Grating lobes suppression in stepped-frequency pulse train

Abstract

Frequency stepping is one of the known techniques employed by modern radars to achieve high range resolution. The main advantage of this approach is that the actual instantaneous bandwidth of a radar is quite small compared with the total processing bandwidth. This allows the transmission of waveforms with extremely wide overall bandwidth (and, as a consequence, the attainment of high range resolution) without the usage of the expensive hardware needed to support wide instantaneous bandwidth. Unfortunately, the autocorrelation function of uniform frequency stepped pulse trains can have undesirable peaks, known as "grating lobes." We address this issue, using an approach which allows us to suppress grating lobes below a desired threshold level in the case of appropriately chosen stepped frequency waveforms. We discuss in detail how to choose relevant parameters in order to produce such waveforms with small grating lobes, and give examples of waveforms with small overlap ratio. We also discuss the issue of high sidelobes in the vicinity of the mainlobe, which are inevitable in a stepped frequency train of linear frequency modulated (LFM) waveforms with small overlap ratio, and show that it is possible to suppress these, as well as the grating lobes, by means of nonlinear frequency modulation (NLFM).