## 4 Experiment Results  
Besides the simulations, the relative experiments are performed as well to verify the proposed approach for pose estimation based on the laser radar system. The schematic diagram of the micro-Doppler detection system is displayed in Figure 8. The system operates based on a coherent laser radar, and optical fibers are used throughout the system from the laser to fiber collimator I and from fiber collimator II to the photodetector. The output power of the optical source is 100 mW, and the laser wavelength is 1064 nm. The laser beam from the fiber laser is directed through a 10/90% fiber optic splitter. Then, the output of 90% laser beam is launched into fiber collimator I. The other output from the coupler is directed to the photodetector via an acoustic-optic modulator and optical combiner, where it is combined with the 10% end of the fiber optic splitter, and the final output serves as the reference light of interference. The diffused light from the dynamic target is collected by the light-collecting system and is coupled into fiber collimator II. The collecting diffused light, together with the reference light, generates the interference on the balanced detector, and then the output signals are transmitted to the digital storage oscilloscope. Through the GPIB interface, the oscilloscope is connected to the computer, and the data collection and final processing are operated by the computer. Figure 6: Estimation result of the attitude angle of the target with unknown size. (**a**) Variation of the absolute error of the attitude angle with the attitude angle. (**b**) Variation of the absolute error of the attitude angle with the spin rate. Figure 7: Estimation error of the side length of the cube-shaped target (SNR = 10 dB). In our experiment, a continuous current (DC) motor is employed as a tool to drive the rotating target and makes it perform a low-speed and uniform motion. Thereby, the experimental data consisting of rotational motion is obtained. In the coherent radar, the phase on the target's return signal is sensitive to motion and micro-motion in the range. A half wavelength's change in radial distance can induce a 360\({}^{\circ}\) phase change. Therefore, the Doppler frequency shift, which stems from the change in the phase function with time, can be utilized to detect vibrations or rotations of structures in the target by the return signal. In the experiment, as the target is spinning driven by the movement of the DC motor, the phase change with time results in the special Doppler frequency shift [6,15]. The interference caused by the joint modulation of the rotational motions translates into electrical signals via the photodetector, and the electrical signals are transmitted to the digital storage oscilloscope. Eventually, the data collection and final processing are managed by the computer. In order to examine the feasibility of our method, the cube surface composed of different materials (i.e., resin, aluminum alloy, and fiber-reinforced polymer) is investigated. The target size is selected as 5 mm \(\times\) 5 mm \(\times\) 5 mm to match the size of the laser spot. The reduction of the target leads to a decrease in the Doppler frequency shift. To ensure the resolution ability in the time and frequency domain, the frequency resolution and temporal resolution are set as 2 kHz and 5 ms, respectively. In addition, other parameter settings are displayed in Table 3. Figure 9 shows the TF diagrams of echo signals of the cube targets with the surface made of different materials for an attitude angle \(\alpha=45^{\circ}\) and a spin rate of 1.4 Hz. It can be seen that due to the different laser scattering properties of different materials, the color of the TF diagram is quite different. Among them, the vertical patterns periodically appeared in Figure 9b stem from specular reflections caused by the surfaces of the aluminum alloy. It should be noted that the outlines of the three TF diagrams are not much different. In particular, there is almost no gap between the outlines of Figure 9a,c.