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AN/APY Side-Looking Airborne Radar (SLAR)

Side-looking airborne radar (SLAR) images are acquired by sending a beam of radar energy to the ground at an angle perpendicular to the aircraft's flight path. Unlike visible and near-infrared wavelengths, radar energy (at the correct wavelength) can penetrate most clouds, making it an especially useful tool where a persistent cloud cover obscures parts of the target.

Using Side-Looking Airborne Radar (SLAR) is an efficient, low-cost method of viewing and mapping terrains over a wide swath of territory on either side of the flight path of the carrier aircraft. Two SLAR antennas on either side of the aircraft illuminate a long, preferably narrow strip of the terrain with a high-powered short radar pulse, normally in the X-band of the microwave spectrum. As the radiated impulse power is reflected by the illuminated terrain and received by the now receiving SLAR antenna, the intensity and times of arrival of the reflections are processed electronically to produce an instantaneous terrain map. As the aircraft proceeds along its path the terrain map is updated.

As an example, a suitable radar pulse repetition frequency of 800 Hz could be used, with a pulse duration of approximately 250 nanoseconds. The quality of the terrain map depends strongly from the precision of the radiated illumination pattern. It is known in the art that a narrow beam in the horizontal plane (a so-called pencil beam in the azimuth plane) having its peak intensity along an axis perpendicular to the flight path and slightly inclined with respect to the horizontal plane, and illuminating the terrain with gradually declining intensity reaching a null underneath the flight path is required. Accordingly, the terrain is approximately uniformly illuminated irrespective of the distance from the antenna. A narrow beam in the horizontal plane is necessary in order to provide good azimuth resolution of the terrain of the strip just under the antenna as an illuminating radar pulse is emitted. Therefore, the far-field azimuth angle of the beam should be as small as possible, and the illumination intensity should decline from its peak at the near horizontal to the near vertical (downward from the aircraft) as uniformly as possible. These characteristics are, of course, desirable in any planar antenna array, and imply minimal side-lobe illumination.

The antenna arrays used in SLAR applications are among those that are required to meet the strictest standards in manufacturing and performance. It is therefore not surprising that the closest prior art to the present invention is a SLAR antenna. Indeed, as will be seen later when describing the preferred embodiment, the latter was realized to physically fit into the same antenna radome.

One existing SLAR antenna comprises sixteen horizontal waveguides, in a single plane each of which is approximately seventeen feet long. The planar front surface of the waveguide array shows the slotted narrow side of the waveguides. The slots are what is known in the art as "edge-wall" slots. The array's waveguides are fed by a tree of T-splitters. As will be appreciated, it is difficult to maintain the waveguide width to within the required extremely narrow tolerance due to the extreme length of the waveguide, particularly because there are sixteen waveguides which could deviate from the nominal and important broad-face width at random. In addition, a substantial support structure is necessary, which, in any event can not provide the uniformity required for a well-shaped beam. But even the support structure would not mitigate non-uniformities inherent in machining a seventeen foot waveguide. Note that the radiating slots in the waveguides are placed approximately half-wave length apart (at X-band about 1.5 cm) and any deviations from their ideal planar position causes beam distortions, which directly affect range and azimuth resolutions. Ideally, each slot must radiate from its appointed relative position within the array the correct amount of power in the correct phase, in order to produce the desired far field illumination pattern.

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