key platform item for all seekers relates to the radome. The radome material must be strong for high g (acceleration) and velocity operations while providing minimal impact (loss or distor-tion) on the MMW waves passing through the material.

The fundamental radar system in the muniton must ‘‘see’’ through the radome. The radome is there to protect the antenna and electronics from weather and potential high-velocity atmospheric impacts while preserving good transmission properties for the MMW signals. In many cases, the antenna inside the radome scans the field of view (either electronically or mechanically). Thus, the radome material must preserve good transmission properties under hostile conditions over all incidence angles in the scanned (and signal return) fields.

Radomes must maintain a high level of ‘‘transparency’’ over all the operating modes of the seeker (scan, frequency, polarization, etc.). In addition, they must be easily and reliably manufacturable at the lowest cost. Guidelines for materials selection and radome performance are shown in Table 3-10. Note that more than transmittance and reflectance must be considered in the selection of an adequate radome [11].

Table 3-11 lists some good materials candidates and their properties for MMW use.

Note that the table also provides a separate set of columns for the material properties at IR. MMW has mated with IR systems on a number of defense programs – mostly seekers – due to the natural complement of MMW and IR (and optical) performance and single-mode countermeasure rejection capability.

Antennas The transduction of the wave to and from the monostatic radar source in the seeker is accomplished by the antenna. For seeker applications, the antenna must be very lightweight to allow low inertial resistance for rapid scanner electronics. A number of antenna approaches are applicable to MMW sensing [12, 13], including:

● a parabolic dish,

● lenses,

● Cassegrain geometries,

● spirals,

● loops,

● wires,

● horns,

● phased (electronically scanned) arrays, and

● many other classical approaches.

A number of MMW antenna types and support and feed structures are illustrated in the following figures. Probably the most common are parabolic dishes and Cassegrain geometry types although two-dimensional array systems such as electronically scanned arrays (ESAs) are maturing rapidly for many applications [14–16]. Figures 3-13 and 3-14 are photographs of MMW parabolic ‘‘dish’’ and Cassegrain antennas, respectively, which are currently available from Deh-Ron Ltd.

Figure 3-15 depicts a microstrip antenna, in which the elements are made much like a printed circuit board, on a low-dielectric stable substrate [17, 18].

TABLE 3-11 ¢ Radome Material Performance Summary (MMW and IR) Fused silica 3mm 90%, falling to

opaque at 5mm

TABLE 3-10 ^¢ MMW Radome Performance Guidelines

Parameter Units Performance Requirement

Transmission Percent >85

Reflection Percent <0.5

Beam deflection mrad 0.05–0.3

Beam deflection error rate mrad/mrad 0.005–0.01

Change in 3-dB beamwidth Percent <5

Sidelobe increase

–20-dB level dB <1

–25 dB level dB <2

–30 dB level dB <4

Axial ratio dB >40 (algorithm driven)

Hardness Knoop 600–800

FIGURE 3-13 ^¢ 35-GHz MMW Antenna (Parabolic Dish).

FIGURE 3-14 ^¢ MMW Cassegrain Reflector Antenna.

Patch

Patch

Microstrip

Microstrip

Screen

Slot Coupling Elements FDF Waveguide (a)

(b) Active Element

FIGURE 3-15 ^¢ Microstrip Patch Antenna.

Figure 3-16 depicts a feed structure for use with a parabolic reflector antenna in a multibeam application. There are several feed horns, one of which can be selected at a given time. Since the horns are somewhat displaced from the parabolic focal point, the beam is scanned by electronic selection of the feed channel [19]. It is easy to see the complexity of a discrete component approach to beam steering at MMW frequencies.

Figure 3-17 shows a two-dimensional array of antenna elements in which the phase of the RF signal into the rows and columns can be individually controlled [18]. By controlling the row and column phases, the beam is electronically scanned in two planes.

Note the significantly reduced complexity of the ESA approach.

In some cases, to reduce cost and inertial for mechanical scanning, the reflector design can be made of ‘‘stamped plastic’’ formed to a parabolic dish and then coated with a high surface roughness tolerance metal surface to preserve the electromagnetic properties at MMW.

Transceivers The key transceiver functions relate to (1) wave transduction, (2) wave generation, (3) wave reception, (4) wave down-conversion, and (5) digitization for signal-processing electronics.

Most of these functions can now be implemented on very small size hosts. A number of millimeter wave integrated circuit (MMIC) and microstrip technologies allow large-scale integration and miniaturization of MMW RF/IF and digital signal-processing devices. The short wavelength allows for entire ‘‘radars’’ being ‘‘grown’’ on single wafers. Figure 3-18 depicts an MMW MMIC device that integrates whole radar trans-ceivers into single modules [20].

FIGURE 3-16 ^¢ Feed System for a Ka-Band

Multibeam Antenna.

FIGURE 3-17 ^¢ Ka- Band MMW Two-Dimensional Electronically Scanned Array Antenna.

3.6.1.2 Seeker Spectra and Modes

A smart weapon or munition may contain either a single sensor or multiple sensors, depending on the application and mission complexity. When only a single sensor type is used, the seeker is considered single spectral. When multiple sensors are used in an application, the seeker is considered multispectral. The current thrust in the military is toward multispectral integration of MMW sensors with optical (IR, laser) to provide good search, terminal, and countermeasure performance across multiple firing platforms (e.g., shooters); each sensor provides its strengths to the application while other sensors compensate for its weaknesses.

For a single-spectral implementation, multiple radar modes are possible. These modes can include active, semi-active, or passive implementations. Each one is described below:

Active – The term active implies the radar is transmitting and receiving its own signal from a common location. From the point of view of a missile seeker, this is sometimes called a fire-and-forget missile.

Semi-active – The term semi-active implies there are separate transmitter and receiver locations in the radar configuration. A typical implementation is a surface-based illuminating radar and missile with a tracking receiver onboard.

Passive – The term passive implies detection of MMW radiometric noise either emitted or reflected from the source patch.

In some cases, all of the above may be used by the seeker onboard the munition during the engagement timeline. When a seeker uses multiple modes it can be called multi-mode. Unfortunately, in some communities the use of multimode implies multispectral seeker implementation. For this chapter, a seeker that contains only one sensor can operate in multiple modes.

The combinations of modes in the use of the seeker during the engagement timeline have traditionally included:

● active-only mode,

● active-and-passive MMW mode,

FIGURE 3-18 ^¢ MMW MMIC Devices Integrate Whole Radar Transceivers into Single Modules.

● passive radiometer only,

● semi-active mode, and

● beamrider mode.

Each mode is fairly self-explanatory. In general, the active modes are preferred in environments with high EM interference. Passive mode operation is generally preferred if the interference levels are low. In most cases, passive modes are confined to satellite platforms, although recent work on concealed-weapon detection indicate passive modes can be used in this application at very short range.

3.6.1.3 Multispectral Implementations

While MMW itself can do a good job with target detection and discrimination, in some cases an optical system can be used for terminal phases in the timeline due to its very narrow beamwidths. The terminal phase requires precise hit-point estimation on complex objects (tanks, buildings, etc.). The MMW system provides broader beams in the common aperture, allowing for efficient search in the timeline. The optical systems can get fine resolution of the target but only at very short ranges and under good weather. Thus, the MMW system is generally onboard to provide efficient target search along with bad weather operations.

Current multispectral munitions are focused on integrating the following:

● dual mode MMW and infrared (MM/IR);

● dual mode MMW and laser radar (LADAR);

● tri-mode MMW, LADAR, and IR; and

● combinations of the above.

Figure 3-19 illustrates an MMW and IR system processing configuration.

Each sensor provides unique – uncorrelated – information on the object (and clutter) of interest in a scene. As illustrated, the data from any sensor suite can be combined at multiple levels, each with a unique performance and cost impact.

The three basic levels of sensor fusion are:

● sensor and pixel level,

● feature level, and

● decision level.

At the sensor level, the raw data from both radar front ends are combined to form a composite data stream to subsequent processes. A good example of this is when the radar sensor provides excellent range measurement while the IR or optical sensor pro-vides excellent azimuth and elevation information. The combination of these pixels can

MMW Video Clutter & Counter Measure Statistical (IR & MMW)

& Microregistration Data (MMW)

FIGURE 3-19 ^¢ Generic Dual Mode (MMW/IR) Radar Processing.

form a 3-D image (r,q, f) in the relative spherical coordinate system afforded by the multispectral seeker. Pixel-level fusion is probably the most costly to implement but has the highest potential performance in the required mission.

In all cases, the two sensors must be microregistered to ensure that the data provided for in subsequent processes are based on the same physical point in 3-D space.

Feature-level fusion of the information is based on feature extraction from the sensor channels. The features can be ‘‘target size’’ or one of many other possible char-acteristics afforded by the individual sensors. Here the data are fundamentally con-volved by each of the sensor channel front ends and the features extracted for combination algorithms. Feature-level fusion is somewhat costly to implement and has a higher performance potential than decision-level fusion.

Decision-level fusion essentially makes a target decision based on the decision made by the onboard sensors. Simple Boolean algorithms such as and and or can be used at this level. Decision-level fusion also is more robust than higher levels in the event of a single sensor failure or degradation. As you might imagine, decision-level fusion is the least costly to implement – from a target-detection algorithm perspective – but probably has the lowest performance potential for the multispectral smart weapon.

The use of MMW components in multispectral (optical) combinations exploits the fact that the wave optics more closely match IR and visible than lower frequency.

The aperture tolerances are similar, and common aperture approaches become feasible.

The component size reduction for the MMW radar sensor also allows more use of restricted volumes in smart weapons.

The combination of multisensor data into a common platform has two distinct advantages:

● false targets and real targets are decorrelated, and

● single-mode countermeasures are easily discriminated.

Each of the MMW radar complements are discussed in this chapter. In some cases, even multispectral seekers must rely on a single spectrum for complete operation. For example, in bad weather or obscurant engagements, the MMW radar may carry the primary load for sensor functions when combined with optical sensors.

3.6.1.4 Typical Waveforms

In general, active MMW radar systems will use either modulated CW or pulsed-type waveforms. For CW waveforms, the key advantages are:

● high average power,

● good clutter smoothing with frequency modulation (FM) (e.g., FMCW), and

● high-range resolution in the frequency domain with FM.

Conversely, pulsed waveforms offer:

● high peak power,

● high-range resolution in time domain, and

● excellent transmit / receive (T/R) isolation for monostatic radar.

Many modulation options are available for either CW or pulsed waveforms. These include variations of frequency modulation, phase modulation, amplitude modulation,

and interleaved waveforms with complex parameters changing on a pulse-to-pulse basis.

Phase modulation variations such as phase-shift keying (PSK) or binary phase codes have also shown promise for very-high-range resolution in military tracking applications.

The basic waveform types discussed are depicted in Figure 3-20.

Pulsed waveform generation can be difficult in some implementations due to require-ments on rise and fall time and pulse shape. For FMCW implementations, the frequency sweep linearity is often the key limitation in some applications. Both types of waveforms are in use by the military and commercial markets today for MMW applications.

With the increased availability of high-rate digital encoders, phase-coded wave-forms such as PSK have also become feasible for MMW waveform use but are not prevalent (yet) in current systems.

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