A. K. AGRAWAL et al. Active Phased Array Antenna Development for Modern Shipboard Radar Systems Ashok K. Agrawal, Bruce A. Kopp, Mark H. Luesse, and Kenneth W. O’Haver Current and future Navy radar requirements are driven by rapidly evolving threats, including both cruise missiles and tactical ballistic missiles. To address these threats, array antennas will have to operate over wider bandwidths with enhanced sensitivity, higher radiated power levels, improved stability, and improved electronic protection to address reduced target radar cross-sections. In addition, there is a growing need for reduced array signatures and a practical need to control costs, including acquisition, operational, and support costs. Active phased array antennas have emerged as a funda- mental technology for addressing these evolving Navy radar system needs. APL’s Air Defense Systems Department has long been at the forefront of phased array antenna development for shipboard radar systems, and the Department is contributing to the development of active array antennas for the new generation of Navy radar systems cur- rently under development. This article provides an overview of the emerging active array antenna technology. INTRODUCTION Shipboard radar systems typically must provide sur- required to perform discrimination and target identifi- veillance of thousands of angular locations and track cation functions. At the same time that radar demands hundreds of targets and guided missiles, all within rela- are increasing, there is a practical need to reduce tively short reaction times. These requirements can be acquisition and operation and support (O&S) costs, met only with phased array antennas that allow elec- improve reliability, and reduce manning requirements. tronic repositioning of radar beams to widely diverse Active phased array antennas are emerging as a funda- angular locations within microseconds. Over a 40-year mental technology for addressing this evolving Navy span, APL’s Air Defense Systems Department has par- radar system need, and APL’s Air Defense Systems ticipated in the develop ment of phased array antennas Department is playing a major role in these develop- for Navy radar systems.1 ment efforts. In addition to enhanced sensitivity, improved system Although the concepts of phased array antennas stability will be required to detect low-flying cruise are fairly straightforward, the factors that determine missiles in sea or land clutter. Wider bandwidths will be the design are extensive and even somewhat complex. 600 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 22, NUMBER 4 (2001) ACTIVE PHASED ARRAY ANTENNA DEVELOPMENT Design factors such as aperture sizing, pattern synthesis, military radar systems. A radio-frequency (RF) block and beamswitching speeds have changed little. On the diagram of a typical passive phased array antenna is other hand, solid-state component technology develop- shown in Fig. 1a. A centralized transmitter, which gen- ment has exploded over the last decade, exhibiting a erally consists of high-power microwave tubes (e.g., continuing and significant impact on the design and traveling wave tubes) or cross-field amplifiers, provides performance of phased array antennas. The heart of the power to the radiating elements through a high- an active phased array antenna is the transmit/receive power beamformer network. High-power ferrite or diode (T/R) module. A T/R module at each radiating ele- phase shifters are controlled at each radiating element ment provides power amplification during transmit and to electronically steer the beam to the desired angle. In low-noise amplification during receive, as well as phase receive mode, the outputs of the radiating elements and shift control for beamsteering. The emergence of gal- phase shifters are combined using a low-power beam- lium arsenide (GaAs) monolithic microwave integrated forming network. Typically, three simultaneous receive circuit (MMIC) technology has enabled the develop- beams are provided to support monopulse tracking. ment of T/R modules with the required performance, Low-noise amplifiers (LNAs) are used to amplify the excellent reliability, and acceptable cost in quantity signal at the output of the beamformers. One of the production. An active phased array radar can (a) Transmit provide orders of magnitude perfor- beamformers mance improvement over its pre- decessor passive phased array radar, Central while at the same time improving transmitter reliability and reducing total own- ership costs. Virtually all high- performance radars under develop- ment today employ an active array antenna. Navy radar development programs for which active phased arrays are a key enabling technol- ∑ AZ delta ogy include the AN/SPY-3 multi- function radar, volume search radar Receive for long-range surveillance, and beamformers advanced radars for Navy Theater Wide Ballistic Missile Defense. EL delta High-power Other active array radars include Delta delta phase shifter the Theater High-Altitude Area Defense, National Missile Defense, and High Power Discriminator radars and the F-22 and Joint (b) Strike Fighter fire control radars. Active array technologies have been used in commercial com- munications applications, includ- T/R ing Iridium and Globalstar systems; module however, these systems have not Sum transmit T/R so far proven to be economically module AZ delta viable. Sum receive T/R Horizontal module combiner Vertical column Radiating ACTIVE PHASED combiner EL delta element ARRAY ANTENNA T/R OVERVIEW Delta delta module To fully appreciate what active array technology has to offer, it is Horizontal useful to first review the conven- combiner tional, or passive, array approach Figure 1. (a) RF block diagram of a passive phased array antenna. (b) Beamformer archi- currently deployed in several fielded tecture of an active phased array antenna. JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 22, NUMBER 4 (2001) 601 A. K. AGRAWAL et al. best examples of passive phased array radars is the reduced, resulting in increased radar sensitivity for a AN/SPY-1 radar (Fig. 2), which has been in service given amount of generated microwave power. for more than 20 years and is the Navy’s highest- Key radar system–level advantages of active phased performing fielded radar. arrays over passive phased arrays are summarized as Passive array systems have several inherent perfor- follows: mance limitations and inefficiencies. For example, the • Increased sensitivity. Lower transmit and receive transmit beamformer typically has significant losses, and beamformer losses, coupled with the ability of solid- the transmitter must generate a large amount of power state T/R modules to operate at higher duty cycles to overcome these losses. In essence, a significant por- than conventional tube-based transmitters, generally tion of the RF power generated by the transmitter is enables order-of-magnitude improve ments in radar dissipated as heat before being radiated. High-power sensitivity. centralized transmitters usually employ microwave tube– • Improved target detection in clutter. In an active based technologies, operate at lower duty factors, array, key sources of transmit noise and instabilities and have limited waveform flexibility. High receive (e.g., T/R modules and power supplies) are distrib- beamformer losses, particularly when low sidelobes are uted at the aperture. Consequently, their noise con- required, significantly degrade receive sensitivity. Also, tributions do not add coherently in the same fashion transmit noise from a centralized source is often a limi- as the transmitted signal, and their contributions to tation in clutter-driven radar applications. Finally, high- pulse–pulse variations undergo an averaging effect. power tube-based transmitters and their attendant high- The result is a significant improvement in the ability voltage power supplies have lower reliability and higher of an active array radar to detect small moving tar- maintenance and replacement costs than solid-state gets in sea or land clutter. technology. This last issue is particularly important for • Improved waveform and pattern flexibility. The shipboard applications that involve relatively long mis- multiple functions of detection, tracking, target iden- sions and a strong desire to avoid at-sea maintenance. tification, illumination, kill assessment, and missile Evolving threats are driving the need for order-of- communications can be better optimized by the magnitude improvements in radar performance. Active waveform flexibility that the solid-state active array array technology offers the capability of achieving the technology facilitates. Also, because both amplitude required performance improvements while at the same and phase control are provided by the T/R modules time offering improvements in reliability, maintainabil- at the element level, radiation patterns are more ity, availability, and life-cycle costs. In active arrays, readily optimized for the radar mode of operation, both transmit and receive functions are moved to the including the use of null synthesis techniques. aperture by placing a T/R module at each radiating ele- • Improved wideband operation. The solid-state tech- ment (Fig. 1b). The T/R modules provide power ampli- nology employed by active arrays can support inher- fication during transmit and low-noise amplification ently wideband microwave frequency operation. In during receive, as well as amplitude and phase control addition, active array architectures are conducive for beamsteering