br Results and discussion br Conclusions

Results and discussion
Conclusions
Acknowledgement
Introduction LIDAR based on laser beam scanning can be applied to several detection and ranging fields, including obstacle avoidance in aerospace navigation [1] as well as real-time surveillance of restricted areas. For example, LIDAR can be used in port areas security to detect crafts in rapid approach, which are not easily revealed by passive optical systems at night, also considering that RF Radar systems can fail in case of non-conductive or small boats. Many laser modulation techniques can be applied, obtaining different measurement ranges and resolutions: The CW-IM-technique is generally implemented by using analog electronic circuits or optical system. Despite its greater operative frequency which can allow a higher resolution, the use of an analog implementation reduces the flexibility and the robustness of the obtained equipment, and does not enable the application of powerful processing techniques that can improve the performance in presence of multiple echoes with very different amplitudes. On the contrary, a digital approach is able to exploit these techniques, increases the cmv and reduces the complexity of the assembling [9]. As a consequence, the resulting devices have reduced costs and increased reliability. The paper is organized as follows: in Section 2 the algorithm is briefly discussed, while in Section 3 the architecture of the fast prototype is illustrated. Section 4 contains a discussion on the digital implementation of the proposed algorithm. Section 5 describes the experimental results of a first version of the LIDAR electronic system, while Section 6 contains the preliminary electrical test of the second release. The last section contains the conclusions and the possible future activities.
CW-IM algorithm CW-IM LIDAR technique is based on linear complex chirp signalwhere fr = Δfreq/T is the increasing rate of chirp frequency, T is the sweep duration and Δfreq=stop_freq-start_freq is the chirp bandwidth. The laser beam is modulated with the component The echo signal S(t) at the output of the photodiode that receives the lights backscattered from the targets is given by S(t) corresponds to RQ(t) delayed by time of flight Δt = 2⋅D/C (where D is the target distance and C is the speed of light). The amplitude of the echo AR depends on the target material, the angle of incidence and the distance. The product can be expressed aswhere The high-frequency terms CIH and CQH are 2 chirps with double chirp rate and different start and stop frequency with respect to R(t). The most of CIH and CQH signals are removed from the C(t) signal using a complex low-pass filter, as shown in Fig. 1. The remaining low-frequency terms CIL and CQL are 2 sinusoids at frequency , they depend on the time of flight Δt. The instantaneous frequencies of the previously described signals are shown in Fig. 1. In case of multiple echoes with different delays , Palindrome is possible to know the amplitude A of the single echo by analyzing the module of the Fast Fourier Transform ( FFT ) of the (CIL+i CQL) signal.
Architecture of the fast prototyping system Two fully digital CW-IM LIDAR electronics have been developed starting from Field Programmable Gate Array (FPGA) fast prototyping system [10]. The first release (see Fig. 2) is based on Stratix II EP2S60 DSP Development Board presenting the following characteristics:Moreover, the hardware contains a signal conditioning circuitry and an optical interface composed by a laser diode and a photoreceiver.
Implementation of CW-IM LIDAR algorithm on digital hardware The proposed algorithm has been implemented through the digital processing of the signals; the main limitations are due to the sampling frequency (Fs) of the A/D converter. If compared with the conventional analog implementations we obtain the following advantages: