ABSTRACT
The radar element requirements are developed in a similar process to the radar system requirements� The key radar elements are the antenna, transmitter, and signal/data processor� Once the target is detected, the radar must be able to accurately determine the position and heading of the target in natural and man-made environments� The position of the target is determined from measuring the range and azimuth and elevation� Each estimate of range and angle is used to update track filters that smooth the target’s position and estimate the target velocity and acceleration� The target track data are used in the engagement solution to determine the target intercept point� The accuracy of the radar estimates will determine the uncertainty volume that the interceptor seeker must search in order to acquire the target and transition into terminal homing� The process for determining the radar element requirements that support target tracking accuracy is illustrated in Figure 8�1�
The key design parameters are instantaneous bandwidth, antenna beamwidth, and target signal-to-noise ratio� These radar parameters affect the accuracy of the target range and angle estimates� The antenna architecture, signal processing, and utilization of enabling technologies optimize radar performance in natural and man-made environments� The antenna architecture is usually selected to provide low sidelobes that mitigate the effects of jamming and clutter in the sidelobes� Mainlobe clutter is removed with signal processing techniques such as moving target indicator (MTI) or pulse Doppler (PD)� Key enabling technologies are digital beamforming, adaptive sidelobe cancellation, and T/R modules�
The radar instantaneous bandwidth (IBW) determines the radar range resolution� The achievable radar range resolution is as follows:
Achievable range resolution = 240/(IBW(MHz)) m
A radar with 100 MHz of instantaneous bandwidth can achieve a range resolution of 0�24 m� The achievable radar resolution includes 2 dB of processing
loss, which is a typical value� In order to resolve two point targets, a good rule of thumb is that they should be separated in range by at least three times the range resolution� For the case of 100 MHz IBW, the two point targets should be at least 0�72 m apart in range�
Range accuracies of 10-20 m are usually adequate to support most target engagements� Instantaneous bandwidths of 12-24 MHz are adequate to support this requirement� Higher instantaneous bandwidths are typically used for target imaging to support target ID� This is typically done in ballistic interceptor defense when it is desirable to locate the reentry vehicle in a cloud of ballistic objects including boosters and separation and thrust termination debris�
The antenna beamwidth sets the radar angular resolution� The antenna beamwidth is inversely proportional to the antenna gain� Therefore, the antenna beamwidth becomes narrower with increasing radar frequency and increasing antenna size� Radar angular resolution is typically in the order of 1/10 of a beamwidth�
Range and angle accuracy are inversely proportional to the signal-to-noise ratio and the number of track updates available to the tracking filter� Therefore, track accuracy improves over time� Figure 8�2 illustrates combinations of antenna beamwidth and number of target updates required to achieve a given track azimuth or elevation angular accuracy of 0�1° and 0�05° for a notional tracking filter design and a fixed target signal-to-noise ratio of 16 dB� An angle accuracy of 0�05° can be achieved using a simple tracking filter using a 2° beamwidth with 25 track updates at a signal-to-noise ratio of 16 dB�
Figure 8�3 shows a similar relationship versus radar frequency� Clearly for a fixed available track time (number of hits), higher frequencies and narrower beamwidths provide better tracking accuracy�
The natural environment can affect the ability of the radar to accurately measure target position and angle� Clutter and multipath can reduce the target signal-to-noise ratio� Multipath can also degrade the angle estimation accuracy particularly for estimates of the target elevation angle�
If the main beam clutter is not canceled below thermal noise by signal processing techniques, such as MTI or PD, the clutter can reduce the effective target signal-to-noise ratio� This will reduce track accuracy and in some cases cause track drops because the target is completely masked by the clutter return� Clutter through the sidelobes must also be minimized� This is typically done with low receive pattern sidelobes and potentially some control of the transmit sidelobes in angular regions where large clutter returns are present� Low receive sidelobes also provide some immunity to sidelobe jamming� The level of clutter rejection needed in the main beam and sidelobes can be traded off in the radar design between the antenna and signal processor requirements�
Low receive channel sidelobes can be achieved with analog or digital beamforming architectures� Digital beamforming provides some additional flexibility for performing adaptive receive sidelobe control� The synthesis of low sidelobe regions in the transmit pattern can be done with the T/R modules in an active phased array� The T/R modules provide the flexibility to synthesize low sidelobe regions using the independent phase control at each element provided by the T/R modules�
Multipath is a target return from a secondary indirect path, which includes a signal reflection from the terrain between the radar and the target� Multipath levels are geometry, terrain, frequency, and polarization dependent� Multipath produces target fading that reduces signal-to-noise ratio and tracking accuracy� In some cases, the multipath is constructive and enhances the target signal-to-noise ratio by up to 6 dB if it is completely constructive�
Multipath can also corrupt the angle estimate by producing a false reflected target at the same range but at a slightly different angle� Most angle estimation techniques are degraded when multiple targets are present in the same range cell because they are unable to distinguish between the two targets� This usually results in a large estimated angle error for the actual target� As mentioned previously, multipath degrades the target elevation angle estimate� Using a narrower beamwidth reduces the effect of multipath errors� Signal processing techniques can be used to improve the target elevation angle estimate in multipath environments�
Jamming in the sidelobes can reduce the target signal-to-noise ratio and tracking accuracy� The best approach to minimizing the effects of sidelobe jamming is to use a low sidelobe receive pattern that essentially has low sidelobes everywhere� In addition, sidelobe cancellation can be employed to further reduce sidelobe jamming levels� Sidelobe cancellation can be easily implemented in digital beamforming antenna architectures� Cancellation of sidelobe jamming becomes more difficult as the bandwidth over which the jamming must be canceled increases� Therefore, cancellation of sidelobe
jamming will generally be more effective for narrowband search and track modes as compared to high-bandwidth target imaging modes�
Mainbeam jamming can potentially be overcome by increasing the effective transmit power� This can be accomplished by integrating over a number of pulses to pull the target return out of the jamming� When the target and jammer are in the main beam but have some angular separation, techniques exist to perform main beam nulling to cancel main beam jamming� Main beam nulling can be incorporated in digital beamforming as well as analog beamforming architectures� In general, good target tracking accuracy performance is difficult to achieve in main beam jamming scenarios�
The target position accuracy is achieved primarily by the antenna and signal processor requirements� The antenna beam must be narrow enough to support angle accuracy requirements for a reasonable number of target updates� The angular accuracy becomes more important with longer target engagement ranges as the magnitude of the cross-range error associated with a fixed angle estimate increases with target range� The signal processor must be capable of canceling main beam clutter while antenna transmit and receive sidelobes must be capable of suppressing the sidelobe clutter� The combination of the antenna architecture and signal processing must also be capable of sufficiently suppressing sidelobe jamming through the combination of low receive sidelobes and adaptive sidelobe nulling�
8.2.1 Terminal Homing and Guidance
The AMD system declares the time or range to go for transition from midcourse guidance to terminal homing for any given engagement� The interceptor terminal homing phase actually begins when the AMD system estimate of time or range to go before the predicted intercept point matches the declared time or range to go� This point in time and range to go defines the end of midcourse guidance and handover� The guidance package is where the flow-down of requirements begins�
The interceptor guidance package consists of a seeker with optics or an antenna, radome/irdome and its associated control and gimbal drive group, and a signal processor� The guidance package contains a rear reference receiver with uplink/downlink capability and its associated rear-facing antenna and a feed line� These components as a set receive RF/IR energy originating at or reflecting from target sets, receive uplink messages, reduce the received energy to target directional information, and provide guidance acceleration commands to the control and steering section to cause the interceptor airframe to fly a minimum energy course that will satisfy the
Pssk requirements� In other words, the objective of the guidance and control system is to bring the interceptor within the specified miss distance in such a manner as to minimize the degrading effects of noise and error influences (e�g�, clutter, spillover, or reflection) and to satisfy the kill probability for a specific kill strategy (e�g�, warhead or hit to kill)�
To allocate guidance accuracy requirements, the interceptor’s miss distance error budget contributors versus interceptor performance measures are itemized� For example, in order to establish accurate heading error estimates, midcourse guidance, control, and navigation must be properly itemized� Heading error is a contributor to miss distance and is folded into the remaining miss distance error sources and so on� Properly accounting for heading error, established at handover, requires accurately predicting available homing time/seeker acquisition range, accurately accounting for the homing loop time constant, accurately predicting interceptor time-dependent speed and acceleration limits, and correctly predicting the guidance commands� Guidance commands will need to be provided in the presence of various noise sources to include target fade, scintillation, glint, and electronic countermeasures, and the guidance package is required to minimize the possibility of homing on unassigned targets, flight associated debris, and debris of targets previously destroyed by another interceptor�
Complications occur such as if the target begins to maneuver, then the homing loop time constant is no longer constant or linear� This requires an additional level of design and modeling to account for the flight control system time response and margins properly capturing the varying and nonlinear effects maneuver will have on the homing loop time constant�
