# Method for Accuracy Assessment of the Length Measurement Unit of Laser Tracking Systems
## 1 Introduction  
In accordance with the assumptions of the Industry 4.0 concept, increasing production efficiency is to take place, e.g., by optimizing the quality control process [1]. Metrology of geometrical values also follows this trend by improving both the methods and technology. Part of the geometric values of metrology is coordinate metrology, which has been developing very dynamically since the end of the 20 th century. A specific part of this field of science is large-scale metrology, which is used where standard coordinate systems are not able to meet the requirements set for them. Devices with the largest measurement area use laser interferometers in their operation. Skillful use of the phenomenon of wave interferometry, through observation and counting of the interference fringes, allows for performing length measurements in the measurement space of up to several dozen meters. One of the most popular systems in the sector that uses a laser interferometer is the laser tracker. This system uses an interferometer to determine the distance of the measurement point from the reference point. There are solutions that use an interferometer (IF) and a fixed reference point (birdbath), but also those that use an absolute distance meter (ADM) instead of a reference point. The latest systems now use only ADM fordistance measurements. Appropriate aiming of the laser beam towards the measuring point is achieved by means of a rotating mirror or the right angular setting of the entire interferometer. In both cases, rotation in two perpendicular axes is performed by integrated drives. The angular positions of the mirror (or interferometer) are read by angular encoders and most often marked as the horizontal angle \(H\) and vertical angle V.  
Like any measuring device, laser tracker systems should be periodically checked and calibrated. A calibration certificate issued by appropriate institutions is a guarantee of the correct operation of the device and allows for assuming that the system performs measurements with the expected accuracy. Institutions dealing with such activities are either state-owned bodies responsible for ensuring measurement consistency at the national level (e.g., in Poland, the Central Office of Measures, in Serbia, the Directorate of Measures and Precious Metals), or bodies with an appropriate accreditation certificate (such as the Laboratory of Coordinate Metrology at the Cracow University of Technology). Several facilities around the world are engaged in research on methods of assessing accuracy in large-scale coordinate metrology. These include the University of North Carolina, where the general concept of laser tracking system is given by [PERSON] et al. in [2] as well as its application in industrial robot accuracy assessment, while in [3], [PERSON] focuses on the dynamic errors of laser trackers which occur during measurements of a moving target; the National Physical Laboratory, where, in [4], the methodology of obtaining a geometrical model of tracker errors is presented by [PERSON] et al., which is based on measurements of a target set in predefined positions from different tracker locations, while [5] describes practical aspects of trackers calibration; the University of Zaragoza, including method for the optimization of tracker location during machine tool verification, which is presented by [PERSON] et al. in [6], and a technique for forecasting the optimal distribution of trackers during multilateration measurements is shown by the same team in [7]; the Cracow University of Technology, where [PERSON] et al. in [8] discuss the influence of point averaging which occurs during laser tracker measurements on the measurement uncertainty, while in [9], a tracker is treated as a reference instrument for checking the accuracy of large-scale optical measuring systems and others. Moreover, there are valuable scientific papers that fully or partially present the discussed issues together with the results of the research carried out.