Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Feature Multichannel Phase-Coherent Microwave Measurements: 5 Things To Consider Engineers working on radar and electronic warfare systems should familiarize themselves with these key elements of phased array operation. By Abhay Samant I n the area of military and defense electronics, electronic warfare (EW) and radar are two of the many applications that rely on multichannel and phase-coherent configura- tions for signal processing, analysis, and generation. These systems enable cutting-edge performance in vital aspects of EW and radar applications, including radar cross-section (RCS) diversity, improved target localization and tracking accuracy, higher angular resolution, increased degree of free- dom, increased waveform diversity, and increased number of uniquely identifiable targets. In essence, they form the back- bone of a new generation of military and defense systems focused on multiple-input, multiple-output (MIMO), and intel- ligent wireless communication. Understanding Military and Defense Application Requirements Radar antenna architectures can be classified into three key categories: conventional rotating antenna (dish antenna), pas- sive phased array, and active phased array. The dish antenna is attractive because of the frequency diversity it offers and its low cost of implementation. However, it has some challenges, including a slow scan rate, high distri- bution loss, and a single point of failure. The MIT LL Millstone Radar is a classic example of dish radar, operating at a center frequency of 1,295 MHz with 8 MHz bandwidth. Passive phased array radars offer advantages like beam agility and effective radar resource management. Phased array radar technology has evolved in the last two decades, start- ing with the B-1B and JSTARS airborne radars, as well as the Patriot surface radars. Differing from conventional rotating radar, the phased array is suited for multimission capabilities, permitting a variety of targets to be observed simultaneously and with a high degree of fidelity. Applications for active phased array radars, which offer increasing beam agility and efficient use of radar resources, are being widely investigated. Each element in an active phased array radar system contains an amplifier and a phase shifter. The latter is added to each antenna element, which helps improve the sensitivity of the direction-finding algo- rithms. Multiple antennas are combined to enhance radiation and shape pattern, a fundamental technique of phased array radars called array beamforming. The array elements are designed in such a way that their electric fields interfere con- structively in desired directions and cancel out each other in the remaining space, as shown in Figure 1. Researchers also have shown how the resolution of a broadside linear array, which is an antenna in the form of an array of radiation elements, can be improved by increasing the number of elements. While 16-channel and 32-channel beam- forming solutions have been deployed in practice, researchers are pushing the boundaries to explore what is possible by increasing the number of antennas by orders of magnitude. In addition to phased array radars, MIMO radars represent another application pushing the need for tightly synchronized multiple channels. The underlying idea — similar to how MIMO wireless devices work — is to simultaneously transmit different (but possibly correlated) signals from each antenna, use transmitted and received signals to produce virtual MIMO response, and then translate MIMO response into a bearing- range image. One of the benefits of the MIMO radar technique is that it enables the use of sparse arrays while maintaining side-lobe levels. Active research and prototyping topics in this area include MIMO aperture improvements and various wave- form optimizations for improved SINR (signal-to-interference- plus-noise ratio) and angle estimations. All of these requirements pose significant challenges to the design, prototyping, and testing phases of MIMO sys- tems. One example of this challenge is demonstrated by the National Weather Radar Testbed (NWRT), developed by engineers and scientists from the University of Oklahoma and the National Severe Storms Laboratory in Norman, OK. This testbed can digitize radar signals from eight channels and enable adaptive fast-scanning techniques and space antenna interferometry measurements. Such measurements are used Electronic Military & Defense Annual Resource, 5th Edition 20 Figure 1: Electromagnetic waves on each array element interfere in a construc- tive or destructive manner to form the desired radiation pattern.