## 2 Robot Environment Perception for Navigation
### Vision and Ranging Sensors
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Techniques** & **Typical Sensors** & **Advantages** & **Disadvantages** \\ \hline \multirow{3}{*}{Structured light} & Kinect 1, Xtion & High accuracy and precision in indoor & Limited range, not suitable for outdoor environment due to noise from ambient light, interference from reflections \\  & SR300 and F200 & environments & and other light sources \\ \hline \multirow{3}{*}{TOF} & \multirow{3}{*}{Kinect 2} & Good for indoor outdoor applications, long range, robust to illumination changes & Lower image resolution than structured light cameras, high power consumption, cost varies with resolution, rain fog can affect sensor performance \\ \hline \multirow{3}{*}{Active infrared stereo} & RealSense R200, RealSense D435i & Compact, lightweight, dense depth images & Stereo matching requires high processing power, struggle at high occlusions and featureless environments, relatively low range especially outdoors \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Depth sensor modalities. ments in different environments. Rotating LiDAR imagers were the first type to successfully achieve acceptable performance using a rotating-mirror mechanical configuration with multiple stacked laser detectors [47]. New trends in LiDAR technology are in the development of low-cost, compact, solid-state commercial LiDAR sensors [48; 49]. The three most widely used LiDAR techniques utilise pulsed, amplitude-modulated, and frequency-modulated laser beams. The most common commercially available method is to use a pulsed laser beam, which directly measures the time taken by the pulsed signal to return to the sensor. In such sensors, time measurements require resolutions in picoseconds (high-speed photon detectors). Therefore, the cost of pulsed LiDAR sensors is comparably higher than the other two methods for equivalent range and resolution [50]. The range of a pulsed LiDAR is limited by the signal-to-noise ratio (SNR) of the sensor. Pulsed LiDAR sensors are suitable in indoor and outdoor environments due to their instantaneous higher peak pulse contrast to ambient irradiance noise. However, Amplitude Modulated Continuous Wave (AMCW) LiDAR sensors with the same average signal power have lower continuous wave peaks and, hence, are vulnerable to solar irradiance. AMCW LiDAR sensors are popular in indoor robot applications due to low SNR. In general, there is no significant depth resolution difference between pulsed LiDAR and AMCW LiDAR, but pulsed LiDAR may outperform AMCW LiDAR accuracy at the same optical power levels because of higher SNR. Increasing the modulation frequency of AMCW LiDAR sensors improves depth resolution but reduces ambiguity distance. Thus, pulsed sensors have higher ranges compared to AMCW sensors. AMCW sensors also have slow frame rates relative to pulsed sensors and usually underperform in dynamic environments. Frequency Modulated Continuous Wave (FMCW) sensors have higher depth resolution in comparison to pulsed and AMCW LiDAR sensors. These sensors can measure the depth and velocity of targets simultaneously, a highly advantageous feature for the autonomous vehicle industry. These sensors can also avoid interference from other LiDAR sensors because of the frequency modulation. FMCW sensors, however, require accurate frequency sweeps to generate emitter signals, which is a challenging task. FMCW sensor technology has been in continuous development but has not yet established itself at the commercial level. A summary of these three LiDAR technologies is provided in Table 3.  
The concept of robotic perception pertains to various robotics applications that utilise sensory information and modern deep learning methods. These applications include identifying objects, creating representations of the environment, comprehending scenes, detecting humans/pedestrians, recognising activities, categorising locations based on meaning, scene reconstruction, and others. Towards the goal of fully autonomous robot navigation, robust and accurate environmental perception is a necessity.