Resolution and Synthetic Aperture Characterization of Sparse Radar Arrays

As evidenced by recent literature [1–4] and by the theme of a recent radar conference, 2001: Radar’s Odyssey into Space [5], there is currently much interest in moving radar technology onto spaceborne platforms. The advantages of moving radar into space are numerous [3, 6–7]. First, spaceborne radars provide global coverage, as opposed to airborne radars that are limited by airspace restrictions. Satellite radars may also reduce the amount of personnel needed to support surveillance operations, since no on-board crew is necessary. Furthermore, support personnel and the radar asset are not as vulnerable to military threats as they would be on an airborne platform. One proposed concept for a space-based radar (SBR) system is to place multiple transmitters and receivers into space, each on their own, small satellite [8–11]. These satellites, called microsats, would fly in a formation called a satellite constellation. Each satellite in the constellation would be able to coherently sample the signal transmitted from each of the transmitters in the constellation. In this way, the constellation would work as a single, virtual radar able to operate in multiple modes including interferometric, synthetic aperture radar (SAR), and moving target indicator (MTI). The advantages of a microsat constellation are numerous [2, 8–9, 12–14]. First, it may be less expensive to launch several microsats than to launch a large satellite with the same overall antenna aperture due partly to the possibility of using microsats to optimize a launch vehicle’s payload capacity. Manufacturing costs are also reduced through the benefits of mass production. In addition, a microsat constellation degrades gracefully as individual microsats fail, either as expected or prematurely, and the constellation can be reconfigured to optimize its configuration after a failure [15]. Failure in a monolithic satellite, however, is catastrophic for the entire radar system. When microsat failure causes system performance to fall below an acceptable level, the hardware and launch costs of a few replacement microsats are much less than an entire replacement system. Furthermore, microsat replacement can be stretched out over time as funding becomes available, and system upgrades can be performed by either replacing failed microsats or augmenting constellations with new, upgraded microsats. The main disadvantage of a microsat constellation is that the constellation will constitute a sparsely populated, nonuniform, three-dimensional array. Although planar, regularly spaced, tightly packed satellite clusters have been discussed, the fuel requirements for maintaining such a constellation are prohibitive. Consequently, radar signal processing algorithms previously developed for SAR and MTI are not applicable. The traditional