On Range Ambiguities From Elevation Grating Lobes in Ship Detection RCM SAR Imagery

As a 2-D spatial filter, the unidirectional electronically steerable planar phased-array synthetic aperture radar (SAR) antenna of the radar constellation mission (RCM) samples the space-time process in azimuth and elevation, respectively, with spatial intervals and visible region defined by the corresponding interelement spacings and wavenumbers. Since position and wavenumber are conjugate Fourier transform variables, interelement spacings need to abide by the Nyquist-Shannon criterion. Furthermore, since the antenna's response is a 2 pi-periodic function, it produces an infinite number of periodically spaced peaks that are equal in strength to the main lobe peak. If any of these peaks is shifted to the visible region, due to azimuth and/or elevation steering, it becomes a grating lobe that warrants attention. This article investigates grating lobe ambiguities due to elevation steering (E-GLAs) in ScanSAR RCM imagery. E-GLAs resemble legitimate targets and they pose a challenging source of false alarms (FAs) for operational ship detection. Using theoretical analysis, simulations, and a real world Ship Detection dataset, it is demonstrated that E-GLAs become visible for off-nadir angles >= 50.38(degrees). A novel algorithm is presented to automatically locate E-GLAs in repeat-pass images. Efficacy of the algorithm is demonstrated by tracking a verified E-GLA in a stack of 26 x 2 RCM images near Gal & aacute;pagos Islands, processed using downlinked and definitive satellite orbits, respectively. Results based on definitive orbits are shown to outperform their downlinked counterparts.