ABSTRACT
A missile defense system essentially has four functional requirements that combine to provide defense against an incoming missile� These elements require balancing and, as such, this presents the missile defense systems with engineering challenges� These elements are intelligence, surveillance, and reconnaissance (ISR); detection and tracking; weapons control; and engagement� Ballistic missile defense is divided into three parts: boost, midcourse, and terminal intercept engagement phases� Cruise missile defense is typically divided into three components: area, self-, and point defense phases� Self-defense and point defense phases may utilize the same system components but have different mission requirements� These elements and missions combine to provide a layered defense capability to maximize defense performance�
One of the most effective defense penetration techniques is to collapse the battlespace by minimizing the engagement time available� The primary techniques available to collapse the battlespace for the offensive missile designer to exploit are [1] speed, altitude, and radar cross section (RCS)� The defense system in turn must utilize faster missiles, elevated and more powerful ISR sensors, and radars with data links and sophisticated signal processing techniques to counter these offensive techniques�
After collapsing the battlespace, the offensive missile designer needs to drive down the probability-of-kill (Pk) or probability-of-raid annihilation (PRA) for the defensive systems that have obtained an engagement opportunity [2]�
Offensive missile tactics and raids can be used to reduce Pk or PRA� Tactics can include jamming and maneuvers either in combination or separately [1-9]� Jamming is employed to delay detection by the radar and missile seeker and to deny the radar and missile seeker accurate range and angle estimates�
Evasive maneuvers are one of the most, if not the most, effective tactics used to evade defensive weapons such as missile and radar-directed gun weapon systems [7-9] and bring down Pk�
Raids are used to saturate and confuse the defensive systems and can be stream or simultaneous [3]� Stream raids are a series of missiles on the same trajectory with some time spacing between the individual missiles� Simultaneous raids are designed such that all offensive missiles arrive at the target almost simultaneously� An example of a simultaneous raid is an azimuth raid where the offensive missiles are separated in azimuth angle but arrive concurrently [3]�
The offensive missile speed is a defense penetration fulcrum that will synergistically add to the weapon’s ability to collapse the battlespace and reduce the Pk/PRA of a defensive system� For example, if an inbound offensive missile is traveling at Mach 1 and is detected by the radar at a slant range of 56 km, when can it be engaged? The system reaction or latency time determines how quickly a missile can be launched against an offensive missile once it is detected� If the system reaction time is 10 seconds, the offensive missile will travel 3�43 km or 1�85 nautical miles (nm) in 10 seconds to a slant range of 52�13 km or 28�15 nm� If the missile used to engage the offensive missile also flies at Mach 1, the offensive missile can be engaged at 26�08 km or 14�08 nm after a 76 second flyout time� If the weapon system can fire a second shot 5 seconds later, a second engagement attempt can occur at 25�21 km or 13�61 nm in 2�5 seconds after the first attempt� The flyout time of the second missile is 73�5 seconds�
If the offensive missile is inbound at Mach 3 and detected at the same range of 56 km or 30 nm, how much is the engagement timeline reduced for the same missile? Now, the offensive missile will travel 10�28 km or 5�55 nm in 10 seconds to a slant range of 42�28 km or 24�45 nm� A Mach 1 missile can intercept the offensive missile after a flyout time of 32�95 seconds at a slant range of 11�30 km or 6�10 nm� The 3-to-1-speed mismatch between the offensive missile and the missile reduces the range of the first engagement opportunity by approximately 14�82-11�30 km (8-6�1 nm)� The second engagement opportunity occurs at 10�0 km or 5�40 nm in 1�2 seconds after the first attempt� The flyout time of the second missile is 29�16 seconds�
Finally, if the offensive missile is inbound at Mach 1 and a Mach 3 missile interceptor is employed, how does the scenario change? Now, the offensive missile is at a slant range of 52�13 or 28�15 nm when the first missile is launched� The offensive missile can be engaged at a slant range of 39�08 km or 21�10 nm after a missile flyout time of 38 seconds� Now, the 3-to-1-speed mismatch increases the first intercept range by 13 km or 7 nm relative to the first example� The second engagement opportunity occurs at 37�78 km or 20�4 nm in 3�8 seconds after the first attempt� The flyout time of the second missile is 36�76 seconds�
