Integrated Sidelobe Level Optimization for the Airborne Phased Array Radar (APAR)

The National Center for Atmospheric Research (NCAR) has proposed an Airborne Phased Array Radar (APAR) to replace the recently retired ELDORA (ELectra DOppler RAdar) system. The APAR approach places four large C-band phased array radars on a C-130 airplane, permitting observations aft, up, and to both sides. A primary mode of operation directs side-looking beams tilted slightly fore and aft, allowing the retrieval of both radial and tangential wind speeds as the plane flies past a weather system. One of the specifications is that the antenna two-way integrated sidelobe ratio (ISLR) must not exceed -65 dB. Furthermore, a desirable operational goal of the system is to observe weak weather details close to the ground, which may require additional suppression of sidelobes in the direction of ground clutter. The present study compares several approaches to array sidelobe suppression, and presents results suggesting how weak, and how close to the ground, weather observations may be made.

We consider one of the side-mounted phased arrays which fits into a 1.9 m tall x 1.8 m wide elliptical airframe cutout, and employ NCAR’s proposed array configuration consisting of 44 panels, or line replaceable units (LRU’s), each containing 8x8 elements placed on a rectangular grid. Using rectangular LRU’s results in a discretized approximation to the available elliptical aperture shape, but is an economical approach to antenna fabrication and maintenance. To take a concrete example, we observe weather targets at a distance of 5 km while flying at 3,500 m altitude. The baseline for comparison uses traditional amplitude weighting such as a 30 dB Taylor taper applied during reception using programmable attenuators at each element, and a 20 dB Taylor taper applied during transmission by using statistical element thinning whereby certain elements are turned off (“thinned”) to approximate a continuous amplitude weighting.

The baseline results are compared to a new approach that uses a genetic algorithm to optimize the taper on receive and the thinned aperture on transmit. Optimization has been used for both array thinning and amplitude tapering, for either transmit or receive apertures individually, but our approach simultaneously optimizes the transmit and receive apertures of a radar array in order to improve the two-way ISLR. Theory and results applicable to APAR system performance will be presented.