## 1 Introduction  
As a new type of spacecraft, CubeSats have become an important part of satellite systems. However, in space, CubeSats often fail due to electromagnetic interference, particle radiation, space collision, and so on. On-orbit inspection and maintenance are required to repair malfunctioning satellites. Among them, pose measurement and motion estimation are the primary links of satellite on-orbit servicing. The CubeSat target may have mechanical vibration or rotation alongside bulk translation, which causes great difficulties for the pose measurement and motion state estimation of space targets [1, 2]. It is worth noting that when there is relative motion between the radar and the target, the carrier frequency of the return signal will be shifted. This phenomenon is known as the Doppler effect [3]. The Doppler frequency shift is associated with the transmitting wavelength and the relative velocity between the radar and the target. The Doppler effect is capable of providing rich information about the target's motion, and it has been widely utilized in various radar applications such as moving-target detection and cluster target resolution [4, 5].  
The target or any structure on the target may have mechanical vibration or rotation in addition to its bulk translation; for instance, the rotation of helicopter rotor blades, the swing of a walking person, or the precession of a cone-shaped target. These motions might induce frequency modulation of the returned signal, which generates sidebands about the target's Doppler frequency shift. This is called the micro-Doppler (MD) effect [6, 7, 8]. Previous studies have revealed that the micro-Doppler effect is relevant to the properties of the target and able to provide detailed information about the target [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Specifically, parameters consisting of spin rate, precession rate, precession angle, and inertia ratio can be estimatedbased on micro-Doppler effect analysis. Accordingly, exploiting the micro-Doppler effect is significant for the application of target identification and parameter estimation.  
In the field of micro-motion feature detection, the research of other scholars is mainly based on electromagnetic waves in the microwave frequency band [20; 21; 22]. Note that for short-wavelength electromagnetic signals, even micro-vibrations with very low vibration rates can induce large phase changes. Therefore, micro-Doppler frequency modulation or phase change with time can be easily detected. A coherent laser radar operating at a shorter wavelength can achieve better velocity precision than that operating at a longer wavelength [6]. As a consequence, a coherent laser radar operating at a short wavelength is needed to obtain high-precision motion estimation. However, micro-Doppler signals with area scattering modulation, rather than multiple scattering centers for microwave cases, will be encountered. For the microwave case, motion estimation can be carried out based on the periodic curves in the TF image [17], while for short-wavelength cases, an appropriate method is required to extract parameters based on the structured broadband (see Section 2) in the TF image caused by area scattering. Therefore, in this paper, a novel framework is proposed to estimate the parameters of CubeSats with micro-motion (i.e., spin motion) based on the micro-Doppler effect by a laser radar system. In particular, the spin rate is estimated based on the 2D Fourier transform of the time-frequency data. The attitude angle can be obtained effectively after threshold processing. The target size is identified through bistatic radar operations. Finally, the proposed method is verified by simulations and experiments. The scheme developed in this work may provide new technical means for the detection of non-cooperative targets and space debris in the future.