Clearly, mismatches in speed can be used to the advantage of the defensive or offensive weapon� The key is to stay technologically ahead of the offensive missile by developing higher speed weapons� The ultimate weapon in terms of speed is a directed energy weapon (DEW)� DEWs, such as a high-power laser or a high-power microwave beam, travel at the speed of light� Of course,
these types of futuristic weapons need to be very accurately directed particularly at longer engagement ranges� Moreover, one of the most important objectives in air and missile defense is to place as much energy on the target as far away from the asset being defended as possible� The energy penalty on DEW with respect to range is a steep cliff� In addition, adverse weather conditions such as fog or rain will likely reduce the lethality of DEWs in all but the shortest ranges�
The offensive missile can also use altitude to its advantage by flying below the radar horizon as long as possible [4,5]� This denies the radar detection of the offensive missile until it is visible above the horizon� The range at which the line of sight between the radar and the offensive missile becomes unobstructed is related to the height of the radar and offensive missile� At lower altitudes, the maximum unobstructed range can be approximated by
R(nm) = 1 xx(hradar + heightoffensive missile)1/2
where the number “1 xx” is an approximation that has to consider earth location and environmental factors such as whether you are over water or land� Clearly, the height of the radar must be increased to extend the radar horizon� If the radar height is increased by a factor of 4, the radar physical horizon can be doubled for low-flying offensive missiles such as “wave skimmers” or terrain following cruise missiles� For land-based and shipborne radars, the ability to increase radar height is limited� Land-based radars can be placed on elevated terrain (e�g�, hilltops) although this makes them more conspicuous to the enemy� Shipborne radar can be placed high on the superstructure or mast� The size and weight of the radar also need to be considered� In general, smaller and lighter weight radars can be placed at higher locations on a ship� Another consideration for the placement of ship-borne radars is placing the radar antenna(s) to minimize obstruction zones, particularly at lower elevation angles�
Radar cross-section reduction can be used to delay radar detection� In clear environments, the radar detection range is proportional to 1/r4 where r is radar slant ranges� This means that the radar must be 16 times (12 dB) more powerful to double the detection range against a fixed radar cross-sectional target� Radar range can be increased through a combination of increased antenna gain (larger antennas or higher frequency for fixed antenna area), increased transmitter power, or reduced losses� Active phased array radars reduce losses compared to passive phased radars by reducing both transmit and receive losses� This is accomplished with the use of transmit/receive (T/R) modules in a distributed architecture� Signal processing techniques can also be used to increase detection range� Noncoherent or coherent integration across multiple pulses can be employed to increase detection range at the expense of slower search frame times� Concentrating radar resources in the offensive missile sector can also be used to increase detection range�
Offensive missiles can reduce their radar cross section by employing shaping or radar-absorbing material (RAM) [4]� Surface shaping causes the energy in the transmitted radar pulse to be reflected in other directions and not directly back to the radar, causing the energy in the radar return to be reduced� The offensive missile can also use composite materials that absorb radar energy to reduce their radar cross section� Other techniques related to the offensive missile seeker involve tilting the seeker when it is not active and the use of a seeker radome constructed of a frequency selective surface (FSS)� Tilting the seeker will reduce the in-band forward aspect radar cross section� In-band refers to the offensive missile seeker band, which may be different from the air defense radar band� This is essentially a simple shaping technique that directs most of the radar energy at bistatic angles to minimize the monostatic radar cross section� The use of an FSS for the seeker radome material reduces the forward aspect radar cross section out of the offensive missile seeker band� The FSS is essentially transparent in the seeker band and reflective out of band� The out-of-band forward aspect RCS is reduced by the radome shape�
