## 2 Literature Review
### Vehicle and Equipment Tracking Using Ultra-Wideband Radios  
In addition to monitoring workers, tracking heavy vehicles and equipment is also critical for automated safety monitoring in road construction environments. There have been several classes of studies on utilizing UWB radios for equipment tracking that can be categorized into relative tracking, absolute tracking, and tracking with auxiliary sensors. In the case of relative tracking, since only the relative position of UWB radio nodes is used, no infrastructure needs to be installed in the environment. Researchers used the idea of relative ranging to avoid collision of tower cranes [39]. Each crane can be equipped with one or more UWB tags. Any two tags which are not on the same crane can be paired in the monitoring system. If any range measurement between any pairs reports a value below a specific threshold, the monitoring system alerts the operators. Building a construction safety system is possible using only the range measurements that monitor the distance between workers and vehicles in construction sites [40]. Furthermore, using two tags on each vehicle and measuring the range between any possible pair of tags across vehicles can provide a relative positioning system for each vehicle along with its relative speed and direction [41].  
Absolute tracking systems require the installation of UWB infrastructure which consists of fixed anchor nodes placed on known positions in the environment. These systems estimate the location of the entity in the field instead of their relative distances. Tracking the location of entities provides more information about the construction environment which is beneficial for some safety tracking applications. Absolute tracking systems have previously tracked the location of the vehicle by installing one UWB radio on the vehicle [42]. However, tracking the orientation of vehicles requires more than one tag [17; 43; 44]. The presence of vehicles and equipment in construction sites may distort the UWB signals sent from the tags. They can also block anchors and prevent them from receiving signals. Despite the high accuracy of UWB localization, their error rises when they are exposed to these deformations or obstructions of the signal [20].  
To reduce the localization error of UWB-based tracking systems, tracking with auxiliary sensors fuses the information obtained from UWB radios with other auxiliary sensors, such as Inertial Measurement Units (IMU). The goal of these approaches is to reduce the localization error by fusing the output of multiple sensors. Research studies in this area either used some variations of the Kalman filter [23; 26; 27; 28] or particle filter [29] to fuse UWB information with other sensing data. Fusing UWB and IMU is one of the popular techniques in this area [23; 26; 28; 29]. Another research study combines IMU, UWB, and real-time kinematic (RTK) positioning (satellite navigation system using signal phase information) [27]. However, fusing data from auxiliary sensors requires reasonable assumptions about the environment, objects, and the trajectory of the movements which cannot be made for construction environments. Therefore, past approaches using auxiliary sensors [23; 26; 27; 28] cannot be used to develop a practical equipment-tracking solution for construction projects.  
To mitigate the localization error of the UWB radios in absolute tracking systems without the need for auxiliary sensors, our previous work called ViPER [44] adapted an error correction mechanism to detect and reduce the error created by the distorted signals using a low-pass filter (LPF). Despite some improvements achieved in this previous work, the design of ViPER has a critical limitation which is the reliance on the continuous flow of data from anchors to correct erroneous inputs which can easily fail when heavy objects completely obstruct radios for several seconds.