Bharatheeyan,

Here is an article that seems to be explaining 360 coverage for Erieye and the degraded performance on aft and fore of beam structure.Not too technical but i guess that should give you an idea what to look for if you wanna investigate further

http://www.ausairpower.net/aew-aesa.html

Another configuration derived from this idea is that of the Swedish Ericsson Erieye, which uses a two sided array in a beam shaped structure, carried above the fuselage of a twin engined commuter airframe. The two sided array used in this arrangement is almost as long as the APY-2 antenna of the AWACS, potentially providing similar angular resolution performance at range, on a very small airframe.

This arrangement however suffers from an obvious and significant operational limitation, as it cannot provide 360 degree coverage, using conventional active phased array technology. With each array scanning a 120 degree sector, the two sided array has a 60 degree blind sector over the nose and the tail of the aircraft, and degraded antenna performance beyond 45 degrees off the beam of the aircraft. With Sweden’s compact geography this would probably not be an issue, as multiple platforms would cover a single area, and operating in pairs, the aircraft could patrol in two racetrack orbits set 90 degrees apart to provide overlapping coverage. The success of this scheme then devolves down to the capability of the computer datalink networking which links the platforms to each other or the ground air defence centre, to ensure that a comprehensive picture of the air situation exists at whatever is the central command post.

In a heavy ECM environment, where platform to platform or platform to ground datalink function is interfered with, the two sided array has thus a major limitation. Producing a three or four sided array with similar array length results in a structure with a size comparable to an E-3 AWACS radome, which in turn requires at least a 737 sized aircraft to carry it, thereby largely defeating the apparent cost advantage of the linear array concept.

A possible resolution would be the use of a supergain array, where the ultimate size of the blind sector would be determined by the array’s module parameters and array length.

Another alternative which exists is the use of a rhombic four sided array geometry, with a 60-120-60-120 degree arrangement of arrays. While the rhombic arrangement will provide full 360 degree coverage, its effective antenna length is halved in the nose and tail sectors. The result is a compromise between the bulky but excellent four or three sided array, and the compact but partially blind two sided array. No publicly discussed proposals to date have involved the rhombic arrangement.

EDIT:*** My bad you said planar phased array and not just planar array***
Also this source seems to be describing Erieye as Active Phased array in which beam can change direction unlike the planar array in which beam has fixed direction. You have repeatedly charcterized Erieye as planar array. Just wondering if there is something that you know contrary to the source (i.e Erieye’s beam cannot change direction)??