Stream and azimuth raids are employed to improve defense penetration capability [2,3]� Both raid types will place increased demands on the defensive systems’ firepower capability in an attempt to overwhelm the air defense system� The defensive systems can potentially counter these techniques with increased magazine capacity, increasing the number of engagements that can be prosecuted simultaneously and by reducing the salvo time (how rapidly missiles can be sequentially fired from a launcher)�
A stream raid is defined as a raid where a group of similar offensive missiles fly the same trajectory, but the trajectories are separated in time [2,3]� Modern radars should have sufficient range resolution to detect and track each offensive missile in the raid� However, sometimes, modern radar may ignore trailing targets that occur in the same beam or at the same indicated angle� An offensive missile can generate false trailing targets if it employs a repeater jammer with digital radio frequency memory (DRFM) [4-6]� The transmitted radar pulse is digitized, stored, and then retransmitted at fixed time delays to mimic false trailing targets� A Doppler shift can be added to the retransmitted pulse that gives the false trailing targets the same apparent inbound velocity as the actual offensive missile�
If the radar identifies each missile in the stream raid as an offensive missile, then special consideration should be given to the engagement solution� If possible, trajectories for the weapons used to engage the trailing offensive missiles should be shaped to avoid debris from the engagement of leading offensive missiles� Trajectory shaping will increase missile flyout times� Stream raid engagements will become more difficult for smaller stream raid time spacings and faster offensive missiles�
An azimuth raid is a raid where the offensive missiles fly toward a common target from different azimuth angles� The raid timing is such that all of the offensive missiles will arrive simultaneously� This scenario is judged
to be a worst-case scenario from an engagement timeline perspective� Most defensive systems do not have the ability to engage multiple offensive missiles in azimuth simultaneously� This is due to several reasons� All offensive missiles in the raid will not be detected simultaneously due to radar search patterns and the stochastic nature of the detection process� Usually, weapons can only be launched one at a time at an interval determined by the missile launcher salvo time�
The environment can and will have significant performance impacts on a defense system� The radar system and RF missile seekers have to contend with multipath, clutter, and jamming environments individually and in combination [2-6]� Multipath signals can interfere with direct path signals causing cancellation or fading for some geometries� In addition, multipath causes errors in the offensive missile elevation angle estimate, resulting in the uncertainty of the offensive missile’s true altitude� Clutter effectively raises the noise floor of the radar making target detection much more difficult without clutter cancellation� Noise jamming also raises the radar noise floor, reducing target detectability� The jamming power at the radar is reduced proportionally to 1/r2 where r is the range between the jammer and the radar� The signal level of the radar return has a 1/r4 since a two-way path is involved between the radar and the offensive missile (transmits and receives)� This relationship gives the jamming platform the ability to standoff at significant distances and still be effective� Other types of jamming, particularly jammers on board the offensive missile, try to deceive the radar and/or missile seekers range and/or angle estimates� These techniques are referred to as deceptive jamming�
The most benign environment is a clear environment, which is defined as a smooth earth surface and standard propagation conditions� In this environment, radar performance is limited by the radar horizon and multipath from the smooth earth surface� Multipath is an indirect path from the offensive missile to radar that involves a reflection from the earth’s surface� The indirect path is longer than the direct path, which results in a phase difference in the radar signals arriving via the direct and indirect paths� There is also a small time delay in the indirect path relative to the direct path due to the increased distance traveled by the radar signal� When the phase difference due to the combination of the path length differences and phase of reflection coefficient at the earth’s surface is 180°, the radar return is essentially canceled� At the other times of the phase, direct and indirect path radar returns can be in the phase, resulting in a 6 dB enhancement in target signal-to-noise ratio relative to the direct path alone� Radars or missile seekers that have wide
