## 1. Introduction

[2] Over-the-horizon radar (OTHR) has been used in the detection and tracking of aircraft targets and surface ship targets in wide-area surveillance at long ranges [see, e.g., *Headrick and Skolink*, 1974; *Trizna and McNeal*, 1985; *Barnum*, 1986; *McNeal*, 1995; *Headrick and Thomason*, 1996; *Root*, 1998a, 1998b, 1998c; *Anderson and Krolik*, 1998, 1999; *Georges and Harlan*, 1998]. In OTHR, target detection is performed at each radar dwell from the radar return signals, and target tracking is then performed by modeling target motions based on the detection across a sequence of dwells. The antenna array in OTHR is always used to detect the direction of a target, which helps the detection and tracking. The detection of slow targets is often difficult with OTHR, due to the spread of the ground or ocean clutter. The existing OTHR algorithms are based on the assumption that the Doppler frequency of each target is constant or approximately constant (i.e., the motion of the target is uniform) during each dwell. A two-dimensional Fourier transform is taken of the received signal. Targets are detected from amplitude peaks away from the zero frequency. The detection capability of an algorithm depends on the SCR or SNR and the Doppler resolution. Since the Doppler resolution depends on the length of the coherent integration time (CIT), if a short CIT is used, then the target spectral power cannot be separated from the clutter spectral power. In order to improve the SCR and Doppler resolution, some algorithms for clutter cancellation, such as the adaptive Fourier transform based clutter rejection method recently proposed by *Root* [1998a, 1998b, 1998c], and superresolution spectrum estimation algorithms have been used, for example, by *Trizna and McNeal* [1985], *Barnum* [1986], *Root* [1998b], and *Barnum and Simpson* [1997].

[3] For a maneuvering target, there is a trade-off between the CIT length, SCR, and the Doppler resolution in the existing Fourier-based techniques. For a slow or uniform moving target, such as a ship, the Fourier-based techniques work well, where a nonshort CIT can be chosen for suppressing the clutter spread. However, for a fast maneuvering target, such as an aircraft and a fast boat, the Fourier-based techniques may not work well as we will see later in simulations. In this paper, we propose an adaptive chirplet transform in the Doppler processing as an alternative of the Fourier transform, where the sinusoidal signal model is replaced by the chirplet signal model because the radar return signals from maneuvering targets have chirplike characteristics. With the adaptive chirplet transform technique, the CIT can be longer and therefore the Doppler resolution may be better than that in the Fourier transform techniques. Since the SCR is low, before implementing the adaptive chirplet transform, we first use the adaptive Fourier transform proposed by *Root* [1998a, 1998b, 1998c] to reject the clutter.

[4] For the chirplet transform, see, for example, *Mann and Haykin* [1995], and for the adaptive chirplet transform, see, for example, *Wang et al.* [1998], *Bao et al.* [1998], *Qian et al.* [1998], and *Wang and Bao* [1999]. The adaptive chirplet transform can be thought of as a time-frequency analysis (TFA) technique. TFA has been found useful in the inverse synthetic aperture radar (ISAR) imaging for a maneuvering aircraft by *Chen* [1994] and [1995], *Chen and Qian* [1998], *Chen et al.* [1996], and *Trintinalia and Ling* [1997] and has also been applied in SAR by *Chen and Miceli* [1998]. This paper is organized as follows. In Section 2, we briefly review the OTHR signal model described by *Anderson and Krolik* [1999] and describe the problem of interest in this paper in more detail. In Section 3, we propose the adaptive chirplet transform for OTHR. In Section 4, we present simulation results. In Section 5, we compare the adaptive chirplet transform with the conventional Fourier transform for the detection of moving targets by applying both of them to a set of real OTHR data received from 372 antenna array elements.