airborne, are uniformly configured as electronic-scan, phased-array architectures with all-solid-state microwave power generation. The SPY-1 Aegis radar, the workhorse of the cooperative engagement capability (CEC), represents a pioneering naval implementation of a phased-array radar, albeit with a conventional centralized microwave tube power source. With four fixed-array faces, its electronic scan provides 360° coverage for ship self-defense via search, track, and weapons control.

Today, the Navy is considering the development of at least four new radars—the multifunction radar (MFR), an X-band radar for short-range ship defense; the volume search radar (VSR), an L-band radar for medium-range search and cueing to replace the SPS-49; the high-power discriminator (HPD; an X-band radar for ship-based theater missile defense); and a possible future long-range multifunction C/S-band replacement for the current Aegis radar. All are envisioned to be electronic-scan, phased-array architectures with active monolithic microwave integrated circuit (MMIC) solid-state transmitter/receiver (T/R) modules.

These surface platform-based sensors are capable of producing “images” of the surrounding air and sea surface space in the traditional radar sense of a georeferenced “map” of the estimated locations of significant observed returns. For an isolated radar, operating in a platform-centric mode, the precision with which the locations of these target reports are defined is of mixed quality—for although a radar usually can provide high-precision range and Doppler measurements, the angular precision is generally poor because of the large radar wavelengths and the dimensions of practically sized radar antennas. Beam widths measured in degrees or finite fractions of degrees are not uncommon. At the range of the target, even for modest ranges of 10 or 20 km, the positional uncertainty perpendicular to the radar beam could be measured in tens to hundreds of meters, whereas the range uncertainty along the beam direction could be less than 1 m. Exploiting the combined measurements of a number of dispersed radars immediately provides a quantum jump in radar-imaging capability without any change in the participating radars’ operational characteristics.

Synthetic aperture radar (SAR) accomplishes much the same thing with a single radar sensor, in a different way—by moving it, with a crucial difference in the point of view—looking down rather than up. When an airborne radar is moved along a linear path and appropriate sequential measurements are made from a number of different spatial positions, the accumulated data can be combined in such a way as to duplicate the performance of a virtual antenna equivalent in size to the distance the platform flew during the collection of the data. The resulting radar images of the ground are of “optical quality,” with uniform meter to submeter resolution in all dimensions, and can be obtained over large surface areas, at ranges up to hundreds of kilometers, through almost any kind of weather. SAR sensors are currently available in the battlefield on joint resources such as the Joint Surveillance and Target Attack Radar System (JSTARS) (APY-3), the



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