Moreover, during the terminal homing phase, the guidance package is required to modify the trajectory and/or guidance law and/or gains for low-, medium-, or high-altitude engagements�
When the interceptor has approached the target sufficiently close to arm, it will enter the intercept phase� During this phase, the fuze or target detecting device (TDD) should detect the target and detonate the warhead assuming that a warhead kill is sought�
8.2.2 Launch and Flyout Phase
We will assume that AMD interceptors are launched from vertical launchers and have a guidance-free flight period and a preprogrammed pitch over period� During these periods, no guidance commands are generated and aerodynamic control and/or the thrust vector control (TVC) actuator systems are disabled for the time required to clear the launcher, and then control is activated to stabilize the interceptor in vertical flight for the time required for the interceptor to clear the launch area� After clearing the launch area, a controlled pitch over places the interceptor on the correct flight path� Assuming a roll attitude-stabilized interceptor after achieving the required pitch over
maneuver and airframe stabilization, the airframe is rotated to the preferred steering orientation normally set prior to launch, and then the roll rate is nulled for roll attitude-stabilized systems� Once this final, nulling of roll rate, launch activity is achieved, the interceptor transitions to either midcourse guidance activation or homing if the interceptor is in a home-all-the-way mode�
During flyout, the rocket motor stack provides the kinematic performance to achieve the time to intercept and the minimum capability against maximum range and maneuvering target requirements� The propulsion system propellant, volumetric properties and hardware design, and number of stages constitute the degrees of freedom for propulsion requirements flowdown� The propulsion section design trade space contains sizing the necessary stages to boost the missile to the high-end speed and sizing the sustainer system to maintain the speed the airframe requires for affecting a successful intercept against the targets at the ranges specified in the requirements�
The flight control system (FCS) is required to stabilize the airframe and affect changes in direction or maneuverability as directed by the guidance computer� The interceptor is designed for achieving directional accelerations with an established magnitude and time requirement set either in a single aerodynamic surface or in a combined plane maneuver� The structural limitation of the interceptor guidance package or other subsystems may require the actual interceptor performance to be limited to a lesser maneuverability capability than the airframe will permit and may drive the flow-down of FCS requirements� The FCS has several components to include the controller computer that provides control and stabilization of the interceptor� The controller section receives commands from the guidance computer (sometimes when combined, these components are called an autopilot; here, we refer to this package as the G&C unit) to command the aerodynamic control fins and/or the TVC vanes to guide the interceptor on a target intercept course� The controller computer provides control authority for pitch, yaw, and roll stabilization� A servo control unit (SCU) contains the control and communications electronics necessary to convert commands from the controller to commands to the actuator system(s) and then provides controller feedback on aerodynamic control fin and/or TVC positions�
The actuator system(s) accept commands from the SCU and transforms these commands to the mechanical sources required to move the aerodynamic control fins and/or TVC vanes so that the interceptor flies the trajectory commanded by the G&C unit�
The inertial reference unit (IRU) package contains a set of gyroscopes and accelerometers used to provide relative position, motion, and accelerations as feedback to the G&C unit� The instrumentation in the IRU package must have the bandwidth to supply feedback to the G&C unit that will enhance achieving the rigorous interceptor stability and control requirements throughout the engagement space� The three IRU package design metrics are accuracy, precision, and bandwidth� The IRU package has to be designed to interface with G&C during both terminal and midcourse guidance�
The interceptor weight and balance requirements are part of the trade space to achieve flyout design requirements� Moreover, the interceptor centers of gravity and moments of inertia parameters are important metrics to the launching system and storage and handling requirements�
The interceptor body diameter and the interceptor length with and without in-line boosting systems and TVC units are part of the flyout design trade space, in that the effects of aerodynamic metrics are impacted to include drag� Moreover, the total interceptor dimensions, including aerodynamic control fins and TVC unit, if used, are required to fit within the volumetric constraints of the launching system� Fixed aerodynamic surfaces are part of the flyout design trade space that can be used to adjust the interceptor center of pressure and improve body lift over the engagement envelope� The aerodynamic control fins are sized to maintain control authority throughout the flight envelope and provide the aerodynamic gain required to stabilize and guide the interceptor to target intercept� The aerodynamic control fins may be foldable such that the interceptor will fit into the launching system�
