Multi-Channel Receiver Concepts for RADARSAT-2 Ground Moving Target Indication

It has been recognized that a two-aperture approach to ground moving target indication is sub-optimum and that target parameter estimation is often compromised by clutter interference or poor signal-to-clutter ratios. This paper explores several concepts for increasing the spatial diversity of RADARSAT-2 by exploiting the very flexible programming capabilities of the RADARSAT-2 antenna, allowing the two-channel SAR system to operate as a three or four channel radar. The realisability of these concepts is demonstrated by both simulation and experimental data gathered by the airborne experimental Environment Canada Convair 580 C-band SAR. 1 RADARSAT-2 Multi-Channel Concepts Efficient ground moving target indication (GMTI) and target parameter estimation can be achieved only after sufficient suppression of interfering stationary clutter, particularly for space-based SARs with typically small exo-clutter regions. In its simplest form, this is accomplished using two radar receiver channels, such as the dual receive antenna mode of RADARSAT-2’s moving object detection experiment (MODEX). In this mode of operation, the full antenna is broken up into two subapertures with two parallel receivers to create two independent phase centers. It is well known, however, that two degrees of freedom are not enough to simultaneously suppress the clutter and to accurately estimate the target’s properties, such as velocity and location [1]. This deficiency has motivated one to explore means of increasing the spatial diversity for RADARSAT-2. 1.1 Multi-channel modes The proposed methods takes advantage of the flexible programming capabilities of the RADARSAT-2 antenna to generate third and fourth virtual channels, as illustrated in Fig. 1. The spatial diversity can be increased either by transmitter toggling between pulses, Fig. 1(a), or via intelligent receiver excitation schemes, Figs. 1(b) and (c). These are only a few ways for achieving multi-channel capability and are by no means exhaustive. Due to transmitter/receiver toggling between pulses, the pulse repetition frequency is cut by one half. This may lead to clutter band aliasing (nonNyquist sampling) if not compensated for by doubling the original PRF. The transmitter toggling approach (between fore and aft sub-apertures), Fig. 1(a), has the advantage of maintaining the same phase-center distance as the dual-channel case, which is 3.75 m for the RADARSAT-2, and is capable of generating three phase centers, shown as upsidedown black triangles. The red arrows denote the transmitter/receiver physical center positions. The two-way beamwidth is significantly broadened (see Fig. 2) for the toggle-transmit case, compared to the dual-channel case, due to the half-aperture transmit and the half-aperture receive. This could potentially lead to some clutter band aliasing even at RADARSAT-2’s maximum PRF, which is 3800 Hz. However, calculations seem to indicate that the 3-dB main-beam can be adequately (Nyquist) sampled at 3112.56 Hz, which is well below the maximum PRF limit of the radar. The transmitter toggling will, however, decrease the transmit power and hence may severely limit the required signal-to-noise ratio (SNR).