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Missile Defense Radars

The term, RADAR, initially from RAdio Detection And Ranging, is defined as a device for transmitting electromagnetic signals and receiving echoes from objects of interest (targets) within its volume of coverage. Presence of a target is revealed by detection of its echo, or a transponder reply. Additional information about a target provided by radar includes one or more of the following: distance (range), by elapsed time between transmission of the signal and reception of the return signal; direction, by use of directive antenna patterns; rate of change of range, by measurement of Doppler shift, description or classification of targets, by analysis of echoes and their variation with time.

Antenna gain and directivity are a function of the physical size of the antenna, and the wavelength of operation. Specifically gain is ratio of the power radiated by an antenna in a given direction relative to the power that would be radiated by an ideal isotropic radiator (i.e., where the power is uniformly distributed without loss over 4p steradians).

A missile may be guided to a target by guidance signals developed from tracking data obtained either at a surface-based radar station or by radar means located totally, or partially, within the missile. The former system is commonly referred to as a command guidance system and the latter as a missile homing guidance system. For example, in a command guidance system where a missile is used to intercept an airborne target, a large, remotely located, high resolution surface-based radar system and high speed digital computer are provided for selecting one of a plurality of targets, tracking both the missile and the selected target, calculating proper guidance signals for the missile from the generated tracking data, and transmitting such signals to the missile to enable the missile to intercept the target.

In a missile homing guidance system, a smaller, light weight, lower power target tracking radar system may be provided for generation of both target tracking data and guidance signals. In one type of missile homing guidance system, both the radar transmitter and receiver are located in the missile and in another type of homing system the radar transmitter is located remotely from the missile while the receiver is located in the missile. The former missile homing system is referred to as an active guidance system and the latter as a semi-active guidance system. Generally, such semi-active guidance system includes a target tracking radar antenna gimballed within the missile and coupled to a control system (i.e., the antenna and control system therefore being referred to collectively as a missile seeker) for tracking the target and to thereby generate guidance signals for the missile.

In one type of missile system, the features of command guidance and semi-active guidance techniques are combined. During the early portion of the missile's flight, the missile is guided by a command guidance phase where the guidance command signals are developed by the digital computer operated in response to signals obtained by tracking both the missile and a selected target with a remote, high resolution, surface-based radar system. During the later portion of the flight, the missile is guided by a semi-active guidance phase where guidance signals are obtained by tracking the target with the missile-borne radar receiver fed by the gimballed antenna system which receives reflected power from target illuminations by the surface-based radar system.

Prior art radar systems have been designed to obtain high discrimination in angle and in range by monopulse of "beam splitting" methods. These methods are valid only if there is only one point target inside the "pulse volume" (pulse length times the cross section of the radar beam). These systems fail when there is more than one target in this volume. Ballistic missiles present at least two targets: the warhead and the last rocket stage. At an oblique angle of observation these may have nearly equal slant range. If many decoys are added, it becomes increasingly probable that monopulse beam sharpening fails because of simultaneous reception of plural target and decoy echoes. It is known that monopulse produces sharp localization of single point targets but does not improve resolution of extended or multiple targets.

The volume of space, in range, azimuth and elevation, over which a Tactical Ballistic Missile (TBM) early warning radar is required to search for incoming missiles is very large. This places very heavy demands on the radar designer, resulting in large, very high power, low mobility radars, with subsequent vulnerability to anti radar missiles and other defence suppression systems. This invention proposes an alternative approach to a TBM early warning radar which considerably reduces both the design and vulnerability problem.

In a ground-based TBM early warning system where the radar is located to the rear of a defended area, at the ranges at which detection is required, lofted trajectory missiles can be at elevation angles of up to 80 degs; depressed trajectory missiles can be at elevation angles as low as 10 degs. In order to keep scan rates high enough to cover the search area in a reasonable time, the radar must have an extremely high Equivalent Radiated Power (ERP) to achieve an adequate detection range.

For shorter range missiles, the wide elevation angle problem can be reduced by using an horizon scan (or "fence") to detect the missiles as they come over the horizon. However, unless extremely powerful radars are used, longer range missiles ascend beyond the range of the beam, and are undetected until they drop into the top of the beam, usually much too late to be engaged. Moreover, this attempt to detect the missiles at low elevation angle and long range extends the required azimuth search angle, greatly reducing the advantage gained from the small elevation search angle.

To increase the altitude coverage, a search radar may be moved further back from the defended area, away from the threat direction. However, it is clear that the range between the radar and the defended area represents an additional range performance demand on the radar ERP. For radars with a limited elevation scan capability, this is usually the only way to provide some degree of TBM detection capability.

As greater and greater ERP and/or dwell time is sought to counter longer and longer range TBMs, which have more and more flexibility in trajectory shaping, significant additional penalties are incurred:

a) The radars become larger and larger, and hence more and more cumbersome and immobile.

b) The large radars become easier targets to find and hit by conventional aircraft attack or visual stand-off weapons.

c) The large ERP requirements make the radars vulnerable to long range anti-radar missiles.

d) The radars are vulnerable to conventional electronic counter measures (ECM) aircraft at low elevation angles, particularly if using an horizon fence search.

For the threat TBMs of current interest (2000 Km maximum range), re-entry velocities up to 4 Km/Sec may be seen for lofted trajectories. However, for depressed trajectories, atmospheric drag limits the degree of practicable depression. It is unlikely, therefore, that 3 Km/Sec will be exceeded for a depressed trajectory. At comparatively high missile target elevation angles, missile time-in-beam is sufficient not to require scanning in this plane. For lower elevation angles however, this is not the case.

Electronically scanned antennas are a critical enabling technology for modern phased array radars. Primary mission applications have included air defense surveillance and track-while-scan radars, tactical ballistic missile defense, and counter-battery location systems. Planar ESA represent the mature state of the art, and so-called passive arrays (that is configurations where the signal is fed at full power to an antenna consisting of passive phase shifters and antenna elements) have been in use since around 1970.

Current emphasis is on the development of active arrays where signals are distributed at low-power and the final stage of amplification is integral to the antenna element itself. Non-planar conformal arrays represent the leading edge of the art, [and the design of broadband arrays with low-side lobe performance (an ECCM feature) is particularly challenging because of the difficulty of characterizing what are known as "mutual impedance effects" in a complex geometry].

The 420-450 MHz band is excellent for long-range search and surveillance and, to a lesser extent, target tracking. The large antennas required for good angular resolution generally limit operations to land or ship-based installations. The Air Force Ballistic Missile Early Warning System (BMEWS) has been the backbone of the U.S. missile defense system for over 30 years. The BMEWS radars are located in Clear, Alaska; Flyingdales, UK; and Thule and Sonderstrom AFB, Greenland, and are used for search and tracking. The radars operate in the 420-450 MHz band, and they have been modernized over the years. Technical improvements on the BMEWS radars may continue over the next 5-10 years. Spectrum requirements will continue over the next 10 years for the BMEWS. In the future, the BMEWS spectrum requirements for the 420-450 MHz band may be reduced if spaceborne radars can perform the early warning function.

The Air Force Pave Paws, or AN/FPS-115, radars are used for detection and tracking of submarine launched ballistic missiles (SLBM), and for satellite tracking. Four phased array radars were installed between 1982 and 1987 in the United States. Other elements of the Pave Paws system are the Perimeter Acquisition Radar which was developed in the 1960's and 1970's as a Safeguard anti-ballistic missile (ABM) radar and the AN/FPS-85 SPACETRACK radar. The AN/FPS-115 Pave Paws radars and the Perimeter Acquisition Radar use the 420-450 MHz band, and are expected to operate for years to come. The older AN/FPS-85 may be phased out. Some modernization may occur to these radars, and the spectrum requirements will likely continue.

The Navy operates the AN/SPS-49(V) as a shipborne air search radar onboard a variety of ships including all aircraft carriers and the AEGIS Ticonderoga class cruisers. The AN/SPS-49(V) is a modern all-solid state radar, and since a large number have been deployed in the fleet, it is likely that operations will continue through the next 10 years. Thus, the 902-928 MHz band will be a spectrum requirement for this radar over the next 10 years. DOD indicated that "Advanced research in radiolocation is being performed in the bands ...390-940... MHz." The Air Force has indicated that it "... uses the 902-928 MHz band to track missiles at test ranges, a capability that is important because it provides real-time tracking capability of manned and unmanned vehicles in test and training areas."

Adequate spectrum space is available in the 2700-3700 MHz bands, and good radar performance and angular resolution can be obtained by radars in this band with reasonably sized antennas. The external noise level is low, enabling long-range air search and surveillance radars to operate in the band. The good angular resolution and narrow antenna beams make the band attractive for military radars for the ability to reduce hostile jamming.

Cobra Judy, or the AN/SPQ-11, is a shipborne phased array radar designed to detect and track ICBM's launched by Russia in their west-to-east missile range. It collects data for SALT treaty verification. The Cobra Judy operates in this band, and a 9-GHz band capability was added in FY85. The spectrum requirements for Cobra Judy are expected to continue for at least the next five years. The AN/SPY-1(V) is part of the Navy shipborne AEGIS weapon system, and considered the Navy's premier fleet air defense system. The AN/SPY-1(V) is a 3-D phased array air defense radar used on guided missile cruisers and destroyers. The AN/SPY-1(V) operates in the 3100-3500 MHz band. Operations and spectrum requirements are likely to extend for many years.

The 5250-5925 MHz frequency range is allocated to the radiolocation service on a primary or secondary basis in six bands. These bands have some physical limitations that reduce their usefulness for long-range air search and surveillance. On the other hand, these bands are used extensively for test range instrumentation radars to track missiles and other targets. The Army's Patriot surface-to-air-missile (SAM) defense system includes the AN/MPQ-53 5-GHz band radar incorporating several phased array antennas. The multi-function radar provides search, surveillance, tracking, and missile guidance.

Good radar performance is achieved in the 8-10.55 GHz band with small antennas permitting numerous mobile applications on aircraft, missiles, ships, tanks, and other vehicles. Small hand-held radar systems are also used extensively. The band is well suited for short range search. The frequency band allocations are wide, permitting the use of narrow pulses with wide emission bandwidth to achieve good target resolution.

The Army's Ground Based Radar (GBR-X) is a new radar evolving out of the Upper Tier Theater Missile Defense Program which is part of the Ballistic Missile Defense (BMD) program, known as the Strategic Defense Initiative (SDI) prior to May 1993. The GBR-X, also referred to as the GBR-T and formerly referred to as the Terminal Imaging Radar, is a transportable radar operating in the 8.55-10 GHz bands. The radar will search and track enemy tactical ballistic missiles, cruise missiles, and other air-breathing threats. It will have fire control capability against such threats. The GBR Theater Missile Defense Radar requires spectrum in the 8.55-10 GHz band, and with adequate funding will likely require spectrum for many years to come.

The 30-300 GHz or EHF part of the spectrum is designated the millimeter wave (mmw) region. Advances in solid state and signal processing technologies in recent years have made the millimeter wave part of the spectrum more attractive for a number of applications. Small diameter antennas are possible offering some advantages such as narrow beamwidths. The narrow antenna beamwidth leads to other operational advantages such as increased immunity to interference and improved resolution.

One of the major disadvantages to operations at millimeter waves is the increased atmospheric attenuation, particularly from water vapor and oxygen. The atmospheric attenuation characteristics in the 10-300 GHz range vary widely and influence the choice of frequency bands. However, there are troughs or atmospheric "windows" of lower attenuation around 35, 90, 140, and 240 GHz, coinciding with some radiolocation service band allocations.

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