# Multifunction RF Systems for Naval Platforms
## 1 Introduction  
The evolving role of modern navies has required increasingly higher levels of capability in the Radio Frequency (RF) shipboard systems that provide radar, communications and Electronic Warfare (EW) functions, including in the latter case both Electronic Attack (EA) and Electronic Support (ES). The result has been a proliferation of topside antennas on naval vessels. It has been estimated that the number of topside antennas has roughly doubled on ships launched in the 1990s relative to those launched in the 1980s, with the antenna count on a typical 1990s-era destroyer for example being on the order of 80 [1]. This has led to a number of problems, including increased mutual electromagnetic interference, larger ship Radar Cross Section (RCS), and higher life-cycle costs associated with the operation of multiple unique RF systems. Since the late 1990s, the idea of MultiFunction RF (MFRF) systems has drawn considerable interest as an approach to addressing this issue. In a MFRF system, several RF functions are consolidated within a shared set of electronics and antenna apertures. Active Electronically Scanned Array (AESA) technology is a key enabler for these systems. A modern AESA employs a separate transmit (Tx) and/or receive (Rx) channel for each of its radiating elements, with a high-power amplifier (HPA) and low-noise amplifier (LNA) in each of the transmit and receive channels respectively. Often, there is also some type of beamforming element in each channel, such as a phase shifter or true time-delay (TTD) circuit. The HPAs, LNAs and beamforming elements are typically packaged into Monolithic Microwave Integrated Circuit (MMIC) modules that are incorporated in the array structure to be as close as possible to the radiating elements, thereby minimizing system losses. In general, theAESA architecture allows dynamic reconfiguration of the antenna aperture, including partitioning of the array elements into subarrays, to form multiple simultaneous transmit and/or receive beams in independent directions with different beam patterns and waveforms. This provides the level of flexibility that is required to support multiple RF functions with the same antenna aperture. The use of shared hardware in a MFRF system facilitates intelligent control of the RF functions with common resource allocation management software. In general terms, an intelligent Resource Allocation Manager (RAM) in a MFRF system performs the critical task of adaptively allocating system assets to the RF functions based on the dynamically changing priorities and resource requirements of these functions within a given mission scenario and sensed RF environment. System assets under RAM control broadly comprise waveform generators, AESAs, receivers, communication modems and signal/data processing resources. The waveform generators, receivers and modems are largely digital and software controlled, which accommodates rapid reconfiguration of these assets to provide the waveform and receiver characteristics required by the supported RF functions. A high-level conceptual diagram of a MFRF system is shown in Figure 1. The general configuration depicted has separate receive and transmit AESAs, the advantages of which are discussed in Section 3. However, for certain MFRF implementations, a single aperture that combines both receive and transmit functions may be more desirable. The figure illustrates the notion that different sections of the apertures can be used to form simultaneous independent beams allocated to different RF functions. In this instance, a transmit beam is being used by the radar to illuminate an incoming anti-ship missile for the purpose of supporting the ship's fire control system, while a second radar transmit beam is tracking a helicopter within its search volume. The EA function is utilizing a third transmit beam to jam the fire control radar of an approaching hostile fighter aircraft. With the receive array, a receive beam is formed in the direction of a satellite to establish a communication link.