operational bandwidths can adjust their transmit frequency as a function of offensive missile range to avoid multipath cancellation� Approximately 30% operational bandwidth is required� Otherwise offensive missile tracks can potentially be coasted during range intervals where multipath nulls occur�
Surface clutter returns, such as those from land or sea clutter, result when the surface area is illuminated by the radar beam� The illuminated area is a function of range from the radar and is bounded by the range by the pulse width and azimuth or cross-range by the azimuth beam width� The contributions from the surface clutter in this illumination region integrate or combine to determine the received clutter-to-noise ratio, which is a function of range and the mean reflectivity of the illuminated clutter patch�
Volume clutter results from rain or chaff in the radar beam� The volume is bounded by the pulse width in range and the antenna beam width in azimuth and elevation� The contributions of the clutter in this volume combine to determine the received clutter-to-noise ratio, which is a function of the range squared and the mean reflectivity of the illuminated clutter volume in m2 per m3�
The clutter returns have a Doppler response� Land clutter is stationary and does not produce a Doppler response� Vegetation on the land such as grasses and trees will sway in the wind and produce small Doppler responses, which have some spectral spread� The Doppler component and spread of sea clutter is determined by the wind and sea state, which are interdependent� Whether the clutter is viewed from downwind, upwind, or crosswind directions also influence the Doppler components� Volume clutter, which is airborne, is also influenced by the wind speed and turbulence� Wind speed generally increases with altitude, therefore the Doppler component from volume clutter returns increases with the altitude of the clutter�
Signal processing techniques are used to cancel clutter� These techniques are based on the Doppler difference between the clutter and the target� Clutter typically has a low Doppler response compared to a high speed inbound offensive missile� Common clutter cancellation techniques are moving target indicator (MTI) and pulse Doppler (PD)� Both techniques require coherent processing of multi-pulse dwells and are ultimately limited by a pulse-to-pulse and intrapulse phase and amplitude instabilities in the radar system hardware�
Wideband noise or barrage noise jammers generally try to cover the radar operational bandwidth� For any given radar pulse or dwell, the instantaneous bandwidth used by the radar is only a fraction of the operational bandwidth� The noise in instantaneous bandwidth of the radar reduces the signal-to-noise ratio of the radar return from a target or offensive missile� If the noise jamming raises the radar noise floor by 10 dB, the signal-to-noise ratio will be reduced by 10 dB relative to a clear environment�
The jamming affects the return through the receiver antenna patterns sidelobes or mainlobe� Sidelobe jamming can potentially be canceled or mitigated by adaptive placing nulls in the receive sidelobes in the jammer
direction� Sidelobe jamming is automatically reduced by how far the sidelobes are below the main beam peak� For low sidelobe phased arrays, the receive sidelobes are typically 40-50 dB below the main beam peak� Sidelobe jamming cancellation becomes more difficult as the number of jammers increases, as the instantaneous bandwidth of the radar increases, and when the jamming occurs in the main beam�
Main beam jamming provides the greatest challenge to the radar designer� Brute force radar signal processing techniques sometimes referred to as burnthrough can be used to form skin tracks on main beam jammers� These techniques require multi-pulse integration to overcome the jamming� Burnthrough techniques can overcome main beam jamming at the expense of radar resources� Other signal processing techniques exist to provide main beam jamming cancellation when there is some angular separation between the target and the jammer� These techniques essentially place a main beam null in the jammer direction while maintaining the gain of the main beam in the target direction�
In real-world scenarios, radars and missile seekers must be able to operate in complex environments that include the combined effects of multipath, clutter, and jamming [2-6]� Missile seekers must also contend with terrain bounce jamming (TBJ) and towed decoys� Both of these deceptive techniques are designed to confuse the seeker angle estimates for the target [2-6]� TBJ is designed to make the seeker fly into the ground instead of engaging the actual target [5]� The towed decoy is designed to capture the seeker and have it attack the trailing decoy versus the actual target [2-6]�
