1. ASCMD,
4.4 BMDO MISSILE DEFENSE R&D PROGRAMS
4.4.2 BMDO Missile Defense R&D Programs for Acquisition and Seeker Sensors
Before a cruise or ballistic missile threat can be negated, the threat first must be detected by one or more sensors. Then it must be tracked well enough so that it can be handed over to the defending weapon’s seeker soon enough to permit successful negation. Because the typical trajectories of the two missile types are different, the acquisition sensors need to be considered separately.
4.4.2.1 Acquisition Sensor Technology Needs and Issues
For ballistic missile defense, initial detection, which involves discrimination of the weapon-bearing RV from the associated cloud of booster fragments and countermeasures (if any) in order to generate fire-control-quality tracks for han- dover of the RV target to the kill vehicle, is normally carried out by one or more long-range, ground-based radar sensors.
Prior to this acquisition, other nonradar sensors (DSP, SBIRS-high, and others) may have provided initial threat warning and cueing to speed up the process. However, acquisition, discrimination, and the establishment of a fire- control-quality track on a specific threat RV are the tasks of the radar. CEC, while superficially appearing to be a collection of separate, communicating ra- dars that might be thought of as cueing one another, in fact functions as a single distributed radar that is ideal for tracking air vehicles where line-of-sight issues can arise.
The radar detection challenges presented by ballistic missiles with lofting trajectories lie in the long ranges at which it is hoped to be able to engage them.
The increasingly small RCSs that can be achieved by RV technology (rather than the terrain masking that may be encountered in the cruise missile scenarios) are a further challenge. Increased radar transmitter power can increase range perfor- mance. All TBMD radars, including SPY-1 and GBR, include high-power mod- ule technologies (e.g., GaAs, GaN, and SiC transmit/receive (T/R) modules) that can increase transmitter power and that are under active development in a num- ber of different science and technology organizations.
Power alone is far from the whole solution. The detection of very small targets requires very low detection thresholds, which inevitably produces a large number of additional “targets” (e.g., false alarms and small pieces of debris) that must be sorted out and distinguished from the actual RV by a discrimination logic. Discrimination of the RV from all the other apparent ballistic objects that may accompany or appear to accompany it is the next step in the threat acquisi- tion process and presents formidable challenges. Clearly the radar must be capa- ble of resolving the individual objects—that is, separating them in all spatial dimensions well enough to allow the RV to be successfully identified and tracked.
Since measurement resolution depends primarily on signal bandwidth, high-band- width radar is needed.
It seems obvious that the greater the number of individual parameters that can be measured about each candidate RV in the cluster (e.g., length, width, body-motion characteristics, reflectivity, effects on polarization), the better the chance that the real RV can be identified. Consequently, BMD radars need to be capable of multimode operation and must have the ability to make a variety of accurate measurements. Algorithms must be developed to exploit these features.
Because of the radar reflection properties (i.e., coherence and isolated scattering effects) of typical objects, increasing bandwidth to gain dimensional resolution finer than that already available in today’s X-band BMD radars does not produce more useful images (e.g., more accurate measurements of length).
Since any given radar waveform produces only some but not all the mea- surements that might be desired, a sequence of different measurements must be carried out. The interesting question then arises of how to optimize the order in which the measurements are attempted. Some BMD radars (e.g., GBR in the THAAD program) address this optimization by considering the radar as an adap- tive measurement system, not just a “radar.” The waveforms and radar modes utilized at any instant are adaptive responses to the results of the previous mea- surements. Such adaptive approaches should be explored and extended since they may prove more robust than preplanned approaches in which unexpected RV or countermeasure characteristics are encountered. This approach may also prove useful for the seeker’s discrimination tasks as well, with active capabilities expected to be added to the passive sensing currently employed in the exo- atmospheric HTK vehicles.
The final step in the acquisition process—that of establishing fire-control- quality tracks on the candidate RV (or RVs, depending on the success of the
ASSESSMENT OF CURRENT AND PROJECTED R&D PROGRAMS 119 discrimination process)—is a straightforward and familiar sensor task given the resolution capabilities needed for discrimination. The longer the objects under track can be observed with high resolution and good signal-to-noise, the better the tracking filter estimates will be. In addition, as track quality im- proves, the volume of the handover basket that is passed on to the HTK vehicle decreases. This in turn minimizes the time required for the onboard seeker to acquire, discriminate, and target the correct RV target. As a result, the kill vehicle divert capability needed to accomplish the intercept will also be mini- mized.
Needed Acquisition Sensor Research
Some obvious sensor improvements (e.g., X-band THAAD equivalent ) are not “research” issues but simply need to be done—their pursuit is a priority for the naval forces. Other needed improvements include power and bandwidth improvements via wide-band-gap materials such as GaN and SiC and low-cost T/R modules based on GaAs (X-band) or Si (S-band) microwave/millimeter wave monolithic integrated circuit technology. Digital radar offers flexibility for adaptive measurements—digital waveform generation is often used, but only for limited sets of waveforms (e.g., linear chirps.) Much more flexibility is possible. Any kind of waveform can be generated and processed digitally. The implications of this kind of flexibility should be investigated.
Needed Research on Adaptive Discrimination
Full digital with analog-to-digital conversion at each element would offer digital phase shifting with no bandwidth limitations when using digital optical communication technology to transfer microwave signals as digital bit streams and cycle-slip plus digital filtering for interpolation to implement the phase shift- ing. This is not practical at present owing to the performance limitations of current analog-to-digital converters (ADCs) and their expense. There is a need to develop high-performance, inexpensive ADCs and digital receivers.
4.4.2.2 Seeker Sensors
The sensor suite onboard the kill vehicle must first acquire the incoming threats or candidate RVs. Missile seekers have an inherently limited field of view and search capabilities. The acquisition of a target (or target complex, as is likely for ballistic targets) requires the designation of a handover “basket” from the long-range sensors. The better this can be accomplished (i.e., the smaller the basket), the better the capabilities of the seeker sensors can be employed to acquire the target as soon as possible. This efficiency, in turn, will maximize the time available for discrimination, aim-point selection, and vehicle end-game
maneuvers, thereby minimizing kill-vehicle divert requirements and maximizing the probability of successful intercept.
For ballistic threats, at handover the missile/kill vehicle seeker is typically presented with a many-object target complex depicted and characterized by the measurement capabilities of the initial acquisition radar, from which the target (i.e., RV) must be correctly identified as soon as possible. Clearly, high-resolution
“imaging” in all dimensions is required to detect and examine each candidate RV for discrimination. Given the limited aperture imposed by typical missile dimensions, high-frequency systems (i.e., electro-optical and millimeter-wave) must be used. While passive sensors can be adequate for precise azimuth- elevation measurements, for precise range and Doppler measurements, active capabilities via LIDAR or wideband radar adjuncts would be desirable. Current candidate Navy TBMD missile seekers (e.g., SM-2 Block IVA and lightweight exo-atmospheric projectiles) rely entirely on passive optical sensors for the terminal phase, although combined passive/active optical seekers are under development.
Because of the range of possibilities, which includes sophisticated counter- measures, it is clear that the more unique measurements a seeker can make on the totality of objects in the target complex, the better the chance of correctly identifying the real RV. Multiband optical (several IR and possibly visible) sensors with laser detection and ranging (LADAR) and/or millimeter-wave (MMW) radar active adjuncts seem to be called for. If affordable and physically realizable, the combination of multiple optical bands with RF measurements offers good decoy discrimination potential. Although decoys may be produced that are excellent replicas of a RV, the designer of decoys finds it difficult to replicate RV signatures precisely in all-sensing modes.
For many years, BMD discrimination research has concentrated on the de- velopment of algorithms derived from observations by a single type of sensor.
The more mature discrimination techniques are based on radar measurements, but there is also a significant body of work on passive optical sensor discrimina- tion. Only recently has serious attention begun to be applied to fusing of radar and optical data to enhance discrimination performance.
The main BMD systems currently under development all have both radar and optical sensors, and the enhanced discrimination potential achieved by com- bining the data they collect on various features of the threat objects is becoming increasingly evident. The radar data, particularly that measurable by the X-band radars being developed, allow the precision measurement of microdynamic fea- tures of threat objects. The passive IR sensors being developed for performing onboard interceptor functions are naturally adept at measuring thermal charac- teristics of threat objects. In addition, there is a large class of features, such as macrodynamic body motions, that both sensors can measure. The potential for a significant improvement in discrimination capability lies in the effective fusion of these feature vectors.
ASSESSMENT OF CURRENT AND PROJECTED R&D PROGRAMS 121 While passive sensors can make simultaneous measurements (e.g., using FPAs) on multiple objects in multiple modes, active sensors typically employ different waveforms sequentially. Here, as for long-range sensors, treating the seeker’s active component (and perhaps the computer resource) as a measure- ment system rather than simply a radar or an imaging system, may permit effec- tive adaptive procedures to be applied.
Needed Seeker Sensor Research
The following seeker sensor research is needed:
• Multiband IR/visible sensors with laser radar or radar adjunct should be developed to address the discrimination issues that are certain to arise as ballistic missiles continue to become more sophisticated. If one looks at the dramatic advances in focal plane materials and mechanization technologies, it is easy to project continued improvements in quantum efficiency, sensitivity, bias and noise suppression, and resolution. The use of additional resolution and narrower de- tector bandwidths opens the possibility of other multispectral discriminants, in- cluding materials and imaging.
• Discrimination algorithms that exploit all the signatures that can be de- tected by multispectral sensors should be developed.
• LIDAR systems with multipixel FPA, which measure range-to-pixel, need to be developed in order to avoid the mechanical complexity associated with scanning optical systems.
• More powerful lasers are probably needed to extend the range of the three-dimensional imaging LIDAR adjuncts.
• RF/MMW adjunct possibilities for enhanced discrimination should be explored, including the possibility of deployable antennas for exo-atmospheric intercepts, to mitigate the limitations of kill-vehicle dimensions.
• Adaptive discrimination algorithms using the active capability of the seeker as a measurement tool need to be developed.
• Multiband LWIR sensors and their associated algorithms are able to re- ject most celestial objects and background by temperature and/or lack of move- ment; however, visible light sensors must deal with this problem. Background obscuration algorithms should be explored to deal with low-cross-section targets against the stellar background, although because the RVs are so small and the sky background is so complex, this approach seems to be less promising than it was for cruise missile threats.
4.4.3 BMDO Missile Defense R&D Programs for Weapons