Continued from page 5 approach landing radar on an aircraft carrier — as compared to a periscope detection radar on a destroyer. Typically a surface warship has at least a surveillance/ search radar and an anti-air-defense/firecontrol radar. These two radar systems provide the ship with the ability to detect, track and engage a variety of threats. Through means of volume search and longrange detection, shipboard surveillance/ search radars provide a total air picture to the surface warship. These systems (first fielded during the Second World War) typically operate at lower frequencies in order to achieve enhanced search capability at a lower system cost. Although the basic function is the same (i.e., detection), these systems have undergone a significant evolution from their first introduction through to the next-generation systems that are currently under development. The requirement to operate in littoral regions, coupled with significant increases in aircraft speed and traffic, has effected this steady evolution, which could only have been realized because of significant advances that took place within RF technologies. The antennae used in these radar systems are no longer mechanically steered, but rather use a phased array with electronic steering, which directs the radar beam itself. A phased-array antenna provides faster beam switching so the system can track more targets while increasing information update rates. Individual tube-based transmitters and receivers are replaced by thousands of 6 RADARS solid-state transmit/receive (T/R) modules embedded in the phased-array antenna, resulting in greatly improved sensitivity. This allows the radar system to detect targets at greater distances. The fidelity of the transmitted and received RF signal is also improved, allowing the radar system to detect smaller cross-section targets. Anti Air Warfare (AAW)/fire-control radars, operating at higher RF frequencies for improved angle accuracy, detect and track low-altitude airborne targets. If the target is classified as a threat, the radar can be used to direct naval fire against that target. The first fire-control radars were fielded during World War II and were used to direct naval gunfire against surface and airborne targets. With the advent of missile technology in the 1970s, fire-control radars moved from directing gunfire to guiding missiles. To support this new requirement, a phasedarray antenna replaced the mechanically steered antenna in the fire-control radar. Adjunct illuminators, used for missile guidance, were added to the system. With the ability to track multiple targets and provide faster update rates, and the ability to guide missiles against airborne targets, the firecontrol radar steadily evolved into its current AAW role. As threats continued to evolve (targets with smaller radar cross section, increased range and greater maneuverability/speed), advanced RF technologies have steadily made their way into AAW radar systems in order to effectively counteract these new threats. Not unlike the next-generation surveillance radar, the next-generation AAW shipboard radar system is under development today with state-of-the-art RF technology. The radar systems for tomorrow’s surface warrior are under development today at Raytheon. These defense systems rely on the latest RF technologies to improve radar performance against an ever-increasing number of threats occurring in operational environments. In addition to achieving improved radar system performance, these advanced RF technologies are enabling next-generation radars to perform a host of multi-function roles. This, in turn, allows the development of a more capable surface defender, with improved survivability at a greatly reduced cost. The multifunctional capability of these next-generation systems also reduces RF interference throughout the ship by sharply reducing the number of operating systems. Airborne Radar Since the third decade of flight, airborne radars have been providing information to pilots about the world surrounding the aircraft. This information has enabled pilots to perform their job better, be that navigation, weather avoidance, or tasks with direct military application and usefulness. From the original 1934 patent by Hyland et al., Raytheon and its various companies have been at the forefront of radar technology development for airborne applications. In the simplest form, the purpose of a sensor is to provide useful data to the user (for example, a pilot). Other examples of useable data are situational awareness, kill-chain support and intelligence, surveillance and reconnaissance (ISR). Raytheon’s airborne radars provide that kind of information today, better than ever before. Situational Awareness consists of information about the environment, and the objects in it, that surround a user. For a pilot user, many kinds of information about the pilot’s surroundings are useful as an aid to navigation. For example, terrain following, terrain avoidance, radar altimetry, precision velocity updating, landing assistance and weather avoidance all assist the pilot in flying the aircraft. Additionally, man-made objects are of primary interest! Raytheon’s airborne radars provide greater detection and tracking ranges of a greater number of targets than ever before achieved. Kill-chain support is another type of useful data provided by advanced, multi-mode Doppler radar systems found on the current generation of fighter and attack aircraft.
Radars aboard the F-15, F-14, F/A-18, AV8B and B2 all provide kill-chain support in addition to situational awareness. The classical kill chain is denoted as find, fix, target, track, engage and assess (referred to as F2T2EA by the user community). The modern multi-mode Raytheon radar finds and fixes targets on the ground and in the air by using Doppler search modes for moving targets, and imaging modes for fixed targets. Once a target is located, it is targeted and tracked using additional waveforms. Targets in track can be engaged, with radar providing targeting information and weapons support. Finally, engagement effectiveness can be assessed through imaging of a fixed site or termination of the track of a moving target. A third type of useful information is intelligence, surveillance and reconnaissance. The user of this data is as likely to be a ground commander as it would be a pilot. Raytheon’s HISAR, ASARS-2A and Global Hawk radars provide imaging and movingtarget information of a region of interest on the ground. Similarly, Raytheon’s APS- 137 radar on the Navy P-3 Orion, as well as the international maritime radar, SeaVue, provide location and tracking information of maritime targets. All of these modern, multi-mode ISR radars provide location, tracking and identification of targets to the battle field commander or the pilot. Airborne radars are undergoing several major, capability-enhancing revolutions. A simple abstraction of a radar system might be to view it as an RF transmitter and receiver, a data processing unit and a directional antenna. Today’s analog transmitters and receivers are being replaced by programmable, digital receiver-exciters, similar to those found on the APG-79. These receiver-exciters offer the ability to support a wide variety of radar functions, with the ability to add growth functions while under development. In the same way, the airborne radar data processor is undergoing a veritable explosion in capability, with the commercial field expanding its capabilities by 100 percent approximately every 18 months (a phenomenon referred to as ‘Moore’s law’). This increase in processing throughput and storage is affording far more sophisticated radar functionality. Finally, the radar antenna itself is also undergoing a major change. Earlier, mechanically steered arrays are being replaced by the Active Electronically Scanned Array (AESA). AESA antennas, as first deployed on the APG-63(v)2, provided inertia-less beam pointing, permitting the radar systems engineer to design functions that can move the beam more rapidly. Advantages such as increased sensitivity and tracking capability result in improved situational awareness. Predicting the future of airborne radars is not difficult. As we extrapolate from the past, the future will require even better quality user information. Greater tracking precision and finer imaging resolutions are currently under development. Larger quantities of hard-to-find targets will populate future battlefields, and Raytheon’s research is addressing those needs. Fused sensors (both Radio Frequency and Electro-Optical,) will allow for enhanced effectiveness as recently demonstrated by Global Hawk during Operation Enduring Freedom and Operation Iraqi Freedom. Additionally, the lines between RF functions are continually blurring, with radars providing Electronic Support Measures and communication functions. The future holds capabilities not envisioned by Roddenberry’s Star Trek. Missile Radar Missile radar seekers were a natural derivative of radar technology developed for fighter aircraft. Once radar was incorporated into fighters, it became quite apparent that the aircraft could locate a target, but it was virtually impossible to destroy the target at any appreciable standoff range, using bullets or unguided missiles. In order to engage the target, some sort of closedloop control of the missile would be needed. The first radar-guided, air-to-air missile developed (in the 1940s and ’50s) was the Falcon missile. The Falcon was guided to the target by ‘homing in’ on RF energy bounced off the target by the fire control radar. This type of missile-seeker radar is referred to as a semi-active radar. The semiactive concept continues to be a valuable operating mode for a number of presentday missiles. But as technology continued to develop, more and more capability was integrated into missiles. Today’s missile radars are closely related to fire-control radars. Modern missile radars adapt the waveform parameters, receiver configuration and signal processing for the mode of operation in use and the missile’s environment (though it should be noted that no one missile does everything). Some missile radars perform air-to-air targeting and others perform air-to-ground. Radar-guided missiles use radar sensors for detecting and tracking both air and surface targets. These radar sensors provide specific target information that is used to guide the missile. The missiles also employ RF communication links, GPS receivers and RF proximity fuzes for detonating the warhead when the missile passes close to the target. Current missile RF-guidance technology operates primarily at microwave frequencies (3-30 GHz). For the guidance function, a forward-looking sensor, employing either a reflector antenna or a waveguide array antenna, is mounted on an electromechanical, gimbal-controlled platform. An aerodynamic nose cone or radome,that is transparent to RF energy protects the antenna. The RF signals originate either from a transmitter on the missile (in an active system), from an illuminating radar on the launch ship, ground system or aircraft (in a semi-active system) or, alternatively, from the target itself (in a passive system). Signals are reflected from the target (or originate from the target), and are received via the missile antenna and Continued on page 8 7